Bogin Evolution 2

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Evolutionary Hypotheses for Human ChildhoodBARRY BOGI N

Dep ar tm en t of B eha vi ora l S cien ces, Un iv ersi ty of Mi chi gan -Dea rbor n,Dea rbor n , M ich iga n 48 128

KEY WORDS childhood; heter ochr ony; neoteny; insert ion m odel

AB S T R ACT The or i gi ns of h uman childhood have fascinated scholar s

f r om many discipl ines. Some r esear cher s ar gue that childhood, and many

other human characteristics, evolved by heterochrony, an evolutionary pro-

cess that alters the timing of growth stages from ancestors to their descen-

dants. Other scholars argue against heterochrony, but so far have not offered a

well-developed alternative hypothesis. This essay presents such an alterna-

t i ve . C h i ld h ood i s d e fin e d a s a u n i qu e d e ve lop m e n t a l s t a ge of h u m a n s .

Childhood is the p eriod following infancy, when t he youn gster is wean ed fromnur sing but st i ll depends on older people f or feeding and pr otection. The

biological constr aints of childhood, which include an immat ure dentition, a

small digestive system, an d a calorie-deman ding brain tha t is both r elatively

l a r ge a n d g r ow in g r a p id ly, n e ce s si t a t e t h e ca r e a n d fe ed in g t h a t ol de r

individuals m ust provide. Evidence is present ed th at childhood evolved as a

new stage hominid life history, first appearing, perhaps, during the time of

H om o h ab il is . The value of childhood is often ascr ibed to lear ning many

asp ects of hu ma n cultu re. It is certa inly tru e th at childhood provides extra

time for brain development and learning. However, the initial selective value

of childhood may be more closely related to parental strategies to increase

repr oductive success. Childhood allows a woma n t o give birth t o new offspring

and provide care for existing dependent young. Understanding the nature of

childhood helps to explain why humans have lengthy development and low

fertility, but greater reproductive su ccess t han any other species. Yrbk PhysAnth ropol 40:6389, 1997. 1997 Wiley-Liss, Inc.

Behold the Child among his newborn blisses,

A six-year sDar ling of a Pygmy size!

See, where mid work of his own h an d he lies,

Fr ett ed by sallies of his m other s kisses,

With light upon h im from h is fat her s eye

W. Words worth: In tima tions of Imm orta lity (18021804)

Childhood fascinates scholars and practi-

tioners from man y disciplines. Virtu ally all

hum an cultures recognize a time of life th at

ma y be called childh ood. Man y h ist orical

s ou r ce s fr om E g yp t ia n t i m es t o t h e 1 9t h

centu ry, including Wordsworth in the poem

above, me nt ion th at childh oodoccupies th e

first 6 to 7 years of life (Boyd, 1980). Some

explanations for the origins and functions of

childhood have been proposed, but none of

these is accepted univer sally. Per haps the

l a c k o f a g r e e m e n t i s d u e t o t h e n a t u r e o f

hu ma n evolutiona ry biology. Allison J olly,

aut hor ofTh e Evolution of Primate B ehavior,

stat es th at hum an evolution is a pa radox.

We have become larger, with long life and

immaturity, and few, much loved offspring,

and yet we ar e more, not less a dapta ble.In

an attempt to resolve the paradox of human

evolution an d our peculiar life history J olly

YE ARBOOK OF P HYSI CAL AN THR OPOLOGY 40:6389 (1997)

1997 WILEY-LISS, INC.


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concludes in th e next sent ence tha t men ta l

agil ity buffer s envir onmental change and

ha s rep laced repr oductive agility (1985, p.

44). The r efer ence to r epr oduct i ve agili ty

means that we ar e a r epr oducti vel y f r ugal

s p eci es com p a r e d w it h t h os e t h a t l a vi sh

dozens, h undr eds, or t housan ds of offspring

on each brood or litter. However, a paradox

rem ains , for t he emph asis on bra in power

r a t h e r t h a n r e p r od u ct i on i s a n a p p a r e n t

exception t o Darwins ru les of na tu ra l selec-

tion. Evolutiona ry success is t raditionally

m e a s u r ed i n t e r m s of t h e n u m b e r of o ff-

spring that survive and reproduce. Biologi-cal a nd behavior al tr aits do not evolve un-

l ess they conf er upon t heir owner s some

degree of reproductive advantage in terms of

survivors a generation or more later. If we

are t ruly interested in our immortality, or a t

l east that of our DNA, then why do we not

pr oduce mor e offspr ing, instead of a few

men ta lly agile offspring?

Another question is, why do our offspring

tak e so long t o reach r eproductive age? After

all, our genetic immortality requires th at we

have gr andchi ldr en, gr eat- gr an dchildr en,

a n d s o o n . Ye t w e t a k e t w o d eca d es of

postnata l development to r each r epr oduc-tively successful adulthood. Moreover, our

p a t h fr om b ir t h t o m a t u r it y is s in u ou s ,

meander ing thr ough alter nat ing per iods of

rapid an d r elatively slow development . This

may be seen in t he examples of the dista nce

and velocity cur ves of gr owth for healthy

boys and girls illustrated in Figure 1. Why

do our offspring not take a more direct and

r apid path to matur ity?

T h is e ss a y a t t em p t s t o a n s we r s t h e se

q u es t ion s a n d p a r a d ox es i n t e r m s of t h e

evolution of h uman gr owth and develop-

ment. Throughout the essay the focus is on

childr en and childhood. I t is ar gued her e

t h a t ch ild h ood is a u n iq u e s t a ge of t h e

hum an life cycle, a st age n ot to be foun d in

the life cycle of any other living mammal. It

is important to define clearly what is meant

by childhood, for often th e ter ms child,

juven ile, and ad olescent are us ed int er-

changeably in the litera tur e.Childhood is defined her e as the period

fol low in g in fa n cy, w h en th e you n gster is

weaned f rom nursi ng but st il l depends on

older people for feeding and protection. In -

fants by definition ar e exclusively or par -

tially breast-fed. One survey of the world-

wide variat ion in the age for t ermina tion of

breast-feeding, that is weaning age, finds it

to occur at a median age of 36 months after

birth (Dettwyler, 1995). Th ough no longer

breast-fed, children are still dependent on

older people for feeding. Severa l biological

and behavioral cha racteristics of the young-

ster necessitate this dependency of child-h o o d . I n t e r m s o f f e e d i n g t h e r e a r e t h r e e

ma jor biological factors (Fig. 2): 1) children

continu e the r apid brain growth experienced

by inf ants and need a diet dense in ener gy

and protein to support this brain growth, 2)

children possess deciduous teeth, with thin

enamel an d shallow r oots, an d cannot pr o-

Fig. 1. Idealized mean velocityan d d istan ce cu rv es of g ro wth inh eigh t for h ealth y g irls (d ash edlines) and boys (solid lines) show-in g th e p o stn atal sta g es of th e p at-t e r n of h u m a n g r ow t h . N ot e t h espurts in growth rate at mid-child-hood and adolescence for both girlsan d b oy s. T h e stag es of p o stn atalare ab b reviated as follow s: I , in -fancy; C, childhood; J, juvenilit y;A,adolescence;M, matur e adult. Datau sed to con stru ct th e cu rv es com efrom Prader (1984) and Bock andThissen (1980).

64 Y EARBO O K O F PH Y S I C AL AN T H RO PO L O G Y [Vol. 40, 1997


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ce s s t h e a d u l t -t y pe d ie t , a n d 3 ) c h il dr e n

have r elatively small body size, and hence asmall digestive system, which l imits total

food intake, furthering the requirement for

nutrient-dense foods. These physical charac-

teristics are coupled with motor and cogni-

tive immaturity, as well as social inexperi-

ence. All of these f actor s r ender the child

dependent on older individuals for care.

The childhood stage precedes the juvenile

stage of the l i f e cycle. A juveni l e stage is

common to most species of social ma mma ls

(Pereira a nd Fa irbanks, 1993), and is a time

of feeding in depen dence from older individu-

als prior to the onset of reproductive matu-

rity (Pereiera a nd Altmann , 1985). Based ona variety of biological, behavioral, and cogni-

tive traits an d abilities tha t ar e described in

detail below, hum an children enter the juve-

nile sta ge at about 7 year s of age. By these

criteria, the human childhood stage of life

span s th e time from a bout 3 to 7 years of age

(Bogin, 1990, 1995; Bogin an d Sm ith, 1996).

More detailed evidence for this biological

definition of childhood is pr esented later in

t his essay. One point t o str ess a t t his junc-

ture, however, is that childhood is likely to

be a n ew life cycle sta ge, tha t wa s evolved de

novo into hominid life history. This view of

childhood stands in diametrical oppositiont o older, but st i ll widely held vi ews that

childhood evolved by altering the develop-

mental timing of the preexisting life stages

of o u r p r im a t e a n c es t or s . T h e t w o m o s t

popular hypotheses i n this r egar d invoke

e it h e r n e ot e n y or h y p er m or p h os is a s t h e

primar y agent of huma n evolution. Neoteny

may be thought of as a slowing down of the

rate of development. Neoteny produces anadult descendant that r etains many imma-

ture characteristics of its ancestor. Hyper-

morphosis may be defined, at this point, as

an extension of the growth an d development

period of the descendant beyond that of the

ancestor. Hyper mor phosis pr oduces a de-

s c e n d a n t w i t h f e a t u r e s t h a t a r e h y p e r m a -

tur e compared with th e ancestor. This essay

will show tha t both th e Peter P an scenar io of

neoteny and the Methuselah- like develop-

ment of hyper mor phosis ar e inadequate t o

account for the evolution of childhood. More-

over, neither neoteny nor hypermorphosis

ca n u n r a v el t h e p a r a d ox of h u m a n e vol u -tion. To resolve th e pu zzle of why we ha ve so

fe w offs p r in g , w h y t h e y t a k e s o l on g t o

develop and reproduce, and why our rate of

growth ta kes a serpent ine path t o adu lthood

r equir es a new, and mor e matur e, view of

childhood a nd its place in hu man evolution.

HUMAN ONTOGENY ANDHETEROCHRONY

Ontogeny refers to t he process of growth,

development, a nd matur ation of the i ndi-

vidual organ ism from conception t o deat h. It

is virtually axiomat ic that every species has

its own u nique pa ttern of ontogeny (Bonner,1965; Gould, 1977). During hominid evolu-

tion th e form an d function of our a ncestor s

s t r u ct u r a l a n d r e gu l a t or y D N A wa s r e -

worked to produce the genetic basis for the

ontogeny of the human species. The litera-

tur e is r eplete with pr oposals f or how the

r ewor king occur r ed. One tr adit ion in the

Fig. 2. Growth curves for differen t bodytissu es. T h e b rain cu rve is for to talweight of the brain (Cabana et al., 1993).The dent itioncurve is the m edian m at u-rity score for girls ba sed on th e seven leftm an d ib u lar teeth (I1 , I2 , C , P M 1 , P M 2 ,M1, M2)us ing the reference data ofDemir-

jia n (197 8). Th e body cu r ve r ep r es en t sg row th in sta tu re o r to tal b o d y w eigh t.The reproductive curve represen ts thew eig h t o f th e g on ad s an d p rim ary rep ro -ductive organs (Scammon, 1930).

65E VO LU T IO N AR Y H Y PO T H E S E S FO R HU M AN CH IL DH OO DBogin]


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study of human evolution looks for a single

major cause or pr ocess. I t h as been ar gued

that humans evolved when we became big-

br ained apes, ter r estr ial apes, ki l ler apes,

hun ting apes, aquatic apes, toolmak ing apes,s y m bol ic a p e s , m o n og a m ou s a p e s , food -

shar i ng apes, and, even, apes wi th ventr al-

ventra l copulatory behavior. None of these,

or an y oth er single-factor h ypothesis, proves

t o be helpf ul to under stand human evolu-

tion, for a non-human primate exception can

always be found. Anot her tr adi t i on looks

i nstead at t he pat ter n of ontogeny. I n the

book Si ze and Cycl e, J .T. Bonner (1965)

develops the idea that the stages of the life

cycle of an individua l organ ism, a colony, or

a s oci et y a r e t h e b a s ic u n i t of n a t u r a l

selection.Bonn er s focus on life cycle st ages

follows from t he research of several 19th-

and 20th-centur y embr yologist s who pr o-

posed t ha t speciation is often achieved by

alter ing rates ofgr owth of existing life stages

and by adding or deleting stages.

A hist ory ofr esear ch on life cycle evolution

w a s p u b li sh e d b y S . J . G ou l d i n t h e b ook

Ontogeny and Phylogeny (1977). Gould hand-

ily su mma rizes th e m echan isms for biologi-

cal chan ge over t ime by sta ting, Evolution

occur s when ontogeny is al t er ed in one of

t wo ways: when new char acter s ar e intr o-

d u ce d a t a n y s t a g e o f d e ve lop m e n t w it h

varying effects upon subsequent stages, orw h en ch a r a c t er s a l r ea d y p r e se n t u n d e r go

changes in developmental timing. Together,

these two processes exhau st the formal con-

tent of phyletic change. Gould conten ds

t hat i t is t he second pr ocess t hat accounts

for h uma n evolution. Th is pr ocess is called

heterochrony. Quoting Gould again, this

book is pr imar ily a long ar gument f or the

evolutionar y importa nce of heterochrony

changes in the r elative t ime of appear ance

and r at e of development for char a cter s al -

ready present in ancestors(au th ors ita lics).

Gould explai ns t hat ther e ar e sever al types

of heter ochr onic pr ocesses, but only oneaccounts for human evolution. This is neo-

ten y, defin ed in th e glossar y of Goulds book

as paed omorphosis (retent ion of forma lly

ju ven ile ch a r a ct er s by a du lt de scen da n t s)

produced by retardation of somatic develop-

ment. In a subsequent publication Gould

provides a somewhat more readable defini-

tion: In neoten y r at es of development slow

down and juvenile stages of ancestor s be-

come adult f eatur es of descendants. Many

centr al features of our an atomy link us with

the fetal an d juvenile sta ges of [non-human ]

pr ima tes . . .(Gould, 198 1, p. 333).

NEOTENY AND HUMAN EVOLUTION

T h a t h u m a n s of a l l a g e s a r e e s se n t ia l ly

child-like in morph ology, beh avior, an d cog-

nitive potential is the essence of the concept

of neoteny. Notions of human neoteny may

be tr aced back as far as biblical wr it ings

(Monta gu, 1989), an d t he concept h as domi-

n a t e d p op u la r a n d s ci en t i fic a t t i t u d es t o-

w a r d t h e e vol u t ion of h u m a n g r ow t h for

hundreds of years. William Wordsworth, for

example, praised the concept of neoteny in

1802 with words of inn ocence an d hope:My heart leaps u p when I behold

A rainbow in th e sky:

So was it when my life began;

So is it now I am a m an ;

So be it when I sha ll grow old,

Or let me die!

The child is fath er of the man ;

And I could wish m y days t o be

Bound each to each by na tur al piety.

The ter m neoten y was coined by J ulius

K ol lm a n i n 1 88 5 t o d e scr i be t h e s e xu a l

mat ura tion of the a xolotl, a u rodele am phib-

ian (salamander), while still in its aquatic,gill breat hing stage of development . Ashley

M on t a g u (1 98 9) s t a t e s t h a t K ol lm a n i n -

tended neoteny to mean reta ining youth

but that Kollman confused the Greek word

teinein (t o s t r et ch ) w it h t h e L a t in w or d

tenere (to reta in). Figure 3 pr esents contem-

porary dictionary definitions, as well as an-

ot h e r cu r r e n t m e d ica l u s a g e o f t h e L a t in

r oot of t he ter m. Despite the scatological

hum or tha t m ay be found in Kollmans ety-

mological error, Montagu and Gould believe

t h a t K o l l m a n h a d t h e r i g h t i d e a , t h a t i s ,

neoteny is th e process for hum an evolution.

T h e i d ea t h a t n e ot e ny is t h e p r im a r yprocess for humanization was first formal-

ized scientifically by Louis Bolk in 1926 (see

Montagu, 1989, for a concise review of his-

torical sources). Gould acknowledges t hat

much of Bolks neoteny is r eally an ar gu-

men t for scientific ra cism an d sexism. Never-

t h e le s s, G ou l d t r i es t o r e t a i n t h e b a by of

neoteny while discarding the bath water of



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racist and sexist science. To do s o, Gould

suggests that the major difference between

h u m a n a n d n on -h u m a n p r im a t e g r ow t h i s

t hat huma ns m atur e sexually while st i l l in

an infanti le or juvenil e sta ge of physical

development.G ou l d e xp r es s ed h i s i de a s on h e t er o-

chr ony via neoteny i n t er ms of a clock

model. The clock has two h ands , one tha t

ca lib r a te s a s iz e a r c a n d t h e ot h e r t h a t

measu res a sha pe arc. The clock sits a bove a

t ype of bar -shaped calendar that r ecor ds

time in th e form of biological a ge, tha t is, age

at s exual mat ura tion. The clock ma y be used

t o c om p a r e a n a n c es t or w it h i t s d e sce n -

dan ts. The a ncestor s clock is fixed with th e

t wo hands at the zenith of the a r cs and age

of sexual ma tur ation at the m id-point of the

calendar. The descendan ts clock measu res

changes in size, shape, and age at matur a-t ion by differ ences in these thr ee vector s.

T h e m a jor p oi n t t h a t G ou l d e m p h a s iz es

with th e clock model is that size, shape, an d

ma tu ra tion are disassociable from each oth er

and can evolve independently. Figure 4a is

Goulds clock for hu ma n evolut ion via n eo-

teny. Hu man s h ave lar ger size (body, brain,

e t c.) a n d m a t u r e a t a l a t er a g e, c om p a r e d

with our an cestor, but reta in the sh ape of an

immature ancestor.

Gould ma kes very clear his inter preta tion

of the consequ ences of neoteny. If hu ma ns

evolved, as I believe, by neoten y, . . . then wea r e, in a m or e t h a n m e ta p h or ica l s en s e,

perm an ent children(1981, p. 333). Could it

be tr ue th at we perm an ent children? Some

eminent s cholars believe it. Benjamin Fr an k-

lin wrote, Our whole life is bu t a greater

and longer childhood. Sigmun d F reud was

more circum spect : In our in ne rm ost soul we

ar e children an d rema in so for the res t of our

lives. Mont agu is th e most emph at ic: We

are intended to remain in many ways child-

like, we were never intended to grow up into

the kinds of adults we h ave become. . . . Our

uniqueness l ies in always r emaining in a

sta te of developmen t(all quotes from Mon-

tagu, 1989).

Given the scientific respectability ofworks

on neoteny by Gould an d Monta gu, the term

and the concept have been popular ized in

western society and adopted into man y other

ar enas. The following ar e some r ecent ex-

amples. Philosopher s posit neoteny as the

pr ocess leading to the human capacity f or

language (Goldsmith, 1993; Brown, 1995).

Psychiatrists allege it is the reason humans

are so playful (Brown, 1995). Some types of

neur ological dementia are described as a

failure of neoteny(Bempr ad, 1991). Hu ma n

female sexual a ttr activeness is claimed to be

a function of neoteny (Jones, 1995). Taking

this cue, the adver tising industr y employs

h u m a n i n fa n t s , ch i ld r e n , a n d n e ot e n ou s

adult women to sell al l sor ts of pr oducts.

Mor eover, t he pr oduct designer s make the

inanima te objects (everything from tea pots

to aut omobiles)more appealing and pur chas-

able by making them seem neotenous, that

is , s h a p ed lik e a h u m a n in fa n t or ch ild

(Boym, 1994, and see Box 1). Finally, a

prominent social an thr opologist claims tha t

hum an evolution via neoteny allows for t he

development of huma n cultur e, especiall y

religion (La Bar re, 1991)!

HYPERMORPHOSIS AND HUMANEVOLUTION

Despite the intellectual weight that biol-

ogy (Gould), social philosophy (Frank lin),

psychoana lysis (Fr eud), an th ropology (Mon-

tagu) , and adver tising br ing to this issue,

Fig. 3. Definitions a nd etymology of the t erm neo-teny.



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and despite the popular i ty of neot eny as acause celebre for s o man y hu ma n tr aits an d

b eh a v ior s , t h e p r op os it i on t h a t a d u lt h u -

m a n s a r e p er m a n en t ch ild r en is n ot a c-

cepted by all scholars. Michael McKinney

a n d K en n e t h M cN a m a r a (1 99 1), i n t h e ir

book Heterochrony: The Evolution of Ontog-

eny, provide a det ailed case a gainst n eoteny

in human evolution. McKinney and McNa-

m a r a a r gu e in s t ea d for a n ot h e r t yp e of

heterochronic process t o account for hum an

growth and evolution, namely hypermorpho-

sis. They stat e their posi t ion as follows:

Neoteny is th e process of growing slower.

Yet h uma ns do not grow par ticularly slow(r e la t i ve t o e it h e r ch i m p or ou r a n ce s -

tors. . .). What we do is delay the offset of

virtually all developmental events (growth

phas es) so tha t ea ch phase is longer. This is

hypermorphosis (p. xi). Figure 4b illus-

t r ates evolut ion by h yper mor phosis using

Goulds clock model. Compar ed with the

neoteny clock, th e h ypermorph osis clock pos-

its that hu man s have larger size, later mat u-

ration, an d a m ore ma tu resh ape t ha n our

putative ancestor. Although McKinney and

M cN a m a r a d o a d m i t t h a t t h e r e a r e a n u m -

ber of non-hyperm orphic feat ur es of hu ma ns

hyper morph osis seems to best explain m ost

o f t h o s e t r a i t s t h a t m a k e u s h u m a n : l a r g e

body size, lar ge br ain, long lear ning stage

an d life spa n(p. xi).

Accor ding to heter ochr ony, huma ns ar e

not permanent children; rat her we are devel-

opmenta lly delayed an d/or growth-prolonged

apes. Adolph Schultz (1960) proposed ex-

a ct ly t h is m or e t h a n t h r ee d eca d es a go.

F i gu r e 5 i s h i s cl a ss ic i ll u st r a t i on of t h e

progressive delay in the onset and offset of

primat e life sta ges. Hypermorphosis, th ere-fore, is har dly a new idea, but is it the r ight

idea?

Sue Taylor Par ker (1996) thinks tha t hy-

permorphosis is the r ight idea, and s he uses

it to a ccount for the evolution of hu man

cognitive capacities. Pa rk er a pplies Pia gets

stage theory (Piaget and Inhelder, 1969) to

analyze the cognitive development of human

and n on-huma n pr imates. She finds that a l l

p r im a t e s s h a r e t h e fi r s t o n e o r t w o s t a g es

(sensorimotor and preoperational). Only the

hum an s pecies progresses to the h igher Pia-

getian stages (concrete operat iona l an d for-

mal oper ations) . Par ker explains that theuniquely human stages were added by hyper-

morphosis, that is, by extending the period

of cognitive development past that of th e

monkeys and apes. This n egates neot eny,

w h ich i n P a r k e r s v ie w w ou l d m e a n t h a t

human cognitive capacities evolved by halt-

ing menta l development at ea rly or interm e-

diate stage of non-human primate develop-

men t. To quote Pa rk er, Cognitively, hu ma ns

ar e over developed r ather than under devel-

oped a pes(p. 377).

Elizabeth Vrba (1996) agrees that hyper-

m or p h os is is t h e k e y p r oce ss in h u m a nevolution, but she a dds a twist to McKinney

an d McNama ra s ar gumen t. Vrba s model of

human evolution is set in the more general

context of mammalian evolution in Africa.

Vr b a t r i es t o s h ow t h a t cl im a t e ch a n g e,

specifically global cooling after 2.9 m illion

y ea r s a g o, r e s u l t ed i n a n e n la r g em e n t of

Fig. 4. a: Clock model for human evolution by neo-teny. b: Clock model for huma n evolution by hyperm or-phosis. Adapted from Godfrey and Sutherland (1996),with permission.



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body size along with a relative decrease in

limb length for several species of African

mamma l s. Such mor phological change in

r esponse to a cooler cl imate accor ds with

Bergma nn s an d Allens ru les. According t oVr b a , t h e Afr i ca n h om i n id s a l iv e a t t h a t

t i m e a l s o c on for m t o t h e s e r u l es , b u t t h e

homini ds a r e notable f or a concur r ent in-

crease in br ain s ize relative to body size. The

g en e r a l m a m m a l ia n a n d t h e s p eci fica l ly

hominid m orphological change is explained

to resu lt from th e sa me evolutionar y event

of growth prolongation, or time hypermor-

phosis, as it acts on char acter s with different

ancestr al gr owth pr ofiles in the same body

plan. Vr ba continues by stating that t hi s

can result in a major reorganizationorshu fflingof body proportions such th at

some char acter s become lar ger and other s

smaller , some hyper adult and other s mor e

ju ven ilize d(199 6, p. 1). Ma n y of t h e a n ces -

tr al gr owth pr ofiles of the hominids ar e,

accor ding to Vr ba, st i l l to be f ound in the

great apes. She stat es, I do not imply th at

the chimpanzee itself is ancestral, but only

that its growth profile resembles that of the

comm on an cestor (1996, p . 17). Vrba pre-

d ict s t h a t b y m a in t a i n in g ch i mp a n ze e

gr owth r ates for legs, ar m s, t or so, skul l,

brain, etc., and prolonging t he total t ime for

g r ow t h i t i s p os s ib le t o d e r iv e a m od e r n

hum an morphology from an African pongid

morphology.

CRITIQUES OF NEOTENYAND HYPERMORPHOSIS

The concepts ofn eoteny and hypermorph o-

sis posit tha t moder n huma ns evolved by

either a delay or an extension of ancest r al

patt erns of growth. Huma n a nat omy, physi-

ology, and behavior, however, cannot be ex-

plained by either of these simple processes.

Criticisms of the application of neoteny to

h u m a n e volu t ion a r e n ot n e w. K u m me r(1953), Bertalanffy (1960), and Starck and

Kummer (1962) argued against neoteny, or

what they called fetalization, on the basis of

hum an craniofacial and postcran ial growth:

. . . th e concept of fetalization is to be re-

fused with r espect to th e ont ogeny and evolu-

t i on of t h e h u m a n . . . f or t h i s i s n ot t h e

r e s u lt of a n a r r e s t o f gr ow t h a t a n e a r ly

phase but of a differ entiation in changed

direction. . . . Hence we may speak of retar-

d a t ion bu t n ot of fe t a li za t i on i n h u m a n

developmen t (Bert ala nffy, 1960, p. 250).

T h e p h r a s e t o e m p h a s i z e i n t h i s q u o t e i s

differe nt iat ion in cha nged dir ection.O n e s p eci fic e xa m p le of t h e fa i lu r e of

neoteny an d h ypermorphosis t o account for

the dir ection of huma n development is th e

ontogeny of cr anial gr owth r elated to lan-

guage development. The huma n newbor n

ca n n o t p r od u ce t h e s p ee ch s ou n d s (p h o-

nemes) used by adult speaker s of any lan-

F ig . 5 . S ch u ltzs d iag ram of th e p ro po rtion al in -cr e a s e i n t h e l en g t h o f l i fe s t a g es a c r os s t h e s ca l a

n atu rae o f l iv in g p rim ates. N o te th at S ch u ltz d id n o trecognize the childhood stage for humans. Indeed, allspecies ha ve the sa me life stages, which just increase inlength from prosimian t o human. The estima tes for totallength of life are based on average expectations rathertha n t heoretical maximum s. The da ta for Ear ly Manare entirely speculative as no species is given and verylittle d ata w ere av ailab le w h en S ch u ltz p rep ared th isfi g u re. A d ap ted fro m S ch u ltz (1 9 6 0 ) an d S m ith et al .(1994).



8/27

guage. Lieberman et a l. (1972) an d Laitma n

and Heimbuch (1982) believe that the sh ape

of the basicr anium is t he r eason f or this.

They argue tha t newborns possess a basicra-

nium with a relatively large angle of flexion(t h e a n g le for m e d b y t h e ju n c t ion of t h e

occipita l and vomer bones). This a ngle influ-

e n ce s t h e s h a p e o f t h e s oft t i ss u e s o f t h e

v oca l t r a ck , e s pe ci a ll y t h e p h a r y n x, a n d

deter mines the natur e of the vocal sounds

t he newbor n can pr oduce. Dur i ng gr owth

the an gle of flexion becomes more a cute, as

t he skull a ssumes child, juvenile, a doles-

cent, an d, fina lly, a dult proportions. As this

growth process takes place a greater range

of linguistically recognizable phonemes is

produced. In this one aspect of growth of the

skull and i ts f uncti onal cor r elates neither

neoteny nor hypermorphosis is a useful con-

cept. Non-huma n pr imates never possess

t h e h u m a n t y pe of b a s icr a n i a l a n a t o m y a t

any stage of their development, nor would

they develop this an atomy if they prolonged

any of their stages of gr owth. As a conse-

quence, non-huma n pr imates cannot pr o-

duce huma n-like phoneme soun ds.

A graphic case against n eoteny and h yper-

mor phosis can be ma de by consider ing th e

ontogeny of hum an body pr oportions. Fr om

f e t u s , t o c h i l d , a n d t o a d u l t , h u m a n b o d y

p r op or t i on s a r e s o m u c h a l t er e d t h a t t h e

matur e mor phol ogy cannot be simply pr e-dicted from earlier stages of growth. Allom-

etry, differential rat es of growth of parts of

t h e b o d y r e l a t i v e t o t h a t o f t h e b o d y a s a

whole (Huxley, 1932), is the rule in primate,

including human, development. Both posi-

tive and negat ive allometry t ake place in the

ontogeny of hu man development (e.g., leg

vs. trun k growth, or hea d vs. body growth).

These allometric changes bring about func-

t ional differ ences between the adult and

child in physical appear ance and per for -

mance.

B r ia n S h e a (1 98 9) p u b li sh e d t h e m os t

cogent al lometr ic analysis to date that r e-ject s bot h n eot en y or h yp er m or ph osis a s a

gran d u nification t heory for all of hum an

g r ow t h a n d e vol u t ion . S h e a i s n ot a n t i -

neoteny or anti-hypermorphosis per se, and

i n f act he al lows t hat the human br ain and

cranium may have evolved by neoteny. But

h e a r g u e s t h a t t h e h u m a n f a c e , j a w s , a n d

the rest of the body did not evolve by neo-

teny. I n Sheas view, a var iety of h et er o-

chronic processes are responsible for human

evolution. The others m ay be h ypermorpho-

sis, acceleration (defined as an increase int h e r a t e of g r ow t h or d e ve lop m e n t ), a n d

hypomorph osis (defined as a dela y in growth

with no delay in t he a ge at matu r ation) . I n

Figure 6 a re illustrat ed Shea s estima tes for

b od y s iz e a n d s h a p e a s a con s e qu e n ce of

neoteny and two types of h yper mor phosis.

None of these acting as a single process can

p r od u ce t h e h u m a n a d u lt s iz e a n d s h a p e

fr om the hu man infant size and shape. The

same holds true for acceleration and hypo-

morphosis. In agreement with Schultz, Shea

stat es th at we [hum an s] have extended a ll

of our l if e histor y per iods, not mer ely the

embr yonic or ju venile ones (pp. 845). Hu -

man s ha ve also altered rates of growth from

those found in other primates and possible

an cestors. To accomplish all th is requ ired, in

Sheas view, several genetic changes or a d-

ju st m en t s du r in g h u m a n evolu t ion . Si n ce

the hor mones that r egulate gr owth and de-

velopment are, virtually, direct products of

DNA activity, Sh ea pr oposes that the best

place t o look for evidence of the evolution of

ontogeny is in the action of the endocr ine

system. According to Shea and others (e.g.

Bogin, 1988) differences in endocrine action

between human s and other pr imates negat eneoteny or hyper mor phosis as unitar y pr o-

cesses and instead argue for a multiprocess

model for human evolution.

In the male chimpanzee, for example, the

concent ra tion of test osterone in blood seru m

pr ior to puber ty ( f r om 1 to 6 year s of age)

averages 13 n g/dl (Mart in et al., 1977). For

t h e h u m a n m a le , t h e p r ep u be r t al s er u m

testosterone concentration (from ages 1 to

12 years) averages 9 ng/dl (Winter, 1978).

The peak velocity in long bone gr owth of

eight male chimpanzees studied by Watt s

and Gavan (1982) occurred at a m ean age of

10.96 year s, with a standar d deviation of 1.31 years. At this age, serum testosterone

a v er a g es a b ou t 4 00 n g /d l (M a r t in e t a l .,

1977). Peak height velocity in human boys

from wester n Eu rope occur s at a m ean a ge of

14.06 year s, with a standar d deviation of

0.92 year s (Mar shall , 1978), when ser um

testosterone levels average about 340 ng/dl



9/27

(Winter, 1978). Thus, the serum testoster-

one concentr at ion of th e chi mpanzee in-

creases about 31-fold from the prepubertal

t o puber t al state. I n t he h uman mal e, ser um

testosterone concentration increases about

38-fold, or 1.23 t imes t he increase for th echimpanzee.

Accor ding t o Watts and Gavan (1982),

ch i m pa n z e es h a v e a r e la t i ve ly s m a ll i n -

crease in t he velocity of growth of individual

l ong bones dur ing puber ty, usually less

t han a centimeter (p. 58), and often less

t h a n 0 .5 cm . C a m e r on e t a l . (1 98 2) p e r -

formed a longitudinal analysis of the growth

of individual limb segments in British boys.

I t was f ound that in contr ast to the negli-

gible velocity change of chimpan zees, th e

p e a k v a lu e i n v el oci t y d u r i n g t h e h u m a nadolescent gr owth spur t r anged between

1.34 cm per yea r for th e forea rm a nd 2.44 cm

per year f or the t ibia. Fr om these findings

one may propose that there are differences

in the effect of testosterone on the skeletal

g r ow t h of c h im p a n z ee s a n d h u m a n s . T h e

gr owth r esponse of the human skeleton t o

r ising testoster one levels is gr eater t han

that of the chimpanzee skeleton, since the

change in the ser um hor mone levels of the

two prima tes differs only by a factor of about

1.23, but th e chan ge in th e velocity of growth

differs by a factor of at least 2.0 and as m uch

as 4.9.

The heterochronic models of neoteny a nd

hyper mor phosis pr edict that a gr eater or

lesser amount of time for growth produces

the dif f er ences in size and shape between

hum ans a nd chimpanzees. But th ese empiri-

ca l d a t a g a t h er e d fr om g r ow t h a n d e n d o-

crine resear ch show tha t it is the sensitivity

of specific skeletal par ts to testoster one,

deter mined by DNA and cellular activity,

that r esults in the dif f er ences in l imb size

and sha pe between adu lt human s and chim-

panzees. The t ime available f or gr owt h is

largely irrelevan t.A closer inspection of huma n cognit i ve

development also f ails to suppor t het er o-

chrony, especially Pa rker s h ypermorph osis

argument. Human cognitive ontogeny from

childhood to a dulthood ent ails t he develop-

ment of new competencies, such as those

related to language and social intelligence.

The stage theory of Piaget (1954; Piaget and

Inhelder, 1969) provides a descriptive and

theor etical under stan ding of the develop-

ment of hum an intell igence. The neoteny

and hypermorphosis a rgument s correctly as-

ser t tha t a dult huma ns possess an intellec-

tual plasticity and cur iosity usually f oundonly in the young of other species. Piagets

stage theory of human development shows,

however, that this is due to the matu r ation

of incr easingly sophisticated and flexibl e

cognitive processes, rather t ha n to the ret en-

tion or extension of infanti le or juvenile

intellectua l abilities. At the physiological

F ig . 6 . S ilh o u ettes of size an d sh a p e ch an g e d u rin ghuman growth. Numbers under silhouettes indicate agein y ears. N u m b ers a b ov e o r o n silh o u ettes in d icaterelative sh ap e. Top left: A ctu al size an d sh a p e ch an g ed u rin g n o rm al h u m an d ev elo p m en t. To p r ig h t : Neo-teny. Note that at adult size shape 3 is still maintained.

Bo t t o m le f t : Time hypermorphosis. The growth per iodis ex ten d ed to 3 6 y ears, y ield in g a p eram o rp h ic g ian t(size an d sh ap e o f th e d escen d an t b ey o n d th at o f th eancestor). Bo t t o m r ig h t : Rate hypermorphosis. Growthen d s at ag e 2 6 b u t p ro ceed s at a faster rat e, p rod u cin gan o th er p eram o rp h ic gian t. N o te th at in b oth cases th ea du lt s h ap e a t 7 i s o u t s id e t h e r a n g e o f n o r m a ldevelopment. Redrawn from Shea (1989), with permis-sion.



10/27

l evel, Eccles (1979) shows that the adult

human potentials for playfulness, creativity,

a n d i n t el le ct u a l a d v a n ce m en t a r e n ot d e -

rived via heterochrony; rather they are new

competen cies derived from a const an t r emod-eling, r est r uctur i ng, and matu r at ion of the

neur ological ar chitectur e in th e central n er-

vous system.

The last word on heterochrony, so far at

least, is the work of Godfrey an d Sut herlan d

(1996). These r esear chers do not a rgu e for or

against one or the other of the m an y hetero-

ch r on i c m e ch a n i s m s t h a t m a y h a v e i n flu -

enced huma n evolut ion. I nst ead, th ey de-

v el op a q u a n t it a t i ve m e t h od for t h e fa i r

t esting of any or al l heter ochr ony hypoth-

eses. Their methodology is a major innova-

t ion in heter ochr ony r esear ch. They fir st

show th at Goulds clock model is ba sed on

three mathematically definable vectors for

size, shape, an d t ime. Godfr ey an d Suther -

land th en sh ow tha t m ovemen t of th e clocks

h a n d s o r a ge b a r m a y b e r e pr e se n t ed a s

linear vector distortions. The tra nslation of

t h e cl ock m od e l t o l in e a r v ect or s m a k e s

quantification relatively easy, and, more im-

portantly, allows for the prediction of exact

differ ences between ancestor and descen-

dent species for the different types of het-

er ochr ony. The auth or s car r y out sever al

pr ediction anal yses and find t hat many of

t h e a s s e r t i o n s m a d e i n t h e p a s t a b o u t t h e

role of heterochrony in hum an evolution are

not cor r ect . I n the end, the aut hor s find no

s u p p or t for a n y cu r r e n t ly p u b li sh e d h e t -

e r och r o n ic m od e ls for h u m a n e vol u t ion .

Please r ead Godfr ey a nd Suther lands ar -

ticle for th e deta ils.

THE OTHER SIDE OF THE COIN

I n the tr i bul ations and tr ials of heter o-

ch r on y t h e ju r y is cu r r en t ly ou t , a n d a

ver dict on what type of heter ochr ony ha s

shaped which aspects of human growth and

development must awai t new r esear ch and

testing. Forgotten by all parties in the litiga-t ion sur r ounding heter ochr ony is a nother

process by which evolution works. I requote

G ou l d, a s h e s t a t e d m os t s u cci n ct l y t h a t

evolution occurs when ontogeny is alter ed

in one of two ways: [the first is] when new

c h a r a c t e r s a r e i n t r o d u c e d a t a n y s t a g e o f

development with varying effects upon sub-

sequent stages; the second is by het er o-

chrony. Much of hum an evolution, especially

the evolution of childhood, is the result of

the introduction of new life stages into the

general pattern ofprimat e growth an d devel-

opment.

C on s id e r fi r s t t h e g en e r a l m a m m a l ia n

a n d p r i m a t e p a t t e r n s o f g r o w t h a n d t h e n

compar e these with the hu man patter n. The

majority of mam mals progress from infancy

to adu lthood sea mlessly, without a ny int er-

vening stages, and while their growth rates

are in decline (Brody, 1945; Berta lanffy,

1960) . This patter n of postnatal gr owth i s

i l l u s t r a t e d i n F i g u r e 7 u s i n g d a t a f o r t h e

m ou s e . H i gh l y s oci a l m a m m a l s , s u c h a s

wolves, wild dogs, lions, elephants , an d the

pr imates, postpone puber ty by inser ting aperiod of juvenile growth and behavior be-

tween infancy and adulthood (Bekoff and

Beyers, 1985; Pereiera a nd Fairba nks , 1993).

Ju veniles may be defined as pr epuber tal

individuals th at a re no longer dependent on

th eir mothers (par ent s) for sur vival(Pereira

and Altma nn, 1985, p. 236). This definition

is derived from ethological research with

social ma mma ls, especially non-huma n pri-

mates, and applies to the hu man species as

well. Ju veniles a re n ot children, as juveniles

ar e independent but childr en st i ll r equi r e

car e fr om older individuals. I n the highl y

social mammals, puber ty occur s while therat e of growth is still decelerating an d th ere

i s n o r e a d il y d e t e ct a b le g r ow t h s p u r t i n

skeletal dimensions (Fig. 8, but see Tanner

e t a l ., 1 9 9 0, w h o fi n d t h a t on e s a m p le of

captive rhesus monkeys have a rate of skel-

etal gr owth at puber ty that is gr eater than

tha t of hu man beings).

H u m a n g r ow t h a n d d e ve lop m e n t fr om

birth to reproductive maturity may be char-

acterized by five stages: 1) infancy, 2) child-

hood, 3) juvenility, 4) a dolescence, a nd 5)

adulth ood (Bogin, 1988, 1995; Bogin an d

Smith, 1996). Thus, humans add childhoodand adolescence to th e patter n found for

primates and other highly social mammals.

Each of the huma n stages of gr owth can be

defined by clear biological an d behavioral

characteristics, especially th ose related to

r ate of gr owth, feeding, and r epr oductive

behavior.



11/27

RATE OF GROWTH

Changes in the vel ocity of gr owth fr om

b ir t h t o a d u l t h ood s ig n a l t h e t r a n s i t ion sbetween these five development al stages.

I deali zed velocity cur ves ar e pr esented in

Figure 1. During infancy growth rate plum-

mets, followed by a period of slower velocity

decline in childhood. The end of childhood is

often ma rk ed by a sm all increas e in velocity,

called the mid-growth spurt (Tanner, 1947).

Following childhood, the ra te of growth decel-

erates du ring the juvenile stage, and eventu-

ally the rate drops to its lowest point since

birth. The onset of adolescence is marked by

a su dden and r apid incr ease in gr owth r a te,

which peak s at a level unequ aled since early

infancy. The mature adult stage begins when

growth of the skeleton stops.

T h e m i d-g r ow t h s p u r t i s a n i m por t a n t

f eatur e of human childhood. This spur t i s

F ig . 7 . Velocity cu rves forweight growth in t he mouse. Inb ot h s e x es p u b e r t y ( v a g in a lop en in g for fem ales o r sp er-m ato cy tes in testes o f m ales)occu rs ju st after w ean in g an dmaximal growth rate. Weaning(W) ta k es p lace b etw een d ay s15 and 20. After Tanner (1962).

Fig. 8. Baboon crown-rum p length velocity. The let-ters indicate the s tages of growth: I, infancy; J , juvenil-ity ; M , sex u al m atu rity . In th e w ild th e w ean in g (W)process begins as early as 4 months of age and ends by1 2 to 1 8 m o n th s (A ltm an n , 1 9 8 0 ). P u b erty b eg in s atabout 3.5 years in females (&) and 4.5 years in m ales (()

and en ds by about 6.0 years in both sexes. Redrawn withsome dat a smoothing from Coelho (1985). The pa tter nsof growth for other primat e species, including chimpan-zees, are sim ilar to th ese for th e b ab oon (see B og in ,1988).



12/27

associated with an endocr ine event called

adrenarche, the progressive increase in the

secr etion of a dr enal andr ogen hor mones.

Adrena l an drogens produce the m id-growth

spur t in height, a t r ansient accel er ation of b on e m a t u r a t ion , a n d t h e a p pe a ra n ce of

axillary an d pubic hair, and seem to regulate

t h e d e ve lop m e n t of b od y fa t n e s s a n d fa t

distr ibution (Kat z et al., 1985; Pa rker, 1991).

T h e r e i s a l i t t l e s t o r y t h a t l i n k s t h e m i d -

g r ow t h s p u r t w it h n e ot e n y. L ou i s B ol k

(1926), th e fath er of t he scientific h ypoth-

esis for human evolution via neoteny, specu-

l a t ed t h a t for ou r e a r ly h u m a n a n c es t or s

sexual mat ura tion took place at a bout 6 t o 8

years of age. The mid-growth s purt was first

reported by Backman (1934) and following

its discovery, several of Bolks followers,

without a ny a dditiona l supporting evidence,

op in e d t h a t t h e m i d-g r ow t h s p u r t a n d a d -

r enar che a r e vestiges of sexual matu r ation

from our evolutionary past. Much research,

however, from clinical m edicine to an th ropo-

logical fieldwork, shows th at th ere is little or

n o con n e ct i on b et w e en a d r e n a r ch e a n d

s e xu a l m a t u r a t i on , a s e a ch a r e i n de p en -

dently controlled events (Smail et al., 1982;

Weir man and Cr owley, 1986; Wor thma n,

1986; Bogin, 1988, in pr ess; Pa rker, 1991).

Bolks idea m ay be wrong, but a connec-

tion with t he evolution of the h uma n pa ttern

of growth is still a possibility. The mecha-nism contr ol l ing adr enar che i s not under -

s t ood a s n o k n ow n h or m on e a p pe a r s t o

cause i t . Ther e ar e connections, however,

b et w ee n t h e p r od u ct i on of a d r e n a l h or -

mones, gr owth, and matur ati on. Cutler et

al. (1978) and Sm ail et al. (1982) measured

t h e p la s m a con ce n t r a t ion of t h e a d r e n a l

andr ogens dehydroepiand osterone (DHA),

dehydroepiandosterone sulfate (DHAS), and

delta 4-an drostenodine (D4 ) before and after

sexual maturation in 14 species. These spe-

cies include sa mples of rodents (rat , guinea

pig, ha mster), domestica nimals (rabbit, dog,

sheep, pig, goat, horse, cow), prima tes (ma-caquesincluding 76 Macaca mulatta a n d

80 M. nemestrina a f ew baboons, and 52

chimpan zees), and the chicken. The plas ma

concentrations of DHA, DHAS, and D 4 were

significantly higher in sexually mature pri-

m a t es s pe cie s t h a n in a n y of t h e ot h e r

animals. However, the serum level of these

adren al andr ogens was not related to sexual

m a t u r a t i o n . R h e s u s m o n k e y s a g e d 1 t o 3

ye a r s, a n d n ot s ex ua lly m a t u r e, h a d t h e

same high concentrations of all three adre-

n a l a n d r og en s a s ol de r, s e xu a l ly m a t u r emonkeys. The same wa s tr ue for ba boons. In

contrast, chimpanzees 7 years old or older

had adr enal andr ogen concentr ations that

w er e , o n a v er a g e, 4 .7 t i m es g r ea t e r t h a n

those for chimpan zees less th an 4 years old.

Thus, among the animals examined so f ar ,

the chimpanzee and the hu man ar e the onl y

species that sh ow adren arche. Only huma ns

are kn own to ha ve a mid-growth s purt .

The primate dat a reported by Cutler et al.

(1 97 8) a n d S m a il e t a l . ( 19 82 ) s u g ge s t a

possible function for adren arche. Chimpa n-

zees and humans have low ser um levels of

adren al andr ogens after infancy an d prior to

adren arche. Moreover, chimpanzees an d h u-

mans have a r elatively slow gr owth and a

long delay in th e onset of sexual ma tur ation.

Perha ps, then, the evolution ofr educed adre-

nal an drogen production prior to adrena rche

m a y b e e xp la in e d a s a m e ch a n is m t h a t

maintains slow epiphyseal matu r ation and

skeletal growth in the face of the prolonga-

tion of the prepubertal stages of growth. In

the hu man , the delay is so protracted tha t it

becomes possible to inser t the childhood

stage of development between the infancy

and juvenile stages.Synthesizing all of these data, i t is pos-

sible t o view th e combination of adrena rche

a n d t h e h u m a n m i d-g r ow t h s p u r t a s l ife

histor y events mar king the tr an sit ion fr om

the childhood to the juvenile stage of growth.

In terms of physical growth, the effects of

the adr enal andr ogens, to incr ease r at e of

skeletal growth, st imulate body hair growth,

and regulate body fat distribution, are sh ort-

lived and quite small. Even so, these physi-

cal chan ges may be n oticed by th e child an d

his/her intimates, such as par ents, an d r ec-

ognized as ma rkers of development al ma tu-

r a t i on . M or e t o t h e p oi n t i s t h e fa ct t h a twhile adr enar che may have only tr ansient

effects on physical development, there is a

m or e p e r m a n en t a n d i m por t a n t e ffe ct on

cognitive function. Psychologists have long

been inter ested in what is called th e 5 t o 7

year old shift in cognition (White, 1965;

Rogoff et al., 1975; Whiting a nd Whiting,



13/27

1975; Weisner an d Gallimore, 1977; Whitin g

an d E dwar ds, 1988; Weisner, 1996). Weisner

(1996, p. 295) sta tes t hat the 57 sh ift

involves changes in intern al sta tes an d com-

petencies of the matur ing child shif ts incognitive capacities, self concept, visual/

perceptual abilities, or social a bilities. The

t r ansit ion mar ks the emer gence of incr eas-

ing capabilities for stra tegic and contr olled

self-regulation, skills at inhibition, the abil-

i ty to maintain attention and t o focus on a

complex problem, and planfullness and re-

flection. Using the term inology of P iaget

(Piaget and I nhelder , 1969) t h e 57 shift

moves the child from the preoperational to

concrete operational stage of cognition. In

short, following the shift, the child becomes

a juvenile capable of much of his or her own

feeding and care. This 57 year old shift is

fou n d i n a l l c u lt u r e s s o f a r i n ve s t ig a t ed

(Rogoff et al., 1975), and thus seems to be a

huma n species phenomenon. I n a ddition to

self-care and -feeding, the juvenile becomes

increasingly involved with domestic work

an d caret ak ing inter actions with oth er chil-

dren (Weisner, 1996, p . 296). The as socia-

t ion of adr ena r che, t he mid-gr owt h spur t ,

a n d t h e 5 7 s h ift a ll s ee m t o m a r k t h e

progression of the child to the juvenile stage

of development .

FEEDING AND BREEDINGChanges in human feeding and reproduc-

t i ve b eh a v ior com p le m en t t h e p a t t e r n of

human growth velocity. The 57 shift is but

one examp le of a cha nge in feeding beha vior

fr om dependency t o independence. Ther e

are other importa nt chan ges related to feed-

ing between birth, childhood, and the juve-

nile stage.

Infancy

As for all mamma ls, human infan cy is the

period when the mother provides all or some

nourishment to her offspring via lactation.T h e in fa n cy s t a ge e n ds w h en t h e y ou n g

mam mal is weaned. Weaning is defined here

as th e terminat ion oflactat ion by the mother

( ot her r esear cher s may define weaning as

t he pr ocess of sh ift ing fr om lactation to

eating solid foods). In human societies the

age a t weani ng var ies gr eatly. I ndustr ial-

ized societies provide a poor indication of

weaning a ge becau se bottle-feeding and the

ma nu facture of baby foods allow either

ear ly t er mination of br east- feeding or no

breast-feeding at all. Preindustrialized hu-man societies pr ovide a better indication of

the age at wean ing, and h ence the tr an sition

from infancy to childhood. One study finds

that the ter mination of br east- feeding oc-

cur s a t a median age of 36 months in th ese

societies (Dettwyler, 1995). Another review

of such r esearch (Lee et a l., 1991) finds t ha t

in so-called food-enh an ced societies, th ose

wher e nutr i t ional intake is good, weaning

takes place as ear ly as 9 months of age. I n

food-limit edsocieties , wh er e chr onic un der -

nutrition occurs, weaning takes place at 36

months. There a re two fascinating corollar-

ies of this compar ison. The fir st is that in

both th e food-enh an ced an d th e food-

limitedsocieties th e mea n weight of wean ed

infants is about the same, 9.0 kg and 9.2 kg

respectively, or about 2.7 times birth weight

( Lee et al . assume a mean bir th weight of

3,400 g for full-term h um an s). The second is

that some solid foods are introduced into the

d ie t w h en t h e i n fa n t a ch i ev es a b ou t 2 .1

times birth weight. Lee a nd colleagues (Lee

et al., 1991; Bowman and Lee, 1995) com-

p a r e t h e h u m a n d a t a w it h d a t a f r om 8 8

species of large-bodied mammals (32 non-

h u m a n p r im a t e s , 2 9 u n g u la t e s , 2 7 p in n i -peds) . They find that f or al l these species

solid food is intr oduced, again, a t about 2.1

times birth weight, but weaning ta kes place

when the inf ant achieves between 3.2 and

4.9 times birth weight. For all primates the

mean value is 4.6 times birth weight (range:

2.3 for Micropithecus talapoin to 9.4 for

Gorolla gorilla). The other great apes wean

at the following mu ltiples of birth weight:

Pan troglodytes, 4.9; P. p aniscus, 6.1; Pongo

py gm aeu s, 6.4.

T h u s h u m a n s a r e s i m il a r t o ot h e r m a m -

m a l s i n t h a t w e i n t r od u ce s ol id food s a t

about 2.1 times birth weight. However, hu-man s are un like other m amm als, even other

species of pr imates, in that pr eindustr ial

and tra ditiona l societies, including food-

l im i t ed gr ou p s s u ch a s !K u n g h u n t e r -

gather ers, wean at a relat ively early stage of

gr owth befor e r eaching 3.0 t imes bir th

weight.



14/27

Childhood

Childhood is the period following wean-

i ng, when the youngster st i ll depends on

older people for feeding and protection. Chil-dren require specially prepar ed foods due t o

t he immatur i ty of thei r denti t ion. The de-

ciduous den tit ion, often called milk t eet h,

have th in ena mel and shallow r oots com-

par ed wit h t h e per manent dent i t ion. Smith

(1991a) and Smith et al. (1994) report that

given this dental morphology, young mam-

mals with only the deciduous dentition can-

n ot p r oce s s t h e a d u l t -t y p e d i e t . S m i t h

(1991a) also finds t hat for virtually all mam-

m a l s t h e d e ci du ou s d e n t it i on i s i n p la ce

during infancy, that is, when the young are

nur sing. With erupt ion an d occlusion of the

fi r s t p e r m a n e n t t e e t h t h e i n f a n t m a m m a lmoves either to adulthood (most mammals

have only two postna tal growth st ages) or t o

the juvenile sta ge (social m amm als). Smith

r e p or t s t h a t m a m m a l s w i t h s o m e p e r m a -

nent teeth are able to process the adult diet

a n d a r e i n de p en d e n t i n t e r m s of f ee d in g .

When hu man infancy ends th e deciduous

dentit ion is st i l l in place; thus t he human

ch i ld s t il l r e q u ir e s a s p eci a l d i et a n d r e -

mains dependent on older individuals for

food.

Children also require a special diet due t o

the small size of their digestive tracts rela-

tive to th at of the adult (Bogin, 1988). This

need can be appreciated most acutely when

it is n ot m e t . B e ha r (1 97 7) s t u die d t h e

causes ofgrowth reta rdation and undernu tri-

tion of children living in r ura l villages in

Guatemala. He f ound t hat the chi ldr en ha d

access to sufficient food, but the traditional

adult diet of corn an d beans did not have th e

caloric density to meet their growth require-

ment s. Bailey et al. (1984)s tu died the growth

of mor e t han 1,000 infants, chi ldr en, a nd

ju ven ile s liv in g in 29 vil la ge s in n or t h er n

T h a ila n d . T h e p a r t icip a n t s in t h e s t u dy

lived in rural agricultural villages, with riceas the basic subsistence cr op. The par tici-

p a n t s w er e b et w ee n t h e a g es of 6 m on t h s

a n d 5 y e a r s a t t h e s t a r t o f t h e s t u d y , a n d

t hey wer e measur ed about ever y 6 months

for 5 y ea r s . I t w a s fou n d t h a t , com p a r e d

with l ocal or inter nati onal r ef er ence stan-

dar ds for gr owth, the r ur al Thai childr en

and juveniles were delayed in growth for all

the dimensions studied. Disease histor ies,

par asite infestations, a nd mor tality dur ing

infancy and childhood were not significantlyassociated with growth in this sample. Bai-

ley et al. concluded t ha t th e delays in growth

were not du e to disease or th e lack of specific

n u t r i en t s , s u ch a s p r ot e in , v it a m i n A, or

ir on; r ather the delays wer e due to a defi-

ciency in the total intake of calor ies. The

most dr ama tic falloff in growth occurred at

18 months of age, which corresponds with

the a verage age at weaning in th ese villages.

Weaning foods were usually watered-down

versions of adult foods. Although there were

no food shortages in the villages, Bailey et

a l . r e a s on t h a t t h e s m a ll g a s t r oi n t es t in a l

t r a ct s of t h e w ea n e d in fa n t s a n d y ou n g

children m ay not ha ve been capa ble of digest-

ing enough food to m eet their calor ic de-

mands f or maintenance and gr owth of the

b od y. T h es e t w o s t u d i es (a n d ot h e r s r e -

viewed in Bogin, 1988) show that without

the use of appropriate weaning foods chil-

dren will suffer calorie in sufficiency leading

to undernu trition, developmental delays, and

growth retardation.

Another r eason tha t childr en n eed a spe-

cial high-energy diet is the rapid growth of

their brain (Fig. 2). Leonar d a nd Robertson

(1992) estimate that due to this rapid braingrowth, a hum an child u nder the age of 5

years u ses 4085 percent of resting m etabo-

lism to maintain his/her br ain [ adults use

1625 percent]. Therefore, th e consequ ences

of even a small calor ic debt in a child ar e

enormous given t he r atio of energy distribu-

tion between bra in a nd body (p. 191). In a

related study, Leonard and Robertson (1994)

also show that the size of the hum an br ai n

r elative to total body size necessitates an

energy-dense diet. At all st ages of life a fter

b i r t h , h u m a n b e i n g s h a v e b r a i n s t h a t a r e

significantly larger than expected given the

hum an body size (Figs. 2 an d 9). Aiello andWheeler (1995) r efine the r elationship be-

t w ee n h u m a n b r a in s iz e a n d b od y s i ze b y

noting th at compar ed with other m amma ls,

including monkeys and apes, human s h ave

exceptionally small gastrointestinal tracts

(g u t s ) r e la t i ve t o b r a in s iz e. Ai el lo a n d



15/27

Wheeler show that both brain t issue and gut

tissue are expensive, mean ing tha t both

t ypes of t i ssue have r elati vely high meta-

bolic r ates. They pr esent est i mat es of the

percent age of tota l body basa l meta bolic ra te

for several tissues utilized by th e typical 65

k g a d u lt h u m a n m a le . F or t h e b r a in t h e

v a l u e i s 1 6 . 1 p e r c e n t a n d f o r t h e g u t t h e

value is 14.8 per cent . Gi ven t hese values,

Aiello and Wheeler propose that during hu-

man evol uti on the gut r educed i n size as a

trade-off allowing, in part, for expansion of

b r a in s iz e. B ot h L eon a r d a n d R ob er t s on(1994) and Aiello and Wheeler conclude th at

given th is brain t o body size/gut size relation-

s h i p , a d u l t h u m a n s r e q u i r e a d i e t t h a t i s

nutr ient- dense, especially in ener gy, and

easy t o di gest . Chi ldr en, of cour se, have a

relatively larger disproportion between br ain

size and body/gut size than do adults (Fig.

2). Added together, each of th ese const ra ints

of childhood, an immature dentition, a small

digestive system, and a calorie-demanding

brain t hat is both r elatively large and grow-

ing rapidly, necessitates a special diet that

must be pr ocur ed, pr epar ed, and pr ovided

by older individua ls.Juvenility

Important developments that allow chil-

d r e n t o p r og r es s t o t h e ju v en i le s t a ge of

growth a nd development are t he eru ption of

t he fir st per manent molar s and completion

of gr owth of th e br ain (in weight). F ir st

molar eruption takes place, on average, be-

tween the ages of 5.5 and 6.5 years in most

huma n populations (J aswal, 1983; Smith,

1992). Fun ctional occlusion occur s some

weeks to months ther eafter. Recent morpho-

l og ica l a n d m a t h e m a t i ca l i n ve s t ig a t ion

shows that br ain gr owth in weight is com-

plete at a mean age of 7 years (Caban a et a l.,

1993). Thus, significant milestones of dental

and br ain matur ation take place at about 7

years of age. At this stage of development

the child becomes much mor e capable of p r oce s s in g d e n t a l ly a n a d u l t -t y p e d ie t

(Smith, 1991a). F ur th er mor e, nu tr ient r e-

q u ir e m en t s for t h e m a i n t en a n c e a n d t h e

growth of both brain and body diminish to

less t han 50 per cent of total ener gy needs.

Finally, the child shifts to a new plateau of

cognitive function.

The child then progresses to the juvenile

stage. As described above, ethnographic and

psychological research shows that juvenile

h u m a n s h a v e t h e p h ys ica l a n d cog n it i ve

abilities to provide much of their own food

and to protect t hemselves from environmen -

tal ha zar ds such as pr edation (childr en a r e

especially vulner able to pr edation due t o

sma ll body size) and diseas e (Weisner, 1987;

Blurton-Jones, 1993). In girls, the juvenile

period ends, on average, at about the age of

10. This is 2 years before it usually ends in

F i g. 9 . Ad u lt b od y w e i gh tand brain weight plotted for 61species of Cercopithecidae (OldWorld m onkeys), apes, a nd h u-mans. The curve is logarithmicregression fit to the data for allsp ecies. T h e illu stratio n is as a gi t a l s e ct i on of t h e h u m a nb rain . E ach p art o f th e h u m anb rain en larg ed d u rin g ev olu -tion, especially th e size of cere-b ral co rtex (d ata fro m H arv eyet a l., 1987).



16/27

boys, the differ ence r eflecting the ear lier

onset of puber ty in girls.

Adolescence

Human adolescence begins with puberty,

mar ked by some visible sign of sexua l mat u-

r ation such a s i ncr eased densit y of pubic

hair ( indeed t he t er m is der ived f r om the

Latin pubescere: to grow hairy). The adoles-

cent stage also includes development of the

s e con d a r y s e xu a l ch a r a c t er i st i cs a n d t h e

onset of i nter est and activity i n adult pat-

terns of sociosexual and economic behavior.

These physical and behavior al changes of

puberty occur in ma ny species of mamma ls.

What m akes h uma n a dolescence different is

t h a t d u r in g t h i s l i fe s t a g e b ot h b oy s a n d

girls experience a rapid acceleration in the

growth of virtually all skeletal tissuethe

a d ol es ce n t g r ow t h s p u r t . H u m a n a d ol es -

ce n ce a n d t h e g r ow t h s p u r t a r e t r e a t e d i n

detail in other publications (Bogin, 1988,

1994a, 1995; Bogin an d Smit h, 1996).

Adulthood

Adolescence ends an d ea rly adu lthood be-

gins with t he completion of th e growth spu rt ,

the attainment of adult stature, the comple-

t ion of dent al matur ation ( er upt ion of the

t hir d molar , i f pr esent ), a nd the achieve-

ment of full reproductive maturity (Figs. 1

and 2). The latter includes both physiologi-cal, socioeconomic, an d psychobehavioral a t-

tributes, which all coincide, on a verage, by

about age 19 in women an d 21 to 25 years of

age in m en (Bogin, 1988, 1993, 1994a). That

i s, these ar e t he median ages at fir st bir th

for women or age at fatherhood for men on a

worldwide ba sis for both contem porary a nd

historic populations.

EMPIRICAL AND MATHEMATICALEVIDENCE AGAINST HETEROCHRONY

C le a r ly, t h e h u m a n p a t t e r n of g r ow t h

fr om bir t h to mat ur it y is qualitati vely an d

quan titat ively different from t he pa tter n forother pr imates. The quan titat ive differences

ca n b e e x pr e s se d i n a m ou n t s , r a t e s , a n d

timing of growth events, and are so reported

in m a n y s t a n d a rd t e xt b ook s of h u m a n

growth (e.g., Tan ner, 1962; Bogin, 1988).

These quantitative differences may also be

expr essed in ter ms of t he t ype and number

of mathematical f unctions that ar e needed

to describe growth. The distance a nd veloc-

ity curves for most mammalian species can

be estimated by a single function, such as a

simple polynomial or exponent ial function.Even the monkeys an d apes, with the a ddi-

t ion of the juvenile sta ge, r equir e no m or e

than two such r elatively simple funct ions

(Laird, 1967; Bogin, 1988). The insertion of

the mid-childhood and adolescent spur t s

i n t o h u m a n o n t o g e n y m e a n s t h a t a t l e a s t

thr ee m athemat ical f unctions ar e needed t o

adequately descr ibe shape of t he velocit y

curve (Fig. 10). Not only more, bu t a lso more

complex functions ar e needed (Bock and

Thissen, 1976; Karlberg, 1985). It is vitally

im p or t a n t t o s t r e ss h e r e t h a t a ll of t h i s

quantitative knowledge of the biology of

hu ma n growth is well established an d widely

available. This information unequ ivocally

negates neoteny, hyper mor phosis, or any

other single h eter ochr onic pr ocess as the

primary pr ocess ofh uma n evolution. Lamen-

tably, the works on neoteny cited above

with the exception of Sheas workmake

little or n o reference to stu dies of the physi-

cal growth a nd developmen t of living people.

HOW AND WHEN DID THE HUMANPATTERN EVOLVE?

The st ages of the life cycle ma y be stu died

directly only for living species. H owever,th ere a re lines of evidence on t he life cycle of

e xt i n ct s p e ci es . S u ch i n fe r e n ce s for t h e

hominids are, of course, hypotheses based

on compar at ive an at omy, compar at ive physi-

ology, compar at ive et hology, a nd ar chaeol-

ogy. Exa mples of th is met hodology ar e foun d

in the work of Martin (1983) and Harvey et

a l . (1 98 7) o n p a t t e r n s of b r a i n a n d b od y

g r o w t h i n a p e s , h u m a n s , a n d t h e i r a n c e s -

tors.

Apes have a patter n of br ain gr owth th at

is r apid befor e bir th and r elatively slower

af ter bir th. I n contr ast , human s ha ve r api d

b r a in g r ow t h b ot h b efor e a n d a ft e r b ir t h(Fig. 11). This d ifference ma y be a ppr eciat ed

by compar ing ra tios of brain weight divided

by total body weight (in gram s).At birth th is

r atio aver ages 0.09 f or the gr eat apes and

0.12 for human neonates. At adulthood the

r atio aver ages 0.008 f or the gr eat a pes an d

0.028 for humans. In other words, relative to



17/27

body size huma n n eonat al bra in size is 1.33

t im e s l a r ge r t h a n t h e g re a t a p es , b u t b y

adulthood t he differ ence i s 3.5 t i mes. The

h u m a n - a pe d iffe r en ce i s n ot d u e t o a n y

single h eterochronic process, tha t is, not th e

resu lt of delay, prolonga tion, or a ccelerat ion

of a basic ape-like pa ttern of growth. Rat her

i t is due to new patter ns of gr owth f or the

h u m a n s p eci es . T h e r a t e of h u m a n b r a ingrowth exceeds th at of most other tissues of

t h e b od y d u r i n g t h e fi r s t f ew y ea r s a ft e r

birth (Fig. 2). Martin (1983) and Harvey et

al . ( 1987) also show that human neonates

have r emar kably large bra ins (corrected for

b od y s iz e) com p a r e d w it h ot h e r p r im a t e

species. Together, r elatively lar ge neonata l

b r a in s iz e a n d t h e h i gh p os t n a t a l g r ow t h

rate give adult humans the largest encepha-

lization quotient (an allometric scaling of

br ain to body size) of al l h igher pr imates

(Fig. 9).

Mart in (1983)h ypothesizes th at a hu ma n-

like patt ern of bra in an d body growth be-comes necessary once adult hominid brain

size reaches about 850 cc. This biological

mar ker is based on a n ana l ysi s of cephalo-

pelvic dimensions of fetuses and their moth-

ers across a wide range of social mammals,

i ncl uding cetaceans, extant pr imates, and

fossil hominids (Martin, 1983). Given the

m e a n r a t e of p o st n a t a l b r a in g r ow t h for

living apes, an 850-cc adult brain size may

be a chieved by all h ominoids, including ex-

t i n ct h om i n id s , b y l en g t h en i n g t h e fe t a l

stage of growth. At brain sizes above 850 cc

t h e s iz e of t h e p e lv ic i n le t of t h e fos s il

hominids, and living people, does not allow

for sufficient fetal growth. Thus, a period of

rapid postnatal brain growth and slow bodygr owth the human patter n is needed t o

reach adu lt brain size.

Martins an alysis is elegant a nd t enable.

Never theless, the differ ence between ape

a n d h u m an b ra in gr ow th is n ot on ly a

mat ter of velocity; it is also a mat ter of life

history stages. Brain growth for both apes

and hu mans ends a t the sta r t of the juvenile

stage, which m eans t hat apes complete brain

growth during infan cy. Hu man s, h owever,

inser t the childhood stage between the i n-

fant and juvenile stages. Childhood may

p r ov id e t h e t i m e a n d t h e con t i n u a t ion of

par ental investment, necessar y t o gr ow th e

lar ger human br ain. Following this l ine of

r easoning, any fossil human , or any of our

fossil hominid ancestors, with an adult brain

size above Mar tins cerebra l Rubicon of

850 cc may ha ve included a childhood st age

of growth a s pa rt of its life history.

Fig. 10. Idealized velocity curve of huma n growth for boys (solid line): I, infancy; C, childhood; J ,

ju ven ili t y; A, a dol es cen ce; M, m a t u r e a du lt . Th e da sh ed lin e is a si xt h- de gr ee poly nom ia l cu r ve fit t o t hevelocity curve data. The polynomial curve does not fit well to real growth data due to the pulses of them id -ch ild h oo d sp u rt (MC S ) an d th e ad o lescen t sp u rt (AS ). T h e h u m an v elo city cu rv e can n o t b e fi tadequat ely by a single continuous mat hema tical function. At a minimum , three functions are requir ed.



18/27

Given t his backgr ound, Fi gur e 12 is my

Schultz-inspired summary of the evolution

of the h uma n pa ttern of growth and develop-

ment from birth to age 20 years (the evolu-

tion of adolescence is not discussed in this

a r t i cl e, b u t s e e B og in , 1 99 3, 1 99 4a , a n d

Bogin and Smith, 1996). Figure 12 must beconsider ed a s a work in progress, as only

t he data f or the fir st and l ast species (Pa n

a n d Homo sapiens) a r e k n o w n w i t h s o m e

ce r t a in t y. T h e p a t t e r n s of g r ow t h of t h e

fossil hominid s pecies a re reconstr uctions

based on published an alyses of skeletal a nd

denta l development of fossil specimens tha t

died before reaching adulthood (see Smith,

1991b, 1992; Bogin and Smith, 1996, for

reviews of this literature). One major mes-

s a ge t o t a k e fr om t h e fi gu r e is t h a t t h e

pr olongation of t he total t ime for gr owth

t h a t p la ys s u ch a p r om in e n t r ole in t h e

hypotheses of neoteny and hypermorphosisi s d e fi n it e ly a p a r t of h u m a n e vol u t ion .

However, time prolonga tion is not sufficient

to account for the inser tion of th e new sta ges

of childhood an d adolescence th at ar e par t of

huma n gr owth.

Au st ra lop it h ecu s af ar en si s is considered

by most paleontologists to be a hominid, but

shar es m any a natomical f eatur es with non-

hominid pongid species including an adult

bra in size of about 400 cc (Simons, 1989) an d

a patter n of dental development indisti n-

guishable from extant apes (Smith, 1991b).

Therefore, the chimpanzee and A. afarensis

a r e d e pi ct e d i n F i gu r e 1 2 a s s h a r i n g t h etypical tr ipartite s tages of postna tal growth

of social mammalsinfant, juvenile, adult

(Per eir a and Fair banks, 1993). To achieve

the lar ger adult br ain size of A. africanus

( 442 cc) may have r equir ed an addition to

the length of the fetal and/or infancy per i-

ods. The ra pid expansion of adult br ain size

during the time ofHomo h abilis (650 to 800

cc) might have been achieved with further

e xp a n s ion of b ot h t h e fe t a l a n d i n fa n cy

per iods, as Ma rt ins cerebr al Ru bicon wa s

not surpassed. However, the insertion of a

brief childhood stage into hominid life his-

tory may have occurred. This conjecture isb a s e d o n a c o m p a r i s o n o f h u m a n a n d a p e

reproductive s tra tegies.

Ther e ar e l imits to amount of delay be-

t w ee n b ir t h a n d s e xu a l m a t u r i t y, a n d b e-

tween successful births, that any species can

tolerat e. The great apes a re examples of this

limit. Chimpanzee females in the wild reach

F i g. 1 1. G r ow t h c u r v e for h u m a n b r a in a n d b o dycom p ared to th a t o f th e ch im p an zee. T h e len g th of th ehuma n fetal phase, in which brain and body grow at th esam e rate for b oth sp ecies, is ex ten d ed fo r h u m a n s.Chimpanzee brain growth slows after birth, but hu man sm a i n t a in t h e h i gh r a t e of b r a i n g r ow t h d u r i n g t h e

p o stn atal p h ase. In co n trast, th e rate o f h u m an b o d yg r ow t h s l ow s a f t er b ir t h . I f t h e h u m a n b r a in /b od yg r ow t h r a t e w e r e e q u a l t o t h e ch i m pa n z ee r a t e , t h e nadult humans would weigh 454 kg and stand nearly 3.1m ta ll (indicated by the Q). After Mar tin (1983).



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menar che ( the fir st menst r uat i on) at 11 to

12 years of age an d have th eir first births a t

an average age of 14 year s (Goodall, 1983).

The avera ge period between su ccessful birth s

in th e wild is 5.6 years an d young chimpan -

z ee s a r e d e pe n d en t on t h e ir m ot h e r s for

about 5 years (Teleki et al., 1976; Goodall,

1983; Nishida et al., 1990). Actuarial data

collected on wild-living animals indicate that

between 35 percent (Goodall, 1983) and 38

percent (Nishida et al., 1990) of all live-born

chimpan zees sur vive t o th eir m id-20s. Al-

though this is a significantly greater percent-

age of sur vival tha n for m ost other s pecies ofanimal s, the chimpanzee i s at a r epr oduc-

tive threshold. Goodall (1983) reports that

f or the per iod 1965 to 1980 ther e wer e 51

bir ths and 49 deaths in one community of

w il d ch i m pa n z ee s a t t h e G om b e S t r e a m

National Park, Tanzania. During a 10-year

per iod at the Mahale Mount ai ns National

P a r k , Ta n z a n ia N is h id a e t a l . ( 19 90 ) o b-

served 74 birth s, 74 death s, 14 immigra-

tions a nd 13 emigra tionsin one comm un ity.

Chimpanzee population growth is, by these

data, effectively equal to zero. Galdikas and

Wood (1990) present data for t he oran gutan

t h a t s h o w t h a t t h e s e a p e s a r e i n a m o r eprecarious situ ation. Compar ed with th e 5.6

years between successful birth s of chimpan -

zees, the or anguta n femal e wai ts up to 7.7

y ea r s , a n d or a n g u t a n p op u la t i on s a r e i n

decline. Lovejoy (1981) calls the plight of

great ape reproduction a demograph ic di-

lem ma (p. 211).

The great apes, and fossil hominids such

a s Australopithecus or early Homo, seem t o

have reached this demographic dilemma by

extendin g infan cy, forcing a dem an d on nu rs-

ing to its limit (Fig. 12). Exten ding infancy

and birth intervals beyond the chimpanzee/

Au st ra lop it h ecu s r ange may not have been

possible if hominids such as Homo habilis

w er e t o r e m a i n e xt a n t . E a r l y Homo m a y

have overcome this demographic dilemma

by reducing the length of infancy an d insert -

ing childhood between the end of inf ancy

and the juvenile per iod. Fr ee f r om the de-

ma nds ofn urs ing and the physiological brakethat fr equent nur sing places on ovulation

(Ellison, 1990), mother s could repr oduce soon

af ter their infants became childr en. Thi s

certa inly occurs among modern hum an s. An

often cited example, the !Kung, are a tradi-

t ion a l h u n t i n g a n d g a t h er in g s ocie t y of

south ern Africa. A !Kun g woman s age at her

first birth avera ges 19 years a nd subsequent

birth s follow about every 3.6 years , result ing

in an aver age f er t i li ty r a te of 4.7 childr en

per woman (Howell , 1979; Shor t , 1976).

Women in another hunt er -gather society,

the Hadza (Blurton-Jones et al., 1992), have

even shor ter inter vals between successfulbirths, stop nursing about 1 year ea rlier, and

aver age 6.15 bir ths per woman. For these

r easons a br ief childhood stage for Homo

habilis is indicated in Figure 12.

Further brain size increase occurred dur-

in g H. erectus t i m es . T h e e a r li es t a d u l t

specimens have brain sizes of 850 to 900 cc.

F i g. 1 2 . T h e e v ol u t i on o f hominid life history during thefirst 20 years of life. Abbrevi-ated n o m en clatu re as follo ws:A. a fa r ., Australopithecusafarensis;A. africa., Australopi-t h e cu s a f r i ca n u s ; H . h ab ilis,

Hom o ha bilis; H. erec. 1, earlyHom o erectu s; H . erec. 2 , lateHom o erectu s; H. sapiens, Ho mosapiens.



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This places H. erectus at or a bove Mar tins

cerebr al Ru biconan d seem s to just ify inser -

tion and/or expansion of the childhood pe-

riod to pr ovide t he biological t ime n eeded for

t h e r a p id , h u m a n -l ik e , p a t t er n of b r a ingr owth. I t should be noted f r om Figur e 12

tha t t he m odel of huma n evolution proposed

here predicts th at from the Australopithecus

t o t h e H. erectus stage the inf ancy per iod

shr inks as the childhood stage expands. I f

t h e i n fa n cy s t a g e d i d i n fa ct s h r in k , H .

erectus, and all later hominids, would have

enjoyed an even greater reproductive advan -

tage over all other hominids. Later H. erec-

tus, with adult brain sizes up to 1,100 cc, are

depicted with fur th er expansi on of child-

h ood a n d t h e i n se r t ion of t h e a d ol es ce n t

stage. I n addition to bigger br ains, the ar -

chaeological record for later H. erectu s shows

increased complexity of technology (tools,

fir e, an d shelter ) and social or ganization

(Klein, 1989). These techno-social advances

a r e l ik e ly t o b e cor r e la t e s of ch a n g es i n

biology and beh avior associat ed with f

Bogin Evolution 2

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