2000 Falk Etal Early Hominid Brain Evolution a New Look at Old Endocasts

7/27/2019 2000 Falk Etal Early Hominid Brain Evolution a New Look at Old Endocasts

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Dean Falk*, John C.Redmond, Jr andJohn GuyerDepartment of Anthropology,University at Albany, SUNY,

Albany, NY 12222, U.S.A.E-mail: [email protected];[email protected];

[email protected]

Glenn C. ConroyDepartments of Anatomy and

Neurobiology/Anthropology,Washington University Schoolof Medicine, St Louis,

MO 63110, U.S.A. E-mail:[email protected]

Wolfgang RecheisDepartment of Radiology II,University of Innsbruck,

Anichstr. 35, A-6020,Innsbruck, Austria. E-mail:[email protected]

Gerhard W. Weberand Horst SeidlerInstitute of Human Biology,University of Vienna,

Althanstrasse 14, A1091,

Vienna, Austria. E-mail:[email protected];[email protected]

Received 6 May 1999Revision received 2 August1999 and accepted5 November 1999

Keywords: Australopithecus,endocasts, frontal lobe,paleoneurology,

Paranthropus, phylogeny,temporal lobe.

Early hominid brain evolution: a new lookat old endocasts

Early hominid brain morphology is reassessed from endocasts ofAustralopithecus africanus and three species of Paranthropus, and newendocast reconstructions and cranial capacities are reported for fourkey specimens from the Paranthropus clade. The brain morphology of

Australopithecus africanus appears more human like than that ofParanthropus in terms of overall frontal and temporal lobe shape.These new data do not support the proposal that increased encephali-zation is a shared feature between Paranthropus and early Homo. Ourfindings are consistent with the hypothesis that Australopithecus afri-canus could have been ancestral to Homo, and have implications forassessing the tempo and mode of early hominid neurological andcognitive evolution.

2000 Academic Press

Journal of Human Evolution (2000) 38, 695717

doi:10.1006/jhev.1999.0378Available online at http://www.idealibrary.com on

Introduction

Much of what is known about hominid brain

evolution has been learned from endocranial

casts (endocasts) that reproduce details of

the external morphology of the brain from

the internal surface of the braincase.

Because these endocasts are usually from

fragmentary pieces of fossilized skulls,

their missing parts must be reconstructed.*To whom correspondence should be addressed.

00472484/00/050695+23$35.00/0 2000 Academic Press


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Discoveries such as KNM-WT 17000 (P.

aethiopicus) and KNM-WT 17400 (P. boisei)

(Leakey & Walker, 1988; Walker et al.,

1986; Brown et al., 1993) provide evidence

of previously unknown parts of the Paran-thropus brain. Prior to these discoveries,

Paranthropus endocasts were sometimes

reconstructed using endocasts of A. africa-

nus (e.g., Sts 5) as a model (see below). In

this study we compare endocasts of Paran-

thropus (including P. robustus, P. aethiopicus,

P. boisei) with those of Australopithecus afri-

canus and identify, quantify, and interpret

previously unknown differences in the fron-

tal and temporal lobe morphology between

these genera. In addition, we provide revised

estimates for the mean cranial capacity of

Paranthropus.

Materials and methods

Previously unknown parts of the cerebral

cortex in Paranthropus were observed and

measured on both the endocast of

KNM-WT 17000 (P. aethiopicus) and on a

silicone endocast prepared from a cast of

KNM-WT 17400 (P. boisei). Corresponding

observations and measurements for Austra-

lopithecus africanus were obtained from sili-

cone endocasts prepared from museum

casts of Sts 5 (Mrs. Ples) and Stw 505 (Mr.

Ples), as well as from a copy of the natural

endocast of Sts 60. Other endocasts used for

comparative purposes included KNM-ER

23000 (P. boisei), Sts 19 (A. africanus) and

the Sterkfontein Number 2 natural endo-cast (A. africanus). Comparative endocast

measurements were taken (by JG) from ten

gorillas (G. gorilla), nine chimpanzees (P.

troglodytes), nine bonobos (P. paniscus), and

ten modern humans. Associated cranial

capacities were obtained with mustard seed

for the gorilla and chimpanzee sample and

from the literature for the human, bonobo,

and early hominid sample.

As a preliminary step, GWW and DFvalidated the size of the silicone endocasts

for two of the specimens (Sts 5 and Stw

505) by comparing several measurements

obtained using calipers with measurements

taken on their corresponding virtual endo-

casts that had been acquired with 3D-CTtechnology from the original skulls (Conroy

et al., 1998). The maximum length, height,

and width obtained by measuring the virtual

endocast of Sts 5 on the computer screen

were 098, 100, and 100 of the respective

measurements obtained with calipers from

the silicone endocast. The length of the

fragmentary Stw 505 virtual endocast and

the distance between its left frontal and

temporal poles were each 098 of the respec-

tive measurements obtained from the sili-

cone endocast. A third measurement on the

virtual endocast of Stw 505 (between its

highest point on the dorsal surface and its

lowest point at the anterior end of the tem-

poral lobe) measured 099 of the compar-

able measurement of the silicone endocast.

Thus, as detailed elsewhere (Weber et al.,

1998), endocasts prepared from museum

quality casts of skulls reproduce measure-

ments obtained with 3D-CT technology

from the braincases of the original

specimens with a high degree of accuracy.

Eight measurements (described below)

were obtained with calipers from basal views

of endocasts and projected onto the basal

plane. The procedure for orienting an endo-

cast in basal view is to first determine the

maximum antero-posterior diameter of the

endocast in left lateral view (using the right

hemisphere, if left is not present) that con-nects the frontal and occipital poles as

described and illustrated by Connolly

(1950:124125). The endocast is then

turned upside down and secured so that the

maximum anterior-posterior diameter is in

the horizontal or basal plane and the mid-

sagittal plane is vertical. In cases of partial

endocasts (e.g., Sts 60, Stw 505; KNM-WT

17400), basal orientations were estimated

by aligning them next to correctly orientedfull endocasts from the same genus (e.g., Sts

696 . ET AL.


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5; KNM-WT 17000). The fossil hominid

measurements were from undistorted and

unreconstructed portions of endocasts. In

order to reduce potential observer error or

bias, measurements were taken together bythree observers (DF, JG, and JCR) on two

different occasions and the results averaged.

JG took the measurements with sliding cali-

pers, while JR and DF confirmed his selec-

tion of landmarks and readings from the

calipers, and made sure the calipers

remained oriented so that measurements

were projected onto the basal plane.

In order to quantify remeasurement error,

the three workers together repeated all of the

measurements on the fossil hominids one

year after the first measurements were

obtained and then compared their results

with the earlier ones. For each of the eight

measurements, remeasurement error was

calculated as the mean of the absolute dif-

ferences (determined for each specimen)

between the first and second sets of

measurements. Remeasurement error was

then expressed as a percentage of the aver-

age length for each measurement. The

results were 1% for measurements 1, 2, 6,

and 8; and ranged from 28% for the other

four measurements. The highest remeasure-

ment errors expressed as percentages of

mean lengths were for the shortest lengths.

Similarly, JG remeasured 12 endocasts

(three each from humans, bonobos, chim-

panzees, and gorillas) one year after taking

the initial measurements from these speci-

mens. The results ranged from 18%, withthe greatest relative remeasurement error

associated with the shortest lengths.

The measurements included (Figure 1):

(1) batbat, the distance between the most

anterior points of the temporal lobes in basal

view; (2) matmat, maximum width of the

frontal lobes at the level of bat; (3) mbat

rof(tan), the shortest distance between the

middle of the line connecting the two bats

and the tangent to the most rostral point onthe orbital surfaces of the frontal lobes (note

that rof should not be confused with the

frontal pole); (4) mcpmbat, the shortest

distance between the middle clinoid process(or anterior border of the sella turcica) and

mbat; (5) mcprof(tan), the shortest dis-

tance between the middle clinoid process

and rof(tan); (6) cobrof(tan), the short-

est distance between the caudal boundary of

the olfactory bulbs (cribriform plate) and

rof(tan); (7) robrof(tan), the shortest dis-

tance between the rostral boundary of the

olfactory bulbs and rof(tan); (8) rof(tan)

bpc(tan), the shortest distance betweenrof(tan) and the tangent to the most

2

1

rof

rob

cob

5mat

bat

mcp

4

mbat

3

6

7

batmat

8

bpc

Figure 1. Measurements obtained from basal views and

projected onto the horizontal (basal) plane from endo-

casts of australopithecines, apes, and humans (see text

for details and Table 1 for data). Landmarks: bat, most

anterior point on temporal lobe from basal view; mat,most lateral point on endocast at the level of bat inbasal plane; mbat; middle of the line connecting thetwo bats; rof, the most rostral point on the orbital

surfaces of the frontal lobes; mcp, middle clinoidprocess; cob, caudal boundary of olfactory bulbs (cri-briform plate) in midline; rob, rostral boundary ofolfactory bulbs in midline; bpc, most posterior point oncerebella in basal view.

697


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posterior point on the cerebella in basal view

(bpc).

Measurements 3, 6, 7, and 8 were used to

calculate three additional lengths for each

specimen: [36] the length between mbatand cob; [67], the length of the olfactory

bulb (cribriform plate); and [83], the

length of the basal aspect of the endocast

caudal to mbat. Indices that express each

measure as a percentage of endocast length

in basal view were calculated by dividing

these three lengths as well as measurements

17 by measurement 8 for the great ape and

human samples, and for the two hominid

endocasts for which measurement 8 was

available (KNM-WT 17000 and Sts 5).

Descriptive statistics were provided for

the lengths (Table 1) and indices (Table 2)

obtained from endocasts of Homo, Gorilla,

Pan, Paranthropus and Australopithecus.

These data were first compared in living

hominoids in order to establish a compara-

tive basis for assessing endocasts represent-

ing Australopithecus and Paranthropus. For all

of the comparisons in this study, there were

significant differences between groups which

were determined by post-hoc analyses of

selected contrast within the general linear

model (GLM) of SPSS (version 8.0). The

alpha level was preset to Pc005 after

correction with Bonferronis method for

multiple comparisons where the indicated

P-values had been adjusted (Tables 2 & 3).

Differences in the mean lengths and indices

were also tested for statistical significance

between the two species of Pan. The onlytwo measurements that were found to differ

significantly between P. troglodytes and P.

paniscus were variables 4 and 4/8. Conse-

quently, for these two variables, results are

reported separately for these two species.

Endocasts of Paranthropus and Australo-

pithecus were compared to each other and

to endocasts from Pan, Gorilla, and Homo

(Table 3) by computing mean differences,

standard errors, and P-values from the dataprovided in Table 1. Finally, the above

observations and statistically significant

results were synthesized and the key features

summarized for endocasts from apes, early

hominids, and humans (Table 4).

Additionally, because previously un-known parts of Paranthropus endocasts are

now available, new endocast reconstructions

were made for Paranthropus specimens

SK 1585 (P. robustus) , O H 5 (P. boisei),

KNM-ER 732 (P. boisei), and KNM-ER

407 (P. boisei), using appropriate unrecon-

structed parts of Paranthropus endocasts as

models. Endocast reconstruction methods

and cranial capacity determinations are

detailed in the Appendix.

Results

Gorilla, Pan, and Homo

Mean measurements from basal views of

endocasts are presented in Table 1, and

means of indices generated by dividing vari-

ables 17 and [36], [67] and [83] by

endocast lengths are provided in Table 2.

Not surprisingly, the means for larger-

brained Homo are significantly greater than

the means for smaller-brained Gorilla and

Pan for variables 18, [67], and [83].

All P-values are


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Tab

le1

Endocastmeasurements

batbat

1

matmat

2

mbat

rof(tan)

3

mcp

mbat4

mcp

rof(tan)

5

cob

rof(tan)

6

rob

rof(tan)

7

rof(tan)

bpc(tan)

8

Cranial

capacity(cm

3)

9

Length

mbatcob

[36]

Length

olf.bulb

[67]

Basecaudal

tobat

[83]

Hom

o(n=10)

M

ean

6970

10890

3890

1600

5389

3030

900

15010

1350

860

2130

11120

S.D.

627

742

376

231

588

474

211

671

17550

392

452

661

Range

(580800)

(9801210)(3

40430)

(120200)

(460640)(2

20370)

(60120)

(14001640)

(10130)

(140270

)

(10101220)

Gorilla(n=10)

M

ean

4900

7760

3400

570

4090

1660

460

12090

48350

1740

1200

8690

S.D.

249

462

291

177

325

190

151

590

7933

227

067

526

Range

(450520)

(710860)(3

10390)

(3080)

(370460)(1

30190)

(2070)

(11401290)

(37505850)

(130200)

(110130

)

(800960)

Pan(n=18)

M

ean

4730

7494

3141

856

4029

1644

479

10756

39267

1476

1126

7633

S.D.

366

522

317

223

399

293

135

388

2500

270

354

468

Range

(400530)

(640830)(2

60360)

(50120)

(320450)(1

00210)

(2080)

(9701140)

(36004450)

(100220)

(10170)

(660840)

Para

nthropus

W

T17000

5000

7400

3400

500

3900

2000

700

11400

41000

1400

1300

8000

W

T17400

4500

6800

2900

400

3300

1400

700

39000

1500

700

M

ean

4750

7100

3150

450

3600

1700

700

40000

1450

1000

8000

S.D.

354

424

354

071

424

424

000

1414

071

424

Range

(450500)

(680740)(2

90340)

(4050)

(330390)(1

40200)

(7070)

(39004100)

(140150)

(70130)

Australopithecus

St

s5

5600

8500

3900

1400

5300

3000

1400

11800

48500

900

1600

7900

St

w505

3300

1100

4400

2900

1100

51500

400

1800

St

s60

6200

3100

1100

4200

42800

M

ean

5900

3433

1200

4633

2950

1250

47600

650

1700

S.D.

424

416

173

586

071

212

4419

250

100

Range

(560620)

(3

10390)

(110140)

(420530)(2

90300)

(110140)

(42805150)

(4090)

(160180

)

N

ote:Descriptivestatisticsformeasu

rements(seeFig.1)takenfromend

ocastsofaustralopithecines,apes,a

ndhumans.Cranialcapacitiesforg

orillasandPan

wereobtainedfromskulls,whilethoseforhumansarefromPakkenberg&Gundersen,1997.AlthoughthemeancranialcapacitylistedforPanisforP.troglodytes

only,ameancranialcapacityof350cm

3

hasbeenpublishedelsewhereforP.paniscus(Cramer,1977).ForS

tw505,mbatwasdeterminedbyt

heintersection

ofthetangenttotheleftbatwiththeintactmidsagittalplane.

699


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Tab

le2

Descriptivestatisticsand

P-valuesforendocastindices

1/8

2/8

3/8

4/8

5/8

6/8

7/8

[36]/8

[67]/8

[83]/8

Hom

o(n=10)

M

ean

046*

073*

026

011*

036

020*

006*

006*

014*

074

S.D.

004

003

002

002

004

003

001

003

003

002

R

ange

(038050)(066076)

(022030)

(008013)

(031042)

(016025)

(00

4008)

(001009)

(010018)

(070078)

Gorilla(n=10)

M

ean

041*

064*

028

005*

034

014*

004*

014*

010*

072

S.D.

002

003

002

001

002

002

001

002

0008

002

R

ange

(037044)(057069)

(025031)

(003006)

(030060)

(010016)

(00

2006)

(011017)

(009011)

(069075)

Pan

(n=18)

M

ean

044

069

029

008

037

015

004

014

010

071

S.D.

003

004

003

002

004

003

001

002

003

003

R

ange

(038049)(062077)

(025034)

(005011)

(030042)

(009020)

(00

2008)

(001020)

(001016)

(066075)

*P

2000 Falk Etal Early Hominid Brain Evolution a New Look at Old Endocasts

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