How humans evolved

CHAPTER 21

HOW HUMANS EVOLVED

THE EVOLUTIONARY PATH TO HUMANS

• The story of human evolution begins around 65 M.Y.A.• This time marks the

explosive radiation of a group of small, arboreal mammals called the Archonta.• They were likely nocturnal and were arboreal and insectivorous.

• Their radiation gave rise to different types of mammals, including:• Bats, tree shrews, and primates.• Primates are the order of mammals that includes

humans.


• Primates are mammals with two distinctive features that allowed them to succeed in the arboreal, insect-eating environment:• Grasping fingers and toes• The first digit in many primates is opposable

and at least some of the digits have nails.• Binocular vision• This permits the brain to judge distance

precisely.


• About 40 M.Y.A. the earliest primates split into two groups:• Prosimians• Surviving representatives today include the

tarsiers, lemurs, and lorises.• Most are nocturnal.

• Anthropoids• These higher primates included monkeys,

apes, and humans.• The early anthropoids, now extinct, likely

evolved in Africa.

A PRIMATE EVOLUTIONARY TREE

Prosimians Anthropoids

Hominoids

HominidsChimpanzeesGorillasOrangutansOld Worldmonkeys

Primate ancestor

New Worldmonkeys

TarsiersLemurs andlorises0

Tim

e (m

illi

on

s o

f ye

ars

ago

)

40

30

20

10

Gibbons


• The monkeys are a very successful group of primates.• New World monkeys• South American descendants of African

ancestors.• All are arboreal, have flat spreading noses,

and prehensile tails.


• Old World monkeys• Descendants of the ancestral anthropoids that

remained in Africa.• None have prehensile tails.• Include both ground-dwelling and arboreal

species

HOW THE APES EVOLVED

• Hominoids evolved from anthropoid ancestors.• Hominoids are comprised of the apes and the

hominids (humans and their direct ancestors).


• Studies of ape DNA have revealed much about how living apes evolved.• Asian apes evolved

first.• Gibbons diverged

from other apes about 15 M.Y.A.

• Orangutans split off about 10 M.Y.A.

• Neither are closely related to humans.


• African apes evolved more recently (between 6–10 M.Y.A.).• These apes are the closest living relatives to

humans.


• Chimpanzees are more closely related to humans than gorillas are.• Chimpanzees diverged from the ape line less

than 6 M.Y.A.• The genes of humans and chimpanzees have

not had time to evolve many differences.• Humans and chimpanzees share 98.6% of

their nuclear DNA.• Gorilla DNA differs from human DNA by about

2.3%.• Gorillas split off from the ape line around 8

M.Y.A.


• The common ancestor of apes and hominids is thought to have been an arboreal climber.• Much of the subsequent evolution of the

hominoids differs with respect to locomotion.• Hominids evolved bipedal walking.• Anatomical features include S-shaped spine,

bowl-shaped pelvis, lower limbs larger than upper limbs.

• Apes evolved knuckle-walking.• Anatomical features include slightly curved

spine, long pelvis, upper limbs larger than lower limbs.

A COMPARISON OF APE AND HOMINID SKELETONS

Chimpanzee

Skull attaches posteriorly

Australopithecine

Spine S-shaped

Arms shorter than legsand not used for walking

Femur angled in

Femur angled out

Long, narrow pelvis

Arms longer than legs andalso used for walking

Spine slightly curved

Skull attaches inferiorly

Bowl-shaped pelvis

WALKING UPRIGHT

• As the world’s climate began to change 5–10 M.Y.A., forests began to be replaced by savannas, and bipedal apes began to evolve.• Hominids include species of the genus Homo,

species of the smaller-brained genus Australopithecus, and several older lineages.• Hominids are bipedal.

WALKING UPRIGHT

• Bipedalism arose 4 million years ago, before brain expansion.• Fossils of hominids, such

as the Laetoli footprints, exhibit bipedalism.• Brain expansion

occurred about 2 million years ago.

THE HOMINID FAMILY TREE

• Recent fossil specimens of early hominids have been found that date as far back as 6 to 7 million years.• These fossils are confusing because they show

a mixture of primitive and modern traits.• There are too few of these very old fossils to

make certain their connections to australopithecines and humans.


• In 1995, hominid fossils dating 4.2 million years old were found in the Rift Valley of Kenya.• They were assigned to the species

Australopithecus anamensis.• They represent an intermediate form between

apes and A. afarensis.• This species represents a base of the family

tree for human evolution.


• Scientists have taken two different approaches in classifying the species of Homo.• “Lumpers” recognize three species of Homo,

focusing on common elements among fossils and attributing the differences to diversity within the group.• “Splitters” recognize at least seven species of

Homo.

A HOMINID EVOLUTIONARY TREE

01234567

H. sapiens

H. heidelbergensis

H. neanderthalensis

H. erectus

H. ergaster

H. habilis

Homo rudolfensis

Kenyanthropus platyops

Ardipithecus ramidus

Orrorin tugenensis

Sahelanthropustchadensis Australopithecus

anamensis A. boisei

A. aethiopicus

A. sediba

A. robustus

A. afarensis

A. africanus

Millions of years ago

Classified by somescientists as the singlespecies Homo habilis

Classified by somescientists as the singlespecies Homo erectus

Classified by somescientists as the singlespecies Homo sapiens

AFRICAN ORIGIN: EARLY HOMO

• The first humans evolved from australopithecine ancestors about 2 M.Y.A.• Homo habilis• Larger brain volume than

Australopithecus but was similarly short in stature.

• Called “handy man” because of its association with tools.


• Homo rudolfensis • Described from a

specimen discovered in 1972 with a larger brain capacity than H. habilis.

• Homo ergaster • A species used to describe

specimens that have a larger brain capacity than H. rudolfensis and a skeleton more like modern humans and less like australopithecines.


• Too few fossils have been found of early Homo to explain with certainty the evolution of Homo.• If two species are accepted, then it would

appear Homo underwent an adaptive radiation.• Because of its modern skeleton, Homo ergaster

is thought to be the most likely ancestor to later species of Homo and is often lumped with Homo erectus.

OUT OF AFRICA: HOMO ERECTUS

• Homo erectus is definitely a true human and has been supported by many specimen finds, including those of Java Man and Peking Man.• Homo erectus was taller and had a larger

brain than H. habilis.• The shape of the skull interior suggests that it

was able to talk.• It was a social species.

WHERE HOMO ERECTUS WAS FIRST DISCOVERED

24

OUT OF AFRICA: HOMO ERECTUS

• The oldest specimen of Homo erectus is from Africa, indicating that H. erectus arose in Africa.• Homo erectus survived for over a million

years, longer than any other species of human.• Homo erectus disappeared about 500,000

years ago in Africa, but the species survived even longer in Asia.

OUR OWN SPECIES ALSO EVOLVED IN AFRICA

• Modern humans first appeared in Africa about 600,000 years ago.• According to some scientists, there have

been three species of modern humans:• Homo heidelbergensis• Homo neanderthalensis• Homo sapiens


• Homo heidelbergensis is the oldest known modern human.• An Ethiopian fossil dates back to 600,000 years

ago.• It coexisted with H. erectus in Africa but had

more advanced features.• It had a bony keel running along the midline of

the skull, a thick ridge over the eye sockets, and a large brain.• Its range included parts of Africa, Europe, and

western Asia.


• About 130,000 years ago, Homo neanderthalensis appeared in Europe.• Compared to modern humans, Neanderthals

were short, stocky, and powerfully built.• Their skulls were massive.


• The oldest known fossil of Homo sapiens is 130,000 years old and occurred in Africa.• Outside of Africa and the Middle East, the

earliest known fossils of H. sapiens are no older than 40,000 years.• This implies that Homo sapiens first

evolved in Africa and then migrated to the rest of the world.


• Recently-Out-of-Africa model• This view of Homo evolution states that Homo

sapiens evolved in Africa and then migrated to Europe and Asia.

• Multiregional hypothesis• This view of Homo evolution states that the

human races evolved independently from Homo erectus in different parts of the world.


• Studies of human mitochondrial DNA (mtDNA) and chromosomal nuclear DNA have helped to clarify the controversy over the origin of Homo sapiens.• Because DNA accumulates mutations over time,

the oldest populations should show the greatest genetic diversity.• All modern humans of different ethnic

backgrounds share a common ancestor dating back 170,000 years ago.• Only 52,000 years ago do Africans separate

from non-Africans.• Essentially the migration of Homo sapiens out

of Africa followed the same paths taken by Homo erectus half a million years before.

OUT OF AFRICA—MANY TIMES

32

Europe

Africa

“Peking man”

Asia

Floresisland

“Java man”


• Evidence has begun to accumulate suggesting that until as recently as 13,000 years ago another species of human existed in Indonesia.• Homo floresiensis

• Even more recently, in 2010, evidence has emerged pointing to yet a fifth species of recent human.• One that coexisted with Neanderthals and

Homo sapiens in Asia 40,000 years ago.

THE ONLY SURVIVING HOMINID

• Neanderthals• This species of modern human were common in

Europe and Asia around 70,000 years ago.• They made diverse tools and lived in huts or

caves.• They did not interbreed with members of Homo

sapiens.• Cro-Magnons• Fossils of these early members of Homo sapiens

date back as late as 100,000 years ago in Europe.• They appear to have completely replaced the

Neanderthals around 34,000 years ago.• They used sophisticated tools and likely had full

language capabilities.

THE ONLY SURVIVING HOMINID

• Modern Homo sapiens• Humans eventually spread across Siberia to

North America about 13,000 years ago.• A recent genomic survey provides clear

evidence.• Human evolution has been characterized by an

increase in brain size and the ability for conceptual thought.

HOMO SAPIENS IS STILL EVOLVING

Modern Genetic Clusters

For example theaverage genome ofthe Uygur people is:

52% East Asian

46% Eurasian2% Other races

*arrows show migration routesof ancestral human populations

Orcadian

French

ItalianMozabite Bedouin

Russian

Uygur Mongol

Yakut

Japanese

HanChinese

Papuan

Homo sapiensoriginated innortheast Africa

Pashtun

Yoruba

Maya

Columbian

Africa

Eurasia

East Asia

Oceania

America

How humans evolved

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