HOW HUMANS EVOLVED
CHAPTER 21
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.
THE EVOLUTIONARY PATH TO
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.
THE EVOLUTIONARY PATH TO
HUMANS
• 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
Hominoids
Time (millions of years ago)
Prosimians
0
Lemurs and
lorises
Tarsiers
Anthropoids
New World
monkeys
10
20
30
40
Primate ancestor
Old World
monkeys
Gibbons
Orangutans
Gorillas
Chimpanzees Hominids
THE EVOLUTIONARY PATH TO
HUMANS
• 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.
THE EVOLUTIONARY PATH TO
HUMANS
• 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).
HOW THE APES EVOLVED
• 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.
HOW THE APES EVOLVED
• African apes evolved more recently (between 6–
10 M.Y.A.).
• These apes are the closest living relatives to
humans.
HOW THE APES EVOLVED
• 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.
HOW THE APES EVOLVED
• 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
Australopithecine
Skull attaches posteriorly
Skull attaches inferiorly
Spine slightly curved
Spine S-shaped
Arms longer than legs and
also used for walking
Long, narrow pelvis
Arms shorter than legs
and not used for walking
Femur angled out
Bowl-shaped pelvis
Femur angled in
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.
THE HOMINID FAMILY TREE
• 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.
THE HOMINID FAMILY TREE
• 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
H. sapiens
Classified by some
scientists as the single
species Homo sapiens
H. heidelbergensis
H. neanderthalensis
Classified by some
scientists as the single
species Homo erectus
Classified by some
scientists as the single
species Homo habilis
H. erectus
H. ergaster
H. habilis
Homo rudolfensis
Kenyanthropus platyops
Ardipithecus ramidus
A. afarensis
A. robustus
Orrorin tugenensis
A. africanus
Sahelanthropus
tchadensis
7
A. sediba
A. aethiopicus
Australopithecus
anamensis
6
5
A. boisei
4
3
Millions of years ago
2
1
0
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.
AFRICAN ORIGIN: EARLY HOMO
• 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.
AFRICAN ORIGIN: EARLY HOMO
• 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
OUR OWN SPECIES ALSO EVOLVED IN
AFRICA
• 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.
OUR OWN SPECIES ALSO EVOLVED IN
AFRICA
• 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.
OUR OWN SPECIES ALSO EVOLVED IN
AFRICA
• 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.
OUR OWN SPECIES ALSO EVOLVED IN
AFRICA
• 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.
OUR OWN SPECIES ALSO EVOLVED IN
AFRICA
• 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
“Peking man”
Europe
Asia
Africa
Flores
island
“Java man”
32
OUR OWN SPECIES ALSO EVOLVED IN
AFRICA
• 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
Africa
Eurasia
Russian
East Asia
Yakut
Orcadian
Oceania
America
For example the
average genome of
the Uygur people is:
French
Italian
Mozabite Bedouin
52% East Asian
46% Eurasian
2% Other races
*arrows show migration routes
of ancestral human populations
Uygur Mongol
Japanese
Pashtun
Han
Chinese
Papuan
Yoruba
Homo sapiens
originated in
northeast Africa
Maya
Columbian