How humans evolved

1
How humans evolved

• Chapter 21

2
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.

3
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.

4
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.

5
A primate evolutionary tree
Hominoids
Prosimians
Anthropoids
New World monkeys
Lemurs and lorises
Old World monkeys
Hominids
Chimpanzees
Gorillas
Orangutans
Tarsiers
Gibbons
0
10
Time (millions of years ago)
20
30
40
Primate ancestor
6
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.

7
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

8
How the Apes Evolved

• Hominoids evolved from anthropoid ancestors.
• Hominoids are comprised of the apes and the
  hominids (humans and their direct ancestors).

9
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.

10
How the Apes Evolved

• African apes evolved more recently (between 610
  M.Y.A.).
• These apes are the closest living relatives to
  humans.

11
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.

12
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.

13
A comparison of ape and hominid skeletons
Chimpanzee
Australopithecine
Skull attaches posteriorly
Skull attaches inferiorly
Spine S-shaped
Spine slightly curved
Arms longer than legs and also used for walking
Arms shorter than legs and not used for walking
Long, narrow pelvis
Femur angled out
Bowl-shaped pelvis
Femur angled in
14
Walking Upright

• As the worlds climate began to change 510
  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.

15
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.

16
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.

17
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.

18
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.

19
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
H. erectus
H. ergaster
Classified by some scientists as the
single species Homo habilis
H. habilis
Homo rudolfensis
Kenyanthropus platyops
Ardipithecus ramidus
A. afarensis
A. robustus
Orrorin tugenensis
A. sediba
A. africanus
Sahelanthropus tchadensis
A. aethiopicus
Australopithecus anamensis
A. boisei
0
1
2
3
4
5
6
7
Millions of years ago
20
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.

21
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.

22
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.

23
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.

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Where Homo erectus was first discovered
25
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.

26
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

27
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.

28
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.

29
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.

30
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.

31
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.

32
Out of Africamany times
Peking man
Europe
Asia
Africa
Flores island
Java man
33
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.

34
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.

35
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.

36
Homo sapiens is still evolving
Modern Genetic Clusters
Africa
Eurasia
Russian
Yakut
East Asia
Orcadian
Oceania
French
America
Uygur
Mongol
For example the average genome of the Uygur
people is
Japanese
Italian
Maya
Mozabite
Han Chinese
Bedouin
Pashtun
52 East Asian
Columbian
Papuan
46 Eurasian
Yoruba
2 Other races
Homo sapiens originated in northeast Africa
arrows show migration routes of ancestral human
populations