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
Lemurs and
lorises
New World
monkeys
Tarsiers
Old World
monkeys
Gibbons
Orangutans
Gorillas
Chimpanzees
Hominids 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 6–10 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 slightly curved
Spine S-shaped
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 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.

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
Orrorin tugenensis
A. robustus
A. africanus
A. sediba
Sahelanthropus
tchadensis
A. aethiopicus
Australopithecus
anamensis
A. boisei 7 6 5 4 3 2 1
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.

24. 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 Africa—many 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
America
French
Uygur
Mongol
For example the
average genome of
the Uygur people is:
Italian
Japanese
Mozabite
Maya
Bedouin
Han
Chinese
Pashtun
52% East Asian
Columbian
46% Eurasian
Papuan
Yoruba
2% Other races
Homo sapiens
originated in
northeast Africa
*arrows show migration routes
of ancestral human populations