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 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

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 Arms shorter than legs and not used for walking Long, narrow pelvis 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 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

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

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”

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 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