Chapter 26 Human Evolution

1. Chapter 26Human Evolution

2. 26.1 A Bit of a Neanderthal • Paleoanthropology is the scientific study of prehistoric humans and their relatives • Neanderthals (Homo neanderthalensis) lived in the Middle East, Europe, and in central Asia as far east as Siberia • They had a short, stocky build, a long, low braincase, and pronounced brow ridges • DNA studies show that Neanderthal genes exist in several groups of modern humans (Homo sapiens)

3. Neanderthal and Modern Human

4. 26.2 Primates: Our Order • Primates are an order of placental mammals that includes humans, apes, monkeys, and close relatives • Primate characteristics: • Hands and feet capable of grasping • Eyes at the front of the head • Large brains with expanded regions for vision and information processing • Varied diet; four types of teeth • Provide extensive care to young

5. Origins and Early Branchings • Primates branched into wet-nosed (Strepsirrhini) and dry-nosed primates (Haplorhini) about 50 million years ago • Anthropoidsinclude monkeys, apes, and humans • New World monkeys are climbers with long, prehensile tails that help them maintain balance • Old World monkeys are larger and have no prehensile tail –many spend their lives on the ground • Apes and humans are tailless hominoids

6. Nonanthropoid Primates: Lemur

7. Nonanthropoid Primates: Old World Monkey

8. Take-Home Message: What are primates? • Primates include lemurs, tarsiers, monkeys, apes, and humans. Most primates are tree-dwellers. Traits such as a flexible shoulder joint and grasping hands with nails are adaptations to a climbing lifestyle. • Compared to other mammals, primates have a relatively large brain with more area devoted to vision and less to smell. • There are two main subgroups. Lemurs and their relatives belong to the subgroup with a wet nose and a fixed upper lip. Tarsiers, monkeys, apes, and humans belong to the subgroup with a dry nose and a movable upper lip.

9. 26.3 The Apes • Hominoids are tailless primates with an upright posture and relatively large brains • They branched off from a common ancestor with Old World monkeys about 30 million years ago • Proconsul species lived in North Africa about 20 mya • In the Miocene, new species evolved and dispersed to Europe and Asia

10. Modern Apes • Gibbons (lesser apes) are small apes with elongated arms that inhabit Southeast Asian forests • Great apes include the Asian orangutan and African gorillas, chimpanzees, and bonobos • Gorillas (Gorilla gorilla), the largest living primates, live in forests and feed mainly on leaves • Chimpanzees (Pan troglodytes) and bonobos (Pan paniscus), are our closest living relatives

11. Gibbon

12. Orangutan

13. Gorilla

14. Chimpanzee

15. A Human–Ape Comparison • Only members of the lineage leading to humans are bipedal – adapted to walking upright • The human backbone has an S-shaped curve that keeps the head centered over the feet • Human feet have a pronounced arch and a non-opposable big toe • An ape’s spinal cord enters the skull’s base near the rear – a human spinal cord enters near the center

16. spinal cord attaches at rear of skull C-shaped curve in backbone legs shorter than arms, thighs angle outward flat foot with opposable big toe Figure 26-5a p443

17. spinal cord attaches at center of skull S-shaped curve in backbone legs longer than arms, thighs angle inward foot with arch, big toe aligned with other toes and not opposable Figure 26-5b p443

18. A Human–Ape Comparison (cont.) • A human thumb is longer, stronger, and more maneuverable than that of a chimpanzee • Many monkeys and apes grasp objects in a power grip, but only humans routinely use a precision grip for fine manipulation of objects

19. A Human–Ape Comparison (cont.) • The lineage leading to humans underwent a great expansion of the brain • A human brain is three times the size of a chimpanzee brain • The human brain also grows for a longer period after birth, and our offspring receive extended parental care

20. Take-Home Message: Which human traits are typical of hominoids and which are novel? • Hominoids share a common ancestor with Old World monkeys, but they are generally larger than monkeys, lack a tail, and have an upright stance. Modern hominoids include Asian apes, African apes, and humans. • Upright walking (bipedalism) evolved in the lineage leading to humans. This lineage also shows a trend toward a more maneuverable thumb, a larger and more complex brain, and a longer period of postnatal development.

21. 26.4 Rise of the Hominins • Hominins are the lineage of humans and all extinct species more closely related to humans than to any other group • Bipedalism is a defining hominin trait – researchers studying hominin origins look for evidence of upright walking • Sahelanthropus tchadensis, 7 mya, western Africa • Orrorin tugenensis, 6 mya, East Africa • Ardipithecus species, 5.8 to 5.2 mya, East Africa

22. The Hominins Sahelanthropus tchadensis Orrorin tugenensis Ardipithecus kadabba Ardipithecus ramidus Australopithecus anamensis Australopithecus afarensis Australopithecus africanus Paranthropus boisei Homo habilis Homo erectus Homo neanderthalensis Homo floresiensis Homo sapiens 7 6 5 4 3 2 1 Millions of years ago

23. Australopiths • Australopiths include two genera of hominins that lived in Africa from 4 to 1.2 mya • Australopithecus afarensis lived in Tanzania and other parts of eastern Africa from 3.9 to 3 mya • Footprints indicate that this species had a pronounced arch and a big toe in line with the other toes • A partial skeleton known as Lucy is the best known representative of this species

24. Bipedal Footprints, 3.6 MYA

25. Paranthropus • Paranthropus descended from an Australopithecus ancestor • Strong teeth and heavily muscled jaws allowed Paranthropus to eat hard and fibrous foods • Paranthropus species are considered close relatives of early humans, but not possible ancestors

26. Australopithecusafricanus

27. Paranthropus boisei

28. Factors Favoring Bipedalism • Bipedal walkers move more efficiently across open ground than chimpanzees who move on all fours • Bipedalism keeps the body cooler • An upright stance makes it easier to see predators • Free hands can be used to gather food and carry it from place to place

29. Take-Home Message:What are hominins? • Hominins include modern humans and extinct members of their lineage. • Possible hominin fossils date back as far as 7 million years ago. Early hominins had small brains. Australopiths are a diverse group of African hominins that probably include human ancestors.

30. 26.5 Early Humans • When classifying fossil s, scientists who are “lumpers” look for similarities and assign fossils to relatively few species • Scientists who are “splitters” focus on differences and often name new species on the basis of subtle differences • Lumpers place the earliest fossil evidence of our genus, Homo, in two species – splitters divide them into four or more

31. Homo habilis • Humans are members of the genus Homo • The oldest species is Homo habilis (2.3 mya), though some scientists classify them as a species of Australopithecus • It overlapped in eastern Africa both with australopiths and at least one other species of Homo

32. Homo habilis

33. Homo erectus • Homo erectus appears in the fossil record about 1.8 mya • H. erectus stood on legs that were longer than its arms and had a relatively big brain • H. erectus left Africa and colonized Indonesia by 1.6 million years ago, and China by 1.15 million years ago • Some scientists split Homo erectus fossils into two species: H. erectus in Asia, and H. ergaster in Africa

34. Homo erectus

35. Early Culture • Culture is a set of learned behaviors passed from one individual to another, and from one generation to the next • The earliest hominin cultural trait for which we have fossil evidence is toolmaking, probably by australopiths • H. erectus used stone tools to cut up animal carcasses, to scrape meat from bones, and to extract marrow

36. Communication • H. habilis and H. erectus probably used gestures and calls to communicate • Thus far, scientists have not found fossil evidence of anatomical modifications necessary for spoken language in either early species of Homo

37. Take-Home Message:What traits characterized early humans? • Homo habilis is the most ancient human species known. It lived in Africa and was similar in many respects to australopiths, but its hands and arms more closely resemble those of modern humans • Homo erectus is known from Africa, Europe, and Asia. It had a significantly larger brain than earlier hominins. Females had a pelvis that facilitated birth of offspring with large heads. • Early humans made stone tools and used them to cut up animal carcasses. Stone tool use may have begun among the australopiths.

38. 26.6 Recent Human Lineages • A 195,000-year-old fossil from Ethiopia is the earliest evidence of our species, Homo sapiens • Compared to H. erectus, we have a higher, rounder skull, a larger brain, flatter face, and smaller jawbones and teeth • We also have a chin, a protruding area of thickened bone in the middle of our lower jawbone

39. Origin of Homo Sapiens • Multiregional model: Populations of H. sapiens in different regions evolved from H. erectus in those regions – with gene flow between populations • Replacement model: H. sapiens evolved in Africa, then migrated to different regions and replaced other hominins in those regions • Genomic studies of global populations support the replacement model

40. Dispersal of Homo Sapiens • Fossils and genetic evidence allow scientists to trace human dispersal routes • About 15,000 years ago, humans crossed land bridge that connected Siberia with North America • Humans devised cultural means to survive in inhospitable environments • A capacity for modifying the habitat and for language aided our cultural evolution

41. Premodern Homo Species • Recent findings suggest that modern humans coexisted with another hominin, Homo floresiensis • Some scientists think these fossils could actually be H. erectus, H. sapiens, or australopiths • Further study of fossils and possibly DNA analysis will help test the competing hypotheses

42. Take-Home Message: How are the most recently evolved hominins related? • Both modern humans and Neanderthals are descendants of H. erectus. • Modern humans originated in Africa, then dispersed into other regions. Early in this dispersal, humans entered the Middle East and mated with Neanderthals they encountered there. • A recently discovered Indonesian hominin has been named Homo floresiensis, but its ancestry and relationship to other species remain a matter of investigation