22.2 Evolutionary History of Humans

22.2 Evolutionary History of Humans

Human evolution sits inside primate evolution. Start with the traits of primates, then follow the fossils on the human branch. Dates below are fossil estimates. Say about when you give a year. A new find can move a date without changing the order of the traits.

Primate traits

Primates share a set of traits used in introductory courses. They have grasping hands, often with a thumb that can meet the fingers, so the hand can hold a branch or a piece of food. They have nails rather than claws on most digits. Their eyes are forward-facing, which gives overlapping views and strong depth perception. They have a large brain for body size compared with many other mammals. Many primates also have flexible shoulders. The four traits to memorize are the grasping hands, the nails, the forward-facing eyes, and the large brain relative to the body.

Living primates include lemurs, monkeys, and apes. Humans are primates. Within that order, humans are apes. A human hand still shows nails rather than claws, the eyes face forward, and the brain is very large for the body's mass. Those features are primate inheritance. They did not appear for the first time in modern humans.

Cousins, not ancestors

Humans did not evolve from living chimpanzees. Humans and chimpanzees share a common ancestor. That ancestor was neither a modern human nor a modern chimpanzee. After the split, each line changed on its own. Estimates of the split commonly fall between about 6 million and 8 million years ago. Chimpanzees are cousins. They are not a living picture of a human ancestor, and they are not a stage that humans passed through. The same point covers gorillas and other living apes. No living ape is the ancestor of Homo sapiens.

Hominins

Hominins are the human branch after that split. The early hallmark of hominins is bipedalism, walking upright on two legs. In the fossil record, bipedal walking appears before very large brains. The common trap reverses that order. A hominin can walk upright and still have a brain far smaller than ours.

The skeleton records bipedalism. The foramen magnum, the opening where the spinal cord leaves the skull, sits farther under the head than it does in a knuckle-walking ape. The pelvis is shorter and broader. The thigh bone angles inward so the knee passes under the body. The foot can push off during a step. One damaged bone is weak evidence. Several of these features together support upright walking long before brain size enters the modern range.

Fossils in teaching order

Australopithecus afarensis, including the fossil called Lucy, lived about 3.2 million years ago. Lucy walked upright and had a small brain, closer to an ape brain than to a modern human brain. The name is a binomial: the genus Australopithecus and the specific epithet afarensis. Lucy is the classroom evidence for trait order. Upright walking was already present. A very large brain was not.

Homo habilis had a larger brain than Australopithecus afarensis and is associated with early stone tools. Introductory courses often call those early flakes Oldowan tools. Brain size had increased compared with the australopiths. It was still well below the size typical of living people. Place Homo habilis after the small-brained upright walkers and before the wider dispersal of later members of Homo.

Dispersal and later brains

Homo erectus had a still larger brain, made more advanced tools, and dispersed from Africa. Those tools often include Acheulean hand axes in introductory accounts. Courses often use Homo erectus in a broad sense for tall, long-legged hominins. Fossils and tools beyond Africa show that the dispersal carried hominins into Eurasia, beginning about 1.8 million years ago. The brain was larger than that of Homo habilis. This dispersal is not the origin of anatomically modern people.

Every date in the table is a fossil estimate. The row order is the teaching sequence.

Anatomically modern humans

Anatomically modern Homo sapiens arose in Africa about 300,000 years ago. Anatomically modern means the skeleton, including a rounded braincase and a face in the living human range. The African origin is a claim about where those fossils appear first. It does not mean that evolution stopped, and it does not mean that later populations were sealed off from every other hominin.

Neanderthals lived in Europe and western Asia. They were close relatives of Homo sapiens, with sturdy bodies and brains at least as large as ours. They interbred with some Homo sapiens populations. In many people whose ancestors lived outside Africa, about 1 to 2 percent of the genome traces to Neanderthals. Neanderthals are not the sole direct ancestor of all living people. Limited interbreeding does not move the origin of anatomically modern humans to Europe.

Which origin account to keep

The better-supported account is a recent African origin of Homo sapiens, with limited interbreeding. Anatomically modern humans appeared in Africa, later spread, and mixed in a limited way with relatives such as Neanderthals. An older multiregional account treated modern humans as evolving from much earlier local populations in several regions, with those regions contributing more equally. This chapter treats the recent African origin, plus that limited interbreeding, as the better-supported account.

Two dispersals are easy to mix up. Homo erectus left Africa about 1.8 million years ago. Homo sapiens arose in Africa about 300,000 years ago and spread later. Neanderthals met some of those later populations and interbred with them. Meeting a relative is not the same thing as descending only from that relative.