Saturday, August 21, 2010

Evolution in Brief

An excerpted and compressed narrative history:

A whirlwind tour of human history on all the continents. For millions of years, from our origins as a species until 13,000 years ago.

Our closest living relatives are three surviving species of great ape: the gorilla, the common chimpanzee, and pygmy chimpanzee (also known as bonobo). Their confinement to Africa, along with abundant fossil evidence, indicates that the earliest stages of human evolution were also played out in Africa.

Human history, as something separate from the history of animals began there about 7 million years ago (estimates range from 5 to 9 million years ago). Around that time, a population of African apes broke up into several populations, of which one evolved into modern gorillas, a second into the two modern chimps, and the third into humans. The gorilla line apparently split off slightly before the split between the chimp and the human lines.

Fossils indicate that the evolutionary line leading to us had achieved a substantially upright posture by around 4 million years ago, then began to increase in body size and in relative brain size around 2.5 million years ago. Those protohumans are generally known as Australopithecus africanus, Homo habilis, and Homo erectus, which apparently evolved into each other in that sequence. Although Homo erectus, the stage reached around 1.7 million years ago, was close to us modern humans in body size, its brain size was still barely half of ours.

Stone tools became common around 2.5 million years ago, but they were merely the crudest of flaked or battered stones. In zoological significance and distinctiveness, Homo erectus was more than an ape, but still much less than a modern human. All of that human history, for the first 5 or 6 million years after our origins about 7 million years ago, remained confined to Africa.

The first human ancestors to spread beyond Africa was Homo erectus, as is attested by fossils discovered on the Southeast Asian island of Java and conventionally known as Java man. The oldest Java "man" fossils -- of course, they may actually have belonged to a Java woman -- have usually been assumed to date from about a million years ago. However, it has recently been argued that they actually date from 1.8 million years ago. ...

By about half a million years ago, human fossils had diverged from older Homo erectus skeletons in their enlarged, rounder, and less angular skulls. African and European skulls of half a million years ago were sufficiently similar to skulls of us moderns that they are classified in our species, Homo sapiens, instead of in Homo erectus. This distinction is arbitrary, since Homo erectus evolved into Homo sapiens. However, these early Homo sapiens still differed from us in skeletal details, had brains significantly smaller than ours, and were grossly different from us in their artifacts and behavior.

The only other significant addition to our ancestors' cultural repertoire that can be documented with confidence around that time is the use of fire. After half a million years ago, the human populations of Africa and western Eurasia diverged from each other and from East Asian populations in skeletal details. The population of Europe and western Asia between 130,000 and 40,000 years ago is represented by especially many skeletons, known as Neanderthals and sometimes classified as a separate species, Homo neanderthalensis. ... The few preserved African skeletal fragments contemporary with the Neanderthals are more similar to our modern skeletons than to Neanderthals skeletons. ... Although those Africans of 100,000 years ago had more modern skeletons than did their Neanderthal contemporaries, they made essentially the same crude stone tools as Neanderthals, still lacking standardized shapes. They had no preserved art. ... They and their Neanderthal contemporaries still rank as less than fully human.

Human history at last took off around 50,000 years ago, at the time of what I have termed our Great Leap Forward. The earliest definite signs of that leap come from East African sites with standardized stone tools and the first preserved jewelry (ostrich-shell beads). Similar developments soon appear in the Near East and in southeastern Europe, then (some 40,000 years ago) in southwest Europe, where abundant artifacts are associated with fully modern skeletons of people termed Cro-Magnons [named after the 1868 discovery of fossil skeletons at Cro-Magnon in SW France]. Thereafter, the garbage preserved at archaeological sites rapidly becomes more and more interesting and leaves no doubt that we are dealing with biologically and behaviorally modern humans. ... Of the Cro-Magnons' products that have been preserved, the best known are their artworks: their magnificent cave paintings, statues, and musical instruments, which we still appreciate as art today. ... Obviously, some momentous change took place in our ancestors' capabilities between about 100,000 and 50,000 years ago. The Great Leap Forward coincides with the first proven major extension of human geographic range since our ancestors' colonization of Eurasia. that extension consisted of the occupation of Australia and New Guinea, joined at that time into a single continent. ... With the settlement of Australia / New Guinea, humans now occupied three of the five habitable continents. ... between about 14,000 and 35,000 years ago, the Americas were first colonized. The oldest unquestioned human remains in the Americas are at sites in Alaska dated around 12,000 B.C., followed by a profusion of sites in the United States south of the Canadian border and in Mexico in the centuries just before 11,000 B.C. The latter sites are called Clovis sites, named after the type site near the town of Clovis, New Mexico, where their characteristic large stone spearpoints were first recognized. ... Unquestioned evidence of human presence appears soon thereafter in Amazonia and in Patagonia" (Diamond 1997:36-52).

J. Diamond, Guns, Germs, and Steel. The Fates of Human Societies (New York and London: W.W. Norton & Co., 1997).
C.B. Stringer, "Evolution of early humans," The Cambridge Encyclopedia of Human Evolution (eds. S. Jones, R. Martin and D. Pilbeam; Cambridge: Cambridge UP, 1992), 241-51.
B.A. Wood, "Evolution of australopithecines," The Cambridge Encyclopedia of Human Evolution (eds. S. Jones, R. Martin and D. Pilbeam; Cambridge: Cambridge UP, 1992), 231-40.

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