Thursday, July 19, 2007

Now we are the only humans

kw: book reviews, human origins, evolution

Imagine for a moment that the legends and myths of "little people", Yeti, "bigfoot", and others were all true; that several species of "alternate humans" were found to exist, not just one here or one there, but by the hundreds, in bands or villages in many places. What would result?

Take a look at the right side of this chart (click to enlarge); focus on the rightmost quarter-inch or less. Between 30,000 and 200,000 years ago, there might have been as many as four to six "human" species alive on Earth at once. The symbols show only the youngest age of each species' known range in time. They existed for 500,000 to a million years or so, some much less (so far as we know).

The data on which this chart is based were gleaned from The Last Human: A Guide to Twenty-two Species of Extinct Humans, created and edited by G.J. Sawyer and Viktor Deak—who performed reconstructions and artwork—with contributions by Esteban Sarmiento, Richard Milner, Donald C. Johanson, Meave Leakey, and Ian Tattersall.

The book is divided into sections by major land areas through time, beginning about seven million years ago in Africa, with three species of three genera (or perhaps two genera), one of which may have been the human-ape ancestor species. All six non-Homo genera presented were at least partly bipedal, to a greater extent than that seen in Chimpanzees, Bonobos, or Gorillas.

The number 22 in the book's title is a bit flexible. It is uncertain whether all the names species are actually distinct, or whether a few of them might represent more than one actual species. Because of this, there are more than 22 dots on the graph, as I split out probable multiples, considering them to be clines, whether in space or time.

To me, the non-Homo genera shown on the graph show a gradual change in brain size, of at most 50% over six million years. Skeletal features other than brain size are used to distinguish them, particularly the size and placement of the face, the shape of the tooth "arch" in the jaw, and the proportions of limbs. The gradual trend toward larger body parallels the larger brains, so no great increase in intelligence is indicated among them; a bigger body needs a bigger brain.

The first Homo specimen, H. rudolfensis 1.8 million years ago, indicates a step change. While other skeletal features clinch the Homo genus, the sudden jump in brain size is remarkable. Interestingly, there were both larger and smaller brains produced in the genus over the next half million years, showing the division between robust and gracile trends...but it is the gracile ones that had larger brains. "Brain versus brawn" meant a lot more in the early Pleistocene than it does in the modern locker room!

By one million years ago, both gracile and robust humans had large brains (nearly a liter, or more). After about 200,000 years ago, four species are known: Neanderthals, H. erectus in Java (probably the third species in a chronocline), H. Floresiensis ("Hobbit" on Flores island, with a 400-cc, or chimp-size, brain), and pre-modern H. sapiens, the immediate precursor to Cro-Magnon.

Homo rudolfensis deep in thoughtThe introduction to each species' "data sheets"—actually very concise, yet comprehensive mini-monographs on each—is an evocative scene "a day in the life of...", and several indicate the writer's favored hypothesis, that H. sapiens not only out-competed the other three species, but ate some of them. Considering that the reconstructions show that two of the three looked more simian than human, this is likely; at least half of Chimp and Gorilla deaths each year are for "bush meat".

Further, given copious historical evidence that hominids take poorly to competition, and that relatively minor differences are taken as sufficient excuse for genocide nearly worldwide, today, it is sometimes amazing that we have still five "races", or geographical somatotypes, as I prefer to state it (Actually, using skeletal evidence alone, there are four non-African body types, and at least eight African ones).

The authors make it clear that, except in a few cases, it is unlikely that any species is definitely ancestral to any later one. Even the 14-16 Homo genera do not form a chain or ladder; rather most are "twigs at the ends of branches", members of small populations that persisted long enough for a few bodies to get partly fossilized and preserved for us to find, thousands or millions of years later. Were the complete history known, and supposing that Earth has until recently had room for four or more simultaneous hominid species and twice as many "near-human" genera such as Australopithecus, and knowing that a species persists only half a million to a million years before evolving beyond recognition into a descendant species (as H rudolfensis probably evolved into H neanderthalensis): I expect fifty to eighty pre-hominid, peri-hominid, and Homo species, at least.

Some forty years ago and more, there was much talk I remember about the search for "missing links". I realize some folks will not be satisfied until a fossil example of every generation from the chimp-human or ape-hominid split is dug up...but that's probably at least a third of a million generations! However, this book lets us learn about, and get a speculative look at a couple dozen links that are no longer missing.

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