ONCE WE WERE NOT ALONE
Today we take for granted that Homo sapiens is the only hominid on Earth. Yet for at least four million years many hominid species shared the planet. What makes us different?

Sharing a single landscape, four kinds of hominds live about 1.8 million years ago in what is now part of northern Kenya. Although paleoanthropologist have no idea how-or if-these different species interacted, they do know that Paranthropus boisei, Homo rudolfensis, H. habilis and H. ergaster foraged in the same area around Lake Turkana.

PARANTHROPUS BOISEI had massive jaws, equipped with huge grinding teeth for a presumed vegetarian diet. Its skullis accordingly strongly built, but it is not know if in body size it was significantly larger than the "gracile" austalopiths.

HOMO RUDOLFENSIS was a relatively largebrained hominid, typified by the famous KNM-ER 1470 cranium. Its skull was distinct from the apparently smaller-brained H. habilis, but its body proportions are effectively unknown.

HOMO HABILIS ("handy man") was so named because it was thought to be the maker of the 1.8 million year old stone tools discovered at Olduvai Gorge in Tanzania. This hominid fashioned sharp flakes by banging one rock cobble against anther.

HOMO ERGASTER sometimes called "African Homo erectus," had a high, rounded cranium and a skeleton broadly similar to that of modern humans. Although Homo ergaster clearly ate meat, its chewing teeth are relatively small. The best specimen of this hominid is that of an adolescen from 1.6 million years ago know as "Turkana boy."



Homo sapiens has had the earth to itself for the past 25,000 year or so, free and clear of competition from other members of the hominid family. This period has evidently been long enough for us to have developed a profound feeling that being alone in the world is an entirely natural and appropriate state of affairs.
So natural and appropriate, indeed, that during the 1950s and 1960s a school of thought emerged that, in essence, claimed that only one species of hominid could have existed at a time becasue there was simply no ecological space on the planet for more than one culture-bearing species. The "single-species hypothesis" was never very convincing-even in terms of the rather sparse hominid fossil record of 35 years ago. But the implicit scenario of the slow, single-mainded transformation of the bent and benighted ancestral hominid into the graceful and gifted modern Homo sapiens proved powerfully seductive-as fables of frogs becoming princes always are.
So seductive that it was only in the late 1970s, following the discovery of incontrovertible fossil evidence that hominid species coexisted some 1.8 million years ago in what is now northern Kenya, that a single-species hypothesis was abandoned. Yet even then, paleoanthropologists continued to cleave to a rather minimalist interpretation of the fossil record. Their tendency was to downplay the number of species and to grop together distinctively different fossils under single, uninformative epithets such as "archaic Homo sapiens." As a result, the they tended to lose sight of the fact that many kinds of hominids had regularly contrived to coexist.
Although the minimalist tendency persists, recent discoveries and fossil reappraisals make clear that the bilogical history of hominids resembles that of most other successful animal families. It is marked by diversity rather than by linear progression. Despite this rich history- during which hominid species developed and lived together and competed and rose and fell- Homo sapiens ultimately emerged as the sole hominid. The reasons for this are generally unkable, but different interations between the last coexisting hominids-Homo sapiens and Homo neanderthalensis- in two distinct gewgraphical regions offer some intriguing insights.

A SUITE OF SPECIES

From the befinning, almost from the very moment the earliest hominid biped-the fist "australopith"- made its initial hesitant steps away from the forest depths, we have evidence for hominid diversity. The oldest-known potential hominid is Ardipithecus ramidus, represented by some fragmentary fossil from the 4.4 million year old site of Aramis in Ethiopia. Only slightly younger is the better-known Australopithecus anamensis, from sites in northern Kenya that are about 4.2 million years old.
Ardipithecus, though claimed on indirect evidence to have been an upright walker, is quite apelike in many respects. In contrast, A.Afarensis looks reassuringly similar to the 3.8- 3.0 million year old Australopithecus afarensis, a small brained, big-faced bipedal species to which the famous "Lucy" belonged. Many remants of A. afarensis has been found in various eastern African sites, but some researchers have suggested that the mass of fossils described as A. afarensis may contain more than one species, and it is only a matter of time until the subject is raised again. In any event, A. afarensis was not alone in Africa. A distinctive jaw, from an australopith named A. bahrelghazali, was recently found in Chad. It is probably between 3.5- 3.0 million years old and is thus roughly coeval with Lucy.
In southern Africa, scientists have just reported evidence of another primitive bipedal hominind species. As yet unnamed and undescribed, this distinctive from is 3.3 million years old. At about 3 million years ago, the same region begins to yield fossils of A. africanus, the first australopith to be s=dicovered in 1924. This species may have persisted until not much more than 2 million years ago. A recently named 2.5 million year old species from Ethiopia, Australopithecus garhi, is claimed to fall in an intermediate position between A. afarensis, on the one hand, and a larger group that includes more recent australopiths and Homo, on the other. Almost exacly the same age is the first representative of the most exacly the same age is the first representative of the "robust" group of australopiths, Paranthropus aethiopicus. This early form is best known from the 2.5 million year old "Black Skull" of northern Kenya, and in the period between about 2 and 1.4 million years ago the robust were represented all over eastern africa by the familiar P. boisei. In South Africa, during the period around 1.6 million years ago, the robusts included the distinctive P. robustus and possibly also a closely related second species, P. crassidens.
I apologize for inflicting this long list of names on you, but in fact it actually underestimates the number of australopith species that existed. What is more, we don't know how long each of these creatures lasted. Nevertheless, even if average species longevity was only a few hundred thousand years, it is clear that from the very befinning the continent of Africa was at least periodically- and most likely continually- host to multiple kinds of hominids.
The appearance of the genus Homo did nothing to perrturb this pattern. The 2.5 - 1.8 million year old fossils from eastern and southern Africa that announce the erliest appear

This article was written in Science America 00

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