Sunday, October 9, 2011 - 0 comments

Once We are NOT Alone : 2. A Suite of Species

By : Ian Tattersall

FROM THE BEGINNING, almost from the very moment the earliest hominid biped—the first “australopith”—made its initial hesitant steps away from the forest depths, we have evidence for hominid
diversity. The oldest-known potential hominid is Sahelanthropus tchadensis, represented by a cranium from the central-western country of Chad [see illustration on page 26]. Better known is Australopithecus anamensis, from sites in northern Kenya that are about 4.2 million years old.

A. anamensis looks reassuringly similar to the 3.8- to 3.0-million-year-old Australopithecus afarensis, a smallbrained, big-faced bipedal species to which the famous “Lucy” belonged. Many remnants of A. afarensis have 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 found in 1995 in Chad. It is probably between 3.5 and 3.0 million years old
and is thus roughly coeval with Lucy, as is the recently named new form Kenyanthropus platyops.

In southern Africa, scientists reported evidence in 1999 of another primitive bipedal hominid species. As yet unnamed and undescribed, this distinctive form is 3.3 million years old. At about three million years ago, the same region begins to yield fossils of A. africanus, the first australopith to be discovered (in 1924). This species may have persisted until not much more than two million years ago. A 2.5-million-year-old species
from Ethiopia, named Australopithecus garhi in 1999, 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 exactly 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 robusts 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 a closely related second species, P. crassidens.

I apologize for inflicting this long list of names on readers, but in fact it actually underestimates the number of australopith species that existed. What is more, scientists 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 beginning 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 perturb this pattern. The 2.5- to 1.8-million-year-old fossils from eastern and southern Africa that announce the earliest appearance of Homo are an oddly assorted lot and probably a lot more diverse than their conventional assignment to the two species H. habilis and H. rudolfensis indicates. Still, at Kenya’s East Turkana, in the period between 1.9 and 1.8 million years ago, these two species were joined not only by the ubiquitous P. boisei but by H. ergaster, the first hominid of essentially modern body form. Here, then, is evidence for four hominid species sharing not just the same continent but the same landscape [see illustration on opposite page and below].

The first exodus of hominids from Africa, presumably in the form of H. ergaster or a close relative, opened a vast prospect for further diversification. One could wish for a better record of this movement, and particularly of its dating, but there are indications that hominids of some kind had reached China and Java by about 1.8 million years ago. A lower jaw that may be about the same age from Dmanisi in ex-Soviet Georgia
is different from anything else yet found [see “Out of Africa Again ... and Again?” by Ian Tattersall, on page 38]. By the million-year mark H. erectus was established in both Java and China, and it is possible that a more robust hominid species was present in Java as well. At the other end of the Eurasian continent, the
oldest-known European hominid fragments— from about 800,000 years ago— are highly distinctive and have been dubbed H. antecessor by their Spanish discoverers.

About 600,000 years ago, in Africa, we begin to pick up evidence for H. heidelbergensis, a species also seen at sites in Europe—and possibly China—between 500,000 to 200,000 years ago. As we learn more about H. heidelbergensis, we are likely to find that more than one species is actually represented in this group of fossils. In Europe, H. heidelbergensis or a relative gave rise to an endemic group of hominids whose bestknown representative was H. neanderthalensis, a European and western Asian species that flourished between about 200,000 and 30,000 years ago. The sparse record from Africa suggests that at this time independent develop-including the emergence of H. sapiens. And in Java, possible H. erectus fossils
from Ngandong were dated to around 40,000 years ago, implying that this area had its own indigenous hominid evolutionary history for perhaps millions of years as well.

The picture of hominid evolution just sketched is a far cry from the “Australopithecus africanus begat Homo erectus begat Homo sapiens” scenario that prevailed 40 years ago—and it is, of course, based to a great extent on fossils that have been discovered since that time. Yet the dead hand of linear thinking still lies heavily on paleoanthropology, and even today quite a few of my colleagues would argue that this scenario overestimates diversity. There are various ways of simplifying the picture, most of them involving the cop-out of stuffing all variants of Homo of the past half a million or even two million years into the species
H. sapiens.

My own view, in contrast, is that the 20 or so hominid species invoked (if not named) above represent a minimum estimate. Not only is the human fossil record as we know it full of largely unacknowledged morphological indications of diversity, but it would be rash to claim that every hominid species that ever existed is represented in one fossil collection or another. And even if only the latter is true, it is still clear that the story of human evolution has not been one of a lone hero’s linear struggle.

Instead it has been the story of nature’s tinkering: of repeated evolutionary experiments. Our biological history
has been one of sporadic events rather than gradual accretions. Over the past five million years, new hominid species have regularly emerged, competed, coexisted, colonized new environments and succeeded—or failed. We have only the dimmest of perceptions of how this dramatic history of innovation and interaction
unfolded, but it is already evident that our species, far from being the pinnacle of the hominid evolutionary
tree, is simply one more of its many terminal twigs.

Next : The Roots of Our Solitude


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