Masters of the Planet (16 page)

Unfortunately this consensus in principle hardly clarifies matters much in practice. For there is no agreement on what the “qualities of a man” actually are, even in the relatively simple terms that necessarily apply to fossil forms known only from bones and teeth. As a result, there is a lot of confusion about exactly which fossils should be placed in the genus
Homo.
To understand the current state of play, we need to return to history for a moment. As we saw in
chapter 4,
back in the 1960s Louis Leakey and his colleagues extended the definition of the genus
Homo
back beyond
Homo erectus
by whatever it took to include in it the gracile “handy man” fossils from those 1.8-million-year-old rocks at the bottom of Olduvai Gorge. Although
the
partial lower jaw that Leakey and his associates deemed the holotype of
Homo habilis
didn't look vastly different from its counterparts among the gracile
Australopithecus
fossils from South Africa, Leakey felt that some fragments of braincase indicated a brain a bit bigger than was typical of the latter (though it was still smallish, at under 700 cc). In addition, the lower jaw was putatively associated with a partial foot of what had clearly been an upright biped, with an in-line great toe and nicely sprung arches. At the time there was nothing remotely comparable to this foot so far back in the hominid fossil record, and its features fit nicely with Leakey's long-established predilection for the notion that the roots of our genus lay way back in time—just as the crude stone tools found in the same sediments matched perfectly with his attraction to the idea of “Man the Toolmaker.” So it was that the morphological concept of the genus
Homo
became stretched to include some very ancient morphologies indeed.

It took a few years for paleoanthropologists to become comfortable with the idea of embracing the rather archaic-looking Olduvai hominid within our own genus. But once they came around to the rather bizarre notion that the genus
Homo
could somehow accommodate a range of morphologies extending all the way from modern
Homo sapiens
to the ancient Tanzanian fossils, the way was open for them to start including in
Homo habilis
a motley assortment of specimens from other African sites. The process started in 1972, when the toothless 1.9-million-year-old cranium KNM-ER 1470 from east Turkana was discovered, and shortly thereafter hailed as the best-preserved
Homo habilis
skull yet. Its allocation to
Homo
was made largely on the basis of an impressive estimated brain volume of about 800 cc (later reduced to 750 cc); but as noted the specimen is rather poorly preserved, and it is still hard to know quite what kind of hominid it represents. The discovery of 1470 was followed by a flowering of other hominid fossil finds in eastern Africa, and diverse cranial and postcranial specimens, from Olduvai Gorge and East Turkana—and as far away as South Africa—were subsequently shoehorned into
Homo habilis.
As each of these fossils was engulfed, the plasticity of the genus
Homo
appeared even greater.

Ironically, even before the extreme untidiness of this assemblage of fossils became too blatant to ignore, our old friend 1470, the very hominid that had convinced most paleoanthropologists that
Homo habilis
was a reality to be accepted, became the standard-bearer of a new name. In the mid-1980s a Russian paleoanthropologist renamed this fossil
Pithecanthropus rudolfensis
(oddly using Eugene Dubois's ancient genus name, rather than the universally accepted
Homo
). Within a few years, other paleoanthropologists started to pick up on the species
Homo rudolfensis;
and, in parallel with the ballooning of
Homo habilis,
this second ancient species of
Homo
acquired new exemplars in Kenya and even as far afield as Malawi. Some of these fossils are as old as 2.5 million years, but most date from around 2.0 million years ago or a little less, and all are pretty
fragmentary
.

The partial skull KNM-ER 1470, from East Turkana, Kenya. Some 1.9 million years old, this individual had boasted a brain of about 750 ml in volume, larger than is typical for australopiths; and its discovery convinced many paleoanthropologists that
Homo habilis
was indeed a real species. Drawing by Don McGranaghan.

The critical time span between two and two and a half million years ago also coincided with that of a number of finds in eastern Africa that their discoverers, presumably a bit worried about the increasing disarray of
Homo habilis
(and indeed, of
Homo rudolfensis
), diplomatically preferred to allocate simply to “early
Homo.
” Before the remarkable finding of those cut-marked bone fragments at Dikika, the 2.5-million-year date of the oldest of these fossils coincided pretty closely with the earliest evidence for the use of stone tools, and this was something that fed back powerfully into both the “early
Homo
” and the “Man the Toolmaker” ideas. Still, putting any of these fossils in our genus is a bit of a stretch,
based
solely on their preserved anatomy; and, as new evidence accumulates, this coincidence is beginning to look less like a red herring than as the basis for a self-fulfilling prophecy that sent paleoanthropologists up a blind alley.

Fortunately, we don't have to wait long after the two-million-year point to start finding fossils that really do merit inclusion in
Homo
because of all the features they specifically share with us. We'll discuss them in a moment; but first I should point out that, unsettlingly, it is hard to know right now where those strikingly new and innovative fossil relatives came from. There is little to connect them directly to any of the “archaic
Homo
” fossils we've just been talking about; and while we know a large array of australopiths—and there can be little doubt that ultimately it was one branch of these early bipedal apes that gave rise to
Homo
—it is really hard to pinpoint where among these diverse creatures the origin of our genus lay. To put the situation in a nutshell, there is not one fossil among all those known in the period before about two million years ago that presents itself as a compelling candidate for the position of direct progenitor of the new hominids to come. All we can say right now is that the period between about 2.5 and two million years ago was clearly a time of continuing evolutionary ferment among members of the hominid family. The ongoing experimentation with the hominid potential that went on in this period is expressed in an intriguing diversity among the fossil hominids we know; but, to add to the uncertainty, it is a diversity that we still glimpse only dimly.

This uncertainty is partly due to the fragmentary nature of the evidence, but there is a good chance the glimpse is also dim because of a general reluctance among paleoanthropologists to accept even in principle that such diversity is indeed out there. One reason for this is that it is difficult to make sense of the abundant but frustratingly incomplete evidence that we have at our disposal. Sorting out species structure within a sample of fossils is the most basic of tasks a paleontologist takes on, but even at the best of times it is also often one of the hardest. The simplest default hypothesis at which you can arrive when you're poring over a table covered in fossil fragments is that everything you're looking at belongs to the same variable species. As such, you don't have to decide just where any possible demarcations lie. But this is only one factor; and
to
a large extent the reluctance to perceive diversity also stems from an underlying expectation about evolutionary pattern. It will require just a little bit more history to understand why, in recent decades, paleoanthropologists have tended to take such an extraordinarily inclusive approach to membership in our genus.

In the half century preceding World War II, paleoanthropology was the province largely of human anatomists, scientists whose training was in the minutiae of human physical variation. They were not forced to confront the riotous diversity of species in nature that other natural historians had to contend with. One byproduct of this insular history was that few paleoanthropologists of the period had much if any training either in evolutionary process, or in the procedures and requirements that should underpin the naming of new species. This led to the liberal description of new hominid genera and species, almost as if each new fossil that showed up needed to be baptized with its own genus and species name, much as Western humans individually receive family and given names. By the time World War II rolled around there were at least 15 hominid genus names commonly in use, and countless species—all for a fossil record that was then of modest size.

In the long run this was bound to be a pretty untenable situation. And it was particularly untenable at a time when a movement that became known as the Evolutionary Synthesis was taking firm hold in most areas of evolutionary biology. The Synthesis was a meeting of previously disparate minds in the fields of genetics, systematics and paleontology, each of which had previously itself harbored multiple versions of the evolutionary process. On the one hand, the Synthesis emphasized the importance of variation
within
populations and species of living creatures, and on the other, it preached the basic continuities of the evolutionary process. The Synthesis allowed for the splitting of evolutionary lineages of organisms (without which we would never have achieved the luxuriant diversity of Nature that we see today). But at the same time it stressed that evolutionary change was the result of slow alterations in the frequencies of genes within established lineages, under the guiding hand of natural selection. Species were hence viewed principally as arbitrary segments of ever-modifying lineages: as transitory units, highly variable at any one point in time. And they were thus expected to slowly evolve
themselves
out of existence. So compelling was this gradualist message that, between the late 1920s and the mid-1940s, the Synthesis became the central paradigm of evolutionary biology in the English-speaking world. Virtually the last holdout, as a result of its peculiar history, was paleoanthropology. But not for long.

Perhaps the most influential architect of the Evolutionary Synthesis was the geneticist Theodosius Dobzhansky, who was declaring as early as 1944 that based on the fossil evidence there had never been more than one (highly variable) hominid species at any one point in time. At an influential conference held at Long Island's Cold Spring Harbor Laboratory in 1950, Dobzhansky was joined by his ornithologist colleague Ernst Mayr, who took this proposition even farther. Mayr argued that culture broadened the human ecological niche to such a degree that, even in principle, there
could
only ever have been one human species at a time. And it's worth remembering at this point that Mayr's notion is intuitively a very attractive proposition to members of a storytelling species that also happens to be the only hominid in the world today. It is somehow inherently appealing to us to believe that uncovering the story of human evolution should involve projecting this one species back into the past: to think that humanity has, like the hero of some ancient epic poem, struggled single-mindedly from primitiveness to its present peak of perfection.

Although he'd probably never seen a hominid fossil in his life, Mayr then took all of the many genera that were cluttering the hominid fossil record, and reduced them to
one:
the genus
Homo.
What's more, he reduced the species involved to a mere three. These formed a single succession:
Homo transvaalensis
(the australopiths) gave rise to the middle stage we call
Homo erectus,
which ultimately transformed into
Homo sapiens
(which included the Neanderthals). Mayr's Cold Spring Harbor declaration hit paleoanthropology like a bombshell. Before long, even he was forced by ongoing discoveries of robust australopiths to admit that there had indeed been at least one side branch from the mainstream of hominid evolution. But Mayr's reductionist view of the human fossil record still held paleoanthropology in thrall for the next several decades. Perhaps because paleoanthropologists had never really paid much attention to evolutionary theory before, paleoanthropology found itself
suddenly
dominated by the Synthesis. Indeed, so thoroughly was the field traumatized by Mayr's remonstrations that, all through the 1950s and well into the 1960s, many paleoanthropologists hardly dared to use zoological names at all, preferring to refer to individual fossils by the names of the sites they came from. That way, they couldn't be accused by their colleagues of being biologically naïve.

Once the trauma had worn off to the point that they felt comfortable with zoological names again, paleoanthropologists lapsed into a style of taxonomic inclusiveness. The attitude seemed to be, if we have to use zoological names, let's use as few as possible. And even though the burgeoning fossil record has since made it impossible to ignore the fact that there was indeed a lot of hominid diversity out there, most paleoanthropologists still hew to the rather minimalist mindset that so thoroughly dominated in the days when most of today's leading practitioners were trained. Of course, paleoanthropologists aren't fools, and nobody denies any more that the human evolutionary tree looks more like a forking bush than a slender sunflower. Even more significantly, a lot of different hominid species are now widely accepted, as the illustration in
chapter 2
shows. Nonetheless, despite wide recognition that there is a great deal more to the evolutionary process than simple lineage modification under natural selection, the gradualist mindset still lingers in a residual reluctance among paleoanthropologists to recognize more branches in that tree than absolutely necessary. Perhaps once this reluctance has faded a bit farther we will be able to look more realistically at the diversity of the “early
Homo
” fossils, and to detect where within that assemblage the roots of our genus actually lie.