Sparing Nature: The Conflict between Human Population Growth and Earth's Biodiversity

Chapter 3 The Human Wedge

3B C H A P T E R The Human Wedge THE warm and wet tropics are a cauldron of biodiversity, so it is little surprise that the ancestry of humans can be traced to the tropical lands of Africa. The vast majority of our primate cousins, such as monkeys and apes, now live in the tropics. Such was the case sometime between six and three million years ago, when one primate lineage now known as the genus Australopithecus made its evolutionary debut and traversed parts of the African continent(fig.3.1).Giventheglobalspreadanddominationthatcharacterizes the eventual descendants of Australopithecus, one might expect to find evidence of a fairly impressive animal, poised to rule the world. But our ancestral line had a most inauspicious beginning with a peculiar and meek creature. What was Australopithecus? It was a somewhat apelike, mostly vegetarian creature with a few evolutionary novelties. For one thing, as best we can tell from the fossil bones, Australopithecus stood upright and walked on two legs. Indeed, the 3.8-million-year-old fossil site of Laetoli in Tanzania has remarkably preserved the footprints of three Australopithecus individuals who traversed some recently lain volcanic ash. But they did not tower over their contemporary animals, for evidence from the famous Lucy skeleton of Ethiopia (3.2 million years old) shows that they were about three and a half feet tall.1 Robert Louis Stevenson wrote: “Despite our wisdom and sensible talking, we on our feet must go plodding and walking.”2 The plodding and walking is our heritage from Australopithecus; the wisdom and sensible talking came only later in our evolutionary history. Stevenson’s apt phraseology becomes apparent if you have ever tried to run from a charging carnivore. Be it a leopard or your neighbor’s dog, you will be aware that two plodding legs don’t move you very fast. Moreover, Australopithecus did not have large threatening canine teeth, such as those of modern baboons, to ward off predators through facial threat gestures. It is a wonder that our ancestors survived at all. Perhaps they were aided by a modicum of brain expansion, but this went little 3 8 B 03-R2435 11/5/02 1:52 PM Page 38 beyond the relative brain size of a modern chimpanzee. Whatever cunning they had, or whatever social organization they used, it was apparently sufficient to allow them to survive among the diverse crowd of animals that formerly inhabited Africa. Nowhere is the rich biodiversity of these early times more evident than in the caves of the Makapansgat Valley, South Africa. The hills of the valley are spotted with caves, some of which are filling up today just like others that filled up long ago, only to be reopened by quarriers in search of limestone. One particularly large cave in-fill produced not only common cave formations such as stalactites, stalagmites, and flowstones for the quarriers but also preserved a rich cache of fossil bones. Interpretations of the fossils help paint a paleoenvironmental picture of a tropical forest some three million years ago, roughly contemporaneous with Lucy and her kin from some of the East African fossil sites. The mammal species represented in the Makapansgat cave fills were more numerous and diverse than those inhabiting the valley today. Only a small percentage of the Makapansgat fossils are of Australopithecus . These were found in a conglomerate of broken bones that appear to have been accumulated by a prehistoric hyena. Alongside were numerous potential competitors of our ancestors, including at least six other primate species. Perhaps Australopithecus existed only in small numbers at the time; alternatively , these upright animals may have been savvy enough to avoid predation more often than their primate cousins. One thing is clear—Australopithecus T h e H u m a n W e d g e A 3 9 Figure 3.1 Skull of Australopithecus africanus from South Africa, dating to about 2.5 million years ago. These meek creatures set the stage for human evolution. 03-R2435 11/5/02 1:52 PM Page 39 was not a huge player in the ecological dynamics of the time. The tropical environment allowed the novel genus to slip into the diverse web of life with little impact. If Australopithecus was one of Darwin’s “wedges,” it was a small wedge hit lightly against the “yielding surface” of ancient Africa. Apparently Australopithecus served as an occasional meal...