The extensive ice deposits that began to appear six million years ago on the poles lowered substantially the level of the seas, dried up part of the earth and erased plant life in many regions. When the ice melted, the initially empty plains were swept with wind. Very fertile dust, i.e. loess, was deposited. Grass that had lately intruded on the world scene, grew and various mammalian herbivores occupied the newly formed niche, with their sequel of predatory carnivores. This set of conditions favored the development of an evolutive line among great apes.
Lucy, a female Australopithecine belonging to Australopithecus afarensis, who lived about 3.6 million years ago in Ethiopia, stood long alone as the first known human ancestor. Her curved fingers revealed grasping hands, whereas apes grasp with feet also. There were no clues about her own ancestors. The idea was that it was a large-toothed, fully bipedal, naked ape living in the Serengeti savanna 6 million years ago. For years, the explanation for the lack of fossils between 3.6 and 9 million years was that the apes went through a bottleneck with only a few species emerging, including the ancestor shared by humans and chimps. It was held that this root ape quickly gave rise to A. afarensis, which in turn led to Pithecanthropus, Neanderthal and Homo (fig. 7.1).
Figure 7.1. The skulls of the four most important Hominid species. The Neanderthal is a European specimen and sapiens is modern man.
The true reason fossils older than 4 million years were not found is because fossil hunters for a long time scoured the open grasslands, which proved to be wrong places. In 1992, an American team found Ardipithecus. It dated 4.4 millions years back. Other Ardipithecus were found, dating back 5.8 million years. Ardipithecus is claimed to “toe off” in a manner seen only in upright walkers. Ardipithecus lived in forests, not in grassy savanna and ate soft fruit. A strong contender is Orrorin, found by a French-Kenyan team. He is 5.7 to 6 million years old. Here, the claim is that Orrorin walked more human-like than Lucy. This implies that Orrorin is the ancestor of Homo, by way of a proposed genus Praeanthropus, taken from fossils initially classified among the Australopithecines. The French claim that Ardipithecus is only the ancestor of chimp while Lucy and other Australopithecines are a dead end. Orrorin was eating soft fruit and leaves, and lived in a wooded environment, like Ardipithecus. None of the two teams considered the possibility that Orrorin and Ardipithecus may be of the same species.
A third team, also French led by Brunet, who was trained as a zoologist specialized in mammal fossils, found a fossil on 19 July 2001 in the Chad desert dating over 6 million years. The skull of Sahel-anthropus tchadensis looks like that of an ape, with a brain the size of a chimpanzee’s, large incisors and widely spaced eyes like a gorilla’s. But the shape and size of its canines, small and unsharpened, and lower face resemble those of human ancestors that came later. The snout proper to chimpanzees was much reduced in Sahel-anthropus. It is in some respects more hominid like than Australopithecus afarensis, dated to 3.4 million years ago. The age of Sahel-anthropus pushes the limits, dating the split between the hominid and chimpanzee lineages to 5–7 million years ago. But what distinguishes a hominid? Is it erect station and upright walking? Then Oririn is a hominid. Is it the reduction in canines? Then, Sahel-anthropus is a hominid.
This discovery killed the thesis defended by the French paleontologist Yves Coppens that hominids evolved solely on the east side of the Rift Valley while apes evolved on the western side. Also, the Chad region, now a desert, was a thick forest in those days, indicating that bipedalism was not acquired after a climate change forced apes out of the trees into the grasslands. Upright walking may have started in the forest. In fact, bipedalism may have evolved more than once.
In 1999, Kenyanthropus was found. It lived from 3.5 million years until 2 million years. It moved between grasslands and wooden habitats.
All these fossils may represent diversity within single species that unfolded into each other in a linear procession. In this view, there is nothing more than one evolving hominid from 6 million years to 2.5 million years. Another opinion is to see these species as a radiation with diversity present from the start. If we follow this line of thought, we have a bushy tree with different hominids hanging off different branches at the same time, making it difficult to draw a clear line of descent.