Looking at the fossil record, one could easily think that our hominid ancestors only lived in Ethiopia, that Egypt was the only happening place 5,000 years ago, that North American dinosaurs were all clustered together in Montana and other falsehoods. While it's true important finds have been made in each of these regions, we cannot discount the role geology has played in the process.
Africa has what are known as "rift valleys." These are valleys with steep sides formed by a rift in Earth's crust. Africa's Great Rift Valley forms a large, triangular depression called the Afar Rift in Ethiopia. As Ardi project co-director Tim White told me, sediments have been accumulating in this depression for millions of years.
"Much of Africa has a high elevation and most surfaces are eroding," he and his team concluded. "This is why fossils are concentrated in the rift valleys of Tanzania, Kenya, Ethiopia, and other African countries."
The Afar Rift is important, in part, because the fossils within the sediment appear in well-defined layers that can be precisely dated. Australopithecus, famous for its "Lucy" skeleton, was found there, and so was Ardi. Last night's Discovery Channel program hinted that even older hominid remains have been found in deeper sediment layers at the site. Scientists can therefore see human evolution at work, as the layers strongly indicate that each of the hominid species at the site are connected, with one species giving rise to another. For example, White believes Ardipithecus likely gave rise to Australopithecus, which down the line gave rise to our own genus, Homo.
So it's not necessarily true that all of our ancestors were somehow huddled together in Ethiopia. Other hominids likely lived elsewhere in Africa. It's just that the geology has permitted scientists to unearth important fossils in Ethiopian soils and to establish probable connections between these fossils.
The excavated Ardipithecus fossils are extremely fragile. Many bones clearly were chewed by hyenas. It goes without saying that a lot can happen to strewn bones over millions of years. It's a minor miracle that any have stayed intact for our generation.
Ethiopia paints a vivid picture of human evolution, but it's likely not the only place where early hominids settled. As White and his team say, "The distribution of plants and animals found today as fossils in eastern Africa, including hominids, is therefore a fortunate accident of geology and where these organisms used to live."
Will another location yield important fossil finds? In future, maybe. At present, though, as White says, "Ethiopia has now taken the lead in human origins studies and discoveries."
For more information, please go to the Discovery site Explore the World of Ardipithecus.
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