The long road to modern man
For a long time it was believed that a sudden dry season on the African continent triggered a period of rapid evolutionary shifts that ultimately led to the emergence of modern-day humans. In an article in Science on November 27, 1997, a contrary opinion was now expressed. Based on a new analysis of East African mammal fossils, American scientists believe modern humans evolved slowly over time after the climate shift several million years ago. The assumed period of very rapid evolution is little more than a scientific artifact. An estimation of evolutionary speeds is difficult. Past environments are very diverse and more bones have been preserved in some environments than in others. A stratum with a particularly large number of fossils could therefore greatly falsify the estimate of the number of new species that appeared at that time. Anna Behrensmeyer and her colleagues at the National Museum of Natural History in Washington suspect that this distortion could be to blame for the so-called "turnover pulse" being mistimed. Many paleontologists believe it may have taken place between 2.8 and 2.5 million years ago. Conventional analysis of the fossil record suggests that Australopithecus evolved into Homo at the time when the climate in Africa was becoming drier and cooler.
To test this thesis, the scientists compiled a database of all published mammalian fossils found in the Lake Turkana area of Ethiopia and Kenya from 4.4 million years ago to the present day. Then they estimated the number of species that emerged or became extinct in each geologic period. When these numbers were compared to the amount of fossils found in each period, the researchers discovered that many of the apparently rapid evolutionary changes -- including the peak between 2.8 million and 2.5 million years ago -- coincided with periods when fossils were abundant were found.
This means, says Behrensmeyer, that mammals - including hominids - have adapted to climate change much more slowly than previously thought. “If we assume there is one beat of the climate drum, and all of a sudden human evolution happened,” she says, “then [Lake Turkana didn't] listen to that drum.”
But there are also critical voices. One of them belongs to Yves Coppens, a paleoanthropologist at the Collège de France in Paris who worked for many years with the fossil record at Lake Turkana. He believes it's too early to drop the turnover pulse thesis.