Fossil Findings by Stony Brook Faculty Challenge Linear Evolution

STONY BROOK, N.Y. – Two new fossils, described on August 9 in the journal Nature, challenge the idea of a more linear succession in human evolution. The fossils were discovered by the Koobi Fora Research Project, an international group of scientists directed by Meave and Louise Leakey, both faculty members with the Department of Anthropology at Stony Brook. The fossils were found in 2000 in the Ileret region east of Lake Turkana, Kenya.

Human evolution over the last two million years has often been portrayed as a linear succession of Homo habilis to Homo erectus to existing Homo sapiens, symbolically represented in the popular illustration of evolution from a knuckle-dragging ape to an upright-walking man. However, the new findings have possibly rearranged major branches thought to be in a straight ancestral line to Homo sapiens because of both their geological ages and physical attributes.

In previous years, the Homo habilis species were found to be 1.65 million years old or older, while some fossils with habilis attributes have been dated as early as 2.33 million years old. Using volcanic ash deposits to date the new specimen, the researchers found that an upper jaw bone of Homo habilis dates from 1.44 million years ago, which is more recent than previously known. Meanwhile, the second fossil, a Homo erectus skull dates 1.55 million years. The fossils were found within only a few minutes of walking distance, suggesting that contrary to previous theory, Homo habilis and Homo erectus lived together in Eastern Africa.

In addition to the mother-daughter pair, the research was conducted by seven other scientists including: lead author Fred Spoor of University College London; Patrick Gathogo, and Frank Brown from the University of Utah; Susan Anton, an anthropologist at New York University; Ian McDougall from the Australian National University; and Christopher Kiarie, and Fredrick Manthi of the National Museums of Kenya (NMK).

In their report, Dr. Spoor and his colleagues wrote, “With the discovery of the new, well dated specimens from Ileret, Homo habilis and Homo erectus can now be shown to have co-occurred in eastern Africa for nearly half a million years.”

Looking at the physical attributes of the second fossil, the Homo erectus fossil was found to be the smallest found anywhere in the world, making it closer in size to the habilis species than previously known. In order to dispel possible doubts, the scientists had to work rigorously to first prove the erectus skull was not another species, nor a freak. The distinctive features of the cranium, jaw and teeth, and the neck allowed it to be identified as an erectus fossil, probably from an 18- or 19-year-old adult female.

In re-examining other erectus skulls and partial fossils, the researchers found that the species have much smaller females than males; that is, greater sexual dimorphism. This is thought to be a primitive stage in evolution related to having multiple sexual partners, leading the scientists to conclude that Homo erectus is less humanlike than had been previously assumed.

Although the findings do not change the relationship of Homo erectus as a direct ancestor of Homo sapiens, if this interpretation is correct, both habilis and erectus likely have originated from a common ancestor between 2 and 3 million years ago. Unfortunately, this period remains as a blank in fossil history, leaving the early evolution of the genus Homo even more masked in mystery.

Palaeontological fieldwork by the Koobi Fora Research Project in the Lake Turkana Basin has been supported by the National Geographic Society since 1968. Meave and Louise Leakey are both Explorers in Residence at the National Geographic Society and affiliated with the NMK.

Pictured Above: Frederick Kyalo Manthi, holds the Homo erectus complete skull he discovered in 2000 near lake Turkana in Kenya. (AP Photo/Karel Prinsloo)