The skull and jawbone of a small, lightly-built individual, discovered at an archeological site in Dmanisi, Georgia, may call into question the prevailing idea that larger brain size was behind the migration of human ancestors out of Africa. An international research team describes their find in the journal Science, published by the American Association for the Advancement of Science.
The scientists found a petite new individual, with a small brain, thin brow ridge, short nose, and huge canine teeth, according to co-author David Lordkipanidze of the Georgian Academy of Sciences, in Tbilisi. This was the third specimen found at the site. By comparison, the other two skulls had room for substantially larger brains.
All three specimens are approximately 1.75 million years old, making them the largest collection of individuals from any one site older than around 800,000 years. Lordkipanidze and his colleagues have tentatively concluded that the three belong to the same species, Homo erectus, thought to be the first hominid species to leave Africa. The Dmanisi fossils most closely resemble the African version of Homo erectus, called Homo ergaster.
"We have now a very rich collection, of three skulls and three jawbones, which gives us a chance to study very properly this question [of how to classify the early hominids,] said Lordkipanidze. "These questions are often based on isolated finds, but here we have the chance to study a population."
The brain of the new Dmanisi specimen was probably around 600 cubic centimeters, while modern human brains are at least twice that much, according to Lordkipanidze. The other two specimen's brains were approximately 800 cubic centimeters.
Scientists have proposed that the evolution of larger brains was directly related to our ancestors' migration out of Africa. According to this scenario, increased intelligence enabled these early humans to adapt to new environments.
The new skull's small brain size "suggests that enlargement of the brain was not the only reason to leave Africa. My feeling is there should be a combination of reasons, not just one reason, that forced people out of Africa," Lordkipanidze said.
The new fossils also offer a rare glimpse into the diversity of a primitive human species. While we take it for granted that modern humans come in all shapes and sizes, scientists know little about individual variation among our ancestors.
"We are seeing a difference mainly in size, not morphology," said Lordkipanidze. "For now, my hypothesis is that we are seeing variability within the population."
Until the discoveries at Dmanisi (Leo Gabunia and colleagues reported the first two in the 12 May 2000 issue of Science), researchers generally believed that the first human species to leave Africa departed only one million years ago, and that they had large brains and relatively advanced stone tools.
The tools found at Dmanisi are of the primitive "pebble-chopper" variety, similar to the Oldowan tools of East Africa.
The skull and jawbone are unusually well-preserved, in part because a solid limestone layer overhead protected them against the usual compaction that occurs over time in sedimentary rocks. Along with the hominid fossils, the Science authors found remains of ancient species of rhinoceros, deer, wolf, horse, and saber-toothed cat.
The other authors of the paper are Abesalom Vekua, Givi Maisuradze, Alexander Mouskhelishvili, Medea Nioradze, and Merab Tvalchrelidze, at the Georgian Academy of Sciences, in Tbilisi, Georgia; G. Philip Rightmire, at Binghamton University, in Binghamton, New York, United States; Jordi Agusti, at the Institut de Paleontologia M. Crusafont, in Sabadell, Spain; Reid Ferring, at U. of North Texas, in Denton, Texas, United States; Marcia Ponce de Leon and Christoph Zollikofer, at U. Zürich-Irchel, in Zürich, Switzerland, Martha Tappen, at U. Minnesota, in Minneapolis, Minnesota, United States.
This research was funded by the Georgian Academy of Sciences, the National Geographic Society, The Leakey Foundation, the Fulbright Foundation, the Spanish Ministry of Science, Generalitat de Catalunya, the University of Zurich, the Eckler Fund of Binghamton University, the American School of Prehistoric Research, and the Peabody Museum of Harvard University.