Between 900,000 and 1.8 million years ago, Homo erectus made an evolutionary leap to develop brains closer in size to the large brains of modern-day man at birth.
Paleontologist Scott Simpson, professor of anatomy at the Case Western Reserve University School of Medicine, is the lead author of research describing a nearly complete female of a 1.2 million-year-old H. erectus pelvic fossils found in the region of Gona, Ethiopia.
Simpson along with Jay Quade (University of Arizona), Naomi Levin (University of Utah/California Institute of Technology), Robert Butler (University of Portland), Guillaume Dupont-Nivet (Utrecht University) and Melanie Everett and Sileshi Semaw (Stone Institute) describe the importance of this pelvis in the article, "A Female Homo erectus Pelvis from Gona, Ethiopia," in the current issue of Science.
Making obstetrical measurements and comparing them to prior published specimens, Simpson determined that the birth canal in the pelvis had adapted to deliver a baby with a brain almost comparable to a modern-day baby at about 36 weeks and weighing approximately four pounds.
The H. erectus brain would have been up to 315 cubic centimeters (cc) at birth based on measurements of the width of the inlet and outlet of the pelvic fossils, making it slightly smaller than today’s babies born with brain sizes averaging about 380 cc.
Working at the Cleveland Museum of Natural History, he also made comparisons with the reconstructed pelvis of "Lucy," an Australopithecus afarensis specimen, which is about 3.2 million years old, to see how the pelvis was evolving as means to deliver offspring with larger brains.
"H. erectus had one foot in the past and another foot in the future," said Simpson.
The pelvic fossils also yielded new information about the size of the female H. erectus. Prior information about hominids living during the Pleistocene era was based primarily on the 1.53 million year old "Turkana boy" skeleton from Kenya. If this young boy had lived into adulthood, current reconstructions of his skeleton predicted he would have grown to a height of nearly six feet and would have been tall and lean, adapted to tropical grassland environment.
Prior information speculated that the pelvis was evolving for endurance running in an arid tropical environment, but Simpson said this new evidence shows it was adapting to birth of a baby with an enlarged brain. The female pelvis showed a small hip joint for the femur indicating she was much shorter in height as compared to than the Turkana boy demonstrating that Homo erectus exhibited marked sexual dimorphism in stature.
Simpson determined that the Busidima pelvis was for a female of approximately 4.5 feet tall. By having a very wide pelvis, this short female would have had a more "diamond-shaped" body form rather than the characteristic shape we see in contemporary women.
During field research in 2001, one of the Afar field assistants named Ali Ma’anda Datto observed a partial sacrum, part of a group of bones that make up the pelvis, protruding from the side of hill in the Busidima Formation in the Afar research area.
After removing the specimen from crumbling and eroding soil, Simpson said, "I could tell the bone was newly fractured and so knew there had to be another part located nearby."
It would be two years before he could return to the area to excavate the other half of the sacrum, and within 10 minutes had found an almost complete, and the first, pelvic bones of a female member of H. erectus. The remaining pelvic bones were embedded in rock and had to be chiseled out. These few fossils have overturned thinking about how Homo erectus females might have looked and lived.
From the fossils and where they were found, Simpson said the female would have been in her early 20’s (approximately late 20’s by modern standards) and may have given birth. These fossils were deposited in an ancient swamp located close to a permanent river that was surrounded by extensive grasslands with numerous acacia bushes.
One million years ago burial of the dead was not practiced, and so the female may have died from being attacked by a carnivore, which would frequent the swampy area because of the water and plentiful food source, of disease or old age.
But by the position of her fossilized bones in the soil, Simpson said she was rapidly covered by soil deposits most likely from a sudden flood. At the time the woman lived about 1.2 million years ago, the area of Ethiopia was geologically active and separating of the rift created a basin that held lakes and a river.
As the river meandered along the basin floor , it created swampy areas which gave rise to lush grasslands and attracted hominids and a host of animals such as water buffalo-like bovines, waterbuck, kudus, zebras, warthogs, and cane rats, which are similar to muskrats. It also attracted predators of these animals as well as hominids that lived on the grasslands near the water.
Simpson suspects carnivores may have eaten the remains and dragged them into a swamp that was covered by a rapid event like a flood and preserved the pelvis in an almost upright position in the sediment.
Over the years, the area remained geologically active and where it once was a depositional area for sediments, it now has become drier and an area where erosion occurs.
"We found the pelvis at the right moment," said Simpson. He plans to the return to the area soon and do more surveying, hopeful that the erosion will continue to work and erode ribs and other skeletal material like the skull from the rock.
Simpson will return to Ethiopia this coming week for two months of field work in the area. Accompanying him will be graduate student Susan Standen and undergraduate Anna Wieser, an anthropology major and geological science minor who will be doing work for her senior capstone project.
The Gona Palaeoanthropological Research Project is directed by Dr. Sileshi Semaw from the Center for Research into the Anthropological Foundations of Technology (CRAFT) at the Stone Institute at Indiana University. The Stone Institute directs the Gona Paleoanthropological Research Project in the Afar Regional State in the Great Rift Valley of Ethiopia.
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