University of Utah geologist Naomi Levin helped date the early Pleistocene fossil, whose large girth indicates H. erectus gave birth to babies with brains not much smaller than those of modern human babies, according to a study published today in the journal Science. Their brains were nearly 40 percent the size of an adult brain, indicating a greater maturity at birth. (Modern newborns' brains are 27 percent their adult size.)
Scientists discovered the pelvis in 2001 sticking out of a badland hillside in Ethiopia's Gona region of East Africa, but it has taken years of excavation and study to develop the fossil's significance. The ancient pelvis gives clues about how our ancestors evolved adaptations for upright walking and larger brains - two signature characteristics of modern humans that have not always been anatomically compatible, says the study's lead author, Scott Simpson, an anatomist at Cleveland's Case Western Reserve University.
During H. erectus' time on Earth, between 2 million and 1 million years ago, its brain size ballooned by nearly a half, from about 750 to 1060 cubic centimeters, as it acquired the ability to use tools, control fire, hunt and gather, and other skills that made it the longest-lived hominid.
"What does this have to do with pelvises?" Simpson said. "The pelvis is the most important part of the anatomy because it has links with locomotion and birthing babies."
Big brains are why human births can be so traumatic. Human ancestors had to evolve a larger pelvis to accommodate the increasing head sizes of their offspring.
Bipedalism and coping with chillier climates required not only more brain power, but an upright posture. This, too, required changes in the bony structure that engages the legs and the spine, and comprises hips, sacrum and coccyx. The Gona pelvis adds a key piece to this evolutionary jump, because most H. erectus finds involve skulls.
"This contributes so much to our knowledge of female anatomy of ancient hominids," says co-author Sileshi Semaw, an archaeologist with the Stone Age Institute and Indiana University. "We need more fossils to understand variations within the same population. If would have been nice to find something above the neck to corroborate our story."
The find builds on the discovery of "Turkana Boy," a juvenile male H. erectus found near Kenya's Turkana Lake in 1984 and dated by U. geologist Frank Brown. The Turkana fossil led researchers to believe that H. erectus had small-headed babies and long, lean bodies built for endurance. But the Gona pelvis indicates the individual was short and squat, hardly a body evolved for efficient two-footed locomotion.
Dating the fossil proved to be a major challenge. Levin, then a U. doctoral candidate, analyzed the sediments in which the specimen was found, but the absence of volcanic ash meant there were few clues to derive precise dates. Levin was able to bracket the fossil's age between 1.4 million and 900,000 years by estimating rates of sedimentation between marker beds whose ages were known.
"Radio carbon dating maxes out at 50,000 years. We used up all our geochronological cards," said Levin, who is headed to Baltimore for a tenure-track position at Johns Hopkins University.
Work by Levin and fellow geologist Jay Quade, another former U. graduate student, helped describe the landscape roamed by the ancient woman. It appears to have been a valley with a large river meandering through forested banks, Simpson said. The woods graded to grasslands populated by large game animals, such as warthogs, impalas and zebras.
bmaffly@sltrib.com
