Homo erectus 鈥� an early ancestor of modern humans 鈥� resembled a squat body builder more than a svelte distance runner, a newly unearthed fossil pelvis suggests.
The roughly 1.2 million-year-old female pelvic bone 鈥� nicknamed the Busidima pelvis after a river near the discovery site in Northern Ethiopia 鈥� points to a shorter, stockier species than thought. Its capacious birth canal shows signs of evolutionary accommodation for a bulging brain.
The most noticeable feature of the Busidima pelvis is its wide birth canal, says , a palaeoanthropologist at Case Western Reserve University in Cleveland, Ohio, who was part of a team that uncovered the nearly complete bone.
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The same location has yielded other fossil signposts in the meandering path to fully modern humans, including a 4.5 million-year-old jaw of a more ape-like species, Ardipithecus ramidus.
However, the history of the more modern Homo erectus, big-brained and fully upright, has been told mainly through skull fossils, says , a palaeoanthroplogist at the Stone Age Institute in Gosport, Indiana, who led the team. The species lived in Africa, Asia and Europe between about 2 million and 100,000 years ago or less.
Bigger birth canals
Researchers have uncovered few below-the-belt fossils, and the most complete one belonged to a roughly 10-year-old male, the Turkana boy. 鈥淲e know literally nothing about the female aspects of the evolution of our ancestors,鈥� Semaw says.
The wide birth canal of the Busidima pelvis is its most obviously human feature, Simpson says. Getting through the birth canal is 鈥渢he most gymnastic thing we ever do,鈥� he says.
To accommodate big-brained babies, humans must have developed larger and wider birth canals over time, but with few pelvic fossils, researchers had little idea when these changes began. The Busidima pelvis shows that a wide birth canal was already in place 1.2 million years ago. It underscores the importance of developing large brains in early human evolution, Simpson says.
鈥淭he most successful individuals in these populations will have positive selection for brains and larger pelvises,鈥� he says. 鈥淏rain size is driving the whole system here.鈥�
Squat proportions
The Busidima pelvis might also shatter another belief: H. erectus鈥榮 supposed prowess as a distance runner. Some researchers have argued that the species was adapted to marathon-like hunts that ended with worn-out prey. Yet Busidima shows few of the adaptations thought to contribute to a runner鈥檚 body, such as a tall stature and a narrow pelvis.
鈥淚t鈥檚 so totally different from anything else we鈥檝e found,鈥� says , an anatomist at Johns Hopkins University in Baltimore, Maryland. In Busidima鈥檚 squat proportions, he sees more traces of Homo鈥榮 past than its future.
鈥淚t really looks Lucy-like,鈥� he says, referring to the nearly complete 3.2 million-year-old Australopithecus fossil discovered in Ethiopia in 1974.
The fossil鈥檚 membership in Homo erectus is even up to question, Ruff says. If anything, it points to greater body shape diversity in the species than previously thought.
鈥淚 would really like to think a little more about whether this thing is Homo erectus or what our definition of Homo erectus is,鈥� he says.
Journal reference: (DOI: 10.1126/science.1163592)
