By John Mangels
The discovery in Ethiopia of a million-year-old female pelvis is forcing scientists to rethink what they thought they knew about the offspring, appearance and lifestyle of the earliest ancestors of modern humans.
The remarkably complete fossil’s shape indicates that females of the tool-using, wide-ranging Homo erectus species were able to give birth to babies with significantly larger brains than previously thought.
The pelvis’ features also raise questions about the prevailing idea that Homo erectus was specially adapted for long-distance running and hot climates.
“This [pelvis] clearly has to be a specimen to be reckoned with in terms of all future analyses,” said Case Western Reserve University paleontologist Scott Simpson, leader of the team that found the fossil and senior author of a paper today in Science announcing the find.
Simpson and his colleagues were scouring a chocolate-colored hillside in the wastelands of Ethiopia’s Gona region in 2001 when a tribesman helping with the expedition found a broken fragment of a human pelvis.
Paleontologists consider the pelvis important because its size and shape provide critical information about our ancestors — their body style, how well they moved on two legs and, if female, the size of the birth canal.
A quick search of the Gona site failed to turn up additional pieces. Political unrest in Ethiopia prevented the team from returning in 2002.
In 2003 the team uncovered pieces that constituted most of the rest of the pelvis.
Simpson and his co-authors determined that the fossil is between 900,000 and 1.4 million years old, and belongs to Homo erectus, the species that began the transition from the more primitive, chimplike australopiths to our own lineage. Homo erectus was starting to expand its brain and was the inventor of stone hand axes, and the first to travel widely outside of Africa.
Pelvis samples from this murky but crucial period of evolution are extraordinarily rare. Much of what scientists have concluded about Homo erectus’ body shape and characteristics is based on a 1.5 million-year-old skeleton from Kenya.
That specimen, nicknamed Turkana Boy, was a 12-year-old youth. The skull indicated he had a large brain, and limb bones showed he was humanlike in many ways and on his way to an adult height of 6 feet.
Although Turkana Boy was a juvenile male, some researchers used its narrow pelvis measurements and known relationships between male and female pelvis sizes to suggest that Homo erectus females were physically unable to deliver babies with brains the size of modern human infants.
That meant small-brained Homo erectus offspring must have been helpless for a time after birth, requiring substantial energy and care from their mothers or other elders to survive until their brains matured — a pattern similar to modern humans.
Some scientists also inferred from Turkana Boy’s long, thin torso that Homo erectus was evolved to efficiently shed body heat. That made sense considering the narrow hips and other features that suggested Homo erectus was capable of “endurance” running.
The ability to run down prey in the hot African grasslands would have given Homo erectus access to a diet that could support rapid brain enlargement, as well as explaining other physical changes.
But the new Gona pelvis seems to conflict with those conclusions.
Measurements show the capacity of the Gona pelvis is comparable to that of women today. It could accommodate a baby with a brain 30 percent larger than predictions based on the Turkana Boy pelvis. That implies Homo erectus grew much of its brain before birth, like modern infants do, but that its post-natal brain development rate fell somewhere between chimps and humans.
A bigger-brained baby would require less care than had been expected for Homo erectus.
“That doesn’t mean it popped out talking,” Simpson said, “but the period of postnatal development was probably reduced.”
Judging by the Gona pelvis’ dimensions, its owner was a stout, short (less than 5 feet) woman with wide hips. “It’s not a pelvis adapted for running or hot weather,” Simpson said.
He and his co-authors conclude that the main evolutionary force driving Homo erectus’ pelvic shape was the need to deliver big-brained babies, not endurance running or heat adaptation, which may have happened later.
Both Alan Walker, the co-discoverer of the Turkana Boy fossil, and Harvard anthropologist Dan Lieberman, a proponent of the endurance running hypothesis, say it’s possible the Gona pelvis belongs to a more primitive species of human ancestor, Australopithecus boisei.
If the Gona pelvis really is from Homo erectus, Lieberman said, why would females not have the heat-shedding and running adaptations present in Turkana Boy and other erectus specimens?
Simpson said the presence of hand axes used by Homo erectus make a strong case that the fossil is properly classified. Only more fossils will answer the criticism, and he’s returning to Ethiopia this weekend to look.
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