Early humans were giving birth to big-brained infants much earlier than previously thought, suggests the most intact pelvis from a Homo erectus female ever found.

The 1.2-million-year-old fossil pelvis was unearthed in Ethiopia in 2001 and is about 85 per cent complete, a new study reports.

Prior to the find, the earliest direct evidence that scientists had of a human or human ancestor - also called a hominin - giving birth to babies with big brains was 400,000 years ago.

Past reconstructions based on less complete fossils had indicated H. erectus females had narrow hips and gave birth to infants with relatively small brains.

But the larger size and broader shape of the new pelvis indicates the skulls of H. erectus infants may have been 30 per cent larger than past skeletal reconstructions suggests.

Consequently, H. erectus would have been more developmentally mature at birth than previously suspected, said study leader Scott Simpson of Case Western Reserve University in Cleveland, Ohio. His team's research appears in the journal Science.

"It's not that they come out walking and talking and fending for themselves, but they were not as helpless as we see in modern humans," he said. H. erectus infants also grew faster than modern babies, and were probably weaned from their mother's milk sooner, Simpson added.

"That means they could be incorporated into the society at very young ages," he said.

The new findings suggest that the brain size of an H. erectus infant was already 30 per cent to 50 per cent that of an adult. By contrast, modern babies are born with brains that are only about a quarter the size of an adult human's. So even though the brains of modern humans are bigger at birth than H. erectus', modern babies are less mature.

SHORT AND STOUT

The researchers think the pelvis belonged to an H. erectus female who was in her early 20s when she died and stood around 4 feet 5 inches tall.

Until now, scientists had thought that H. erectus adults had tall, slender bodies, possibly as adaptations to living in a hot, tropical environment and for endurance running. H. erectus females were assumed to have had narrow hips and relatively small birth canals that could allow the passage of only small-brained infants.

But such theories were largely based on measurements of the pelvis of "Turkana Boy", a 1.5-million-year-old juvenile male H. erectus fossil discovered in Kenya in 1984.

Experts have always considered pelvis reconstructions based on Turkana Boy rough drafts. Aside from the problems associated with extrapolating from male to female pelvis sizes, the Turkana Boy specimen was very fragmented.

"Unfortunately, the parts that are missing are crucial to the reconstruction," Simpson said.

GORILLA-LIKE DIFFERENCES

Dan Lieberman is an anthropologist at Harvard University who was also not involved in the study. "This (new) pelvis is a nice addition to the fossil record," Lieberman said. But he added that the discovery "raises many more questions than it answers."

For example, it was known that H. erectus was probably sexually dimorphic, meaning males and females had different body sizes and shapes. The new fossil suggests the size difference may have been akin to that of gorillas, where males are much bigger than females - an idea Lieberman isn't sold on.

"For this pelvis to be a female H. erectus, then we need to accommodate really considerable sexual dimorphism with very tall and narrow males and very short and wide females," Lieberman said. "I need to be convinced."

There is also the question of how shorter and stouter H. erectus females would have managed to shed heat in the hot African climate. Long, slim bodies shed heat better than short, stout bodies.

A slender frame has more surface area relative to body volume, allowing sweat to cool the body more effectively.

"We know from many other skeletal features that H. erectus were active hominins in the midday sun in hot East Africa," Lieberman said. "If so, were males but not females adapted to heat?"

Chris Ruff, an anatomist at Johns Hopkins University who was not involved in the study, called the discovery a "landmark" that could set a new standard for how complete a hominin fossil can be.

But Ruff wonders if the pelvis might not have belonged to another hominin that lived at the same time as H. erectus.

"They state that only H. erectus remains have been found in this area for this time period," he said, but "there were other things around in East Africa, such as Homo habilis, which lived until 1.4 million years ago at least."

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