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Newborn babies may be a bundle of joy, but they are a heavy bundle of joy. Scientists say human babies weigh proportionally more at birth than the babies of any other primate species. Now, an anthropologist in Boston has shown that our earliest human ancestors probably had big babies, too. And it's something that may have influenced the development of modern human societies. NPR's Joe Palca investigates.

JOE PALCA: You can admire the human species, but you have to be realistic.

Dr. JEREMY DESILVA (Anthropologist, Boston University): Humans are strange in all sorts of ways.

PPALCA: Jeremy DeSilva is an anthropologist at Boston University. How are humans strange? Well, walking upright on two legs is strange; our newborns are almost totally helpless; and they're big.

Dr. DESILVA: Our babies are unusually large. They have unusually large heads. They have unusually large bodies compared to other primates.

PALCA: A newborn ape typically weighs about 3 percent of what its mother weighs. For humans, that jumps to 6 percent. DeSilva wondered if this were true for a species like Australopithecus, that came millions of years before modern humans. But there's a fundamental difficulty answering that question.

Dr. DESILVA: We don't have fossilized remains of newborns.

PALCA: DeSilva came up with a clever way around that problem. It's a two-step process. First, you use adult skulls to estimate the newborn's skull size. That can be done very accurately for all primates. DeSilva was able to analyze a dozen Australopithecus skulls.

Dr. DESILVA: So once you have the size of the head, there is what researchers have called the 12-percent rule.

PALCA: The 12-percent rule says that the brain represents 12 percent of the total body weight.

Dr. DESILVA: It's not exactly 12 percent. In fact, in the apes, it tends to be more 10 percent.

PALCA: Even with that margin of error, DeSilva says it was clear that the birth weight of Australopithecus infants was much closer to the 6 percent of modern humans than the 3 percent of apes.

He's published these results in the journal PNAS.

This change in relative birth size was a critical development. DeSilva believes birthing large babies probably influenced human culture.

Dr. DESILVA: The whole expression that it takes a village is, in part, rooted in the fact that we have really big infants that are pretty helpless. If we wanted to get anything done, we have to hand them off.

PALCA: So stable communities developed where some took care of babies while others hunted or did the taxes.

Life was changing in other ways for Australopithecus. Two and a half million years ago, these guys began swapping a life in trees for one on solid ground. Owen Lovejoy is an anthropologist at Kent State University in Ohio.

Dr. OWEN LOVEJOY (Anthropologist, Kent State University): One of the things, I think, that goes along with the switch to upright walking and terrestriality is a switch to a more substantive infant that has a higher chance of survival.

PALCA: If the trend to proportionally larger babies was nearly complete two and a half million years ago when Australopithecus showed up, when did it begin? Anthropologist Robert Martin, of the Field Museum in Chicago, thinks the answer is an even older human relative called Sahelanthropus.

Dr. ROBERT MARTIN (Field Museum): I did some quick calculations after you sent me the paper, and it looks as though Sahelanthropus was intermediate between the great apes and Australopithecus. So it looks as though Sahelanthropus might already have started along that pathway 7 million years ago.

PALCA: Martin says there's only one Sahelanthropus skull discovered so far, so that conclusion is preliminary. But it appears that big babies are something humans - and our ancestors - have been dealing with for a long time.

Joe Palca, NPR News, Washington.

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