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How big babies shaped society

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Relatively speaking, human babies are heavier than other infant primates - and that may have played a role in shaping us as social animals.

When it comes to size, human babies are more of a handful than other infant primates, and scientists say that may have played a role in shaping us as social animals.

Now an anthropologist is making the case that the socialization process could have started much earlier than previously thought — perhaps more than 3 million years ago, when Lucy and her Australopithecus brethren roamed Africa.

Boston University's Jeremy DeSilva came to that conclusion after running the numbers on bones from a wide variety of primate species, extinct and living, and determining that Australopithecus babies were probably just as much a handful for Lucy's kin as modern babies are for us. "I didn't expect to see that," he told me.

DeSilva's findings were published online today in the Proceedings of the National Academy of Sciences.

The idea behind DeSilva's research is that human mothers gave birth to relatively large babies, weighing roughly 6 percent of adult body mass. Chimpanzees and gorillas, in contrast, give birth to young that weigh 3 percent of the mother's body mass. "Carrying a relatively large infant both pre- and postnatally has important ramifications for birthing strategies, social systems, energetics and locomotion," DeSilva wrote.

Scientists have long observed that bigger babies mean human mothers need more help than chimp mothers to give birth, take care of their babies and carry them around. In prehistoric times, that could have been a factor behind the development of extended family ties and other characteristics of human social organization. But how far back did that trend go?

Guessing their weight
DeSilva reviewed the body-mass studies for humans, chimps and gorillas — and he also gathered up bone measurements for extinct hominid species ranging from Homo erectus (which lived 700,000 to 1.8 million years ago) to Ardipithecus ramidus (which existed 4.4 million years ago).

"Estimating infant body size when you don't have a body isn't easy," DeSilva acknowledged. He used estimates of the size of an adult hominid brain to come up with an estimate for the size of the brain at birth. Then he used a standard formula to extrapolate from the infant brain size to the total body mass. (A human infant's brain is 12 percent of body mass; for a chimp, the corresponding figure is 10 percent.) Finally, he used another statistical method, based on the load-bearing capacity of leg bones, to estimate the adult mass of the now-extinct hominids.

DeSilva expected that the baby-size estimates would get bigger around the time of Homo erectus. But instead, the figures indicated that the weight of Australopithecus infants was 5 to 6 percent the weight of their moms.

"The difference between a chimplike 3 percent and an estimated 5 or 6 percent is a big deal," DeSilva observied. "I think it's a pretty substantial cost to the mother."

Going farther back on the evolutionary timeline, DeSilva found that Ardipithecus' weight estimates were more in line with the chimps, in the 2 to 3 percent range. Since Ardipithecus is seen as being close to the common ancestor of chimps and humans, the figures suggest that infant body proportions increased significantly by the time Lucy lived, 1.2 million years later.

It takes a village?
DeSilva speculated that Australopithecus babies would have been unable to walk on their own for their first 6 to 7 months. Their mothers would have faced the challenge of finding nutrients for themselves as well as breast-feeding the babies, "and would have benefited from the help of pair-bonded males, older children or siblings, or a combination of these."

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A reconstruction shows how Lucy, a member of the species Australopithecus afarensis, might have looked 3.2 million years ago.

"The expression that 'it takes a village to raise a child' may actually go back pretty far back into this Australopithecus group," DeSilva told me.

Although the idea that Australopithecus was more of a social animal takes some getting used to, DeSilva said it actually fits with other evidence about the species' group behavior — including studies being done on the "First Family," a collection of fossils from at least 13 Australopithecus individuals found at a site in Ethiopia, near the place where Lucy was found. But DeSilva emphasized that much more study would be needed to confirm the relationship between bigger babies and social organization. 

"The causality arrow on this, I'm not sure," he told me. "The data I played around with just shows that this group, Australopithecus, was birthing bigger kids than we thought. I think that has implications for reconstructing their biology."

And it also has the effect of making Lucy, the 3.2 million-year-old creature who is thought to be our distant cousin, seem more ... well, more human. But what do you think? Does DeSilva's speculation make sense? Feel free to weigh in with your comments below.

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