Did big babies help bring human ancestors down from the trees?


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Relative to our ape brethren, humans give birth to really big babies. This especially substantial infant size — along with newborns’ large heads and general helplessness — helped to spur the development of more advanced social systems to help mother and child safe, researchers think.

Chimpanzee with infant [Image courtesy of iStockphoto/markrhiggins] A new study examines the evolution of this trend to try to pinpoint when in human evolution this growth spurt occurred—and how it might have signaled a shift in social dynamics as well.

Modern humans give birth to babies that are generally about 6.1 percent of the mother’s body weight, whereas chimpanzee babies are usually closer to 3.3 percent of the mother’s mass.

And as anthropologist Jeremy DeSilva of Boston Univeristy pointed out in his new paper, “carrying a relatively large infant both pre- and postnatally has important ramifications for birthing strategies, social systems, energetics, and locomotion.”

Using models that estimate neonatal brain and body mass, DeSliva estimated that the 4.4-million-year-old hominin Ardi (Ardipithecus ramidus) likely would have borne an infant between 2.1 and 3.2 percent of the mother’s body mass, which would place it closer to more primitive hominin traits. By about 3.2 million years ago, however, “females of the genus Australopithecus were giving birth to relatively large infants” of about 5 to 6 percent of maternal body mass, DeSilva noted in his paper.

For modern chimpanzees — and likely early human ancestors — with relatively small infants, strong grasping toes and long, clingable body hair, taking babies into the trees with them both before and after birth is not too taxing.

But as these traits seem to disappear with Australopithecus, arboreal living would be “a more dangerous activity,” DeSilva wrote, noting that bipedal, less hairy mothers would have to park their young or actively carry them.

And these bigger, probably more helpless babies of Australopithecus might not have started walking until six or seven months after birth, he estimated.

Saddled with a large, nursing infant that is unable to walk, Australopithecus mothers could have used assistance from males as well as juveniles in caring for themselves and their babies (grandmothers are thought to have come into the picture only after lifespan lengthened with the emergence of the Homo genus).

This new need for extended postnatal care may have driven the emergence of a social structure different from that of chimpanzees, which “rarely will share their infants with other members of the group,” DeSilva noted.

And despite assertions that more modern rearing practices did not emerge until the evolution of early Homo species, the new assessment indicates that “the increased levels of shared infant care critical to infant survival in modern humans could have its roots in the genus Australopithecus,” rather than in Homo, DeSilva hypothesized

DeSilva cautioned, however, that many of the adult body mass estimates derive from “only a small number of often taxonomically ambiguous fossil specimens,” and he suggests his calculations be retested as new hominin bones are uncovered.

His analysis was published online January 3 in Proceedings of the National Academy of Sciences.

Author: Katherine Harmon | Source: Scientific American [January 03, 2011]



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