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

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smaller chimpanzee baby riding on mother's backRelative 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.

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.

Image courtesy of iStockphoto/markrhiggins

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  1. 1. jtdwyer 7:47 pm 01/3/2011

    Interesting. My guess is that the effective development and operation of a larger brain mass requires a proportionally larger respiratory and digest system just to provide necessary nutritional support. Body development would necessarily follow, at least, producing the larger infant. As I understand, the supply of neurons is pretty much constant after birth, so if a larger brain is advantageous…

    I also imagine that Neanderthal births were quite difficult – for the mothers, anyway…

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  2. 2. juxtapose82 4:16 pm 01/4/2011

    Good observation. I agree that may have a purpose. I submit also that gestation periods in humans are closer to that of a gorilla than a chimp yet the opposite is true in body mass. Possibly we were made more to harbor these brains than survive outright so evolution gave us an extra incubation period to gain those few extra pounds and for protection.

    Or, we are comparing today’s birth weights to those of the apes when it is widely scrutinized that diet has been making each generation larger, maybe that is happening starting in utero, hence the rapid spike.

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  3. 3. jtdwyer 4:48 pm 01/4/2011

    Ah that’s another interesting point. Not to cast aspersions on mothers, as difficult as pregnancy must be, but perhaps in our post-industrial society of (seemingly) plenty, are mothers-to-be understandably overindulging in nutritional cravings?

    Perhaps even such a trend could be contributing to childhood obesity – but that’s purely speculation.

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  4. 4. juxtapose82 8:23 am 01/5/2011


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  6. 6. sunnystrobe 1:15 pm 01/11/2011

    A more meaty diet may have made all the difference sizewise! ( cf. the significantly taller-sized recent generation of young Chinese, due to a much more meaty Western diet). But , come Neandertal times, the still rather restricted width of the birth canal, combined with a nutritional vitamin C deficiency -due to an Ice Age with NO fruits & veggies in the permafrost, might well have been contributing to their ultimate destinction. ( A much higher risk of giving birth to spina bifida babies, caused by maternal folic acid deficiency, comes to mind; folic meaning ‘leafy’; not to forget: we primates lost the ability to produce our own vitamin C millions of years ago, so we are nutritionally dependent on raw plant food to boost our immune system.)
    The motto: Go veg, go green, suddenly makes sense , even now, when it comes to our survival chances as a neotenous ‘baby face’ race!
    See for an evolution-based aspect of life styles, past & present!

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