The Birth of Childhood

That's the title of a fascinating, four-page story in the current Science by my partner on the journal's anthropology beat, Ann Gibbons. The story, which is available at this link for those with online access to Science, explores a critical question about human evolution. As the article's "dek" puts it: "Unlike other apes, humans depend on their parents for a long period after weaning. But when--and why--did our long childhood evolve?"

Ann goes on to give us the latest news and views on this topic. A few tidbits, to keep within the bounds of copyright restrictions:

Unlike our close cousins the chimpanzees, we have a prolonged period of development after weaning, when children depend on their parents to feed them, until at least age 6 or 7. Street children from Kathmandu to Rio de Janeiro do not survive on their own unless they are at least 6. "There's no society where children can feed themselves after weaning," says anthropologist Kristen Hawkes of the University of Utah in Salt Lake City. By contrast, "chimpanzees don't have childhoods. They are independent soon after weaning," says anthropologist Barry Bogin of Loughborough University in Leicestershire, U.K.

Humans are also the only animals that stretch out the teenage years, having a final growth spurt and delaying reproduction until about 6 years after puberty. On average, women's first babies arrive at age 19, with a worldwide peak of first babies at age 22.5. This lengthy period of development--comprised of infancy, juvenile years, and adolescence--is a hallmark of the human condition; researchers have known since the 1930s that we take twice as long as chimpanzees to reach adulthood. Even though we are only a bit bigger than chimpanzees, we mature and reproduce a decade later and live 2 to 3 decades longer, says Bogin.

New research into the evolution of childhood, Ann reports,

...is creating some surprises. One direct human ancestor, whose skeleton looks much like our own, turns out to have grown up much faster than we do. The life histories of our closest evolutionary cousins, the Neandertals, remain controversial, but some researchers suspect that they may have had the longest childhoods of all. The new lines of evidence are helping researchers close in on the time when childhood began to lengthen. "Evidence suggests that much of what makes our life history unique took shape during the evolution of the genus Homo and not before," says anthropologist Holly Smith of the University of Michigan, Ann Arbor.

Ann also writes about a new fossil discovery, reported in the same issue of Science:

The new, remarkably complete female pelvis... suggests that life history changes had begun in H. erectus. Researchers led by Sileshi Semaw of the Stone Age Institute at Indiana University, Bloomington, found the pelvis in the badlands of Gona, Ethiopia. They present a chain of inference that leads from pelvis, to brain size, to life history strategy.

The pelvis is particularly wide. Ann explains what that means:

The wide pelvis suggests H. erectus got a head start on its brain development, putting on extra gray matter in utero rather than later in childhood. That's similar to living people, whose brains grow rapidly before birth... But if H. erectus's fetal growth approached that of modern humans, it built proportionately more of its brain before birth, because its brain never became as massive as our own.

Thus, H. erectus grew its brain before birth like a modern human, while during childhood it grew up faster like an ape. With a brain developing early, H. erectus toddlers may have spent less time as helpless children than modern humans do...

There's much more, so check out this issue of Science online or in your library for all the details. But in the meantime, you can listen to Ann discussing her story on the Science Podcast.