Two summers ago in Ireland I watched a mare give birth. Within minutes the foal was finding its legs. Soon it was scampering about. This past summer it was hard to tell the mother from her offspring.
Meanwhile, for all of that time and more, human parents would be cuddling, feeding, burping, changing diapers, and otherwise tending a child toward eventual independence. Street children in big Third World cities cannot survive on their own unless they are at least six years old. In the developed nations we continue to coddle our kids until they have made it through puberty and adolescence.
What's the deal? Why do we get stuck with such a long period of maturation on the part of our kids? Our nearest relatives, the chimps, are pretty much on their own by age 4. Female chimps are moms at 11, half the age at which the average human female breeds. Of course, chimps are dead of old age at 45, so they pay at the end for their faster start.
Paleontologists exhaustively study the fossil fragments of our humanoid ancestors, especially children, for clues to childhood. The result seems to be that our nearest ancestors were more like chimps than like modern humans in the duration of dependence. An extended childhood appears to be unique to us, and the big question is why.
One possibility is that delayed reproduction creates higher quality moms. Also, humans wean their infants twice as fast as chimps, which means human moms can pop out successive babies more quickly. No wonder then that we live so much longer that chimps and are so overwhelmingly numerous.
I would guess that prolonged childhood has more to do with the development of culture. That little foal galloping around the meadow on day two pretty much already knew all it needs to know. We are still teaching our human kids at age 20, not just stuff like speech and the three Rs, but also Shakespeare, constitutional law, computer science, and the difference between right and wrong. Perhaps extended childhood and acculturation evolved together.
By the way, I wrote last year about the Homo erectus fossil youth I called Nari, one of the most complete hominid fossil skeletons ever found. I said he was thought to be 11 or 12 years old, an estimate based on his height compared to modern humans. Now, microscopic study of Nari's tooth enamel suggests he was 8 years old. Kids grew up faster in those days.
(These thoughts inspired by an article by -- appropriately -- Ann Gibbons called The Birth of Childhood, in the November 14 issue of Science. Nari photo credit: John Gurche.)