為什么人類的嬰幼期如此漫長? Why are children so, well, so helpless? Why did I spend a recent Sunday morning putti
Why are children so, well, so helpless? Why did I spend a recent Sunday morning putting blueberry pancake bits on my 1-year-old grandson's fork and then picking them up again off the floor? And why are toddlers most helpless when they're trying to be helpful? Augie's vigorous efforts to sweep up the pancake detritus with a much-too-large broom ('I clean!') were adorable but not exactly effective.
This isn't just a caregiver's cri de coeur -- it's also an important scientific question. Human babies and young children are an evolutionary paradox. Why must big animals invest so much time and energy just keeping the little ones alive? This is especially true of our human young, helpless and needy for far longer than the young of other primates.
One idea is that our distinctive long childhood helps to develop our equally distinctive intelligence. We have both a much longer childhood and a much larger brain than other primates. Restless humans have to learn about more different physical environments than stay-at-home chimps, and with our propensity for culture, we constantly create new social environments. Childhood gives us a protected time to master new physical and social tools, from a whisk broom to a winning comment, before we have to use them to survive.
The usual museum diorama of our evolutionary origins features brave hunters pursuing a rearing mammoth. But a Pleistocene version of the scene in my kitchen, with ground cassava roots instead of pancakes, might be more accurate, if less exciting.
Of course, many scientists are justifiably skeptical about such 'just-so stories' in evolutionary psychology. The idea that our useless babies are really useful learners is appealing, but what kind of evidence could support (or refute) it? There's still controversy, but two recent studies at least show how we might go about proving the idea empirically.
One of the problems with much evolutionary psychology is that it just concentrates on humans, or sometimes on humans and chimps. To really make an evolutionary argument, you need to study a much wider variety of animals. Is it just a coincidence that we humans have both needy children and big brains? Or will we find the same evolutionary pattern in animals who are very different from us? In 2010, Vera Weisbecker of Cambridge University and a colleague found a correlation between brain size and dependence across 52 different species of marsupials, from familiar ones like kangaroos and opossums to more exotic ones like quokkas.
Quokkas are about the same size as Virginia opossums, but baby quokkas nurse for three times as long, their parents invest more in each baby, and their brains are twice as big.
But do animals actually use their big brains and long childhoods to learn? In 2011, Jenny Holzhaider of the University of Auckland, New Zealand, and her colleagues looked at an even more distantly related species, New Caledonian crows. These brilliant big-brained birds make sophisticated insect-digging tools from palm leaves -- and are fledglings for much longer than not-so-bright birds like chickens.
At first, the baby crows are about as good at digging as my Augie is at sweeping -- they hold the leaves by the wrong end and trim them into the wrong shape. But the parents tolerate this blundering and keep the young crows full of bugs (rather than blueberries) until they eventually learn to master the leaves themselves.
Studying the development of quokkas and crows is one way to go beyond just-so stories in trying to understand how we got to be human. Our useless, needy offspring may be at least one secret of our success. The unglamorous work of caregiving may give human beings the chance to figure out just how those darned brooms work.