A theory portraying children as start-up companies and middle-aged adults as their investors has been proposed to explain why humans have such big brains and long life spans.
Evolutionary biologists have puzzled for decades over why humans live twice as long as chimpanzees and gorillas and have brains three to four times larger than their closest living relatives.
"We're thinking of the brain as an investment," says economist Arthur Robson, at the University of Western Ontario. Robson and anthropologist Hillard Kaplan, at the University of New Mexico, believe this investment is so substantial that it requires a longer human life span to give it the time to pay off.
"The combination of issues that they raise is novel - a useful first step," says evolutionary biologist Michael Rose, at the University of California at Irvine. But he warns against putting too much confidence in a mathematical model: "If you're a good applied mathematician, you can come up with a model to give you any conclusion you want."
Nonetheless, Rose is intrigued by the explanation of why human children are so unproductive for so long. "This is the best paper on the evolution of teenagers I've ever read," he says.
The inspiration for Robson and Kaplan's theory came from the observation that children in modern day hunter-gatherer societies consume more calories than they produce, accumulating an enormous debt that peaks at age 20.
As active young adults, they produce more than they consume, but it takes decades to pay off their childhood debt. Only at age 50 do they start moving out of the red and into the black. They then begin to make a net contribution of resources to their society, offsetting the debt of the children in the next generation.
The researchers noticed the parallels between the energy flows in these societies and cash flows in start-up companies, and applied an economic model to human evolution. They treated physical and mental capacities as "embodied capital", with the brain representing a special form of capital that increases in value over time.
According to the model, the brain requires such an enormous investment of energy during childhood that human ancestors must have evolved long life spans to make that initial investment worthwhile.
Robson and Kaplan believe these early humans must have lived in an environment where food gathering was complicated, favouring the development of big brains. They also suspect that the environment reduced the risk of premature death by affording some protection from predators.
The model therefore assumes that the environment was the key factor shaping human evolution. But, Rose argues, this leaves out the sophisticated social interactions that must have also contributed to development.
Journal reference: Proceedings of the National Academy of Sciences (DOI: 10.1073/pnas.152502899)