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JON HAMILTON: I'm Jon Hamilton, and I need caffeine. But my kid doesn't. He starts his day like this.

(Soundbite of screaming child)

HAMILTON: Which got me thinking: why did evolution give children so much energy and their parents so little? I asked Michael Rose. He's the father of four children. He's also an evolutionary biologist at the University of California, Irvine.

Professor MICHAEL ROSE (University of California, Irvine): Well, it's fairly clear that the human evolution has been strongly shaped by very powerful selection pressures over the last two million years to build a bigger brain.

HAMILTON: That big brain doesn't have much in it when we're born. So Rose says children need all that energy to explore the world and devour information.

Prof. ROSE: Play and activity and doing all kinds of things including things, including things your parents and teachers don't like, is a big part of developing a functional human brain.

HAMILTON: Brain development pays off in the long run. Kids eventually get smart enough to survive on their own.

But while they're going full throttle through childhood, they put themselves at risk. So evolution has equipped children with parents who are slower, but perhaps wiser.

Steven Lima, of Indiana State University, knows a lot about the value of parents. He's an expert on predators and prey. He says that evolutionarily speaking, energetic children make easy prey.

Professor STEVEN LIMA (Indiana State University): They spend a lot of time roughhousing, running around, screaming, and all that sort of thing. Well, a lot of animals do that. And you know, that's one of the most ridiculous things you can do from the predator point of view. You just, it's a giant Eat at Joe's sign. You know, just come kill me. I'm running around, I'm not paying attention, I'm making a lot of noise.

HAMILTON: Parents are much more alert to danger, even if we're not very perky. So we keep an eye out for things like tigers and traffic. And eventually, Lima says, children grow up.

Prof. LIMA: Playing around like that just becomes ridiculous. You don't get anything out of it anymore. It just becomes dangerous.

HAMILTON: What slips away, of course, is youthful energy. Michael Rose, at UC Irvine, says he's reminded of that each year, when a new crop of incoming students arrives on campus.

Prof. ROSE: In terms of, you know, energetic physiology, in terms of your ability to get up and go and do things, you can't beat an 18-year-old when they're motivated. And it's strictly downhill from there. And I've experienced a lot of that downhill myself.

HAMILTON: Me too. And sometimes it takes a jolt of caffeine to temporarily slow the descent.

But there's an upside to having less energy. Rose says it means we don't take as many risks, and that improves our odds of living long enough to influence the next generation.

Prof. ROSE: Fifty and 60-year-old humans can be very relevant to the future of their offspring and grand-offspring. And for that reason natural selection may indeed still have some force in keeping us alive in middle age.

HAMILTON: Provided we don't fall asleep on the job.

So as a father, I know that coffee isn't just about the buzz. It's about preserving the future of our species.

(Soundbite of baby)

HAMILTON: Jon Hamilton, NPR News.

RENEE MONTAGNE, host:

And want to know the average caffeine content of your favorite pick-me-up? It's there at our Web site, npr.org/your health.

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