Tamyra Ames describes her oldest son as a “heart attack waiting to happen.”

When Luke was only 18 months old, he climbed out of bed, unlocked the door, opened the gate and toddled into the street.

Six months later, he sneaked through a sliding door and climbed into the pool. When he was 8, he pushed through the screen and tumbled out a second-story window, surviving with minor bruises.

Her twins, Kathryn and Jason, were even worse, thwarting every baby-proofing device that she bought, says Ames, of Tucson. They greedily gobbled the dog food and even boosted each other over the baby gate.

“Toddlers are so excited by the world, and they’re experimenting all the time,” says pediatrician Harvey Karp, who describes toddlers as “little cavemen” in his book, TheHappiest Toddler on the Block (Bantam, $15). “But their abilities leap ahead of their judgment and their ability to think things through.”

After a day spent rescuing their young daredevils from self-destruction, experts say parents can’t be blamed for wondering: How on earth has the human race survived? If natural selection promotes the survival of the fittest, shouldn’t that process have chosen less dangerous tendencies in the smallest, softest and most clueless of humans?

A different kind of danger long ago

In fact, these common childhood behaviors may have been less hazardous in humanity’s early days, says Joyce Benenson, a biological anthropologist at Harvard University.

She notes that the typical baby’s environment has changed radically since the days of hunting and gathering.

In the few remaining hunter-gatherer cultures, women carry their children in slings for their first few years of life, nursing them through the wildest days of toddlerhood. That keeps babies from running off or putting dangerous objects in their mouths, Benenson says. Extended families and neighbors in these cultures help parents keep a close eye on children.

“These horribly dangerous behaviors just wouldn’t have had a chance to hurt too many of us,” Benenson says.

Though the natural world presents hazards of its own, babies who graduated from the maternal sling may have had fewer opportunities for mischief than toddlers today, says Meredith Small, an anthropology professor at Cornell University in Ithaca, N.Y., and author of Our Babies, Ourselves.

“Our homes have more stuff that can be a danger to kids,” Small says.

In humanity’s early days, babies were more vulnerable to the elements and diseases that are now easily treated. But there were no toys with small, detachable parts, no plastic wrap in which babies could suffocate, no windows blinds with dangling cords, no caustic cleaning supplies.

Some toddler behavior may reflect evolution at work. Babies’ tastes — such as a marked preference for sweets and fats — also may have protected them through the ag