Thirty thousand years ago, North and South America were home to the woolly mammoth, the saber-toothed tiger, and a giant ground sloth the size of an SUV. What happened to them?

It is becoming increasingly clear that what happened to them was us. Humans, crossing a land bridge from Siberia where the Bering Sea now lies, arrived on this continent somewhere between 11,000 and 16,000 years ago. The more recent figure was accepted wisdom until very recently, when evidence of human presence at least 11,000 years ago was discovered in South America. It's a long walk there from Alaska, so the time of the arrival of Homo sapiens in the Americas has been pushed back.

When humans arrived, it was bad news for the existing fauna. Perhaps our ancestors decided that the largest animals were dangerous, or maybe they just tasted good. Either way, we systematically did them in, even the biggest and most ferocious. There is no doubt that humans, not lions, tigers, killer whales or grizzly bears, are and have been for at least the past 100,000 years the top predators on this planet.

This has given us an inflated view of ourselves. We think that we are, to use a confusing expression, top dogs. It is therefore particularly irritating when the "dumb beasts" of the animal world do with ease things that we supposedly super-smart and super-capable humans cannot. I don't mean actions based on specialization of body structure, such as flying like an eagle, swimming like a dolphin, or jumping like a flea. I'm referring to things which, by all logic, our bodies should be able to manage without modification. For instance, why can't we grow a new finger or foot if we lose one, or connect a spinal cord severed by injury? We grow new skin without any problem, so some regeneration capability is clearly built in to us. Also, amphibians grow whole new limbs and some lizards new tails, which means that the capacity to regenerate nerve cells is a natural function in some animals.

I believe we will understand how to make nerve cells grow and reconnect within 20 years, but that is not what I want to talk about in this column. I want to discuss something that, on the face of it, should be easy: sleeping for a long time.

Humans are unfortunate when it comes to sleep. On the one hand, sleep is absolutely essential to us. Deprived of it, humans become deranged and die after a couple of weeks. Other animals, such as seals and dolphins, seem to sleep very little if at all, and the dolphins may have developed the curious ability to sleep with only one side of the brain at a time.

On the other hand, we can't follow the example of some other animals by slowing our body functions in times when food or water is short. That must once have been a valuable survival mechanism for humans, too, even if food for many of us now seems almost too easily available. Yet when winter approaches, we sleep little more than during summer, while a variety of warm-blooded animals, such as bats, dormice, marmots, woodchucks, and ground squirrels, head for a sheltered hiding place and lapse into a kind of coma. Their bodies cool until their temperature is close to that of their surroundings, even to just a few degrees above freezing. (No warm-blooded creature can stand actual freezing, though insects can and do.) The heartbeat slows by as much as a factor of 50, breathing becomes slow and irregular, and the metabolic rate drops by up to a factor of 100. Nervous reflexes slow and stop below about 64 degrees Fahrenheit, while brain electrical activity cannot be detected below 52 degrees.

Long winter sleep is called hibernation, while long summer sleep, usually induced by drought rather than heat, is called estivation. Warm-blooded animals don't estivate, but some reptiles and amphibians do. Crocodiles, alligators, and turtles will bury themselves in mud until the rainy season returns. Some Australian frogs do even better, remaining under the clay beds of dried-up ponds for as long as it takes until the next rare shower of rain brings them out to feed and breed.

Burying yourself in mud seems like an undue price to pay for the ability to sleep for as long as you choose, and in our case it should not be necessary. Our model should be the warm-blooded animal, where the key variable is the reduction of body temperature. And in this area, there are powerful reasons for investing the necessary research funds. Major surgery, particularly heart surgery, is often better performed when a patient's body temperature is reduced by as much as 20 degrees. Unconsciousness automatically results from such a decrease, making other anesthesia unnecessary while the body is chilled. We also know, because it has happened accidentally to certain unfortunate children, that the human body temperature can dip as low as 58 degrees, and return again to normal with no ill-effects. What we do not know is how long such a low temperature can be sustained and permit full recovery.

What is missing today is an adequate body of experimental evidence. Most of our experience with lowered body temperatures and extended human unconsciousness comes from one of two situations. Either we are dealing with a sick person on an operating table, when no one wants to extend unconsciousness longer than necessary and hibernation research is far from anyone's interest; or reduced body temperature results from an accident, such as plunging through the ice of a frozen pond, where again the objective is to restore normal body functions as rapidly as possible.

What we need is a group of volunteers willing to participate in controlled experiments that explore the unknown territory relating temperature, unconsciousness, and human metabolism. Since there is a good chance that in at least some experiments, the unconsciousness will be permanent, I expect the pool of volunteers to remain small.

Which is a pity. Ben Franklin once said that he wished he could be pickled in a barrel for a couple of hundred years, so that he could see what the world was like when he came out. I feel the same way. We are a long, long way from a human hibernation of two months, let alone two centuries, but I still have hopes.

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