Tom Griefnow, age 12, of Inver Grove Heights, Minnesota, for his question:

It is true that dolphins once lived on land?

Modern biologists accept this theory. All the evidence indicates that the ancestors of the dolphins were land animals who went to live in the sea. Everything points to this theory and nothing contradicts it. This sort of evidence, however; is not scientific proof. To prove the popular dolphin theory, we need a continuous fossil record of their ancestors, back through millions of years.

We know the family history of the horse because we have the fossils of his ancestors to prove it. Step by step; this fossil record traces his development back through 50 million years. We have the fossilized skeletons to prove when hooves replaced toes, when teeth grew stronger, legs longer and all the other improvements in the horse family. But so far, this sort of proveable evidence regarding the dolphin family tree has not been found.

However, there are facts and more facts about the fascinating modern dolphins that now live in the world oceans. Obviously these highly intelligent animals are happily at home in the sea. What's more, these fatties with flippers could not cope with life on land. On the other hand, they have some outstanding features that seem very much out of place. When we take a general view of the world of nature, we notice that every animal is designed to live conveniently in a certain environment. The modern dolphin is an exception to this general rule    and we wonder why.

The creatures that rightfully belong to the sea have gills or other devices to extract dissolved oxygen from the water. They are cold blooded, creatures, for life in the ocean is not at the mercy of extreme seasonal changes.

There is other provable evidence that a few sea creatures ventured onto the land, more than 300 million years ago. Gradually, through countless generations, their bodies changed to make life on land more convenient. In time, some land species became air breathing, warm blooded mammals who gave birth to live infants and fed them on mother's milk.

The dolphins are mammals    and there is no known way that they could have developed their mammalian features while living in the sea. So far as we know, there can be only one way to explain them and all the other sea going mammals. Ages ago their ancestors left the sea and mastered life on the land. Later their ancestors went back to live in the sea.

Nobody can say exactly when or why the ocean going mammals deserted the land. They may have fled into the water to escape their .enemies    but more likely the reason was hunger. In the past, there were long global droughts that made life on land almost unbearable. Some experts suggest that this is when the dolphin ancestors returned to the sea. Long periods of drought also may have driven ancestors of the larger whales, the seals and their kinfolk back to live in the sea.