Holly Williams, age 11, of Gastonia, N.C., for her question:

WHAT WERE THE FIRST SIGNS OF LIFE ON EARTH?

Nobody knows exactly when the miracle of life first stirred on earth. But scientists have a good idea of what sort of living things there were and also where they appeared. Most experts suspect that the great adventure began more than 2 billion years ago, when the earth was about half its present age.

Imagine a world with one huge continent, where barren rocks were touched only by the weather. The ocean covered more than half of the surface, and its brimming fresh waters slopped over the shores. This is how things were when life began on earth. The sun shone, the rains descended and the stormy lightning flashed through the clouds.

The smallest unit of life is a living cell, too small for human eyes to see. Yet is is built from highly special molecules, endowed with the miraculous abilities to convert food into energy, to multiply and produce more units of life.  Compared with nonliving rocks, these qualities were miracles indeed.

Yet the living and nonliving substances were made from the selfsame chemical elements already present in the earth. All living cells, then and now, contain assorted protein molecules made mostly from common elements such as carbon and hydrogen, nitrogen and oxygen. Each protein molecule is assembled from a large assortment of smaller chemical units called amino acids.

We assume that the first signs of life began with the formation of amino acids from simple chemicals in the ancient freshwater seas. Nobody knows how this was arranged or how long it took. Eventually certain amino acids were assembled to build protein molecules, and at long last assorted protein molecules were assembled to build cells.

Then the true magic began. For the living cells were not entirely dependent upon chancy chemical activities. They could consume raw material and produce their own energy ¬and multiply themselves. Most experts agree that the very first forms of life were plant cells, probably quite similar to the blue green algae that still teem in the global oceans.

Later, certain life forms specialized to form animal  type cells. All these mini cells were too small and frail to leave fossil remains. However, as they lived and died, they caused changes in the rocks. It is assumed that the earliest algae caused changes in certain carbonized rocks of Canada, Rhodesia and Australia.