Paige Martin, age 10, of Calgary, Alberta, Canada, for his question:

When did life on earth begin?

Many living things of the past left fossils in the ground. The big dinosaurs left their bulky bones. Trees left petrified wood and imprints of their leaves. Scientists can date these fossil records and trace the story of life back through the ages. For example, they know that there were plenty of dinosaurs around 100 million years ago. They also know that tiny algae lived in the seas more than a billion years ago.

Most experts think that the very first living things were so soft and so tiny that they left no fossils in the ground. But they did cause changes in certain rocks. This is the best early evidence we have to go on. It suggests that small simple ,creatures may have lived on earth 600 million years ago. Maybe this is when life on earth really began. Or maybe someday we shall find proof that it began at an even earlier date. At present, nobody can say for sure.