when did life begin on Earth?
Prehistoric Life… For billions of years on Earth, the only living things were microscopic, single-celled organisms. Then about 700 million years ago, the first real animals, such as jellyfish and sponges, appeared in the sea. They were entirely soft-bodied, but rare fossils give us a glimpse of how they looked. Over the next two hundred million years, creatures with hard parts (shells and bones) appeared. From this time on, the beginning of the Cambrian Period, the fossil record becomes much more detailed. Some kinds of animals, like sharks and crocodiles, have survived for long periods. Others died out rapidly as conditions changed. Still others have gradually changed or evolved from prehistoric into modern forms. Geological Time… The Earth is about 4.6 billion years old. Just as time on a clock is divided into hours, minutes, and seconds, so Earth’s history is divided into four huge chunks called eons. The first three of these, lasting four billion years are often grouped togethe
