For hundreds of millions of years, animal life resided only in the oceans. And then about 400 million years ago, fossil tracks suggest that an animal called a eurypterid left the water to walk on land. Maybe it was fleeing enemies, maybe it was searching for an easy meal, or maybe it was seeking a safe place to lay its eggs. Eurypterids were ...

In the 4.5 billion year history of Earth, a mere 10 million years seems rather insignificant, the equivalent of two months in the life of a 75 year-old man. Yet, during a 10 to 20 million year stretch of time, beginning about 540 million years ago, life evolved at an explosive rate. Scientists call the period the ''Cambrian Explosion.'' ...

When we think of animals, we think of movement. Surprisingly, the diverse and graceful ballet of animal movement may have started with cnidarians (pronounced ''ny-DAIR-ee-ans), a group that includes corals, sea anemones, sea pens and jellyfish. All of these animals, with few exceptions, have nerves and muscles. Because cnidarians are the ...