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 members of a larger group that would invade the land many times over -- a group known as the arthropods. Defined by segmented bodies, jointed legs and hard exoskeletons, arthropods comprise over 80% of the animal species living today. Many are familiar, such as crabs, lobsters and of course, insects.
Paleontologists have recently discovered that arthropods invaded land not once but many times. Fossil evidence shows that different groups including insects, millipedes and centipedes, spiders and scorpions -- all came ashore on their own at different times.
With appendages adapted for walking, breathing, pinching, sensing and ultimately flying, arthropods have achieved amazing success on land. Indeed, the development of flight was the tour de force of the arthropod insect branch and perhaps the single most important adaptation that allowed them to eventually dominate every habitable ecosystem on Earth.
