How many seconds ago on the "cosmic calendar" did the two halves of the world find each other?

What are some possible short and long term medical concerns for crew members on longer space exploration trips

What was the land like over 500 million years ago? what was under the sea?

Aristotle classified animals according to their a. habitat and mating behavior c. mating behavior and relatedness b. habitat and physical differences d. physical differences and mating behavior