The Life's Little Mysteries blog is in the midst of a string of posts that are, basically, like Marvel Comics "What If?" series as applied to the scientific history of Earth.
For example: What if humans had evolved to include more than two sexes, or to need three or more sex cells in order to procreate? What if Pangea (everybody's favorite supercontinent) had never split into chunks? What if Earth had never been in a massive collision with another, huge space object—meaning, what if the Moon didn't exist?
Now, if you've read very many of the comics you know that the answer to "What If?" is almost always "everybody dies". This series of posts is a bit less fatalistic. But, still, the point is made—these changes would radically alter life as we know it, and not necessarily in ways that sound like a lot of fun.
Take that question about the Moon. The implications of a Moon-less Earth are farther-reaching than you might guess:
Huge tides generated by the moon – which orbited much closer to Earth when it formed – washed the chemical building blocks for life from land into the oceans and helped "stir up the primordial soup," said Neil Comins, a professor of physics at the University of Maine.
The moon's gravity has helped slow Earth's rotation from an initial six-hour day to our current 24-hour day, while also stabilizing the tilt of our planet's axis, and thereby moderating the seasons. Life forms on a moonless Earth would therefore have different patterns of activity per the short days and nights, Comins told Life's Little Mysteries. These creatures might need to migrate more frequently to cope with extreme climate swings as well.