Roughly 540 million years ago, toward the start of the Cambrian Period, the planet was mostly ocean, and life was both alien and vaguely familiar. Small, phallic-looking worms rummaged through ocean-floor sediments while blind swimming beasts flung out whiplike tentacles to ensnare prey. Meanwhile, early versions of mollusks and sponges populated the seafloor as jellyfish floated above.Source