The invasion of land by ocean-dwelling plants and animals was one of the most revolutionary events in the evolution of life on Earth, yet the animal invasion almost failed—twice—because of the twin mass extinctions of the Late Devonian Epoch. Some 359 to 375 million years ago, these catastrophic events dealt our ancestors a blow that almost drove them back into the sea. If those extinctions had been just a bit more severe, spiders and insects might have become the ecologically dominant forms of animal life on land. This book examines the profound evolutionary consequences of the Late Devonian extinctions, which shaped the composition of the modern terrestrial ecosystem. Only one group of four-limbed vertebrates now live on Earth while other tetrapod-like fishes are extinct. This gap is why the idea of “fish with feet" seems so peculiar yet these animals were once a vital part of our world.