The only way scientists know about the earliest land animals on Earth is through fossils found in ancient rock. And from these fossils, we know that almost all species of animals have changed dramatically since the first animals walked on land. The changes became necessary as the Earth has gone though such environmentally changing events as volcanic eruptions, the movement of the continents, changes in climate, and massive extinctions. (For more about early animals, see the chapter “Basics of Biology.”)

But what animals were responsible for first walking on land remains a highly debated topic. One possibility was suggested in 2013, in which researchers suggested that a toothy creature called the Ichthyostega represented one of the first transitions between fish and terrestrial animals. It lived about 374 to 359 million years ago and is thought to have lived in the shallow water of swampy areas—either along coastlines or waterways. The best guess is that the animal was looking for something to eat and was lured by the possible food on land. The four-legged creature probably dragged itself out of the water and on land by using its front legs and dragging its back legs, much like how modern seals move. The creature also had a string of bones in its chest that may have been the precursors to the sternum that holds the ribcage—necessary for supporting the weight of its chest as it moved on land.