The first animals to walk on land are unknown. There is evidence that soft-bodied arthropods and slug-like animals visited the land as far back as 510 million years ago, in the Cambrian era, leaving behind mysterious tracks called Climactichnites and Diplichnites. These tracks are mysterious because no fossils have been found of the animals that made them. Some of these trace fossils are as wide as four inches. Perhaps these animals did not actually breathe air, and only slimed along on land for short periods as a way of moving from pond to pond.

According to scientific consensus, the first verified land animal was a one-centimeter myriapod. Present-day examples of myriapods include millipedes and centipedes. This myriapod, discovered in 2003 in Scotland and named Pneumodesmus newmani, is dated to 428 million years ago. Paleontologists can tell it lived on land because its fossil shows it possessed spiracles; holes that insects, spiders, rays, and sharks use for breathing air. Prior to the discovery of newmani, the oldest known air-breathing creature was a spider-like organism from 410 million years ago.

The first land-walking animals are often incorrectly cited as Devonian transitional forms called “fishapods” because they are intermediate between fish and true tetrapods. An example is the fish Tikaalik, which lived approximately 375 million years ago, during the Devonian period. It is remarkable that such organisms are so frequently cited as the first land animals when land animals from more than 50 million years before, such as Pneumodesmus newmani, are now widely known. The effect may have something to do with a bias in favor of the more familiar vertebrates over invertebrates.

The earliest land animals probably lived in oxygen-poor shallow pools near land. As the first vascular plants developed, they would have choked the areas around these pools with weeds, making it evolutionarily advantageous to climb over and around them via quick forays onto land areas. The land at that time would have been much more nutrient-rich than the water, as plants colonized the land before animals and left their decaying plant matter everywhere. Bacteria and fungi broke down much of the plant matter but it still would have been appealing to a hungry fish. Around 365 million years ago, some fish (so-called “fishapods”) developed limbs and climbed onto the land. The appearance of the first true trees about 370 million years ago would have helped this along, by depositing more nutrients into the soil and making the environment more habitable.