If we turn back the evolutionary clock around 420 million years, nearly all life on Earth was confined to the water. There weren’t even any leafy plants or insects on land, let alone animals with backbones, legs and lungs. But over the next 100 million years, there was an explosion of new life that evolved to include the first land-dwelling “tetrapods,” four-legged, salamander-like creatures that were the earliest common ancestors of all reptiles, amphibians, birds and mammals—including humans . That momentous evolutionary leap from water to land is known as the “fish-to-tetrapod transition,” and while scientists have a big-picture understanding of how it happened, the details remain elusive. Paleontologists have unearthed remarkable fossils from the Devonian Period (420 to 359 million years ago) of creatures that are somewhere on the evolutionary spectrum between fish and four-legged animals.…