Water gave birth to life, and guarded it jealously. For billions of years, the land was barren, while life proliferated in the buoyant, nurturing bath of the seas, ignorant of such terrestrial concerns as falling down. The first invaders were plants, which began creeping upland from the streams and swamps some 450 million years ago, followed by arthropods and a few brave mollusks, which became the land snails. But waiting in the shallows was a nine-foot-long, crocodile-headed fish with both gills and, on the top of its head, air-breathing nostrils called spiracles. With a fateful lunge landward, it changed the course of life on earth forever.

In 2004, when the fossil bones of Tiktaalik roseae were dug from the ground of Ellesmere Island, in the Canadian Arctic, the discovery was hailed as a breakthrough not just for paleontology, but for beleaguered science teachers trying to keep creationism out of their classrooms. A fish (with scales and gills) clearly resembling a tetrapod (with a flat head, a neck and prototypes of terrestrial limb bones in its lobelike fins), it precisely filled one of the gaps in the fossil record that creationists cited as evidence against Darwinian evolution.

Scientists can’t say whether Tiktaalik itself is the ancestor of any species alive today; there were likely several related genera making the same transition around the same time. But the marvelously preserved fossil sheds new light on how the vertebrate invasion of land took place, some 375 million years ago.

Until this year, Tiktaalik was known only from its front half, but in January, evolutionary biologist Neil Shubin of the University of Chicago and his colleagues reported excavating the posterior skeleton of their original specimen. The hip and pelvis were surprisingly robust, suggesting more powerful rear limbs than previously believed. Although almost certainly still encased in fleshy lobes, appendages could have helped support or even propel the animal in shallow water or mud flats. If so, it changes our view of the evolution of tetrapods, whose ancestors were believed to drag themselves by their forefins, only developing useful hind legs once ensconced on land.

As for what drove this epochal migration, “it’s extremely bloody obvious: There were resources on land, plants and insects, and sooner or later something would evolve to exploit them,” says vertebrate paleontologist Mike Benton of the University of Bristol. It’s also possible, says Shubin, that fear played a part. “If you look at the other fish in the water at the time, they’re big monstrous predators,” he says. Some exceeded 20 feet in length. Even for Tiktaalik, a toothy carnivore itself, this was a “predator-rich, competitive environment.” If you can’t be the biggest fish in the pond, maybe it’s better to get out of the water altogether.

And from those first lumbering steps, it appears, came the whole parade of terrestrial vertebrates: amphibians and reptiles and birds and mammals, including those that later returned to the oceans. The process set in motion by Tiktaalik (or its cousins) was necessary for the great variety of animal life we see today.

On land, animals faced all new challenges. Nothing in the history of life to that point would have prepared them for the rude experience of slipping off a branch, or the shocking necessity of copulation as a substitute for broadcast spawning. Challenges, though, were also opportunities, to expand and diversify; Benton estimates that the land holds perhaps ten times as many species as the oceans.

Human intelligence is unique on the planet, and even by a generous definition of language, only a few mammals and birds seem to have mastered it. Simon Conway Morris of Cambridge, England, author of Life’s Solution, believes that evolution inevitably converges on certain traits, including intelligence. The octopus, which can manipulate objects with its arms and solve problems, is an example of an intelligent animal whose ancestors (as far as we know) never lived on land. But it’s hard to imagine anything like our technology developing underwater.

The emergence of intelligence remains a mystery, Benton says: “Arguably, a coral reef is equally complex as a forest. But why primates developed big brains to navigate around and find food, but not, say, clown fish—I couldn’t say.”

Shubin’s book, Your Inner Fish, recently adapted as a PBS series, traces the evolutionary history of the human body back through the time of Tiktaalik, showing, for instance, how a bone adjacent to the spiracle evolved into a bone in the tetrapod’s middle ear. A fish that hauled itself out of the water on its fleshy fins, for reasons we can only speculate on, bequeathed us our limbs, backbone, teeth and sense organs—and, perhaps, our expansive curiosity and restlessness.