Out of the water

A fish with a neck

In the summer of 2004, on Ellesmere Island in the Canadian Arctic, a team led by Neil Shubin of the University of Chicago, Edward Daeschler of the Academy of Natural Sciences in Philadelphia, and Farish Jenkins of Harvard found the fossils of a fish with a flat, crocodile-like head. The team had been coming to the Arctic since 2000, for a reason that sounds like a puzzle. They wanted a fish that was becoming an animal with legs, and from earlier fossils they knew roughly when to look: about 375 million years ago, in the Late Devonian. They needed rock of that age, laid down in fresh water, and exposed at the surface instead of buried. In Nunavut, a treeless place with almost no soil, the old rock sticks out of the ground.

The animal they found, announced in Nature in April 2006, was named Tiktaalik roseae, a name from Inuktitut that means “large freshwater fish,” chosen with Inuit input in Nunavut. It was a predator up to about 2.7 meters long, and it was a fish: it had scales, gills, and fins with fin rays. But the head was flat, with the eyes on top, like a crocodile’s. It was the earliest known fish to have a neck, with the shoulder bones separated from the skull so that it could turn its head. Its ribs were robust, perhaps helping to prop up its body in shallow water. It had lungs as well as gills. And inside its fin, instead of a fan of thin rays, was a sturdy set of bones with a joint that works like a wrist.

Fins into fingers

Tiktaalik belonged to the lobe-finned fishes, a group named for fins that have a fleshy base with bones and muscles inside, unlike the thin rays of a goldfish’s fin. In the Devonian, lobe-finned fish like Eusthenopteron were common and successful. Only eight species survive today: two coelacanths, which live in the deep sea of the western Indian Ocean and near Indonesia, and six lungfish, in Africa, South America, and Australia. A museum curator named Marjorie Courtenay-Latimer saw the first living coelacanth in the catch of a fishing boat at East London, South Africa, in December 1938, a fish that scientists believed had died out 66 million years ago. Lungfish, though, are the lobe-finned fish most closely related to us.

One point about fins is easy to miss. Look at how many paired fins a fish has: two pairs, a pair at the chest and a pair at the hips. Everything else, the back fin, the belly fin, the tail, comes singly. Those two pairs became our arms and our legs, and they are why almost every four-limbed animal on earth has exactly four limbs. Evolution does not design from scratch; it modifies what is there. A fish with three pairs of paired fins might have had six-limbed descendants. We turn the pages of a book with a rebuilt chest fin and walk on rebuilt hip fins.

The change was not sudden. In 2020, a team described a complete specimen of Elpistostege, a flat-headed fish from Quebec about 380 million years old, and scanned its fin with X-rays: inside was a set of bones that line up in rows like a wrist, and, at the end, small bones arranged like fingers, still wrapped in a fin. The pattern of bones that became our hand was already present in a fish 380 million years ago.

The first steps

The next animals have names that sound like a joke: Ichthyostega, the “fish roof,” and Acanthostega, the “spiny roof,” both from East Greenland rock about 365 million years old, found on Danish-led expeditions in the 1930s and studied for decades since. They were limbed animals about 60 centimeters to 1.5 meters long, with a tail fin and, in Ichthyostega, seven toes on each hind foot. Acanthostega had eight fingers on each hand. Not five. The five-fingered hand we have is a later simplification; the first limbs with digits had more and varied.

Here is the surprising part. When Jennifer Clack of Cambridge studied Acanthostega, she found that its limbs could not carry its weight on land: the elbow could not bend forward, the ribs were short, and it still had gills. It had limbs and digits, yet it lived almost entirely in the water. Limbs with fingers did not evolve for walking on land. They evolved in water, perhaps for pushing through plants and mud in shallow streams, and only later turned out to be useful on land. Ichthyostega was more land-ready; a 2012 study of its shoulder and forelimb bones concluded that it could haul itself across mud in short jerks with both front limbs at once, the way a mudskipper or a seal does, instead of striding.

The footprints tell an older story. In 2010, a Polish team reported tracks from a quarry at Zachełmie, in the Holy Cross Mountains, with clear prints of fingers and toes, made by animals more than two and a half meters long. Dated to about 390 to 395 million years ago, they are some 15 to 20 million years older than Tiktaalik. Not everyone agrees on what made them: some scientists doubt that tetrapods did, and the site, first read as a marine shoreline, may have been a set of shallow lakes. If the tracks are what they seem, Tiktaalik is not our grandparent but a late-surviving cousin, an example of the kind of animal that came before. That does not make it less valuable: a cousin that survived shows what the family was like.

China has a place in this chapter too. The earliest limbed animal found in Asia, Sinostega, was identified in 2002 from that jaw fragment in the Zhongning region of Ningxia, a dry area of the Yellow River valley. When it lived, the same place was a landscape of lakes and rivers. A fossil is also a record of the way a place has changed.

Why did some fish start to leave the water? Nobody knows for certain. The popular answers are that the shallows were full of food, plants and insects were already on land, there was less competition, and fewer big predators. The animals did not “decide” to go; each generation lived in shallower water than the last, and those with stronger fins and better lungs did better. And none of them gave up the water completely: even today, most amphibians go back to it to breed.

Which came first?

Everyone knows the riddle: which came first, the chicken or the egg? Paleontologists have an answer, though it depends on how you read the question.

Look at the eggs laid by a frog. They are small jelly-coated globes, laid in water or in damp places, and if you scoop them out and leave them on a dry bank, they die. The eggs of reptiles and birds are different. They have a shell, and, inside, a set of membranes that keep the embryo in a sealed, watery world of its own: the amnion, a sac of fluid around the embryo; the yolk sac, which feeds it; the allantois, which stores its waste; and the chorion, which lets oxygen in. This is the amniotic egg, and the animals that have it, or whose ancestors did, are called amniotes: reptiles, birds, and mammals.

The amniotic egg was a second escape from the water. The first escape, in the last section, let animals walk out; but they still had to go back to the water to lay their eggs, and their skins had to stay moist. The amniotic egg packs the pond into the egg. It let animals lay eggs far from water and, together with watertight skin, spread across dry continents. The oldest known amniote fossils are about 318 million years old, from the coal-age forests of Joggins in Nova Scotia. There, in 1852, Charles Lyell and William Dawson found the bones of early land animals inside the hollow stumps of fossil trees, where they had fallen or sheltered and been buried; Dawson later described Hylonomus from the same kind of stump, a lizard-like creature about 20 centimeters long that hunted millipedes and insects. Some recent analyses place it just outside the group that contains all living reptiles, but it shows the way of life.

Eggs fossilize poorly, and early ones were probably leathery, not hard. In 2020, an international team studied embryo-bearing eggs of two dinosaurs, Protoceratops from Mongolia and Mussaurus from Argentina, and found that they were soft-shelled, like a turtle’s. Their conclusion was that the first dinosaur egg was soft, and that hard shells evolved independently several times (a later study disputed this for the long-necked group). Whatever the details, there were amniote eggs on earth hundreds of millions of years before there was a chicken.

Dinosaur eggs once led to a famous mistake. In 1923, an American Museum of Natural History expedition in the Gobi Desert in Mongolia found the skeleton of a small beaked dinosaur lying on a nest of eggs, and named it Oviraptor, “egg thief,” assuming it was stealing. About seventy years later, in 1993, scientists found an embryo of the same kind of animal inside a similar egg, and other skeletons were found sitting on their nests with their arms around them, like a brooding hen. The “thief” had been guarding its own eggs. The name stuck anyway.

So which came first? If “the egg” means the amniotic egg, there is no contest: eggs came first, by some 300 million years. If you mean the chicken’s egg, the puzzle depends on a definition. Species do not appear on a particular day. The animal that was the “first chicken” had to hatch from an egg laid by something that was almost, but not quite, a chicken, because the change in its genes would have been passed from its parents into that egg. So the first chicken was inside an egg, and that egg came first. If, on the other hand, you define a “chicken egg” as an egg laid by a chicken, then the chicken came first. The better lesson is that nature draws no sharp lines.

Back to the sea

The fifth turning point is an odd one, a return. The first vertebrates lived in the sea. Then some left it, grew limbs and lungs, and laid eggs on land. And then, over and over, some of those land animals went back.

The first was a small reptile called Mesosaurus, about 299 to 280 million years ago, in the early Permian. It was slender, with a long snout full of fine teeth, a long tail for swimming, and webbed feet. Its fossils are found only in South America (Brazil and Uruguay) and southern Africa, in the deposits of shallow, probably fresh or brackish inland waters, not the open ocean. A freshwater animal cannot cross the Atlantic. In the early 1900s, the German scientist Alfred Wegener took the same fossils on two continents as evidence that the continents had once been joined, which was an early step toward the theory of plate tectonics.

Others went much farther out. The ichthyosaurs, or “fish lizards,” appeared about 250 million years ago, a few million years after the worst mass extinction in history, and they came to look like dolphins, with a vertical tail fin, flippers, and some with eyes among the largest of any vertebrate. They breathed air and did not lay eggs. In 2014, a fossil from Chaohu in Anhui, China, showed an early ichthyosaur with young still inside her and one emerging head first. Later ichthyosaurs from Germany were preserved giving birth tail first, as dolphins do. In the Triassic, what is now Guizhou and Yunnan lay under a warm, shallow sea, and its fossil beds have yielded a rich zoo of marine reptiles. Keichousaurus, a small swimming reptile with paddle limbs, has been found with embryos inside the mother; Odontochelys, about 220 million years old, was an early turtle relative with a belly shell and teeth, but no shell on its back.

The sea reptiles kept coming. Long-necked plesiosaurs rowed with four flippers. Mosasaurs, huge marine lizards related to monitors and snakes, ruled the last 20 million years or so of the dinosaur age. Sea turtles have been at sea for over 100 million years, and still crawl up the beach to bury their eggs. After the dinosaurs came the mammals and birds. Whales descend from four-legged, wolf-sized animals such as Pakicetus, which waded in the rivers of what is now Pakistan about 50 million years ago; their closest living relatives are hippos. Seals and sea lions descended from carnivores. And penguins, whose ancestors were flying birds, traded their wings for flippers more than 60 million years ago: a penguin “flies” through the water.

A return is not a reversal. A whale is not a fish. Its tail flukes beat up and down, because it inherits the flexible back of a running mammal; a fish’s tail swings from side to side. A whale keeps its lungs, its warm blood, its milk, and the bones of a hand inside its flipper. Evolution does not go backward. It goes forward from wherever it stands, and what a lineage keeps from its past, here a lung or a limb bone, is the best evidence of where it has been.

Why go back? The likeliest reasons are food, in a sea full of fish and ammonites, and safety from the big predators of the land. There was a price. Air-breathers must come to the surface, and their young must be born alive in the water or on a beach. Many of these lineages paid it for tens of millions of years, and several still do.