The first amphibians

The story

A scientist who knew where to dig

In 2004 the American palaeontologist Neil Shubin and his colleagues Ted Daeschler and Farish Jenkins were looking for something nobody had ever found: an animal halfway between a fish and a four-legged animal. They did not search at random. They knew that in the Devonian, about 375 million years ago, there had been a move from water to land. They also knew they needed a rock of the right age, formed in ancient rivers and swamps. They found it on Ellesmere Island, in the Canadian Arctic, where it is extremely cold today but where the climate was then warm.

After years of expeditions, in the summer of 2004 one of the researchers saw a flat snout sticking out of a rock. It was the fossil of a big fish up to about two and a half metres long. They named it Tiktaalik roseae. “Tiktaalik” is a word from Inuktitut, the language of the Inuit of Canada, and means “large freshwater fish”: the elders of the local community were consulted to choose the name. The discovery was announced in 2006 in the journal Nature.

A fish that looks like a crocodile

Tiktaalik had fish features: scales, gills, fins with rays. But it also had features of a four-legged animal. Its head was flat, with the eyes on top, like a crocodile's, and the nostrils opening on the snout; it had a neck, which fish do not have: it could turn its head without moving its whole body. It had strong ribs, which supported the body, and probably primitive lungs as well as gills.

The most important part was the front fins. Inside they had bones that match the upper arm, forearm and wrist of our limbs: one upper bone, two lower down and small wrist bones. The fins could bend, like an elbow, and let the animal push its body up from the bottom. They were not yet legs with toes, but they could let the animal prop itself up in shallow water, or drag itself for a moment out onto the mud. Scientists have called animals of this halfway kind “transitional forms” or, with a slightly jokey name, “fishapods”.

From water to land, by degrees

Tiktaalik is not the direct ancestor of all four-legged animals, but a species close to that group and an example of what it may have been like. Other fossils tell of the later steps. Acanthostega, found in Greenland and studied in depth from the 1980s by the British palaeontologist Jenny Clack, already had true legs with eight toes, but lived mostly in water and still had gills. Ichthyostega, also from Greenland, had sturdier legs and perhaps dragged itself on land. These animals are called early tetrapods, which means “four-footed”.

A recent discovery has complicated the picture a little. In 2010 a team of palaeontologists, including some Polish ones, described, in Poland, footprints of about 390 to 395 million years ago: they would be older than Tiktaalik. If they really are tetrapod footprints, the first four-legged animals already existed before the fish Tiktaalik, which would then be a surviving “cousin” of older forms. Scientists are still debating it.

Why leave the water?

To us it looks like an enormous step, but perhaps it was not. The shallow waters of the Devonian were warm, short of oxygen and full of predators. A fish with lungs and sturdy fins could breathe air at the surface and move between pools. Over time, little by little, those that could stay out of the water longer found food without competitors: insects, worms and other small animals that lived among the plants. It was not a choice, but a slow advantage that built up over millions of years.

The descendants of those first four-legged animals are the amphibians, reptiles, birds and mammals, including us. When you move your hand and wrist, you are using a pattern of bones that arose in fins like those of Tiktaalik.