In 1999, paleontologist Per Ahlberg, sifting through an old museum drawer, discovered a fragment of jawbone that appears to have belonged to a creature that was the missing link between sea and land animals.

The fish-to-four-legs transition occurred about 370 million years ago.

It was one of the most crucial transitions in the evolution of life on Earth, without which humans, for one, would never have evolved.

These early land animals had backbones and four legs, hence their name tetrapods, which is Greek for "four-footed."

They are the ancestors of all tetrapods—from dinosaurs to dogs, horses to humans.

Even snakes are tetrapods, because their ancestors had four legs.

Dolphins and whales are also tetrapods, having evolved from a race of wolflike creatures that began to adapt themselves to water about 50 million years ago.

The fish-to-tetrapod transition took place during the Devonian Period (408 to 360 million years ago), which is known as the Age of Fishes for the abundance of fish fossils known from that time.

It was long thought that the first tetrapods were fish that first clambered ashore, then evolved limbs to stand on.

One theory, advanced by the late paleontologist Alfred Romer, held that tetrapods arose from fish living in ponds that dried up periodically. When a pond evaporated, these fish used their fins to drag themselves to new ponds.

Recent discoveries by Jenny Clack and other paleontologists, however, reveal that fish evolved limbs first, then came ashore.

Careful study of fossil anatomy shows that those rudimentary limbs could not support their owner's weight. Rather they were used to support the animal in the shallow swamp edges in which it lived.

One such early tetrapod, a creature known as Acanthostega, had eight fingers on the end of each of its four limbs.

Clack believes Acanthostega used its limbs to steady itself in the water and its fingers to clutch swamp plants for support.

Those early tetrapods evolved from the so-called lobe-finned fishes, a group of fish that had stout, strong fins whose bones foreshadowed in basic arrangement those of arms and legs.

Surviving lobe-finned fishes, including lungfishes and the coelacanth, are our closest living relatives among the fishes.

Until a fisherman brought up a coelacanth off the coast of Africa in 1938, the species was thought to have gone extinct with the dinosaurs.

Fossil remains of the first early tetrapod to be discovered, Ichthyostega, were unearthed in Greenland in 1929. For most of the 20th century, it was the only example of an early tetrapod.

In 1987, also in Greenland, Jenny Clack and her team discovered remains of Acanthostega, including the most complete early tetrapod ever found.

It took three years to clean and otherwise prepare this fossil, which Clack dubbed Boris.