When you consider the years paleontologists often spend in daily and intimate contact with their fossils, it’s not very surprising that they come to regard their long-gone animals as pets. Some work on the show dogs of the fossil world; they brag about how fast their velociraptor ran or how efficiently their saber-toothed tiger could sever a spinal cord. But when you listen to Jenny Clack talk about her pet, a fossil creature named Acanthostega that she has been working on for seven years, she sounds like the owner of a sweet, homely mongrel. It wasn’t very smart, she says. It probably spent a lot of its time sitting at the bottom of lagoons, hidden in the muck, waiting for something to come by it could eat. Clack has helped make a reconstruction of it; its salamander-like body has big glassy eyes sitting on top of a flat, Muppet-style head, its mouth permanently caught in a foolish smile. Acanthostega mixes the anatomy of a newt with the charm of a mutt.
But though it may not have their flair, in terms of evolutionary significance Acanthostega can easily go nose to nose with any of its fossilized companions. Velociraptors and sabertooths are both tetrapods-- that is, they have four limbs, along with fingers and toes, hips and shoulders. We humans are also tetrapods, as are iguanas, ravens, bullfrogs, porcupines, and every vertebrate that has ever walked on dry land. Even whales and snakes are tetrapods, although they shed their legs long ago. Acanthostega was a tetrapod, too, but at 360 million years old, it has a special distinction: aside from creatures suggested by a few older fossil fragments, it is the most primitive tetrapod known. That means that Jenny Clack’s sweet, unprepossessing pet holds answers to the great mystery of how our ancestors changed from fish and hauled their bodies out of the water.
Clack, who works at the University of Cambridge’s Museum of Zoology, discovered the bulk of Acanthostega’s skeleton in 1987 and has been carefully reconstructing it ever since with fellow paleontologist Michael Coates. They are just finishing up their monographs on the creature, and some of the conclusions they’ve drawn from its body are surprising other paleontologists. For a long time it was assumed that our limbs and feet, which work so well for walking on land, evolved for that exact purpose. But Acanthostega has convinced Clack and Coates otherwise; tetrapod anatomy evolved while our ancestors lived exclusively underwater-- and it evolved for life underwater. The first vertebrate that walked onto land didn’t crawl on fish fins; it had evolved well-turned legs millions of years beforehand.
Before she found Acanthostega, Clack had studied early tetrapods for ten years, but she never expected to have the privilege of studying their actual origin. It’s not something you build your career on, she says. You may hear some paleontologists bemoan the rarity of dinosaurs, but compared with the first tetrapods, they’re as common as gravel. For most of this century, in fact, only one primitive specimen was known in any detail: a bulky, dog-size beast from Greenland named Ichthyostega.
At the end of the 1800s, Greenland, like most of the Arctic, was still pretty much terra incognita. The quest for the undiscovered North Pole had drawn a number of explorers, but many of them--and whatever knowledge they had gained--were lost in seas of ice. In 1895 an ambitious Swedish engineer named Salomon Andrée decided that instead of traveling over the ice by sled, he would soar to the pole in a hot-air balloon. Two years later he and two companions first sailed as far north as they could, to the island of Spitsbergen, 600 miles south of the pole. There they filled their balloon with hydrogen and rose into the sky. If the winds had been with them, they would have reached the pole in two days. As it turned out, no human ever saw them touch land again.
