Science: tell me about ... Archaeopteryx

THE peculiar way that evolution works in its blind way through the years was demonstrated again last week, with the announcement by a team of American scientists that they had found fossils in the Gobi Desert which really did show that birds are descended from dinosaurs. Didn't we know that already, from the fossils of the Archaeopteryx - which people usually think of as the "dino-bird" from which all modern birds are descended?

The first thing about Archaeopteryx is that it describes a genus, not a species - which means that it covers a range of different animals. The first of its fossils was discovered in 1861, in a limestone quarry near Langenaltheim in Germany. All seven fossils so far discovered come from Germany, and are of animals which lived about 140 million years ago, during the Jurassic period. That means that they're only one indicator of how birds evolved from dinosaurs - but they definitely show that they did.

Archaeopteryx had teeth in both jaws, a long, feathered tail and three clawed fingers in its front limbs. The feather structure appears identical to that of modern birds; but they also had long tails (with feathers down the sides), which is a distinctly dinosaur-ish characteristic. So are the teeth and claws.

So what animal is it? "Most people would accept that it's the earliest ancestral bird," says Angela Milner, head of the fossil vertebrates division at the Natural History Museum. "It's a snapshot of how birds evolved from meat-eating dinosaurs." The reckoning is that Archaeopteryx is closely related to another dinosaur of the Jurassic period, the chicken-sized meat-eating Compsognathus.

The key thing that tells us Archaeopteryx is a bird, not a dinosaur? The feathers. Those are the single defining characteristic. "Archaeopteryx is a transitional stage between dinosaurs and birds," says Milner.

The Gobi Desert animals, which lived in a different place and at a different time (about 70 million years ago) show a number of different characteristics. Notably, while Archaeopteryx may have flown, the Gobi Desert ones probably didn't. "They seem to have lost the ability to fly," says Milner. "But they are more like modern birds than Archaeopteryx."

How come, if they can't fly? "There are modern birds which can't fly," points out Milner. "It seems to be an ability which has been lost and found many times over evolution." Which is a useful reminder that natural selection is not about direction, just survival.