Between Swimming and Crawling.

Byline: Temma Ehrenfeld

Imagine a grotesque salamander-like creature, nearly a meter long, weighing 20 kilograms or more. It stands in a shallow stream on massive arms with its elbows permanently bent, balancing on the tips of its fingers. Like an athlete doing morning push-ups, the creature bobs its head above the current. Amphibians like this one--in the large category of four-limbed creatures called tetrapods--stand near the base of the evolutionary tree, between the fish that came before them and the birds and mammals that followed. Last week researchers announced in the journal Science that they'd identified the fossil of an arm bone of just such a creature, the oldest indisputable tetrapod fossil yet found--a discovery that sheds light on just how nature got from fins to fingers, from swimmers to crawlers.

The 365 million-year-old fossil, discovered in the mountains of Pennsylvania, is unique in another way: it's far more massive than the arm bones of other tetrapods. What this means is that evolution wasn't a simple ladder of progress, with a single creature plopping somehow onto the mud and evolving ashore. The "fish-tetrapod transition" was characterized by a panoply of competing adaptations, each struggling to outdo the others. ...