SCIENTISTS believe they have found the long-sought missing link between fishes and land animals in the shape of the oldest known arm bone, it was revealed today.
The 365-million-year-old humerus belonged to a creature that was just learning to prop up its body.
It is believed to bridge the gap between the fins of fish and the limbs of amphibians. Precisely how land-living animals evolved from fish has been a scientific mystery.
An important part of the transition is the change from fins to arms and legs.
The newly discovered fossil, found in an unlikely site beside a busy American highway, is the earliest arm bone known to belong to any limbed animal.
While possessing features seen in primitive fish fins, it also has the characteristics of a true limb bone.
The action of these muscles would have produced a motion similar to a "push-up or a bench-press", indicating that the bone served to prop up the body.
The findings are published today in the journal Science.
The specimen was unearthed in 1993 from a spot beside the Clinton County Road in north-central Pennsylvania.
Researcher Dr Michael Coates, from the University of Chicago, said: "This bone is a lot more robust than a humerus from any of the ancient species."
Dr Coates said it was now clear that the ability to prop the body dated back further than was previously thought.