DavidKramer.DK

The advent of limbs for walking on land, and so of the tetrapods, was one of the main events in life's history. The traditional idea is that limbs arose from the fins of fish that were already making a living, at least in part, on land. There are, however, indications from the fossil record for the aquatic transition of fins to limbs which had the function of propping up the body in shallow waters.

Neil H. Shubin et al. now add to that evidence with their interpretation of a fossil limb bone, a humerus that belonged to a creature that lived about 370 million years ago. The authors compared the humerus with the equivalent bones from tetrapod and fish fossils of the same era. They point to its unique features which, they say, represent an intermediate between fins designed as swimming aids and limbs for walking on land or in water.

Shubin et al. conclude that the humerus was not necessarily a component of a limb adapted for locomotion on land. But they argue that there were evidently important transitions occurring in fish before the origin of the tetrapod limb with its characteristic elaboration of digits.