Evolution from fins to legs didn't trigger much anatomical change

Overturning long held views that the origin of vertebrates with legs triggered an increase in the anatomical diversity of their skeletons, new research has found that limbs of the earliest vertebrates, dating back more than 360 million years ago, were no more structurally diverse than the fins of their aquatic ancestors.

The researchers found that fish and early four-legged vertebrates known as tetrapods developed similar levels of anatomical diversity within their fins and limbs, despite the fact that their skeletons were constructed in very different ways.

"It has usually been assumed that when organisms evolve novel attributes that enable them to colonise fundamentally new environments - as in the move from water to land - this should trigger rapid evolutionary diversification and be accompanied by an increase in structural variety," said Matthew Wills, professor at the University of Bath in Britain.

"Our work challenges this received wisdom, and shows that, at least in the case of the evolution of early tetrapods, key innovations did not quickly lead to greater anatomical variety," Wills noted.

The study was published in the journal Palaeontology.

The evolution of limbs was thought to have opened up a whole new realm of possibilities for tetrapods, so the scientists set out to examine just how substantial the evolutionary transition from fish to tetrapods really was by analysing a variety of different fin and limb skeletons from the fossil record.

"Our work investigated how quickly the first legged vertebrates blossomed out to explore new skeletal constructions, with surprising results. We might expect that early tetrapods evolved limbs that were more complex and diverse than the fins of their aquatic predecessors," said Marcello Ruta from the University of Lincoln in England.

"However, although radically different from limbs, the fins of the distant fish-like forerunners of tetrapods display a remarkable array of subtly varying traits," Ruta pointed out.