New research discovered that 360-million-year-old vertebrates with four legs did not vary significantly in structure from the fins of their aquatic ancestors.
The findings challenge old views that the origin of tetrapods, vertebrates with legs, stimulated anatomical variation of their skeletons.
The study was led by Marcello Ruta, of the School of Life Sciences at University of Lincoln, and Matthew Wills a professor at University of Bath's Milner Centre for Evolution in the UK. The study authors discovered that early tetrapods and fish experienced the same levels of anatomical variety within their limbs and fins even though they had differently built skeletons.
The findings, published in the journal Paleontology, overturn some long-held beliefs regarding evolution. Traditionally, it is believed that when organisms develop key features, enabling them to explore new environments, the rate at which evolution and diversification happens will accelerate. This is said to have happened with the transition of birds from dinosaurs and in this case, from fish to limbed tetrapods.
Scientists believed that this evolution opens up a whole new world of prospects for tetrapods. Hence, they began to analyze the magnitude of the evolutionary transition that happened from fish to tetrapods. They started digging deep into the fossil records and examined several different fin and limb skeletons.
"Our work investigated how quickly the first legged vertebrates blossomed out to explore new skeletal constructions, with surprising results," Ruta said. "We might expect that early tetrapods evolved limbs that were more complex and diverse than the fins of their aquatic predecessors. However, although radically different from limbs, the fins of the distant fish-like forerunners of tetrapods display a remarkable array of subtly varying traits. This variation may point to a previously unsuspected range of biomechanical functions in their fins, despite the fact that those ancestors lived exclusively in water."
Wills explained, "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. 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. For the first time, legs had evolved to fulfill new functions. Not only must they be able to support the weight of the body on land, but they also needed to enable the animal to walk. Perhaps these dual requirements limited the number of ways in which these first legs could function and evolve, thereby constraining their range of variability."
