What do lizards, birds, horses, frogs, dogs, lions, and humans have in common? That they are creatures with four limbs; or in other words, they belong to the clade of tetrapods . This anatomical feature appeared 360 million years ago , when life developed in the oceans and the fins of aquatic beings were transformed into legs . Until now it was thought that the change implied a qualitative leap in the complexity of tetrapod skeletons, but according to a study published in the journal Palaeontology there was no such revolution.
The new work challenges one of evolution’s most established concepts: When key innovation occurs, the rate of diversification and change accelerates. Life explores new anatomical possibilities and becomes, so to speak, more “creative”. The four legs were a classic example.
And yet, by analyzing the fossil record, researchers have found that members of tetrapods are not much more sophisticated than the fins of their immediate ancestors. On the contrary, aquatic beings also developed subtle biomechanical transformations, despite moving only through water, while the diversity of four-legged animals did not explode at the beginning , as was predictable.
Marcello Ruta, a researcher at the University of Lincoln (Great Britain) and co-author of the study, ventures an explanation: perhaps the four members had to fully adapt to their two new functions , walking and supporting the weight of the animal, which limited their possibilities of innovation .
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