Wednesday, 28 December 2011

New Research Sheds Light Upon Where The First Amphibians May Have Evolved

The classic representation of the first amphibian
The classic image of the amphibian evolution is of a small creature, neither fish nor amphibian, hauling itself out of a fast shrinking pool of water in the middle of a desert on its fleshy fins. It has fast become an emblem of evolution, in the sense that it depicts adaption to changing environments in order to survive. Yet this celebrated image may be wrong. The current scenario was created by Alfred Romer, a palaeontologist from the University of Chicago.

He believed that bridging the gap from fish to amphibian would have been a period of struggle and escape in order to survive. However a study conducted by Gregory.J.Retallack, from the University of Oregon, has raised a major challenge to Romer's theory. He argues that the various fossils associated with the transitional phase between the two vertebrate groups, from footprints to complete fossils, have all been found in areas which were flooded woodland, when the respective creatures were alive.

'These transitional fossils were not associated with drying ponds or deserts, but consistently were found with humid woodland soils,' he said. 'Remains of drying ponds and desert soils also are known and are littered with fossil fish but none of our distant ancestors. Judging from where their fossils were found, transitional forms between fish and amphibians lived in wooded floodplains. Our distant ancestors were not so much foolhardy, as opportunistic, taking advantage of floodplains and lakes choked with roots and logs for the first time in geological history.'

A comparison between Romer's theory (left) and Retallack's theory (right)
This new theory has fast gained support as it makes sense in relation to the well established facts of amphibian evolution and explains some of the more mysterious points. Romer's desert theory hypothesis states that limbs evolved solely to allow amphibians to move about on the land. Fish had to change their entire skeletal structure and respiratory system in order to be able to do this. Yet the time gap involved, around 30 million years, is far longer than would have been needed if, as Romer's theory suggests, the evolution of limbs was a frantic evolutionary change from water to land.

Retallack's scenario allows for a much longer timespan and explains the morphology of the fins of the transitional forms. Instead of evolving to allow mobility on land, the front fins/legs would have helped the creature negotiate obstacles in the water such as tree trunks. In a recent blog post, I described how lungfish were able to move their fins independently to produce a walking-like motion along the bottom of a river or pond or even locomotion similar to running if it moved with speed.

The fins were still not strong enough to allow the creature to support itself on dry land. Yet the buoyancy of the water would have made up for the lack of strength. Such an ability would have been very useful in flooded swamps. The creature would have been able to escape predators by swimming and then 'walking' to shallow water where they could not follow. Simultaneously, the flattened heads and flexible necks of amphibians and their close ancestors would have made feeding in the shallows far easier.
The fossil of Tiktaalik roseae, the link between fish and the first amphibians

A second hypothesis for amphibian evolution was proposed a few months ago by Polish scientists led by Grzegorz Niedbwiedski, which relied upon the tides. They stated that the rising tides would have carried the amphibian ancestor up to nutrient-rich, algae covered rocks. Their fins would have allowed them to grip the rocks and feed until the tide began to regress, allowing them to return to warmer and deeper waters. This theory does explain the way the fins evolved into legs.

Yet the problem, as with Romer's hypothesis, is a lack of fossils from what were once tidal beaches. Romer's and Niedbwiedski's theories both explain some of the facts, but Retallack's proposition seems to cover all points, from the the situation of the sites where the transitional fossils were found to the way we think the bodies of fish evolved to a life on land. We must wait for future fossil evidence to confirm one of the three scenarios.