June 26, 2008

Fossil May Explain Man’s Evolutionary Transition From Water To Land

Scientists believe the discovery of well-preserved fossils in Latvia may explain the evolutionary history of how our ancestors moved from water to land. 

Swedish researchers have reconstructed parts of the animal, which had a fish-like body but a head that appears better suited to land than water. The four-legged fish, known as Ventastega curonica, would have looked similar to a small alligator, the scientists say, and may in part explain the process of evolution.

Researchers Per Ahlberg and Henning Blom from Uppsala University in Sweden said the 365-million-year-old species eventually went on to became an evolutionary dead end.

The first backboned land animals, known as tetrapods, were the ancestors of modern day amphibians, reptiles, birds and mammals such as humans.  It has long been known that tetrapods, animals with limbs rather than paired fins, evolved from a group of fishes during the Devonian period about 370 million years ago. However, fossils of tetrapod-like fishes and fish-like tetrapods from this time were still rather different from each other and do not provide a comprehensive picture of the intermediate steps in the transition.

The discovery in 2006 of an intermediate fish-tetrapod, known as Tiktaalik, advanced theories about the transition, but unexplained gaps remained between Tiktaalik and the earliest of the true tetrapods. But the new fossils of the very primitive Ventastega curonica seem to complete the story on this phase of the transition. 

"From a distance, it [Ventastega curonica] would have looked like an alligator. But closer up, you would have noticed a real tail fin at the back end, a gill flap at the side of the head; also lines of pores snaking across head and body," Alhberg said.

"In terms of construction, it had already undergone most of the changes from fish towards land animal, but in terms of lifestyle you are still looking at an animal that is habitually aquatic."

"Ventastega was first described from fragmentary material in 1994; since then, excavations have produced lots of new superbly preserved fossils, allowing us to reconstruct the whole head, shoulder girdle and part of the pelvis", Ahlberg said.

The reconstructions made by Ahlberg and Blom together show that Ventastega was more fish-like than any of its contemporaries. Its teeth and skull shape are neatly intermediate between those of Tiktaalik and Acanthostega (another transitional species).

"However, the shoulder girdle and pelvis are almost identical to those of Acanthostega, and the shoulder girdle is quite different from that of Tiktaalik (the pelvis of Tiktaalik is unknown), suggesting that the transformation from paired fins to limbs had already occurred. It appears that different parts of the body evolved at different speeds during the transition from water to land", said Ahlberg.

"I would draw the inference that Ventastega probably had limbs very much like Acanthostega.  These were little things sticking out of the sides, with a strangely high number of digits. You would have seven, eight, maybe even nine toes per foot, rather than five or so which you would expect to find in modern day animals," he explained.

Ventastega eventually died out, and other creatures went on to become our distant land-living ancestors.

Image Courtesy Arthur Weasley (Wikipedia)


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Uppsala University

The research is published in the journal Nature. An abstract can be viewed here.