Monday 22 December, 2014

Fish may have started ‘walking’ underwater before land

Published On: Tue, Dec 13th, 2011 | Marine Biology | By BioNews

Scientists have discovered that an African lungfish can use its scrawny limbs to ‘walk’, which redraws the evolutionary course of life on Earth from water to land.

Extensive video analysis revealed that the lungfish commonly uses its thin pelvic or hind limbs to not only lift its body off the bottom surface but also propel itself forward.

Though the forelimbs look similar to the hindlimbs, they were not involved in locomotion, the authors found.

Both abilities were previously thought to originate in early tetrapods, the limbed original land-dwellers that appeared later than the lungfish’s ancestors.

The observation reshuffles the order of evolutionary events leading up to terrestriality, the adaptation to living on land.

It also suggests that fossil tracks long believed to be the work of early tetrapods could have been produced instead by lobe-finned ancestors of the lungfish.

“In a number of these trackways, the animals alternate their limbs, which suggested that they must have been made by tetrapods walking on a solid substrate,” said Melina Hale, PhD, associate professor of Organismal Biology and Anatomy.

“We’ve found that aquatic animals with fundamentally different morphologies and that aren’t tetrapods could potentially make very similar track patterns.”

Lungfish are a popular pet in the paleontological community, treasured for their unique evolutionary heritage.

“Lungfish are very closely related to the animals that were able to evolve and come out of the water and onto land, but that was so long ago that almost everything except the lungfish has gone extinct,” said Heather King, lead author of the study.

King and her colleagues designed a special tank in which the motions of lungfish could be videotaped from the side and below for in-depth analysis.

Lungfish also demonstrated both ‘bounding’ motions, where both limbs moved at once, and ‘walking,’ marked by alternating limbs. Coupled with the ability of the lungfish to fully rotate the limb and place each subsequent footfall in front of the joint, the motion suggests that similar creatures would have been capable of producing some of the fossil tracks credited to tetrapods.

“This shows us — pardon the pun — the steps that are involved in the origin of walking,” said co-author Neil Shubin.

“What we’re seeing in lungfish is a very nice example of how bottom-walking in fish living in water can easily come about in a very tetrapod-like pattern,” Shubin added.

The study has been published in the Proceedings of the National Academy of Sciences.

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