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Dec. 13, 2011

A Fish out of Water?

by Kaitlyn Gerber

Click to enlarge images

By Kaitlyn Gerber, Carleton College

The African Lungfish can display primitive but highly improbable walking behavior, as observed by University of Chicago scientists. Photo Credit: Yen-Chyi Liu, University of Chicago


Scientists have long known that the lungfish, a four-legged freshwater fish that can breathe through lungs as well as gills, is evolutionarily unique. However, a team of researchers at the University of Chicago have made another startling discovery: lungfish can "walk," using their thin limbs to lift their bodies and propel themselves along the bottoms of streams and lakes.

Published online in the Proceedings of the National Academy of Scientists, a recent paper has the potential to reshuffle evolutionary history. Previously, scientists had believed that the ability to walk had originated in tetrapods, animals with a backbone and four limbs, who were through to be the first land-dwellers. However, the ability of lungfish to "walk" along the bottom suggests that the motions of walking actually occurred underwater, before transitioning to land. As a result, fossil tracks originally attributed to early tetrapods may actually be from the ancestors of the lungfish.

The study originated when Heather King, a doctoral student at the University of Chicago, observed strange movements in an African lungfish belonging to Dr. Michael Coates, the study's co-author. "If you just look at the lungfish," said King in an interview with Scientific American, "you would think it was impossible for it to walk. It doesn’t have a sacrum, which was thought necessary for the animal to lift itself off the ground, and it doesn't have anatomical feet." King and her colleagues designed a special tank that monitored the lungfish's every move with video cameras, which revealed that the lungfish commonly use their hind, or pelvic, limbs to elevate themselves and push their bodies forward.

"This is all information we can only get from a living animal," explained King in an interview. "Because if you were just to look at the bones, like you would with a fossil, you might not ever know these motions could occur."

At various points, the lungfish both "bounded," using both pelvic limbs at the same time, and "walked," moving its limbs in alternating motion. Because it can apparently rotate each limb and place each footfall in front of the joint, the motion suggests that some creatures would have been capable of producing some of the fossil tracks that had been attributed to primitive tetrapods. In particular, sets of tracks discovered in early 2010 surprised scientists because they were 395 million years old -- 20 million years older than the earliest tetrapods in the fossil record. If these footprints do, in fact, belong to a common ancestor of both the lungfish and the tetrapods, they may indicate that when the tetrapods appeared, the ability to walk had already evolved.

"It's tempting to attribute alternating impressions to something like the footfalls of an early tetrapod with digits, and yet here we've got good evidence that living lungfish can leave similar sequences of similar gait," Michael Coates, professor of Organismal Biology and Anatomy at the University of Chicago, explained in a statement. However, "the fin or limb use thought to be unique to tetrapods is actually more general."

Below is a segment of King's footage, in which the lungfish "walks" forwards with its pelvic limbs.

Video Credit: The University of Chicago
____________________________

Kaitlyn Gerber is a sophomore at Carleton College, where she plans to major in biology. Originally from Ridgefield, CT, she is an active soccer player and science fan, especially of ecology and astronomy.

About Kaitlyn Gerber

The views expressed are those of the author and are not necessarily those of Science Friday.

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