From fin to limb: how our ancestors moved out of the sea
Somewhere between 390 to 360 million years ago, a four-legged vertebrate, or tetrapod, crawled out of the water and gave rise to the amphibians, reptiles and mammals we see today.
Scientists have established that this creature descended from fish and evolved its limbs and digits underwater, before its transition to dry ground. Life on land was accompanied by major modifications of the vertebrate skeleton, such as the evolution of a neck.
Sandy Kawano, a graduate student at Clemson University, wondered how that transition from surf to turf might have happened — and she turned to modern animals to figure it out.
Fossils of such science fiction muses as Ichthyostega, an early tetrapod, provide information on these organisms’ appearance, but you can’t get behavior out of old bones, Kawano said. So to look to the past, she turned to the present, figuring that examining present-day tiger salamanders and mudskippers — amphibious creatures whose limbs resemble those of early tetrapods— might offer clues. By analyzing their bodies and locomotion, Kawano thought she might be able to decipher how the early tetrapods managed the move to land.
“We’re using living animals to breathe life into the fossils,” she said.
Salamanders are tetrapods. Mudskippers are bizarre fish, with stalked eyes and late-career Marlon Brando-esque jowls that function as gill chambers, storing water. The salamander represents the winner in the race to land; the mudskipper, the loser, because it branched off the evolutionary tree in a different direction than the tetrapods.
To study why the mudskipper lost out, Kawano took live tiger salamanders and mudskippers and placed them on a surface called a force platform, which measures the forces exerted as an animal walks. It’s akin to standing on a scale, except a scale only measures force in the vertical direction, while the force platform measures vertical, medial (side-to-side) and forward and backward forces.
The salamanders used their approximately equal-sized limbs to walk, with the hind limbs providing most of the acceleration. Although the forelimbs provided little propulsion, they handled a similar amount of vertical force as the hind limbs, indicating that the organism was well-adapted to walking on land.
The mudskippers, on the other hand, used their pectoral fins to move forward and their suction-like pelvic fins for support. As they skipped forward, however, most of the force exerted was in the medial, rather than vertical direction. Kawano thinks this may explain why the mudskippers didn’t sire the tetrapod lineage: their fin bones could not have withstood the vertical forces of life on land.
This is only a hypothesis, Kawano cautions. Her next step is to dissect salamanders and mudskippers, remove their skeletons and apply different kinds of forces to test the strength of their bones. Then she’ll compare what she learns with observations of fossils of early tetrapods, integrating past and present to better understand how life got on all fours.
“It’s like a crime scene,” Kawano said. “You get little pieces of what happened, and then your job is to stitch it all together.”
Kawano’s study was published online in May in the journal Integrative and Comparative Biology, and will be presented tomorrow at the Society for Experimental Biology’s meeting in Valencia, Spain. You can read a summary here.
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