Human Limbs Prewired in Genetic Toolkit of Ancient Fish
Primitive fish already may have possessed the genetic wiring needed to grow hands and feet well before the appearance of the first animals with limbs, scientists said on Wednesday.
Tiktaalik is the lilting name of a newly discovered fossil fish with fingers. It lived 365 million years ago in the northern reaches of Canada, back when the northern reaches of Canada were tropical coastal wetlands not far from the equator.
How land-living animals evolved from fish has long been a scientific puzzle. A key to mystery of the transformation of the fins of fish into the arms and legs of our ancestors has been found by University of Chicago paleontologists Neil Shubin and Michael Coates and Ted Daeschler from the Academy of Natural Sciences in Philadelphia with the discovery of the fossil.
University of Chicago researchers were seeking clues behind a momentous milestone in the evolution of life on Earth -- when four-legged amphibians that descended from fish first colonized dry land. These first amphibians paved the way for reptiles, birds and mammals, including people.
"What we're interested in here is the transition from fin to limb -- a great evolutionary event," paleontologist Neil Shubin, an author of the research with colleagues Marcus Davis and Randall Dahn, said in a telephone interview.
They studied one of the most primitive types of fish on Earth -- the long-snouted paddlefish Polyodon spathula -- and found the fish that predated the first land vertebrates may have possessed genetic underpinnings for limb development.
"What we found is that aspects of the genetic program and the patterns of gene activity that serve to make hands and feet are actually found in the fins of fish -- not just any fish but in primitive living fish," Shubin added.
In the April 2, 2004, issue of Science, the scientists describe the bone, a humerus, from the Late Devonian Period found in Pennsylvania that bridges the gap between the fins of fish and the limbs of amphibians.
“It’s a mosaic of primitive fish and derived amphibian,” said Shubin, lead author of the paper. This integral piece of the evolutionary puzzle not only shares features with primitive fish fins, but also has characteristics of a true limb bone.
“This bone is a lot more robust than a humerus from any of the ancient species,” said Coates, who specializes in fish-tetrapod transition and the divergence of modern lineages. “Relative to other tetrapods, this is almost over-engineered. There’s a massive space for the attachment of substantial muscle going across to the chest.” Action of these muscles would have produced a motion similar to a pushup or a benchpress.
Which is why the researchers believe that the arm bone served to prop up the body. They argue that “this function represents the intermediate condition between primitive steering and braking functions in fins and the derived aquatic or terrestrial walking gait.”
“When this humerus is compared to those of closely related fish, it becomes clear that the ability to prop the body is more ancient than we previously thought,” Coates said. “This means that many of the features that we thought evolved to enable life on land originally evolved in fish living in aquatic ecosystems.”
"So it seems like you had the genetic tool kit (for limbs) for a long period of time," Shubin said.
Posted by Casey Kazan.
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