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Primitive fish had genetic wiring for limbs
2007-05-23
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 roughly 365 million years ago, scientists said on Wednesday. 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. The research was published in the journal Nature. Paddlefish, found in freshwater locales in the United States and China, are early "ray-finned" fish. Their fleshy fins are structurally similar to fish predating the first land creatures. Their fins contain cartilage thought to correspond to the upper arm bone of land vertebrates, Shubin said. While paddlefish are ancient, they did not exist at the time of the vertebrate conquest of land, but are seen as an evolutionary offshoot of some fish around at that time. GENE PATTERNS The researchers looked at so-called Hox genes -- which play an important role in limb development -- in paddlefish pectoral fins. They inserted molecular markers to track where these genes are active in the fin, and found the activity pattern resembled what these genes do in limbs of land dwellers. The findings run counter to the theory that the appearance of limbs was a novel evolutionary occurrence requiring great genetic changes to enable the first limbed creatures to adapt to their new environments of streams and swamps. The first forests sprouted up roughly 385 million years ago, with towering trees resembling modern-day palms, helping give rise to new freshwater ecosystems. Shubin and other scientists last year announced the discovery of the remains of a creature called Tiktaalik dating back to 375 million years ago and seen as a missing evolutionary link between fish and the first land vertebrates. It had fish-like characteristics, but it boasted a skull, neck, ribs and parts of limbs resembling the first amphibians such as Acanthostega that arose 5 to 10 million years later. "So it seems like you had the genetic tool kit (for limbs) for a long period of time," Shubin said. "And then, when the new ecosystems appear at around the time of Tiktaalik and slightly before, that's when forms started to use that to make true fingers and toes and stuff like that."
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