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Fish DNA Tie Fins to Limb Evolution

Scientists have traced the development of limbs back 400 million years and think that our arms and legs evolved from the fins of fish. A new study published in the journal Nature analyzed the genes of the fish believed to have been the closest relatives of land-walkers, and reported evidence of DNA that closely resembles appendage-forming DNA in animals today.

Lobe-finned fish, or sarcopterygians—meaning “fleshy fins”—are the most likely candidates for the first life forms to develop walking appendages and subsequently lead to the evolution of the superclass Tetrapoda—comprising vertebrates with four limbs and their descendants. But scientists have long debated which of today’s lobe-finned fish was the closest relative to the ancient fish that first developed land-walking limbs. Two kinds of fish could have taken the cake: lungfish or coelacanths. Both types of fish still exist today.

The Nature study seems to have settled this debate. Led by Chris T. Amemiya from the University of Washington in Seattle, a group of researchers from around the world put their heads together to map out the coelacanth genome. Coelacanths lived in the Devonian period, and were thought to have gone excint in the Late Cretaceous until an extanct species was found in 1938 (there are now two known coelacanth species living today.) A coelacanth’s DNA consists of 2.8 billion nucleotides, whereas a lungfish’s has 100 billion—the scientists knew decoding a lungfish genome would be near impossible, so they focused on the coelacanth.

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Coelacanths are endangered, but the researchers found a mistakenly captured specimen from 2003 whose tissues had been preserved, for their analysis. They determined that lungfish were actually closer in relation to the first tetrapods, but found new evidence in the coelacanth genome that is helping scientists better understand tetrapod evolution.

Coelacanths developed certain characteristics that helped their descendants adapt to life on land. Amemiya’s team found a piece of DNA that acted in the embryo to drive the development of limbs. This specific gene sequence, a type of DNA called enhancer DNA, is found in other animals as well, but not in other fish—indication that coelacanths were on their way to becoming land-walking animals. To test the activity of this gene, the researchers plugged it into mice DNA. Their experiment resulted in the development of limbs in mice embryos.

The team also found the DNA segment that produces eggs—similar to the gene that drive the formation of the placenta in land animals.

This study reports significant evidence that lobe-finned fish were the predecessors of tetrapods—that’s not to say they were the first animals to walk on land. A recent study in the Journal of Paleontology found that the first life form to ever emerge from the ocean to live terrestrially was actually an ancient arthropod known as euthycarcinoid Mosineia macnaughtoni.

Using this new gene information about the first limbs, scientists hope to piece together the story of why, how, and when some animals first came to be terrestrial creatures.

Read the full study on the coelacanth genome in the journal Nature.

The Paleontological Research Institution, Ithaca, New York, is pleased to sponsor Paleontology content for This View of Life. Founded in 1932, PRI has outstanding programs in research, collections, and publications, and is a national leader in development of informal Earth science education resources for educators and the general public.



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