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Scientists: Fish DNA may reveal how fins evolved into feet

The African coelacanth is a fish, but researchers say that it is actually more closely related to humans and other mammals than to many other present-day fish. That’s one reason that Chris Amemiya, a University of Washington-Seattle biologist, is hopeful that the recent sequencing of the entire African coelacanth genome could tell us much about not only the fish’s history, but ours, as well.

Amemiya was a member of the international team that sequenced the genome, and he led a study of some preliminary findings, which was published April 17 in Nature. The study notes that one fish has an even closer genetic similarity to humans and other land-based life: the lungfish. However, the lungfish’s genome is too large and complex to be sequenced in like fashion any time soon.

The coelacanth’s genome sequencing is an arguably significant breakthrough in the meantime, however. The fish has been swimming in deep-sea areas for 300 million years—it is essentially a living relic of some of the earliest eras of life on Earth.

Furthermore, according to Amemiya, its genes have scarcely changed at all in that long time frame. They apparently change very slowly, which gives the fish a more “primitive” appearance than many fish, including having four fins, each one with bones and joints. This unique fin formation could possibly a precursor to the four legs that certain fish, including maybe the coelecanth’s ancestors, gradually evolved en route to becoming tetrapods, the prehistoric ancestors of amphibians, reptiles, and mammals.

Coelecanths were not tetrapods themselves, nor were they the fish that first ventured onto land. That land-venturing distinction may go to the lungfish, according to University of Chicago scientists who note that the lungfish has limb-like structures with which it can “walk.”

Nonetheless, the coelecanth’s 300 million years of genetic history place them fairly close to the tetrapods’ emergence. Also noteworthy, Amemiya and colleagues found that coelacanths and tetrapods share one gene sequence that plays a role in limb development. Further study of the coelecanth genome could therefore lead to significant new discoveries about the tetrapods, according to Amemiya.

The study comes as researchers announced earlier this month that they have discovered the oldest immediate ancestor of the human lineage. The specimen, discovered in South Africa in 2008, is thought to be nearly 2 million years old. According to researchers, it contains a blend of human and ape-like traits that may provide insights into how humans evolved over hundreds of thousands of years.

The coclecanth was discovered by a fisherman in 1938, and since then, researchers have identified two species: the African coelacanth and an Indonesian coelacanth. Genetic differences between the two exist but are very slight, according to researchers, and in all, the fish’s genes are exceptionally slow to change. Some researchers attribute the genes’ stability in part to the fish’s habitat: it lives far down underwater, at depths of 100 meters or more, an environment that is relatively stable and presents few predators. The fish has had less pressure to adapt than most species in Earth’s history.