These fish are 'living fossils'—among the most primitive animals on Earth

If you could hop into a time machine and travel back 150 million years, the world would have looked very different. The supercontinent known as Pangaea was just beginning to break up, stegosaurs plodded across the land, and ichthyosaurs plied the seas.

But if you stuck your head into a stream, you might have spotted a familiar face.

Gars.

Known as “living fossils,” gars are a group of toothy, torpedo-shaped fish that have remained relatively unchanged across vast expanses of time. Ancient gar fossils show a surprising number of similarities to the seven gar species alive today.

Of course, many species meet the criteria of “living fossil” since Charles Darwin first coined the term in 1859. (Read more: “These 5 ‘living fossils’ still roam the Earth.”)

But now, a new study shows that, at the molecular level, gars are the most living fossil-y of all living fossils. And it’s not even close. 

Of 481 vertebrate species, the researchers—led by Chase Brownstein and Thomas Near of Yale University and Dan MacGuigan of the University of Buffalo—found that gars have the slowest rate of molecular evolution known to science. 

Even across millions of years, their DNA and RNA have changed up to three orders of magnitude more slowly than any other major group of vertebrates, including other classic living fossils such as coelacanths and sharks, says Solomon David, an aquatic ecologist at the University of Minnesota and co-author of the study, published recently in the journal Evolution.

The scientists believe the gars’ sluggish evolution may be due to an over-active DNA repair mechanism—a genetic quirk that could lead to advances in human medicine.

Slow evolution, “extraordinary” hybrids

To arrive at these conclusions, the authors first had to assemble an extensive family tree of species with published genomes.

Then, by zeroing in upon exons, or coding regions of DNA for all the species within that tree, they were able to estimate the rate at which a given species changed over evolutionary time.

They discovered that placental mammals, such as humans, had mutation rates of about 0.02 mutations per million years, whereas amphibians evolved much more slowly at a rate of 0.007 mutations per million years.

But gars? They averaged only 0.00009 mutations per million years at each exon site.

The study authors also report a related discovery: Gars are the most distantly diverged organisms known to hybridize—a record previously held by two species of ferns separated by some 60 million years. 

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For instance, alligator gar and longnose gar, whose territories overlap in the southern United States, last shared a common ancestor a hundred million years ago, but the species can still interbreed. What’s more, their hybridized offspring are fertile, says David. (Read more: “Ligers, zorses, and pizzlies: How animals hybrids happen.”)

Interestingly, alligator and longnose gar hybridization isn’t some hypothetical experiment. Earlier this month, Kati Wright, a master’s student at Nicholls State University in Louisiana, hauled a six-foot, alligator-longnose hybrid out of the Trinity River in Texas, an extremely rare find.

“When you look at their snout, it’s obvious,” says Wright, explaining the hybrid’s nose is wider than that of a longnose gar but not as wide as an alligator’s.

It may be that the gar family’s extremely slow evolutionary trajectory is what allows these distantly related cousins to continue producing offspring, since their molecular makeup is so similar, David suggests.

Carl Rothfels, an evolutionary biologist at Utah State University who discovered the hybridization between ferns and was not involved in the new study, says the level of hybridization shown in the new study is even more extreme than a human and a lemur producing fertile offspring. 

It's "extraordinary,” says Rothfels, who was not involved in the research, in an email. “Off the charts!”

Help for humans?

Aside from setting new records all over the place, the scientists believe that the gars’ evolutionary secrets, such as their efficient DNA-repair mechanism, could benefit human health­.

“As you copy DNA over and over and over again, you can get mistakes or changes,” says David. “But gars have something in there that when a mutation pops up, it gets corrected.”

David likens the process like a game of telephone that plays out over the millennia. When most organisms play it, the phrase whispered at the beginning changes over time until it takes on a totally different character by the end. But when gar play, the phrase remains nearly the same.

“If we can isolate what that is—and we’ve got some ideas as to what gene that might be—we can then take it to the next step of thinking about implications for human medicine and disease,” says David, who already breeds gar for use as model organisms.

For instance, if whatever is correcting those mutations can be replicated in human bodies, it could potentially prevent or counteract diseases such as cancer, the result of shortcomings in DNA repair and cell growth run amok.

If it all works out, there would be a delicious bit of irony for David, who spends a lot of time trying to change minds about fish that have long been persecuted for being ugly and of no commercial value.

“These fish that have been mistreated and considered ‘trash fish,’” he says, “may end up turning around and actually being extremely valuable to us from a human health perspective.”