New Genome Map Reveals Crocodilians Evolve Slowly

First Posted: Dec 12, 2014 09:00 AM EST

Crocodiles are some of the largest and most fearsome reptiles on our planet today. Now, scientists are learning a bit more about these animals after they mapped the genomes of three crocodilians in a new study.

"One of the major finds in our case was that crocodilian genomes change very slowly when compared to birds," said David Ray, one of the researchers, in a news release. "We compared both birds and crocodilians to turtles, which are the closest living relatives of the group that includes both birds and crocodilians. We found that they've evolved slowly also. The best explanation for this is that the common ancestor of all three was a 'slow evolver,' which in turn suggests that rapid evolution is something that evolved independently in birds."

Birds' closest relatives are actually the dinosaurs, which means that scientists can only look at their fossils, which provides limited information on their biology when compared to organisms alive today. Yet crocodilians also are related to birds, which means that studying them is an important part of learning more about avian species.

So what did they find with their new study? IT turns out that alligators are genetically uniform, surprisingly enough. While scientists initially believed that severe hunting during most of the 20th century may be to blame, they later discovered that this probably wasn't the case.

"Because alligators underwent a severe population decline, we first thought that might be what happened," said Ray. "However, we see the same pattern in all three species and the likelihood that all three were subject to the same genetic bottlenecks is small. We suggested instead that change just occurs slowly in crocodilians. In other words, it wasn't that the genetic differences were reduced because of overhunting. Rather, the amount of variation in crocodilians is low because change simply occurs slowly in these genomes."

In fact, the DNA in alligators, crocodiles and gharials is about 93 percent identical across the genome. By comparison, a human shares about 93 percent of his DNA with a macaque.

"The difference is that humans and macaques shared a common ancestor around 23 million years ago while alligators and crocodiles shared a common ancestor in the much more distant past, around 90 million years ago," said Ray. "That means that things are changing in primate genomes about four times faster than in crocodilians."

The findings reveal a bit more about crocodilians and show a bit more about their evolution. This, in turn, may be important for future studies.

The findings are published in the journal Science.

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