A new study has unraveled a mutation that gives clues on how ancient sea vertebrates transitioned to land.


Zebrafish
(Photo : Brian Gratwicke / Wikimedia Commons)

Limbs are incredibly useful. They have been adapted for different functions in different ecosystems through the course of evolution. 

The earliest limbs found date back to roughly 375 million years ago. Fossil records document how fleshy fins of prehistoric fish became more limb like that allowed amphibious ancestors ashore.

These creatures are known as tetrapods or four limbs. Now, a new study on modern-day fish provides new insight into the genetic underpinning that paved the way for evolutionary change.

How Fish Grew Limbs

Life began on in the water. Animals slowly moved onto land and switched from fins to limbs. But how exactly it happened millions of years ago has been an evolutionary mystery for scientists until today.

A new study published in the journal Cell by researchers from Boston Children's Hospital demonstrates mutations of either of two zebrafish genes that can create limb-like fins in these fish.

By using the latest gene-editing techniques, researchers were able to replay the mutation in the lab, and scientists were able to pinpoint how zebrafish grow fins that resemble human arms.

Brent Hawkins, co-author and a postdoctoral researcher at Harvard University says, "Most surprising to me is that such a dramatic change to the fin skeleton and musculature is possible with just a single mutation.

Hawkins adds that previously fin-to-limb transitions were presumed to involve myriads of gene changes.

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Studying Fish Mutations

Researchers screened over 10,000 mutated animals for skeletal defects in order to find the relevant genes that triggered the mutation in fish. Among those were zebrafish that had developed extra bones in their fins. 

Much like fruit flies and mice, zebrafish are classic study animals for understanding development and genetics.

Classified as teleosts, or bony fish that support their fins on pointed rays. Some of the mutant zebrafish had fish with extra bones. The bones were attached to muscled and even formed joints just like human limbs.

Hawkins says, "Finding a fish with extra fin bones that should never be there was quite the Éureka!' moment."

The most striking observation was how the new bones required other changes with the fish's anatomy. Researchers explain that the development of the mutation brings changes in musculature.

With a single mutation, the fish's fins became more like human arms. With that Hawkins and his colleagues set out to find what could have been responsible for the change.

Ancient fish had the potential to make human-like limbs before the event happened, with a bit of luck ancient mutants pioneered a new way of life that brought them ashore. But the mutation isn't exclusive to fish since humans have also evolved to form limbs.

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