Human Hands Evolved From Fish Fins, New Study Surprises Scientists

A genetic evolutionary link between fish fins and human hands surprised a team of researchers at the University of Chicago, where evolutionary biologist Dr. Neil H. Shubin runs a lab comparing embryonic development of four footed animals and fish.

Two decades of study have led Dr. Shubin and his team to discover a direct genetic link between the development of human hands and of fish fins, as announced on Wednesday. Evidence of direct evolution of humans from an aquatic ancestor has previously been shown through the fossil record.

Human hands and fish fins appear outwardly too dissimilar to have a direct line of evolution. Internal structures are different as well.

Both human hands and fish fins have a type of cartilaginous, blood vessel filled tissue called endochondral bone, but a fish's endochondral bones remain at the base of its fin. The rest of a fish fin has a significantly different type of tissue lacking cartilage and blood vessel called dermal bone.

In 1996 French researchers were able to shut down development of wrist and ankle bones in mice by turning off their Hoxa-13 and Hoxd-13 genes. Dr. Shubin knew that fish had related genes, but needed more advanced technology to test his theories.

With cutting edge CRISPR-Cas9 technology, scientists now have the ability to edit the genome. Dr. Shubin and his colleagues inserted bits of DNA into zebrafish Hoxa-13 and Hoxd-13 genes. Fish with manipulated genomes did not develop fins.

Additionally, glowing red molecules were added to cells that activate Hoxa-13. In mice, cells that grow the wrist glow red. Surprisingly, fish's fin rays glowed red.

"Here we're finding that the digits and the fin rays have some sort of equivalence at the level of the cells that make them," Dr. Shubin said to the New York Times.

"Honestly, you could have knocked me over with a feather - it ran counter to everything that I was expecting after working on this problem for decades."

Human hands, mice limbs, and fish fins all have the same molecular blueprint. Understanding the development of organisms like mice and fish leads to understanding about the genes and cells in our own bodies.