Did the hands of animals evolve from the fin rays of fish?

It has long been accepted that descendants of fish transformed into creatures that can walk on land, including amphibians and reptiles. With this transformation, scientists say massive changes in the creatures' physiology also occurred.

In a new study, biologists from the University of Chicago have proposed that as these ancient fish evolved, their long and elegant fin rays were replaced by the fingers and toes that most modern mammals possess today.

In fact, the report suggests that the same cells that comprise fin rays in fish is actually crucial in the formation of fingers and toes of four-legged creatures.

Changing Current Perceptions

For three years, University of Chicago researchers performed countless experiments using new gene-editing techniques as well as sensitive fate mapping to determine the link.

The team was able to track and label the developing cells in fish, and describe how the small bones at the end of the fins are linked to hands, which are features far more suitable for land-dwellers.

Neil Shubin, senior author of the study, says that scientists have believed that fin rays were completely different from toes and fingers, especially because one type of bone is formed out of cartilage and the other is formed in simple connective tissue.

Now, Shubin and his team's recent findings may change this entire notion, causing experts to re-think what they know about fingers and toes.

Examining The Genes Of The Zebrafish

Shubin and his colleagues examined the zebrafish to find out how fins are linked to fingers and wrists. Postdoc scholar Tetsuya Nakamura used CRISPR-Cas9 to delete important genes in zebrafish that are associated with limb-building and selectively bred zebrafish that underwent gene deletions.

At the same time, former graduate student Andrew Gehrke adjusted cell-labeling techniques to determine where and when specific embryonic cells migrated as the zebrafish grew and matured.

In the end, researchers discovered that the cells that mark the fingers and wrists of mice and people were exclusively in these fin rays. They focused on the Hox genes, which control the physiology of a growing embryo along the shoulder-to-fingertip and head-to-tail axis. Many of these Hox genes were found to be crucial for the development of limbs.

To confirm their study, Shubin and his team used CT scan to view the minute structures within the fin of the adult zebrafish. The scans show that the fish that lacks certain genes did not develop fin rays, but the number of small bones made of cartilage fin increased.

Researchers suspect that the zebrafish with deleted genes caused cells to stop traveling from the base of the fin to their typical position close to the tip.

This inability to move meant that fewer cells were present to create fin rays, leaving more cells at the fin base to create cartilage elements.

"When I first saw these results you could have knocked me over with a feather," says Shubin.

The findings of the study are featured in the journal Nature.