Scientists believe that many millions of years ago, our aquatic ancestors made the leap from inhabiting the oceans to living on dry land, marking the evolution of the first four-limbed vertebrates known as tetrapods. Although the tetrapods that inhabit the Earth today – including humans – possess fragments of this aquatic past deep in the genetics that make up their bodies, researchers are still struggling to determine how fish developed limbs in the first place. Now, a new study by University of Chicago researchers furthers our understanding of this evolution by shedding light on the relationship between fish fins and fingers.

One of the biggest sources of confusion in this area of research lies in the fact that fin rays form from connective tissue, while the fingers and toes observed in tetrapods stem from cartilage. Their dissimilar nature led many scientists to dismiss the possibility that fish fins were related to fingers and toes and focus elsewhere for the answer to this evolutionary riddle.

Neil Shubin, a professor at the University of Chicago and senior author of the study, spent three years of experimentation to force a rethink to this notion. Using genetic editing and mapping techniques, Shubin and his team were able to track fish cells as they developed, revealing that the tiny bones on the ends of their fins are genetically related to the fingers and toes that allow for life on land.

Shubin's team used the CRISPR/Cas9 gene-editing technique to delete specific genes in the zebrafish genome known to play a role in limb development and then subsequently crossbred numerous fish with these genetic deletions. Meanwhile, lab member Andrew Gehrke worked on refining fate mapping, the cell-labeling technique that would allow the team to pinpoint the movement of specific embryonic cells in the fish as they developed.

Using fate mapping, the team focused on the Hox genes, which are known to play a big role in limb development. Previous research has shown that the deletion of HoxD and HoxA genes halts the development of wrists and digits in mice, and in the current experiment, the team discovered that the deletion of these same genes led to a reduction in the long fin rays of the zebrafish.

"It was one of those eureka moments," says Gehrke. "We found that the cells that mark the wrists and fingers of mice and people were exclusively in the fin rays of fish." In addition, the team used a high-energy computerized tomography (CT) scanner to get a close look at the smallest structures lurking within the fins of adult zebrafish, structures that standard microscopes can't pick up.

The observations from the study suggest that the wrists and digits of tetrapods stem from the same cluster of cells as fish fins, supporting the idea that the hands and fingers that we use today are the equivalent of the fins that ancient fish used to traverse the oceans before they crawled onto the terrestrial landscape.

Future research will further examine the Hox genes to determine why these cell clusters lead to such different structures in fish and humans and continue to study fossils of the Tiktaalik, a 375-million-year-old fish that developed strong hind limbs despite the fact that it did not live on land.

The findings were published in the journal Nature.