Clever experiment using a method that wasn't available 5 years ago -- CRISPR/Cas -- shows that fins and fingers come from the same genes. Even they were surprised, despite the fact that these are the researchers that discovered the Tiktaalik "fishapod".

What's striking is how CRISPR enabled them to be meticulous AND fast.

"What matters is not what happens when you knock out a single gene, but when you do it in combination,” Nakamura explained. “That’s where the magic happens.”

The researchers also used a high-energy CT scanner to see the minute structures within the adult zebrafish fin. These can be invisible, even to most traditional microscopes. The scans revealed that fish lacking certain genes lost fin rays, but the small bones made of cartilage fin increased in number.

The authors suspect that the mutants that Nakamura made caused cells to stop migrating from the base of the fin to their usual position near the tip. This inability to migrate meant that there were fewer cells to make fin rays, leaving more cells at the fin base to produce cartilage elements.

“It really took the combination of labeling and knockouts to convince us that this cellular relationship between fins and limbs was real,” Gehrke said.

Future research includes new expeditions to find more fossil intermediates–such as Tiktaalik, a link between primitive fish and the first four-legged animals, discovered by Shubin and colleagues in 2006–in the transition from fins to limbs. They are also planning experiments with Hox genes to learn how a common population of cells can form such different structures in fish and people.