New research reveals that the DNA switches controlling human finger development originated from genes that regulated fish cloacas 380 million years ago[^1].

"Rather than building a new regulatory system for the digits, nature has repurposed an existing mechanism, initially active in the cloaca," explains developmental geneticist Denis Duboule from the University of Geneva[^1].

The research team discovered this by comparing fish and mouse genomes, focusing on Hoxd genes. When they tagged DNA switches with fluorescent markers, these lit up in mouse digits but in fish cloacas. Deleting these genetic elements using CRISPR-Cas9 disrupted finger development in mice and cloaca formation in fish[^1].

"The common feature between the cloaca and the digits is that they represent terminal parts," notes geneticist Aurélie Hintermann. "Sometimes they are the end of tubes in the digestive system, sometimes the end of feet and hands"[^1].

[^1]: ScienceAlert - Fish Buttholes May Be The Reason We Now Have Fingers, Study Finds