The next time you use your fingers to, say, text someone or flip someone off, be sure to thank a 380-million-year-old fish butthole for printing you these spectacularly versatile digits.
Scientists have traced the genetic origins of human fingers to…well, not fish fins, exactly, but to the ancient cloacas of prehistoric fish. I’m sorry to report that your fingers may have evolved from the same developmental system that once managed poop and procreation in our aquatic ancestors.
Researchers from the University of Geneva and the U.S., publishing their work in Nature, examined the genomes of zebrafish and mice. They hoped to identify the DNA “switches” that activate finger and toe development.
Eventually, they discovered the same regulatory elements that light up our digits in embryos once helped form the cloaca, a fish’s all-purpose orifice for waste and reproduction.
A Prehistoric Fish’s Need to Poop Gave You Fingers
Evolution doesn’t like reinventing the wheel. If it works, it works, and it will keep a system roughly the same, only making minor tweaks over time. In this case, that applies to a regulatory gene network called Hoxd.
In zebrafish, these switches don’t lead to fins. The researchers found that the budding fingers in the toes were the regions lit up by fluorescent markers, which helped them identify the tagged switches.
In mice, they illuminate the budding fingers and toes. Scientists used the gene editing tool CRISPR-Cas9 to snip these elements out and observed immediate effects, manifesting as malformed digits in mice and deformed cloacas in fish.
The ends of the animal went wonky. It might sound a little cruel, but sometimes science has a lot in common with a mechanic making a minor adjustment under a car’s hood and then telling a buddy to turn the ignition to see if it starts.
All of this means that your fingers owe their existence to a long-dead fish’s need for pee and poop. It’s also a reminder that evolution would rather remix something it already made than build a whole new system from scratch.
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