You may never have wondered why you have five fingers at the end of each arm (or why you have fingers at all, for that matter) but the answer is surprisingly interesting and a little cheeky. A recent study published in the journal Nature suggests that the DNA triggers responsible for controlling the development of digits (fingers and toes) trace their origins back to the fish cloaca, a multi-purpose opening which handles both reproduction and waste excretion.
If we want to know why our fingers look and work the way they do, we have to look closely at an ancient fish’s butt.
How the cloaca became the foundation of fingers
About 380 million years ago, a handful of fishy vertebrates took their first steps onto terra firma. Scientists have long hypothesized that the digits of land animals likely evolved from fins, slowly transforming over generations through the power of natural selection. The new study suggests instead that the birthplace of fingers lies not at the sides, but in the rear.
“The common feature between the cloaca and the digits is that they represent terminal parts,” explained study co-author Aurélie Hintermann. “Sometimes they are the end of tubes in the digestive system, sometimes the end of feet and hands, i.e. digits. Therefore, both mark the end of something.”
Researchers compared the genomes of mice and zebra fish, separated by hundreds of millions of years of evolution, fishing for common genetic ground. They looked at the genes involved in mousy digit development but also at the non-coding regions of the genome, areas which control the expression and activation of other genes. These are sometimes known as regulatory landscapes and they can apparently make a serious career change when evolution calls for it.
The same regulatory landscape which influences the development of fingers is present in fish, despite their distinct lack of digits. When researchers used CRISPR (a common gene editing tool) to fiddle with those genes in zebra fish, they noticed changes in the cloaca, instead of in the fins where they expected it.
These regulatory genes control the activation of Hox genes, sort of blueprints of the body plan. Hox genes are what tell your body to grow a torso with a head at one end, with four limbs, two eyes, and so on. Hox genes lay out all of the major body segments and how they relate to one another. Fiddling with the Hox genes in fruit flies, for instance, can cause them to grow extra legs where their antennae should be. While forehead legs might be a horrifying genetic malfunction, this new study makes clear that evolution can be flexible and doesn’t always take what might seem like the clearest path.
Evolution happens not just at the coding surface level but also at the foundational architectural level. Regulatory genes can turn on their heads and what was behind you yesterday might be at the tip of your fingers tomorrow.
Looking for more of the weird and unexpected? Catch The Twilight Zone, streaming now on SYFY.