Onions Made Pre-Human Ancestors Cry Too, Study Suggests

The sensors in your body that make you tear up when you're cutting onions have been around for 500 million years, a new study finds.

Foods like wasabi and onions, as well as substances like tear gas and cigarette smoke, contain tissue-damaging and irritating chemicals. When you get a taste or waft of the substances, a protein found throughout your body is thought to sense these irritating chemicals and send signals to your nervous system. The result is pain, which is why slicing onions makes you cry.

In the new study, scientists found this chemical-sensing protein, called TRAPA1, is present in flies and for exactly the same purpose. Even more surprising, the team thinks the protein could date back millions of years to the common ancestor of all the varied creatures in the animal kingdom.

"While many aspects of other chemical senses like taste and smell have been independently invented multiple times over the course of animal evolution, the chemical sense that detects these reactive compounds is different," said study author Paul Garrity, a biologist at Brandeis University in Waltham, Mass. "It uses a detector we have inherited in largely unaltered form from an organism that lived a half-billion years ago, an organism that is not only our ancestor, but the ancestor of every vertebrate and invertebrate alive today."

During the Cambrian Period, which lasted from 543 million to 490 million years ago, life forms included primitive marine organisms, such as echinoderms (a group that now includes sea stars and sea cucumbers), annelid worms and sponge-like organisms.

History of chemical sensing

Garrity and his colleagues reconstructed TRPA1's family tree back some 700 million years using a variety of bioinformatic methods (bioinformactics applies computer programs and statistic techniques to study biological data).

For instance, the researchers compared the TRPA1 protein from different organisms to see how similar they were. They then used several computer programs to figure out how the proteins would relate to each other in terms of evolution.

"We discovered that a new branch split off the tree at least 500 million years ago, and that this new branch, the TRPA1 branch, appeared to have had all the features needed for chemical sensing even back then," Garrity said. "Since that time, it appears that most animals, including humans, have maintained this same ancient system for detecting reactive chemicals."

The ability to detect such harmful compounds, known as reactive electrophiles, would have given animals an evolutionary advantage, as they'd be able to avoid potentially toxic food or dangerous situations.

Medical benefits?

Since TRPA1 is so widely dispersed throughout the animal kingdom, it holds promise both as a target for therapeutics and deterrents. Further investigation might reveal new ways to turn TRPA1 off in humans to treat pain and inflammation, Garrity said.

Research might also reveal how to turn the protein on in pests like malaria-carrying mosquitoes to hinder them from transmitting disease, he said.

The results were published online March 17 in the journal Nature.