A half-billion years of irritation

Just a couple of years ago, I wrote a column about the advent of tearless onions that included some background on why onions make us cry in the first place. Ordinarily I wouldn’t revisit a topic quite so soon, but you know how it is with science: things change fast, and just this week there was breaking news in the field of onion-induced tears.

Well, as breaking as any news can be when it deals with something that’s been around for half a billion years.

Onions have always made humans cry, or at least for as long as humans have been eating them, which seems to be a long time indeed—so far back in pre-history that we can’t even say for sure where onions originated. Central Asia? Iran?  West Pakistan?

In any event, once humans started farming, onions were quite likely one of their very first crops. As I wrote in my column back in 2008, “They’re less perishable than many others, can be easily transported, and will grow in a variety of soils and climates. They’re full of water, so they help prevent thirst, but they can also be dried and stored for eating later when food is scarce.

“Chinese gardens had onions 5,000 years ago. The Egyptians, for whom the onion symbolized eternity, buried them with their Pharaohs and frequently depicted them in religious imagery. Onions are mentioned in the Bible The ancient Greeks fed onions to athletes to fortify them for the Olympic Games.

“Onions were a staple in the Middle Ages in Europe, and the first Europeans brought onions with them to North America—only to discover the First Nations people were already using onions in a variety of ways.”

But even while humans were eating onions, they were crying about it. That’s because breaking open the cells of an onion releases enzymes that decompose some of the other substances that escape from the broken cells. That produces sulfenic acids, which escape the onion as a volatile gas.

This gas reacts with the water in your eyes, and among the products of that reaction is a mild sulphuric acid, and very irritating. Irritated eyes produce extra tears in an effort to wash away or at least dilute whatever it is that’s irritating them.

But…why do our eyes find sulphuric acid, or any other kind of chemical, irritating in the first place?

That’s where the new research, reported in the March 17 issue of the science magazine Nature, provides enlightenment.

In order for chemical irritation to occur, a protein has to sense the offending chemical and send a signal to your nervous system that the brain interprets as pain. The protein that does this is called TRPA1, and scientists have discovered that this very same protein is present in fruit flies, and serves exactly the same purpose. Not only that, the researchers (from Brandeis University in Waltham, Mass.), think the protein could date back so far in evolutionary history that it was present in the common ancestor of all the 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…uses a detector we have inherited in largely unaltered form from an organism that lived a half-billion years ago,” says study author Paul Garrity.

What kind of organism? Something primitive and marine: a worm, a sponge, or something like a sea cucumber.

Using statistics and computers, a process called bioinformatics, the researcher compared various versions of TRPA1 in assorted organisms today and figuring out how those variations relate to each other evolutionarily. They believe that the branch of the animal kingdom containing TRPA1 split off about 500 million years ago, and “since that time…most animals, including humans, have maintained this same ancient system for detecting reactive chemicals.”

Something that has stuck around that long obviously has important survival benefits, and indeed TRPA1 allows organisms to avoid harmful compounds potentially present in food, fumes or liquid.

Knowing more about TRPA1 could potentially lead to drugs that could turn off its function, helping to treat pain and inflammation in humans.

But never mind that. Personally, I just find it cool to imagine a giant herbivorous dinosaur, something the length of an airliner, chowing down on a field of onions…and crying the same tears I do when I slice them for a stir-fry.

Permanent link to this article: https://edwardwillett.com/2010/03/a-half-billion-years-of-irritation/

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