Around 500 million years ago, an ancient fish was out for a leisurely swim.

Suddenly, it stopped and quickly changed course, sensing danger up ahead.

It hadn’t seen a predator.

In fact, it wasn't acting on a visual cue at all.

It had tasted danger - an increase in the acidity of the water that it was crucial to avoid.

And this ancient ability to gauge acidity is something we - and as far as we know, all other vertebrates - still have today, hundreds of millions of years later.

We know it as sour taste, and it may be the oldest taste of them all.

But while its original purpose was to warn vertebrates of danger, in a few animal groups, including us, its role has reversed.

The taste of danger became something it was dangerous for us to avoid, instead.

The evolution of sour taste is actually pretty mysterious when you think about how it compares to the other tastes.

Bitter acts as a warning of potentially toxic compounds, sweet and salty help us to identify valuable sugars and salts, and umami - aka savory - is often associated with protein-rich foods.

But sour, which essentially just signals acidity, doesn't have the same clear connection.

Acidic foods aren’t nutritionally vital like sugar, salt, or protein, and they’re not essential to avoid, like toxic compounds.

To solve the mystery of sour, in 2022 a team of biologists investigated the distribution of sour taste perception across vertebrates, and tried to figure out both when and why sour taste evolved.

Initially, the researchers thought that they’d find at least a few species that couldn’t taste sour.

After all, there are many examples of species losing the other four tastes as their diets have shifted over evolutionary time.

Some carnivores, like cats, for example, have lost their ability to taste sweet.

On the flip side, pandas - having switched to an herbivorous diet - can no longer taste umami.

Both species still carry the genes for perceiving those tastes, they just don’t function any more.

But they didn’t.

The researchers couldn't find a single example of a vertebrate that’s lost the ability to taste sour.

It’s not obvious why sour taste would be important, yet it also seems to have been so important that no vertebrates are known to have lost it!

And based on the fact that the ability to detect acidity via sour taste is found in everything from ancient vertebrate lineages like lampreys to us apes, they concluded that the most likely explanation is that this ability was present in the common ancestor of all vertebrates.

Because!

In evolutionary analyses, we work from the principle of parsimony - the most likely explanation is the one that requires the fewest changes or assumptions.

So if the ability to taste acidity is at least as old as vertebrates themselves, what was its original purpose?

Well, the researchers hypothesized that it was probably important for ancestral vertebrates - ancient fish - to gauge the acidity of their aquatic environment.

Now, water can become dangerously acidic when carbon dioxide dissolves in it, forming carbonic acid, which lowers the water’s pH.

And straying into particularly acidic waters can be bad for living organisms - throwing the balance of many aspects of their physiology out of whack.

So the ability to sense the acidity of an environment through taste would have been really important for our earliest vertebrate ancestors.

And there’s even reason to think that it would have been present, and needed, well before any of the other tastes.

Because, the earliest vertebrates are thought to have been filter feeders - consuming whatever food particles floated their way.

So the ability to perceive the other tastes would only have become useful much later, when vertebrates became more selective and started actively assessing their food.

But tasting environmental acidity?

That would have been useful and important right from the start.

So it’s likely that this taste was the first one to evolve, functioning initially as a way of monitoring environmental acidity, and eventually transitioning to a food-assessment role.

And this transition is probably why it stuck around in terrestrial vertebrates once they’d left the water and started living on land.

While they no longer had to worry about straying into acidic waters, being able to detect and avoid acidic foods might still have been pretty useful.

For example, high levels of acidity can disrupt an animal’s gut microbiome.

This can be especially bad for species that rely on those bacteria to help them digest their food, like sheep and deer.

So we have some ideas about why sour taste evolved and stuck around, but they all have to do with sour taste acting as a warning.

And while the researchers did find that most vertebrate species have an innate aversion to anything sour, some species actually seem to like it and actively seek it out.

Humans and the other great apes, as well as several other primates, for example, really enjoy eating sour foods, at least, up to a certain level of sourness.

And we even have fossil evidence of acidic food consumption in early Homo.

Fossils attributed to Homo habilis found in Tanzania, dating to nearly 2 million years ago, show evidence of dental wear consistent with the consumption of highly acidic foods.

So why do us apes and many of our primate relatives like that tart flavor?

Well, the researchers think this shift might be a consequence of some quirks of our biology and behavior.

See, our branch of the primate family tree is pretty odd among vertebrates because we can’t make our own vitamin C, also known as ascorbic acid.

We lost this ability around 61 million years ago when a key gene for producing the vitamin mutated and became non-functional in our lineage, turning into a pseudogene that we still carry on our 8th chromosome.

Ever since then, we’ve needed to get vitamin C from our food instead, mostly by foraging for fruit.

Without it, we risk suffering from things like scurvy.

And vitamin C has no other taste than sour, being an acid.

So the switch from sour taste being off-putting to being enjoyable might’ve been a pretty useful adaptation that encouraged us to seek out foods rich in vitamin C. And in addition, there’s good evidence that our early ape ancestors were actively consuming lots of acidic, fermented fruits, which offered some major advantages.

Like, during fermentation, specific microbes - like lactic acid bacteria - produce acids that prevent the growth of disease-causing microbes.

Plus, fermentation can even increase the nutritional value of food, making it easier to digest.

So our preference for sour taste might have also been driven, or at least reinforced, by the advantages of eating fermented fruits - a valuable resource that we otherwise wouldn't have been inclined to seek out.

We still have a lot to learn about the evolution of sour taste, perhaps the strangest and oldest taste of them all.

But we’re beginning to understand that its ancient roots and varied functions have shaped our evolution and guided our ancestors for hundreds of millions of years.

First away from danger, and later, towards opportunity.