Earlier this year, we all had our minds torn apart by a dress that was clearly blue and black to some people, and 100 percent white and gold to others. But what’s more mind-blowing is that there’s actually evidence that, until modern times, humans didn’t see the colour blue at all.

In a fascinating feature over at Business Insider, Kevin Loria breaks down the evidence behind the claim, which dates all the way back to the 1800s, when scholar William Gladstone, who later went on to be the Prime Minister of Great Britain, noticed that, in the Odyssey, Homer describes the ocean as “wine-dark” and other strange hues, but he never uses the word ‘blue’.

A few years later, a philologist called Lazarus Geiger decided to follow up on this discovery, and analysed ancient Icelandic, Hindu, Chinese, Arabic and Hebrew texts, to find no mention of the word blue. And, when you think about it, why would they need one? Other than the sky, there isn’t really much in nature that is inherently a vibrant blue.

In fact, the first society to have a word for the colour blue was the Egyptians, the only culture that could produce blue dyes. From then, it seems that awareness of the colour spread throughout the modern world.

But just because there was no word for blue, does that mean our ancestors couldn’t see it?

There have been various studies conducted to try to work this out, which you can read more about in Loria’s feature, but one of the most compelling was conducted by Jules Davidoff, a psychologist from Goldsmiths University of London, who worked with the Himba tribe from Namibia. In their language, there is no word for blue and no real distinction between green and blue.

To test whether that meant they couldn’t actually see blue, he showed them a circle with 11 green squares and one painfully obvious blue square. Well, obvious to us, at least, as you can see below. But the Himba tribe struggled to tell Davidoff which of the squares was a different colour to the others. Those who did hazard a guess at which square was different took a long time to get the right answer, and there were a lot of mistakes.

But, interestingly, the Himba have lots more words for green than we do. So to reverse the experiment, Davidoff showed English speakers this same circle experiment with 11 squares of one shade of green, and then one odd square of a different shade. As you can see below, it’s pretty tough for us to distinguish which square is different. In fact, I really just can’t see any differences at all.

The Himba tribe, on the other hand, could spot the odd square out straight away. FYI, it’s this one:

Another study by MIT scientists in 2007 showed that native Russian speakers, who don’t have one single word for blue, but instead have a word for light blue (goluboy) and dark blue (siniy), can discriminate between light and dark shades of blue much faster than English speakers.

This all suggests that, until they had a word from it, it’s likely that our ancestors didn’t see blue at all. Or, more accurately, they probably saw it as we do now, but they never really noticed it. And that’s pretty cool.

Find out more about how language shapes our ability to detect colour in Loria’s article over at Business Insider, and in this fascinating RadioLab episode, which inspired Loria’s feature.