Tetrachromacy: Subject cDa29, Mosaic Retinas and the Fourth Color Cone

SPEAKER_1 (0:00)

Are you, like, absolutely sure the colors you see are all the colors that actually exist?

Gabriele Jordan (0:04)

I mean, most people are.

SPEAKER_1 (0:06)

Right. Think about looking at a rainbow, or a bright red apple, or a clear blue sky. You just assume you're seeing the full picture, you know? The entire spectrum of reality rendered in perfect high definition.

Gabriele Jordan (0:17)

Yeah, that's the standard assumption.

SPEAKER_1 (0:19)

But what if you aren't? What if human vision is essentially confined to this, like, standard three-channel setup, a biological limitation we just accept, while a hidden genetic condition actually unlocks an entirely new dimension of color?

Gabriele Jordan (0:33)

It's wild to think about.

SPEAKER_1 (0:35)

Right, like a secret level of human reality, hiding right in plain sight.

Gabriele Jordan (0:39)

Yeah, and the idea of walking around with a visual superpower, it sounds like science fiction, but it is deeply grounded in genetics and optics.

SPEAKER_1 (0:49)

Definitely.

Gabriele Jordan (0:49)

We're talking about a condition known as tetrachromacy. Most humans are trichromats, meaning our color vision is built on three independent channels.

Tetrachromats have four.

SPEAKER_1 (1:00)

Four channels. Wow.

Gabriele Jordan (1:02)

Unlocking how this biological phenomenon works requires tracing this wild evolutionary history, looking at the structural hardware inside the eye, and facing the tantalizing possibility that people who literally see colors that you and I cannot even imagine are walking among us right now.

SPEAKER_1 (1:20)

Okay. Let's unpack this. Because before we can understand how someone gets a fourth color channel, we really have to understand how we ended up with three.

Right.

Gabriele Jordan (1:29)

You have to go back.

SPEAKER_1 (1:30)

Exactly. And that requires going way back in the evolutionary family tree to the foundational science of human vision.

Gabriele Jordan (1:36)

So the best place to start is the physical hardware inside your retina, that thin layer of tissue at the back of your eye. Okay. We have two main types of light receptors, rods and cones. Rods are highly sensitive to low-intensity light.

SPEAKER_1 (1:50)

So, like night vision.

Gabriele Jordan (1:51)

Exactly. They are the workhorses of your night vision. But they lack the cellular machinery to differentiate wavelengths, so they don't really contribute to color perception.

SPEAKER_1 (2:01)

Oh, interesting. So they just see in black and white, basically.

Gabriele Jordan (2:04)

Pretty much. Cones, however, operate in higher intensity light and are entirely responsible for our color vision.

SPEAKER_1 (2:10)

Okay. Got it.

Gabriele Jordan (2:11)

A typical human has three types of cone cells, categorized by the specific wavelengths of light their opsin proteins are most sensitive to. So S for short, M for medium, and L for long wavelengths.

SPEAKER_1 (2:24)

Which map out to the blue, green, and red parts of the spectrum, right?

Gabriele Jordan (2:28)

Spot on.

SPEAKER_1 (2:29)

If I think about my eye like a computer screen, a standard monitor renders millions of colors using just RGB, you know, red, green, blue pixels. Yeah.

Gabriele Jordan (2:39)

That's a great way to picture it.

SPEAKER_1 (2:40)

So our three cones are essentially doing the same thing, sending three distinct data streams that the brain mixes together to create a three-dimensional color space.

Gabriele Jordan (2:50)

Right. And that three-dimensional space feels incredibly rich to us. But if you look at the evolutionary record and trace our lineage back to the common ancestor of all vertebrates.

SPEAKER_1 (3:00)

Okay.

Gabriele Jordan (3:01)

That creature was not a trichromat. It was a tetrachromat.

SPEAKER_1 (3:05)

Wait, really?

Gabriele Jordan (3:06)

Yeah. It possessed a four-pixel system with a whole extra cone type picking up wavelengths we cannot perceive today.

SPEAKER_1 (3:14)

So wait, if our ancestors had this four-cone hardware, they must have lost it somewhere along the line.

Gabriele Jordan (3:18)

They did.

SPEAKER_1 (3:19)

Given that early mammals emerged during the era of the dinosaurs, I imagine that surviving meant staying out of the way of massive daytime predators.

Gabriele Jordan (3:28)

Oh, absolutely.

SPEAKER_1 (3:29)

So they had to choose between seeing a beautiful, highly detailed color spectrum in the daylight or simply surviving the night.

Gabriele Jordan (3:36)

That period is referred to as the nocturnal bottleneck.

SPEAKER_1 (3:39)

The nocturnal bottleneck. Okay.

Gabriele Jordan (3:43)

Early mammals retreated into the shadows. For millions of years, they were navigating a dark world where rod cells were far more critical for survival than cone cells.

SPEAKER_1 (3:52)

Makes sense. You don't need color in the dark.

Gabriele Jordan (3:55)

Exactly. And evolution operates on a harsh principle of energy conservation. Maintaining complex cellular hardware that you aren't using is a massive biological waste.

SPEAKER_1 (4:06)

Right.

Gabriele Jordan (4:06)

So over countless generations in the dark, mammals lost the genetic instructions for two of those original four cone types.