Imagine being one of your ancestors, hundreds of millions of years ago.

You look out of your cave, through the towering trees, and there is danger everywhere. Through the forests run the two-legged thecodonts: large, fast, and always happy to add you to their meal. Then there are the plant-eating, lumbering scelidosaurs with their armoured, spike-covered bodies. They are no more interested in you than a piece of rock in their path: more benevolent than the thecodonts, but no less dangerous.

Out on the sandy beach, the trees and shrubs are smaller and more suited to your size. But the low cover also makes you more visible to hungry eyes. The pterosaurs, with their huge leathery wings, soar through the skies. The ones in the distant sea are watching for fish, but there are others, closer by, snapping up insects to pass their time. These ones are watching for you.

Should you risk wandering outside, in search of insects to fill your empty belly? You decide against it. Better to wait for dusk, when it is harder to be seen, even if it also makes it harder to hunt. Fewer predators will be on the lookout at dusk. Those that are will be very hungry, and very desperate.

You wake up in the evening, having gathered as much strength as you can possible gather without any refreshment. You open your eyes wide, and the little light there is gets focused through onto your retina. This is the back surface of your eye, covered with light-detecting “cone” cells. Each cone cell can only see one-third of the light coming in: either the red, blue or green part. This is what allows creatures to see things in colour, but at twilight it makes things very dim.

Turning your head slowly, you try to search for movement in the fallen leaves. There is none to be found. You have stayed in this same cave for too long; the insects now know enough to keep away. It’s time for you to move on.

It is getting almost impossible to see now. Luckily, you have other senses to keep you going. Your nose sniffs out for the scent of predators and prey; your sensitive ears are alert to the slightest movement.

Everybody is asleep; finally it is your nose that saves the night. You find a trail left by a cockroach barely half an hour ago, and soon you track your way down to where they live. They must have found a good food-source here; there are so many of them! You will need to find them, though.

Three cockroaches had taken shelter under the roots of a tree to settle for the night. Now, you make sure they will never wake up.

Halfway through the last cockroach, you hear a soft crunching of leaves. You are not the only hungry one tonight, and the other one is much bigger. It must have followed your trail, just like you followed the cockroaches. You try to decide the best route of escape, but it’s too dark to see anything. With no option left, you pick a route at random. Surprisingly, your pursuer does not follow.

Unknown to you, you have an advantage. Some of the cone cells in your eye are faulty, and allow a few extra bits of light to come through. This extra light means you can see better than most people in the dark, though you also become slightly more colourblind. These faulty cones will later evolve into the completely different “rod” cells, which are not at all picky about what light they accept. The rod cells cannot tell colours apart at all, but they are much better at seeing in the dark. It is these cells that enabled mammals like you to survive in the corners when the huge reptiles ruled the planet, and these cells are still present in all mammals today.

The hunter has been shaken off, but now you have lost your bearings. It’s too dark to find your way back to the cave. It looks like you’ll have to spend your night in the open. A twig snaps behind you, and you scurry deeper into the shadows.

It’s a long night ahead….

This story was originally published in the 16–29 October 2016 issue of Sirius. It was meant to be an essay about the origin of rod cells in mammalian eyes, but I got a bit sidetracked.

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