So life appeared on planet earth about 3.6 billion years ago. But for the bulk of that time, organisms, life on the planet, was not able to see. Around 500 million years ago there was a revolution in the technology of life and the sense of vision appeared, and it appeared very quickly across a number of species and it led to a great diversity in the forms of life. So at this point 500 million years ago called the Cambrian explosion when suddenly all these vision-enabled life forms started to roam the planet.
One of the earliest and perhaps most successful of those life forms is the trilobite, and here I have a fossilised trilobite. They appeared 500 something million years ago and they lived on the planet for something like 270 million years. So they are an animal with a segmented body and they had two fairly primitive, by modern standards, compound eyes. And the sense of vision gave animals a competitive advantage. If an animal could see another animal, perhaps it was a hunting animal and it wanted to prey on that other animal, if it had a sense of vision it had a real advantage, it didn’t just have to blindly bump into it. He can actually see where that animal is and go after it and consume it. And similarly if you were a prey animal, the sort of animal that someone might want to eat, then if you had a sense of vision and you had eyes then you could sense a predator coming and you could take some evasive action. So vision led to this… almost an arms race on planet earth as predators and prey animals gain the sense of vision and they improved the sense of vision until we have the sort of very sophisticated vision in very, very many animals today.
So the sense of vision appeared on planet earth about 540 million years ago. And when it appeared it was a game changer. In fact, the sense of vision was so effective that, over time, ten different sorts of eye designs have evolved. For instance, the lensed eye that we have, the compound eye that a insect has, the strange reflector based eyes that scallops have and so on. So there are ten different designs of eyes on the planet. And the lensed eye, like the one we have, has, in fact, been invented seven times across different animal species and across time. So to me these facts argue that vision is a really important and really effective way of sensing what is going on in the world around you.
Now of course vision needs light in order to operate, we need the scene to be illuminated with ambient light. Photons reflected from the scene into our eyes, they fall on the retina and create some neural stimulus. Absolutely has to have a source of light, but of course we have evolved on a planet next to a really bright star. So light is in abundance. It is a very logical solution to the problem. We have an abundant source of light and we have evolved the sensor which has got many advantages, but primarily the ability to sense what is going on at a distance beyond our fingertips.
So how did the sense of vision evolve? There is a lot of speculation about this, but generally it is accepted that light sensitive cells appeared on the bodies of very primitive life forms a long time ago, more than 500 million years ago, and they have the ability to sense brightness or light. And perhaps this was useful to work out which way was up and which way was down and maybe you could see the shadow of something going across and that was a useful thing to have. So these photoreceptors could sense light and they were connected by some sort of primitive nerve fibres to some sort of primitive brain.
This was clearly a useful idea because over time the photoreceptors were arranged into a curve, or in three dimensions, into a cup. And this allowed the photoreceptors to respond to rays that came from a particular direction in space. And this was more useful than just knowing that there was something bright out there, now we knew the direction of the bright thing and this conferred some advantage. This design was refined over time, the chamber became filled with water, perhaps they had better optical properties, and then there was sort of coverings evolved. And then lenses evolved. Until we get to something like the human eye today were we have, you know, protective layers, and we have lenses. We have an iris, which regulates the amount of light which falls onto the retinas. So we can operate quite well in low light and bright light conditions. The photoreceptors, which are this yellow layer shown here, are connected by a bunch of nerve fibres, the optic nerve, which carry those stimuli to the optical or visual parts of the brain. And that is actually the part of the brain located at the back of our head.
The sense of vision evolved over 540 million years ago and ushered in the Cambrian explosion and complex life forms, such as trilobites, with eyes. How did such an amazing sense come to exist?