They appeared in an evolutionary blink and changed the rules of life for ever. Before eyes, life was gentler and tamer, dominated by sluggish soft-bodied creatures lolling around in the sea. The invention of the eye ushered in a more brutal and competitive world. Vision made it possible for animals to become active hunters, and sparked an evolutionary arms race that transformed the planet.
When did eyes evolve?
The first eyes appeared about 541 million years ago – at the very beginning of the Cambrian period when complex multicellular life really took off – in a group of now extinct animals called trilobites which looked a bit like large marine woodlice. Their eyes were compound, similar to those of modern insects. And their appearance in the fossil record is strikingly sudden. Trilobite ancestors from 544 million years ago don’t have eyes. So what happened in that magic million years? Surely eyes, with their interconnected assemblage of retina, lens, pupil and optic nerve, are just too complex to appear all of a sudden?
HOW DID EYES EVOLVE?
The complexity of the eye has long been an evolutionary battleground. Ever since William Paley came up with the watchmaker analogy in 1802 – which claimed that something as complex as a watch must have a maker – creationists have used it to make the “argument from design”. Eyes are so intricate, they say, that it stretches credibility to suggest they evolved through the selection and accumulation of random mutations.
Charles Darwin was well aware of the argument. In On the Origin of Species he admitted that eyes were so complex that their evolution seemed “absurd to the highest degree”. But he went on to convincingly argue that it only seemed absurd. Complex eyes could have evolved from very simple ones by natural selection as long as each gradation was useful. The key to the puzzle, Darwin said, was to find eyes of intermediate complexity in the animal kingdom that would demonstrate a possible path from simple to sophisticated.
Those intermediate forms have now been found. According to evolutionary biologists, it would have taken less than half a million years for the most rudimentary eye to evolve into a complex “camera” eye like ours.
The first step is to evolve light-sensitive cells. This appears to be a trivial matter. Many single-celled organisms have eyespots made of light-sensitive pigments. Some can even swim towards or away from light. Such rudimentary light-sensing abilities confer an obvious survival advantage.
The next step was for multicellular organisms to concentrate their light-sensitive cells into a single location. Patches of photosensitive cells were probably common long before the Cambrian, allowing early animals to detect light and sense what direction it was coming from. Such rudimentary visual organs are still used by jellyfish and flatworms and other primitive groups, and are clearly better than nothing.
THE FIRST EYES IN NATURE
The simplest organisms with photosensitive patches are hydras – freshwater creatures related to jellyfish. They have no eyes but will contract into a ball when exposed to bright light. Hydras are interesting from an evolutionary perspective because their basic light-sensing equipment is very similar to that seen in other evolutionary lineages, including mammals. It is based on two types of protein: opsins, which change shape when light strikes them, and ion channels, which respond to the shape-shifting by generating an electrical signal. Genetic research suggests that all opsin/ion channel systems evolved from a common ancestor similar to hydras, pointing to a single evolutionary origin of all visual systems.
The next step is to evolve a small depression containing the light-sensitive cells. This makes it easier to discriminate the direction the light is coming from and hence sense movement. The deeper the pit, the sharper the discrimination.
Further improvement can then be made by narrowing the opening of the pit so that light enters through a small aperture, like a pinhole camera. With this sort of equipment it becomes possible for the retina to resolve images – a vast improvement on previous models. Pinhole camera eyes, lacking a lens and cornea, are found in the nautilus today.
The final big change is to evolve a lens. This probably started out as a protective layer of skin that grew over the opening. But it evolved into an optical instrument capable of focusing light on to the retina. Once that happened, the effectiveness of the eye as an imaging system went through the roof, from about 1 per cent to 100 per cent.
Eyes of this kind are still found in cubozoans, highly mobile and venomous marine predators similar to jellyfish. They have 24 eyes arranged in four clusters; 16 are simply light-sensitive pits, but one pair in each cluster is complex, with a sophisticated lens, retina, iris and cornea.
Trilobites went down a slightly different route, evolving compound eyes with multiple lenses. But the basic sequence of events was the same.
Trilobites weren’t the only animals to stumble across this invention, although they were the first. Biologists believe that eyes evolved independently on many, possibly hundreds, of occasions.
And what a difference it made. In the sightless world of the Early Cambrian, vision was tantamount to a superpower. Trilobites became the first active predators, able to seek out and chase down prey like no animal before. Unsurprisingly, their victims counter-evolved. Just a few million years later, eyes were everywhere and animals were more active and bristling with armour. This burst of evolutionary innovation is what we now know as the Cambrian Explosion.
However, sight is not universal. Of 37 phyla of multicellular animals, only six have evolved it. But these six – including our own phylum, chordates, plus arthropods and molluscs – are the most abundant, widespread and successful animals on the planet.