Soon after he presented overwhelming evidence for evolution by natural selection in On the Origin of Species, Charles Darwin confided in a letter to a close colleague: “The eye, to this day, gives me a cold shudder”. He was concerned whether such a complex, integrated organ as the eye could have evolved in incremental steps by random mutation and blind natural selection.
He need not have worried: there are examples of animals living today showing virtually all stages in eye evolution, from those with nothing more than a tiny, single light-sensitive cell to those with large, intricate eyeballs.
Sophisticated eyes capable of forming images have evolved more than 50 times in the animal kingdom, and simulation studies have shown that they can evolve very rapidly indeed. In fact, it has been suggested that a fairly simple focused lens eye can evolve within only a few hundred thousand years – a geological blink in time.
However, when it comes to the fossil record, Darwin’s dilemma was well-founded: the eyes of most animals are constructed of soft tissue that doesn’t fossilise well, although there are some notable exceptions. For instance, trilobites have so much armour that even their eye surfaces are hardened, so we can infer trilobite vision in very fine detail. Similarly, many reptiles and birds have a ring of tiny bones around the eye, called the...
