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Rapid Evolution? The Eyes Have It

A fossil compound eye, around 515 million years old, from the Emu Bay Shale.

A fossil compound eye, around 515 million years old, from the Emu Bay Shale on Kangaroo Island, South Australia. The individual lenses would have numbered over 3000, with the largest in the centre forming a light-sensitive “bright zone”. Image credit: John Paterson

By Michael Lee & John Paterson

The discovery of exquisite fossils on Kangaroo Island reveal that complex eyes evolved very rapidly during evolution’s Big Bang, the Cambrian explosion, half a billion years ago.

Michael Lee is senior research scientist at the South Australian Museum and University of Adelaide. John Paterson is senior lecturer at the University of New England in Armidale, NSW.

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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...

The full text of this article can be purchased from Informit.