"In the country of the blind, the one-eyed man is king." HG Wells's famous dictum tells us something that may seem self-evident: sight matters. But imagine for a moment that the country of the blind is in fact the whole world, 550 million years ago. It's a world where life is primitive and aimless, and evolution slow and painstaking. Then something remarkable happens. Over the next five million years, the process of evolution kicks into overdrive.
For the first time, animals evolve hard external parts. Both hunters and prey develop armaments and defenses. And in this short space of time - the blink of an eye, in geological terms - the number of different classifications of animals, or phyla, mushrooms from three to 38, the number we still have today.
The "when" and the "what" of this extraordinary event, known as the Cambrian Explosion, have been known for some time, and were made famous in Stephen Jay Gould's Wonderful Life. What has - until now - been speculation is the "why." Why did this "Big Bang" of biology happen when it did? What caused it?
It was the development of vision in primitive animals that caused the explosion. Precambrian creatures were unable to see, making it impossible to find friend or foe. With the evolution of the eye, the size, shape, colour, and behaviour of animals was suddenly revealed for the first time. Once the lights were "turned on," there was enormous pressure to evolve hard external parts as defences, and clasping limbs to grab prey.
The animal kingdom exploded into life, and the country of the blind became a teeming mass of hunters and hunted, all scrambling for their place on the evolutionary tree.