544 million years ago, in the geological period called the Precambrian, there existed 34 different phyla of animals.  A phylum is a classification based on the animal's internal body plan.  Some phyla have one entry/exit from the gut, some have two.  Some have a two layers of tissue, some have three or more.  Some phyla have a distributed nervous system, some have a central one, and some have none at all.  34 different styles of animal swam in the seas 544 million years ago, but the untrained eye would have had a hard time telling them apart.  They all looked like pale, blobby things drifting in the currents.  Even the chordates, the phylum to which we belong, was represented by a transparent sack of cells with a rudimentary nervous system.  Most of these animals had the same niche, filter feeders.  There were no true predators, just creatures that managed to drift into each other and eat each other.

543 million years ago the Earth's oceans still contained 34 phyla, but what a difference a million years makes.  The seas abounded with all sorts of creatures.  Crabs, cephalopods, shrimps, and a dizzying variety of worms swam around, holding down dozens of different niches.  Some were still filter feeders, but now the ocean saw real competition between different styles of predators.  Animals were no longer soft and shapeless but came with an array of shells and carapaces, complete with claws, pincers, spines, saws and tentacles.  The landscape became much more colorful, too.  Animals of 543 million years ago could probably make pigments and diffraction gratings and prisms to produce any color in the rainbow.

So what happened in that million year span, the period scientists call the Cambrian explosion?  According to the book In the Blink of an Eye by Andrew Parker, vision happened.  At some point in that million years a patch of light sensitive tissue on the front end of a proto-trilobite infolded and got capped off with a lens.  Nerves connected this new organ to the creature's brain, and for the first time in Earth's history, an animal saw something.

I want to just stop and consider that scene.  Up to that point, all the animals were just drifting about, eating what they happened upon.  They could move in the direction of a chemical gradient or react to changes in water pressure, but they couldn't plan a course of action.  They couldn't fix themselves upon a distant objective and move themselves over there to eat it.  Success or failure as a Precambrian life form was largely a matter of luck.

Until that first trilobite opened it's eyes.  Imagine the advantage that creature would have had.  It had opened a channel to a flood of information that all other animals were still oblivious to.  It could see what it wanted to eat.  And what it wanted to eat could not see it.

The natural response to such an imbalance in abilities was to spur the development of eyes in other animals.  The crustaceans, led by the trilobites, ruled the seas for a long time after the Cambrian explosion.  But ammonoids, early cephalopods, soon grabbed space in the open seas.  Chordates, making up for a late start, used their central nervous system to optimize predation strategies.  Prey animals, and in the ocean almost all animals can end up as prey to someone else, had to mount a defense of some kind.  That's where the shells and spines come in.

And now that life has exploded into thousands of new species, you have to sort out who is who to avoid an embarrassing and fruitless mating event.  Animals in today's oceans use color and light to sort out the dance cards and it's no stretch to imagine the Cambrian creatures doing the same.

Parker tells an interesting, if sometimes disjointed, story.  He makes a good case for his theory that vision spurred the Cambrian explosion, bringing in modern marine biology, physics, chemistry and paleontology to bolster his case.  In the last chapter he speculates on the environmental trigger that spurred development of the first eyes, including a change in Earth's atmosphere that let in more light, a change in ocean transparency, or a change in the sun's output.

And most importantly, the book made me think.  Not only about that lone trilobite staring about in wonder (and hunger) for the first time.  It also got me thinking about the next leap in evolution.  What unassuming creature will one day open its new organ and perceive a new channel of information.  Maybe the organ that can perceive dark energy is already lying dormant in the humblest of animals, just waiting for the right nerve connections to bring it online.  What advantages might that creature have over the rest of us? 

I'd say you'd better start being nice to those crustaceans.