by Andrew Parker
“Too many notes.” That’s the memorable response of the Austrian Emperor in the movie “Amadeus” when asked by Mozart what he found objectionable in one of Mozart’s operas. I’m going to use that phrase to summarize my reaction to this book: “Too many words”.
The basic thesis of the book is clear and simple: it was the development of vision that triggered the Cambrian Explosion, the astounding period 540 million years ago when there was a sudden and gigantic multiplication of life forms. “Explosion” truly is the perfect word to describe this period; nearly three dozen completely new phyla appeared. A phylum is a fundamental branch in the tree of life; our own phylum, the chordates, consists of all animals with a backbone. That includes all mammals, birds, amphibians, reptiles, and fishes. All of those animals together constitute just one phylum -- and more than thirty new phyla appeared in the span of perhaps ten million years during the Cambrian Explosion!
Obviously an event as extraordinary as the Cambrian Explosion was created by some extraordinary cause -- and Mr. Parker provides that cause: the development of vision. To support this hypothesis, four basic questions must be answered:
1. Could vision have developed in that short period of time leading up to the Cambrian Explosion?
2. Do we have evidence that vision did, in fact, develop during that period?
3. How could vision have generated so many different phyla?
To answer these questions, Mr. Parker cites some fascinating research into the likely course of development of vision. It would have started with the development of a light-sensitive patch of skin; this patch would have been useful in warning animals on the sea floor when a larger animal swam overhead. The next step was the depression of that skin patch into something like a shallow pit; this would have provided some simple directional resolution, because the light-sensitive cells on one side of the pit would be more receptive to the light coming from the opposite direction, like so:
Of course, if a shallow pit provides some directional information, a deeper pit provides even more directional resolution. This would entail additional light-sensitive cells:
Ultimately this leads us to the pinhole camera, in which all the light enters through a small aperture; this gives excellent directional resolution:
But such a deep recess could get clogged up with dirt and such, so the next step was to fill it with transparent goo and provide a clear skin covering:
Lastly, the eye could be made more light-sensitive by thickening that clear covering into the shape of a lens to focus the light precisely:
Further research showed that, given typical rates of protein mutation inside cells, this process could have been carried out in about half a million years. Supporting this result is the fact that vision has developed independently numerous times during the history of life. There are, of course, other ways to build an eye, but the existence of such alternatives does not call this line of reasoning into question; those alternatives merely make it easier to build a visual system.
So the answer to question #1 is a solid “Yes, vision could easily have developed during the short period preceding the Cambrian Explosion.”
Question #2 is more difficult to answer because the fossil record from half a billion years ago is rather limited. There is plenty of evidence that animals during the Cambrian Explosion, such as trilobites, possessed vision. There are even some fossilized cases of eyes on stalks. But our evidence from before the Cambrian Explosion is very weak, because animals during that period sported no hard parts; they were vaguely like jellyfish and worms. It would be almost impossible for any eyes from that period to make it into the fossil record. Moreover, absence of evidence is not evidence of absence; we can never prove that eyes didn’t exist prior to the Cambrian Explosion, we can only prove that they DID exist during those times -- if and only if we discovered such a fossil, which hasn’t happened. Nevertheless, the evidence does suggest that vision did develop right around that time. So we can answer Question #2 as follows: “The evidence suggests that vision did develop just prior to the Cambrian Explosion.”
Question #3 can only be answered in a hypothetical fashion; our evidence is nowhere near good enough to actually show us the evolution of all those phyla in enough detail to answer Question #3. However, it is fairly easy to provide an explanation for the impact of vision on the biosphere. Before animals had vision, they just wandered around blindly hoping to sweep up enough organic matter to feed themselves. They were, in effect, filter feeders, trolling the seas sucking up seawater and gleaning the tiny plankton. Another strategy was to patrol the sea floor looking for worms, other surface life, or the bodies of dead creatures. None of these strategies is very efficient; you have to patrol a lot of territory to gather up enough food to survive. This kept most animals small; a large creature would have had to cover far more territory than was physically possible.
But the development of vision changed everything: with vision, you could actually identify and locate prey. Before vision, all animals were playing “pin the tail on the donkey”; with vision, they removed their blindfolds and suddenly there were donkeys everywhere! Of course, vision is a two-way street: if a predator can see a prey animal, the prey animal can see it and take defensive action. This also explains why defensive armor first appeared in the Cambrian Explosion. There’s no need for armor when the odds of a blind predator finding you are minimal; but predators equipped with vision will surely see you, and you need some protection. That’s how the trilobites came to be:
Thus started a kind of arms race between predators and prey. A variety of strategies for both predation and defense against predation developed: armor, camouflage, nocturnal lifestyle, digging down, horns, and on and on. It was the plethora of defense strategies and predatory strategies that triggered the Cambrian Explosion. And the starting point for the explosion was the development of vision.
This entire thesis could have been developed and explained in full in about a hundred pages, but this book is 300 pages. What are the additional 200 pages devoted to? Nothing of importance. There are several chapters explaining how light affects modern species, such as cave-dwelling animals who dispense entirely with their eyes. There’s also a long, long digression talking about various means by which animals are able to create iridescent color effects by building microscopic structures that are, in effect, diffraction gratings. This is all well and good, but it has nothing whatsoever to do with the book’s thesis. Those extra 200 pages are chock full of extraneous information that the author picked up during a long career. The end result is that you must wade through a lot of irrelevant stuff to get to the real meat. For example, how about this bit of erudition:
“We have established that the earth is said to be 4,600 million years old, as is the sun. So sunlight would, to some degree, have struck the earth’s surface well into the Precambrian...”
Isn’t that astounding! The sun actually shone half a billion years ago!
Now, if you just want to learn sundry facts about the evolution of life on earth, this book would be nice to read. But the subtitle of this book is “How vision kick-started the big bang of evolution”. If you read it with the deliberate purpose of finding out “how vision kick-started the big bang of evolution”, you’ll have to put up with a lot of irrelevant stuff before you eventually get to the point.