February 6. In the blink of an eye

Today is a follow up to the discussion Colleen Elliot and I had about the Cambrian Explosion a few days ago. In 2004, A book by Oxford zoologist Andrew Parker was published entitled In the Blink of an Eye – how vision sparked the big bang of evolution. It offers arguments for the idea that the development of eyes led to increases in predatory behavior, and was the ultimate cause for the Cambrian explosion.

Trilobite eye (NASA photo)

First, Parker defines the explosion not as the proliferation of animals, but as the proliferation of calcareous and phosphatic shells and exoskeletons in animals. He makes the argument that with new discoveries, we see that most or all of the existing phyla of animals had already been present for some millions of years when the explosion happened. So what we see as an explosion is really just those organisms figuring out a new way of living, inside various types of armor.

The focus of In the Blink of an Eye is why – why did this change happen? Parker objects to most of the possible causes that Colleen and I discussed the other day because they would probably have generated a gradual change, not an abrupt one as is observed. Or they were too far removed in time, as we mentioned regarding the end of Snowball Earth. So he makes the case for the development of eyes as the diving force behind the change.

It’s not hard to imagine that if, suddenly, some animals became able to sense movement, or to recognize patterns that indicated other animals – FOOD! – that the “arms race” Colleen and I mentioned in passing could have developed. There is pretty good evidence that animals DID develop eyes at the time of the Cambrian diversification, about 542 to 530 million years ago. It’s reasonable, but in my opinion not absolutely conclusive that it might have driven the increase in diversity that we see happening then.

For this to happen across species, in fact across phyla, at about the same time, there must have been some common reason. Maybe the development of eyes drove the Cambrian explosion, but why did eyes develop in so many kinds of animals at the same time?

Parker analyzes this question carefully, and ultimately suggests that there could have been a dramatic, and sudden increase in the amount of light reaching those animals, so that sensitivity to light became a useful survival mechanism. What could cause a sudden increase in light on earth?

Well, the sun could have suddenly become more luminous, or the atmosphere and oceans could have become more transparent. While those might seem far-fetched, there’s actually good reason to think they might have been possible, especially the changes on earth. There’s some evidence that the atmosphere, even while it was becoming increasingly oxygen-rich, might have been fog-like during the Precambrian. If something happened – a threshold was reached, or conceivably, a nearby supernova event drenched the earth in enough radiation to change it – it might have become more transparent. Or the water could have become more transparent because of changes in mineral content. Such changes certainly DID occur – but were they enough to suddenly change the amount of light reaching primitive animals, enough to make them all, suddenly, develop sensitivity that led to arms and armor as the world became an eat or be eaten kind of place?

I don’t know. The Blink of an Eye is a fascinating idea, very much worth considering, and the book is a good read filled with lots of earth history. Personally I think the jury is still out on why the Cambrian Explosion happened, and it remains perhaps the most intriguing and poorly understood event in the history of life since it originated on earth.

—Richard I. Gibson

 Further reading:

In the Blink of an Eye (Amazon)
In the Blink of an Eye (B&N) 
Calcite in the oceans (NASA - source of trilobite eye photo)
Trilobite eyes