The Cambrian and the evolution of vision

Hi all,

Just finished reading a book called 'In the Blink of an Eye'. Basically the author claims to have figured out the cause of the Cambrian explosion. His premises are:

1) Many phyla simultaneously appeared in the fossil record about 543 million years ago. 2) However, genetic evidence together with the Ediacaran fossils suggests the phyla themselves originated at least tens and probably hundreds of millions of years earlier (perhaps after the previous Snowball Earth episode?). 3) The question then becomes, why did so many phyla all independently evolve shells/skeletons at the same time? 4) Eyes (proper eyes capable of forming images rather than just detecting light/dark) also originated 543 million years ago. 5) Although hard body parts appeared in many phyla at the same time, eyes appeared in only one phylum, and for that matter probably in only one species (the ancestor of the trilobites). Other phyla by and large do not have eyes - the main exceptions are chordates and molluscs, but they only evolved eyes much later.

His conclusion is that the appearance of eyes in trilobites made active predation possible, therefore predatory trilobites were the selective pressure that drove all the other phyla to evolve hard body parts at that time.

This seems quite a plausible argument, but premise 5 worries me a bit, because it seems to be implying that eyes evolved maybe three times independently in the history of life, whereas I've seen other estimates that eyes evolved at least 40 and perhaps 60 times independently. Am I missing something here?

Not actually true. More like 520.

'So many' may be exaggerating. Arthropods, molluscs, brachiopods (the last two possibly a single event), echinoderms, cnidarians. That's four or five out of thirty-odd animal phyla. Did I miss any? Also, some of the shelled forms, including perhaps inarticulate brachiopods and some cnidarians, appear before the Cambrian explosion, and others, chordates and bryozoans, got hard parts after it.

How is it possible to say this? Fossil evidence would be trilobites and some of the Chengjiang fossils, again more like 520ma. Molecular evidence would suggest earlier, around the time of the initial divergence dates varously given by molecular estimates.

Not true. A number of the Chengjiang and Burgess organisms have visible eyes. Most of these are arthropods or something close to arthropods, but they clearly aren't all trilobites. And some, like Nectocaris, are of unknown relationships. At any rate, eyes would appear to precede trilobites within arthropods, at least by inference from phylogeny. But perhaps the entire burden of the hypothesis could be shifted to anomalocarids instead. Trilobites are merely the first predators to develop calcified shells.

No. Eyes have evolved many more than three times. But the premise isn't affected if the first evolution of eyes in a predator happened about that time. This is really just a refinement

To clarify since I also just finished reading this book, the author postulates that eyes were first developed by a soft-bodied prototrilobite. He defines 'eyes' as organs capable of making out an image rather than simple light detectors. Predation by this animal then served as the stimulus for the 'arms race' between predator and prey to get under way. Tens of millions of years later, we see the result in the Cambrian fossils. I'd also point out that he goes to great lengths to point out that the 'explosion' was only the evolution of hard parts and, thus, better fossil preservation. He does not say that the phylum actually diversified from each other during that time. In fact, he goes to pains to prove the opposite. There was a Precambian 'surge' for tens of millions of years prior to the fossils that we know and love that saw the rise of the various phlya, first as soft-bodied forms, only later evolving hard parts. Now, personally, I'm not 100% convinced but it is an interesting read and he makes a pretty good case, much better than I'm expressing here.

Dave Avery

If all this is speculation, then how can he place the evolution of eyes so accurately at 543ma, which coincidentally is the beginning of the Cambrian (though not by a long shot of the explosion)?

I think that's true also, but the evidence for that view is certainly not conclusive. There are no diverse soft-bodied faunas known predating the explosion, and it would certainly be nice to have one or two.

Trilobites are only one arthropod subphylum, and taxonomists generally regard the Arthropoda as being polyphyletic (the closest reference I have to hand is a textbook edited by Caroline Pond (2001)), and so having no common ancestor. The compoind eyes of trilobites and insects are missing from arachnids and crustaceans, and eyes may well have evolved anew in at least these groups in or following the Cambrian period.

Also, the 40-60 estimate for the evolution of eyes includes primitive eyes, like those of many arachnids, capable of little more than detecting differences between light and dark. If the author is using a narrower definition this no doubt reduces his estimate for the number of times eyes evolved.

Philip Bowles

I don't think this is true. Textbooks are notoriously out of date when published, and certainly arthropod monophyly is either assumed or successfully tested in most recent publications I know about.

Of course that's just crown-group arthropods for the most part. Did you have another definition in which stem-arthropods are polyphyletic?