Sigh. This article is a little too mystical about the Cambrian Explosion for my taste. "[W]hen life, for no obvious reason, burst into a crazy display of weird new fantastic forms"—this sort of thing happens when evolution comes across a new trick, like triploblasty, or skeletons, or ecommerce. Suddenly new species appear that use the new trick; and the first players will be much more varied than the later ones, since the later ones will all be descended from the species that won out in the first round.<p>(My layman's hypothesis is that the Cambrian Explosion is an illusion: there was a lot of diversity before, but it was all soft-bodied species that don't fossilize. Then some unicellular life form evolved that made use of calcium, perhaps as a shell to resist predators. It wasn't a perfect defense, so the predators managed to eat <i>some</i> of them, which got calcium into the food chain. This triggered an arms race, as predators evolved weapons, and prey evolved shells and skeletons. Suddenly all the existing lineages were evolving into species that could appear in the fossil record.)<p>Edit: here's a Wikipedia article that says I'm not the first to think of this: <a href="https://secure.wikimedia.org/wikipedia/en/wiki/Small_shelly_fauna" rel="nofollow">https://secure.wikimedia.org/wikipedia/en/wiki/Small_shelly_...</a>