Necessity is the Mother of all Speciation

As a cancer biologist, I don't get too much exposure to evolutionary research. At least, not as much as I'd like to. It's a fascinating field, and one that I'd like to keep up with more than I do. One specific topic that I have always found mysterious, and frankly quite beautiful, is the evolution of the eye. The latest issue of Nature magazine highlights one recent study that seems to characterize a structure that Darwin predicted should exist: a "proto-eye," or a fundamental precursor to all modern "eyes." The investigators characterize a two celled system that exists in zooplankton, whereby these invertebrates are able to respond to changes in light by beating their little cilia. What is extraordinary about this study is the mechanism for beating the cilia. It turns out that the sunlight itself is what triggers the cilia. The authors argue that this staggeringly simple system may have given rise to all forms of the eye that we observe in nature today.

The reason for my interest in this topic is that eye evolution has never seemed so simple to me. Mind you, I am no evolutionary biologist. I am a cell biologist, though. And, on a cellular level, there are at least two mechanisms for light sensing. Not surprisingly, these mechanisms are phylogenetically segregated. That is, all vertebrates have similar - cilia based - mechansims, whereas invertebrates sense light at the membrane of the cell (actually, this isn't entirely true... the mechanisms are actually segregated among deuterostomes and protostomes respectively, but who's really reading this blog anyway?). This may seem like a subtle distinction, but it really sets the stage for all subsequent eye evolution. Think for a moment about the variety of eyes represented in the animal kingdom: the mammalian "eye ball" such as our own, or the fish eye, the bird eye, the reptilian or amphibian eye, which can be contrasted with the "eyes" on some snails, or the eye of an octopus or squid, or the compound eyes that are common among some insects. Now consider that, on a cellular level, the eyes of an ant and a squid are more similar than the eyes of a squid and a fish. Even though the structures are vastly similar between cephalopods and vertebrates, on a cellular level, they're quite distinct. Taken a step further, a squid eye is even more akin to a snail eye, than it is to a fish or a frog.

If that's not enough to induce a head scratch, consider that the most common ancestor between vertebrates and invertebrates had no real eye at all! It was something of a worm (probably something like a planarian) with just a few light sensing cells, but no eye structure to speak of, and no brain to match. That means the eye structures observed between vertebrates and invertebrates - to say nothing of the ability to integrate light-sensing cells with a complex neural system, let alone a brain - must have evolved completely independently. This is known as convergent evolution. In other words, two seemingly similar structures or traits that have evolved through independent lineages, and in this case, have developed through independent mechanisms. I can't stress enough how beautiful this observation is. It's worth a moment to stop and consider the selective pressure for such a trait. It could be said that the ability to sense and process light is so critical to survival that the evolution of vision was inevitable. So inevitable, it would seem, that it happened at least twice.