image from here
From this. It blows my mind (and I'm pretending to be this guy a bit this morning:
The presence of a body plan in the genome, whether of a fly, a whale, or a human, was unexpected by embryologists. Previously, most of them did not think that development of embryos was controlled by genes; they had assumed that the different parts of developing embryos were determined by physical interaction between neighboring cells and that there was no overall division of the embryo according to a genetic plan. Experiments had shown, for example, that removing developing wing tissue from one part of an embryo and implanting it elsewhere still gave rise to a wing, although an abnormal one. Scientists attributed the abnormality to the effects of the neighboring cells in the embryo. This was wrong. In fact it was caused by the disruption of the body plan.
and this:
Hox genes, as Carroll explains, are in fact one of many kinds of genes that direct embryo development by a mechanism of switches. One example that is not a Hox gene is the gene that controls the development of the eye in fruit flies. If this gene (called Pax 6) is damaged when it mutates, the fly fails to develop eyes. We now know from the experiments described in Carroll's book that the Pax 6 gene is also found in butterflies, mice, and humans. Indeed, Pax 6 genes are interchangeable. The Pax 6 gene from a fly will turn on genes that make eyes in mice, and the Pax 6 gene from a mouse will turn on genes that make eyes in flies.
It had long been assumed that eyes had evolved independently in different species. The structures of mammalian eyes and insect eyes are very different and it would seem most unlikely that they had followed a similar evolutionary path. Mammalian eyes, for example, have a single lens that focuses an image on the retina. Insects have eyes with many tube-like structures, each tube having its own lens and retina. Yet the discovery of the Pax 6 gene gives us reason to believe that the evolution of the eye in all the animals followed related, and to some extent common, paths, though we cannot completely exclude the possibility that each kind of eye evolved following completely independent pathways.
That means we have a lot more in common with frogs and fish (and banjos?) than previously thought.
