Reclusland

- The dorsal-ventral boundaries of the vertebrate limb bud and the insect wing disc are established by a gene called fringe; other axes and boundaries are defined by genes in the wingless, apterous, hedgehog, and decapentaplegic families, and they operate in roughly similar ways in both groups of animals. This is like discovering that two cultures have independently invented a game like basketball, and then finding that while the games look vastly different in play, the court dimensions are the same, right down to the distance of the three-point line and the diameter of the center circle. It should make one question how independent their origins actually are.

- The octopus eye and the human eye superficially resemble each other from the outside, but structurally the tissues of each are organized in profoundly different ways, with different arrangements of cells in the retina, different kinds of photosensitive cells, and very different optic nerves. Dig deeper still, however, and all these eyes—octopus eyes, human eyes, fly eyes, spider eyes, flatworm eyespots—have a common master regulator gene, called Pax-6 in vertebrates and eyeless in flies.

- And the genes are interchangeable! Extract Pax-6 from a mouse, inject it and express it in the limb of a fly, and the confused tissues of the fly will respond by assembling an eye—a fly’s eye—on its limb.

- Evolution is a tinkerer that cobbles together new functions from old ones, and the genome is a kind of parts bin of recyclable elements. When new features evolve, the parts in the bin are co-opted to operate in new roles.

- This makes these master genes precisely analogous to the stock of goods found in a hobbyist’s electronics store. Standard subunits—oscillators, op-amps, field effect transistors, switches, rheostats, and so forth—will get incorporated into many different kinds of projects; whether she is building a radio or a synthesizer or a burglar alarm, the hobbyist will find it easier to just grab an oscillator integrated circuit off the shelf than to design her own.

- This is what we’re seeing in biology, too. We find an evolutionary novelty, like the vertebrate limb, and we can determine that it arose uniquely in our lineage. At the same time, we find a deeper heritage of shared genes that we hold in common with all other animals—a metazoan tool kit upon which we all draw to evolve.

Can you tell I used to love Lego kits as a kid?