Modularity and Symmetry

Wherein Captain Imperio meets evo devo (Hat tip to Changcho)

Modularity is the law in biology, and indeed in the universe. Somehow those ancient Greeks rightly guessed that the world was made of universal tinkertoys. The funny thing is that it works at seemingly every level. Everything is made of atoms, and atoms are made of even fewer and simpler parts: electrons, protons, and neutrons, each of which is identical to all the other members of its species. At larger scales are stars and their solar systems, which assemble to build Galaxies, which in turn form clusters. Above and below, we can't quite be sure - though protons and neutrons are made of quarks.

But this post is about life, because I have started reading Endless Forms Most Beautiful: The New Science Of Evo Devo And The Making Of The Animal Kingdom by Sean B. Carroll. All the life we know inhabits one planet around one star, and it, like everything else, is made of atoms, but there is a lot more to its modularity. All that life is made of cells, the atoms of life, and all those cells have an elaborate set of molecular tools and factories, the most crucial of which are common to all.

Carroll, so far at any rate, is mainly concerned with yet higher levels of modularity. Vertebrates, arthropods, and many other large life forms are built of multiple segments that show clear homologies within individuals and across species.

Modularity is a kind of symmetry, but it tends to be a broken symmetry. Fingers and toes are homologs but different. Ditto arms, wings, and fins. It's a fascinating story, and I'm just getting to the good part - I need to get back to it.

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