Gonna Build Me a Mountain/Some Assembly Required
Some unicellular creatures called dinoflagellates are remarkable for their ability to change shape and mode of existence in response to environmental conditions. Their shape changing skills are nothing compared to those of a more familiar cell, however. Each human (or mouse, elephant, or frog) cell has the instruction set for turning itself into a blood, bone, brain, or skin cell. The story of how they manage these feats is the tale of embryonic development, and it is a remarkable tale indeed.
When people set out to build something, we gather parts from afield, and bring them together in carefully (in my case, usually not carefully enough!)planned sequence and pattern, but an embryo assembles itself from the inside. The method is both elegant and instructive. First, the egg and early embryo establish a set of internal coordinates. These internal coordinates are specified in terms of chemical gradients, and are initially simple (up-down, forward-back, right-left), but as the embryo develops, are progressively refined by further chemical gradients to exquisite positional precision. The chemicals that make up these gradients are signals that turn on or off certain specified genes in the locations specified. Those genes, in turn, are controlled by a pattern of switches which can give them great specificity. A typical develomental gene has ten or more switches which turn it on or off in response to rather complex signals.
This fact gives rise to modularity and reusability for genes: the same bone morphogenesis proteins turned on in your finger bone were turned on in many or most of the other bones of your body, but the precision of control made possible by the the switch mechanism allows the tissues they produce to be sculpted and otherwise specialized for the specific uses intended.