Back in the early days of Science Fiction, when the task at hand was to subdue a recalcitrant universe, the Shape-shifter was a stock character, representing the ultimate foe. As you drew your trusty ray gun to eliminate the slavering dragon, he promptly turned into a nuclear blast which effectively trumped your pathetic weapon.
What really put him out of business, though, was an increasingly sophisticated readership insisting on the credible in place of the unimaginable. SF degenerated into a study of conceivable relationships, and the Shape-shifter turned into a memory.
It occurs to me, however, that while he specialized in rapid transformations we are all in a sense shape-shifters, at least in the first few weeks of our lives, albeit at a somewhat slower pace than the fictional character. We all started out as a gamete, a single cell with a lot of potential looking for a mate to unlock its powers.
In a fertilization process our father contributes his own gamete, actually several millions of them of which only one succeeds, to create a single-celled Zygote which can be called an Embryo because it now contains all the instructions necessary to build and thereafter maintain a complete human being. This is an exciting time, as within hours the fertilized cell starts to divide into two identical cells then four and eight until it transforms into a hollow ball called a Blastocyst and starts looking for a place to stay.
During a week of house-hunting, the Blastocyst follows a chemical trail until it reaches an attractive site where it digs in and connects up to a convenient blood supply and waste disposal facility. Once installed, the cell replication rate increases enormously and the cells begin to differentiate into the many different forms required for bone, muscle, brain and lungs, which then fold in on themselves to form a legitimate Fetus.
The odd thing is that the Fetus as it grows, it adopts a succession of different shapes which over the next few weeks seem to recapitulate the evolution of our species, from fish to saurian to ape to man. And even after birth we retain the opposable thumb, vestigial tail and vermiform appendix characteristic of our arboreal ancestors.
What is even more remarkable is that while every mature cell has a specialized function, it also contains a complete set of instructions for building another human. What Darwin glimpsed in 1859 and Watson and Crick triumphantly confirmed in the 1950s is that all living things, from bacteria to whales to trees and tomatoes and us, are related because we all sprang from the first RNA (ribonucleic acid) molecule which, having learned to replicate itself, became subject to the shape-shifting process called Natural Selection. This, quite automatically, selects for whatever works best in the environment of its time, without favoring any particular shape.
It all seems highly improbable, but if any mother can create a human out of a couple of cells in nine months, think what Nature can do in the two or three billion years since that first RNA molecule turned into two.