Our alt modes--like my ambulance--are locked in when we're sparked, and we grow into whatever we're supposed to be. It's something to do with the way Vector Sigma interacts with the planet's surface that decides what we are. We figured out a way to artificially grow people after a while and things got really dicey once the government was deciding what people's alt modes were--there was a lot more redundancy, more people out of work. Before that we just trusted our planet to make whatever--whoever--we needed. We're back to doing it that way now.