Welcome to the Company (Ftrain.com):
Recapitulation theory ("ontogeny recapitulates phylogeny") puts forth that incubating humans act out evolution as they grow from zygote to baby. This was a popular idea a century ago, but it's turned out the science isn't that simple. Yet the principle holds that the dividing fetal cells are engaged in a kind of performance of all of evolution—from simple to complex, from general form to specific form. The developing human loses its tail early, gains a cerebrum later.
Thus newborns are time boiled down, and every ounce gained is another 20 or 30 million years of life; they compress the three billion years since abiogenesis into a nine- or ten-month performance that runs from conception to birth. By the time they arrive they have gone for rides on comets, teased dinosaurs with sticks, come down from the trees, and run across the savannah.