Humanus is found directly in rotten meat, but can also be centrifuged from several other aspects, notably Instrumentum (which itself has many common compounds), Messis, and Pannus (which also has Instrumentum).
Humanus, along with Motus and Spiritus, is needed to make the bodies for Golems. It can also be centrifuged to yield many other aspects. In particular, double-centrifuging it will yield both of the other "golem aspects" (and Victus as well). Since the 4.2.2 changes, Humanus contains all the primal aspects (and therefore, so do its compounds).