In the early days of human colonization it became clear that human soldiers were not cut out for fighting the endless hordes of alien aggressors. The spirit was willing, but the flesh was found wanting. Humans were not strong enough, tough enough or fast enough to compete with the countless xeno breeds that desired the eradication of the human race. Humanity was saved by two technologies: the ability to grow an engineered human body to maturity in a few months and the ability to transfer a consciousness from an old body to the new one (provided that the brains are identical).