just really have to clarify: no one believes that all life evolved from an amoeba, or that anything evolved from any modern amoeba. An amoeba is a present-day protozoan, which is a protist. The mainstream view of a monophyletic story of life involves a complex biochemical history leading up to a primitive prokaryote and thence diverging over billions of years into the diversity of life we see today, including modern prokaryotes (bacteria), amoebas and humans.