Radiation Or Convergence

Did human beings originate on a single planet, then spread to many planets throughout the Galaxy, or, alternatively,  did they originate on many planets, then converge? This question arises not in Poul Anderson's Terran Empire, four hundred light-years across, but in Isaac Asimov's Galactic Empire, literally filling the Galaxy although spreading no further.

The question is absurd. Asimov assumes a Galaxy full of terrestroid but uninhabited planets easily colonized by human beings without any need for terraforming or adaptation. He does not describe any extra-solar organisms from which human beings might have evolved. In fact, he does not describe any extra-solar organisms - whereas, for every exoplanet in his Technic History, Anderson always presents a full ecology of plants and animals. He also goes into detail about the evolution of some of extra-solar intelligent species. See Speculution. 

Anderson deserves to be read more widely than Asimov.