Van Rijn deduces:
"'Ah-ha! Natural selection... It does give one notion of what made your race get brains. Hibernate or migrate! And if you migrate, then be smart enough to meet all kinds trouble, by damn!'" (IX, pp. 396-397) (For full reference, see here.)
I find that I have summarized from The Man Who Counts in Diomedean Evolution. In Diomedean Evolution II, I completed the account, then commented on human childhood and education, then mentioned Anderson's treatment of this issue in his earlier future history. On Diomedes, polar species hibernate and did not become intelligent although there are intelligent hibernators and estivators on Talwin.
Here I quoted Anderson's explanation of how intelligence evolved in the sea on Starkad. Ythrian evolution is summarized here.
there was a long hot spell millions of years ago;
ancestors of nogas had eaten soft plants made scarce by drought;
instead, they caught leaves torn loose by ancestral rukas gathering fruit;
they also had a tickbird relationship with proto-krippos;
krippos spied forage and guided nogas to it;
rukas got protection and kept stripping trees;
a giant blood-sucking bug injected a microbe to keep wounds open in nogas;
rukas and krippos swatted and ate the bugs, then sipped the blood themselves;
the three species began to link up and formed the tripartite Didonians. See also here.