Most of those millions of years when the schism affected/(unconsciously)directed their development might have been during their most earliest stages, when they were still simple organisms in the Gallifreyan seas.
Sorta like that Thing Q tried to put into the soup on Earth in All Good Things.
To think that they were post-atomic and only then began to mutate slowly over the course of millions of years, seems like a fate they would be savvy enough to fight, direct or ward against similar to the decisions Davros had to make as his people were turning into Jellyfish, and he "NO! FUCK THAT! We should be Octopuses!" It's almost like the schism was something the future did to Gallifrey to make to derail some unwanted future where they were not up to snuff..
Which matches what happened to the Daleks early life on Skaro. They searched space in all directions for 8 galaxies only to find out that they were alone and that it was fantastic to be the only life form in the universe and that any change tot hat status quo would be unsavoury... Methinks the Daleks at the end of time steam rolled over 8 galaxies worth of life to make sure that their cradle was adequately and beautifully barren.