From the dinosaurs to "rat-dogs" and Syai h(r)umans ...
Recent advances in knowledge of our evolutionary past establishes that a 10 km wide rock that fell on Earth millions of years ago led to the end of dinosaur dominance. On the face of it, it seems that had this not happened, we would not be here today. Does this undermine the plausibility of intelligent life on other planets that are physically similar to Earth, like the Syai planet?
Syai "experts" on evolution (according to Terres) don't deny that outer-space rocks fell and damaged large regions of evolving life, but none seem to have been as big and devastating as our Yucatan one. Smallish dinosaurs have survived into the Syai present: Terres calls two of the varieties "lizard-eels" and "swamp-dwelling billled-waders". There were larger ones even in comparatively recent times, but as with almost all of the very large mammals on Earth, "intelligent life" in its more primitive state wiped them out. It's also worth noting in support of my argument that Earth's giant plant-eaters were apparently on the wane around the time of the Yucatan collision.
On the Syai planet, mammals evolved and grew larger where and when conditions such as volcanic eruptions left regions almost empty of larger creatures. A species that had originally more connections to rodents rather than apes, and which Terres calls "sea-beavers", began to build fishing platforms along coast-lines out of gnawed branches and natural flotsam and jetsam. Occasionally these would detach and drift with wind and current. The sea-beavers and other small animals that happened to be on these rafts - if they were lucky - would be washed up on distant lands suitable for their habitation. Along with the other factors mentioned, this facilitated the eventual evolution of the rodent-like yet human-like form of intelligent life, as well as the large rodent-like yet dog-like animal that Terres named "rat-dogs".