Despite what humans think, they’re not the first sentient species on planet earth; the reptiles came first. Dinosaurs didn’t just come into existence and then die out to make way for mammals; they reigned supreme, changing and evolving over time, until after many thousands of years, the Silurians ruled the planet.
The trouble is, humans believe earth to be theirs while the remaining Silurians see them as apes, invaders and usurpers of what rightfully belongs to them. After all, they were here first.
Once again, it looks like the Doctor will have to broker a peace between the two species.