Cracks In The Window
This version of Earth was in just the right spot to have the Chicxulub meteor break up on impact with earth, sparing the dinosaurs and primitive mammals. What the denizens didn’t know was that the pieces of the meteor carried an alien techno-organic virus, which caused certain animal species to fully convert over to machine. While many of the mammals were spared, many of the insects and reptiles were not. By the time proper homo-sapiens evolved, the dinosaur and insect populations were nearly entirely converted to robotic organisms.
Humans still occupy this world, but it is in its Pangea configuration. Small tribes and settlements mostly eke out a living through nomadic hunting and gathering.