T-Rex: One day everyone on Earth became immortal. They could no longer be killed! And by "everyone" I mean "only the dinosaurs", of course!
T-Rex: I DON'T MAKE THE SAME MISTAKES TWICE.
T-Rex: This was super great, until we started having kids and discovered THEY didn't share our immortality. They lived, they grew old, and they died. Watching our children die before us became depressing, and soon we stopped having babies. Our offspring didn't, of course, and so Earth divided into classes: the immortals and our descendants, who lived and died in what felt - to us - like a heartbeat.
Utahraptor: This sounds actually kinda hellish.
T-Rex: Oh, it gets worse!!
T-Rex: Since WE could now focus on a problem for centuries, technology developed quickly. But society stagnated, as new generations with new ideas never came to power. The embarrassing prejudices your grandparents had? That was US, only we never died, so our weird racisms never got cleared away.
T-Rex: Later on aliens were like, man, this immortal planet of racists would make EXCELLENT workers in the bowels of our extremely unpleasant warp engines.
T-Rex: THE END.