I Won't Live Forever Because of the Dinosaurs!

Mammals were at the bottom of the food chain when dinosaurs were the top predators and therefore to survive mammals became small, nocturnal and short-lived. They had to reproduce rapidly and did not need processes and genes related to long life such as repair and regeneration.

The same cannot be said for reptiles, amphibians and fish. They die from being eaten, not from old age! Many of these continue to reproduce throughout their lives.

I read about this fascinating subject in THIS ARTICLE.