The Chicken man population lives in a river and eats chicken. His only predator is the upside down dinosaur
I am chicken man and I love to swim. I have plenty of chicken and water.
The Chicken man population is running out of food because his predator ate it all. The skinny chicken men are able to survive on less food and they happen to be faster now. The upside down dino can't catch skinny chicken men so the Skinny Chicken men survive.
I survived because I am skinny so I don't have to eat as much.
The skinny chicken men migrated to the mountains far from the dinos and took all the chicken with them. They grew to be strong and fast. They built shelters and learned how to stay safe and survived the meteor attack because they had shelter.
Eventually, the dinos had no more food but the Chicken men survived because they learned how to work together and survive the harsh conditions.