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I can barely remember the details but here it goes. Aliens were about to destroy earth and kill all humans so a group set out to escape to another planet so human kind could survive. They land on another planet and are left without technology and fight animals that come at night with sticks. They move into a large cave and over many many generations they pass down knowledge and stories to the children and eventually build a ship and go to the alien planet to kill them. The humans were set on revenge. One other detail I remember was there were these small animals that at first the humans did not get along with and over many generations they eventually found the animals to be telepathic and were friendly. This has been eating at me for days, what is this book?