Ha, the old white-haired tribal elder was practicing primitive eugenics

Ha, ha...wouldn't you know it. The white-haired tribal elder chief - wearing the funky straw mask, was practicing primitive eugenics with his new guest, our hero of the Ulam tribe. Having found a perfectly healthy, strong, male specimen from outside the marshland village tribe, the tribal chief thought it a good idea to add the man's genes into their village population. Actually it was a good idea.

Prehistoric humans, somehow by intuition and instinct, avoided things and invented cultural customs to prevent inbreeding and diversify the gene pool within their tribes and villages. In their time local human populations were very small. You could be living in a tribal group of say, 30 people. Avoiding in-breeding was a serious concern. Hence some ancient humans adopted the custom of greeting a friendly stranger by offering him their wives for the night. The possible resulting pregnancy was accepted as one of their own.

By the way, this movie was a good lesson in fire starting by employing the ancient but still effective method of the fireboard and swizzle stick. The movie was accurate in depicting the simplicity of the technique YET it's difficulty in execution. It took practice. The movie should have shown a closer look at the key element to using the fireboard and stick. There has to be very fine tinder at the hole where the swizzle stick rubs, creating the friction heat to ignite it. But it did show where the young tribesman put the glowing ember into a pod of larger tinder to build the ember into a fire. It works, but this method is time-consuming and takes practice and skill. It was no surprise that the easier and more efficient flint and hard stone (later steel) technique became the main choice of fire starting. The marsh tribe rewards our Ulam tribe hero for his stud services by showing him how to make fire.