I agree that the advancements of prehistoric people reflect intellectual acheivements that make our technological wonders look pathetic.
And that they have survival value.
But in the huge span from the brain of a badger to that of something able to grok the value of saving a good pointed stick for later, much less sharpening it, a big brain is good only for dealing with other members of your pack. Not that the advantage is entirely sexual - a smarter social animal is often able to get a bigger share of the food, a sleeping spot next to a cliff where there is less rain, etc. But for raw survival against the rest of the world? Nah.