There is evidence to suggest that our cave-dwelling ancestors polished many of their flints to a degree far surpassing what was necessary for hunting purposes.
Conclusion (therefore)
It seems that early humans possessed an aesthetic sense.
Evidence
Our cave-dwelling ancestors polished many of their flints to a degree far surpassing what was necessary for hunting purposes.
Evaluate
Most of the time that arguments deal with archaeological discoveries (found artifacts / bones / etc), the author tries to interpret the significance of the discovery, so we end up with an Explain the Curious Fact type argument. A flint, in case you don't know, is a type of rock that cavemen would use to make weapons or tools. It's basically a chunk of rock that has a pointy spot that humans could sharpen and affix to a spear.
Curious Fact ... Why were these flints polished beyond what was necessary for hunting purposes?
Author's Explanation ... They polished them like that simply because they found them beautiful that way.
This conclusion is one of those implied causal conclusions --- the author doesn't explicitly say,
We have to put all that together in our brains, simply by thinking about the connection between the premise and the conclusion and filling in the blanks.
When an author concludes a causal explanation for a curious fact, we always evaluate it two ways: 1. Could there be some Alternate Explanation for the curious fact? 2. How plausible is the Author's Explanation?
Goal
Since this is a Weaken question, we either want 1. to suggest an Alternate Explanation for why these flints were polished so much.
or
2. to undermine the plausibility of the story that cave-dwellers possessed an aesthetic sense that favored shiny things and that's why they polished their flints.