GMAT Sentence Correction (SC) | Re: Researchers in Germany have unearthed 400,000-year-old

Note that "stunning evidence ..." is not modifying "hunting ground".
What is actually the stunning evidence? It is that "400,000-year-old wooden spears have been unearthed from an ancient hunting ground". The entire clause is the stunning evidence.

An absolute phrase is a modifier. It can be a 'noun + noun modifier' when it modifies the entire preceding clause as happens in this case.
An absolute phrase has some other usage too.