I know I am not supposed to edit GMAT officially correct answers, but I thought that the sentence should have been
Researchers in Germany have unearthed 400,000-year-old wooden spears from what it appears to be an ancient lakeshore hunting ground, a stunning evidence that human ancestors who systematically hunted big game much earlier than believed.
...no. you don't have "an evidence"... you just have evidence.
for nouns that aren't countable, you don't use "a"/"an".
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Tim is bald, but his brother still has hair. (if you wrote "has a hair", that would mean tim's brother has exactly 1 hair left on his head.)
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To eat this food, I need silverware. (clearly you can't say "a silverware")
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I bought leather with which to sew handbags. (...not "a leather")
etc.