Good question! You are correct that we should accept premises as true on the LSAT. We do not need to question them. However, we do need to question whether or not the given premises are sufficient evidence for the conclusion.
In this case, the conclusion is the last sentence, stating that these species would have gone extinct even without human intervention.
Based on what evidence? Premise: Scientists estimate that over half of the species that have ever existed went extinct before humans even had tools.
I have no idea if this is true in real life, but I am accepting it. This is why you are correct to some degree. We accept premises. I think your confusion comes from failing to recognize the conclusion.
Assuming this premise is true, is not enough to draw the conclusion. This is the error. Maybe lots of species go extinct naturally. But how do we know which ones would have in the absence of humans? The author presents no information to answer this question.
