The comparison is of the struggle - that of cavemen [or some other early human community] and that of more modern humans. So you could say either of these:
- "I wanted to know if that struggle was any harder than that of more modern humans.”
- "I wanted to know if that struggle was any harder than more modern humans' [struggle].”
Colloquially, one doesn't hear the apostrophe anyway. But in exams and formal writing it (or "that of") is required.
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