I just noticed this was in the BB section. I'm going to assume there'll be different answers to this here than in the strength section.
People who lift for vanity wouldnt bother to lift if there was no one to look at them in my opinion.
People who train for life would. If there was no one else on earth, you'd have to fend for yourself - chase & hunt food etc. You need to be strong and fast for that. But maybe that'd be enough training and wouldnt need to lift on top?