[xenadu02]

If groups with many longer lived members suffered a disadvantage then natural selection would favor dying earlier, but not too early. You have to think about survival of genes in groups, not just individual survival.

FWIW my guess is it just didn't matter much. Surviving to 50 was plenty enough to have offspring yet not so long you compete with you grandkids for resources or become too much of a burden. That's already much much longer than most species so if anything evolution has already optimized humans to live fairly long lives. Once that got to be "long enough" there just wasn't enough selective pressure to matter.