That has a lot more to do with whether the cave started out containing anything "more complex than insects." 5 million years isn't, like, an evolve-monkeys-from-scratch number of years. It's arguably an evolve-humans-from-chimps number of years. But if your cave doesn't contain any chimps... or even if it does but they all starve to death in Year 2 out of 5,000,000...

Humans didn’t evolve from chimps.