Caves were some of humanity's first shelters. Who knows what our distant ancestors were thinking as they sought refuge there, huddling and cooking meat over a fire, maybe drawing animals on the walls. Caves protected our ancient ancestors from the elements, and from predators and rivals, back when sticks, stones, furs and fire were our only technologies. So there's a poetic parallel between early humans and us. We're visiting the Moon again, and lunar caves could shelter us the way caves sheltered our ancestors on Earth. On the Moon, astronauts will need protection from a different set of hazards. They'll…