Summary: alien rebuilds its body, constantly healing damage

Observation: "We pumped four grenades up onto the catwalk – no direct hits, but the Fade was slammed over the railing. The thing was on fire, for chrissakes. It goes down behind the generator, into a big pile of spent fuel canisters. Made a hell of a racket. We figure – grenade blast, on fire, fell a couple of stories onto cast-metal tubes … let's kick back, get some medkits, and set up a forward base. A minute later, we hear a canister roll across the floor. We all stand up like, 'Nah … couldn't be …'. Next thing we know we're dodging acid and screaming for ammo."

Thesis: the Kharaa can harvest our resources, and use them to build various organic structures. It looks like some aliens evolve the ability to re-build themselves, probably creating an organ that teaches the local bacteria to regard it as a structure that requires construction. So it is not actually healing itself, but literally being built anew (the Gorge's healing spray likely operates on the same principle). Absorbing raw material from the surrounding bacteria requires subdermal conduits that interfere with carapace evolution, and as near as we can tell, the redemption evolution as well.

The following chart shows how many points per second are regained by an alien with the regeneration evolution: