Tardigrades

So there's an animal living in ponds and forests across the world, with eight little stubbly legs and a tiny little round mouth. They're called moss piglets, or waterbears (or tardigrades if you're boring), and they're microscopic but plump and wrinkly. They generally eat plants or bacteria, and just kind of float around not doing a whole lot on the grand stage of ecology.

They're also completely impervious to just about everything, including ionizing radiation, the vacuum of space, extremes of temperature, starvation, and dehydration. They also don't appear to have evolved from anything other than each other. So, yeah, not saying invincible alien vanguard species, but I may be heavily implying it.