Being able to survive unearthly conditions may just be a side effect of being able to survive extreme, yet common conditions, such as dehydration.

Also, note that they don't thrive in these conditions, they're merely able to survive for a relatively long time.

They thrive in water, but adapted to be able to withstand dry periods by desiccating and stopping their metabolism (you could say their life is suspended, really) and resuming when water returns. In their dry state (anhydrobiosis/cryptobiosis) there's not really anything that would violently evaporate in a vacuum and thus destroy cell structure, like there is in live cells, and they repair DNA damage after they rehydrate, so they're not harmed much by vacuum or ionizing radiation.

This is an argument for panspermia according to Neil deGrasse Tyson. (Real Time with Bill Maher, S13E29)

I was going to say the same thing. Such a hardy species I would think would be the product of an incredible amount of evolutionary progress. How do they fit in with the rest of the living species taxonomy?

Don't get too excited - the Wikipedia article (very good) explores their genome and taxonomy. Fossilised records hard to come by but cambrian and Cretaceous records have been found. So probably boringly terrestrial

Or it could simply mean that they came to Earth a long time ago.

Sure, but as of now, there is no positive indication they are extraterrestrial. They fit into the species cladogram, they don't seem to suddenly appear as a time anomaly, they just seem like extremely tough little guys.

I read somewhere that they can absorb DNA from foreign sources (other animals, etc) into their cells when they rehydrate, leading to an accelerated rate of evolution.

Here's an article about that: http://phys.org/news/2015-11-huge-chunk-tardigrade-genome-fo...

Doesn't really solve the problem of how life first arose. It just pushed the whole thing back into space.

But pushing it into space does open up some interesting possibilities. There must be some very exotic chemistry during and after a supernova, or when a comet has an unusually close encounter with a star.

Basically dehydration and radiation have the same effect on DNA and other large molecules. The major effect of a hard vacuum is dehydrations. Being able to survive both of these is just a side effect of being able survive dehydration.

I remember reading a"theory" that they might have been alien. Arrived on a meteor or something like that. Would make a cool animated movie with characters.