In effect you did say life migrated throughout the planet. My question is, given the radically different environments on earth how do you get a worm to migrate from a deep ocean 400C black smoker to a sub-zero polar ice pack without killing it? Did it want to go north and adapt or did it have no choice. In that case what was causal for that 6000 mile migration from one deadly environment to another totally opposite deadly environment?
As to Tardigrades, I can see how they migrated via rivers, hitching a ride on flotsam in the oceans, via rivers, and possible even via storms.
But that organism must have already had extraordinary survival properties to be spread so widely.
Another Tardigrade Fossil Discovered
Another interesting application lies in biogeography. “Because this is such an ancient group, tardigrades can be used to understand how organisms move over the plant over periods of time,” added Barden.