We know that putting humans in zero gravity CAN have serious long term impacts to their physiology... but- as Raoul points out- those can be potentially overcome...

What I find surprising is that removing gravity is not more IMMEDIATELY fatal - that in over 3 billion years not one single essential metabolic process evolved a critical dependence on gravity, or the physical processes that gravity enables.

Convection, separation of fluids by density, gravitational organization
of the distribution of fluids and solids in the body... apparently none of that is critically important to our ability to live. Sure there might be some effects that could shorten lifespan, not a lot of long term data, but people have spent over a year in microgravity and survived. In fact, not one person has died from a health issue in orbit.

I find it surprising the effects are so benign.

Probably my surprise is simply a result of my lack of knowledge about biology- because it isn't just humans that can survive zero gravity- pretty much any Earth organism seems to be able to survive it with varying degrees of impairment... The processes of life must be, for some reason I don't understand, fairly agnostic to gravity- I find that odd, given it is pretty much the ONLY constant in our over 3 billion years of evolution.

In over 3 billion years, something as fundamental as the way fluids of different densities behave when mixed in the presence of gravity NEVER became critical for cellular functioning.

I find that rather shocking... which probably means I am overlooking something obvious, I would like to know what that is. It certainly seems to be the case that it's just not that critical for Earth life- but is that just an astronomical fluke? It seems easy to come up with countless ways evolutionary shortcuts on other planets might lead to a life form that cannot survive even brief exposure to zero-Gravity...