Over the course of evolution on our planet, billions of years, many things in our environment have changed- temperature, O2 content of the atmosphere, atmospheric pressure, the amount of sunlight, radiation levels- life started in the oceans and eventually made its way on land… life has experienced wild swings in many parameters of the environment, and so it evolved to be flexible and be able to adapt to even quite significant swings in all of these… sure there were mass extinctions when Ice ages hit, and mass extinctions now with global warming… but life can tolerate significant changes in the parameters because it has had to over the course of billions of years of evolution.

It is not surprising- when the environment is prone to change these parameters unpredictably, evolution would select for adaptability… tolerance to those changes.

But over the course of evolution there has been one unchanging constant- from the first cells to the present – all of evolution has taken place in a fixed gravitational field- this is perhaps the one constant over those billions of years.

It has always struck me as odd that we can survive the complete absence of gravity- that zero-G does not cause immediate death… at no point in evolution was the one absolute constant taken advantage of for some biological purpose critical for life.

When I have mentioned this to co-workers the first response is “Well, life started off floating ‘weightless’ in the ocean, so naturally we would be evolved to tolerate ‘weightless’ conditions” – and at first glance that may seem like a reasonable counter-argument, but upon a deeper examination it really starts to fall apart.

Neutral Buoyancy is NOT the same as zero-gravity- A whale floating in the ocean still has gravity acting on it tugging on its internal organs and the food in its gut- the distribution of viscera and fluids is determined by gravity- organisms can change their orientation relative to the gravity vector, but it is still there acting on their bodies…

But perhaps you could argue that because organisms are often changing orientation with respect to the gravity vector they have had to evolve to be insensitive to it- a reasonable answer, but still does not resolve the issue- even at the level of a single cell the absence of gravity has what I would consider to be pretty profound consequences- convection ceases in zero-G- the separation of fluids of different density is no longer neatly arranged by a gravity field.

If you light a candle on the ISS it burns in a very alien way- there is no convection to carry away the combustion products- you get a smoldering globe of  flame that sits there – only burning due to the Oxygen that is able to slowly diffuse through the combustion gasses building up around the wick.

For billions of years the only way an organism could experience free-fall was, well, to fall freely- and for larger organisms there would be no point evolving to be able to survive that for more than a second or so, because hitting the ground would be fatal anyway.

Despite these rather fundamental changes people, and so far pretty much any other organism from Earth, can rapidly acclimate to zero-g and spend extended time there. Sure there are some negative health effects- muscle and bone loss, nausea, etc… but it is something we easily survive.

If, in the preceding billions  of years of evolution, just ONE survival-critical metabolic function had evolved to become dependent on gravity, our ability to venture beyond the Earth would be greatly impaired- exploration would be solely the domain of robotic probes- sending back data to organisms that were doomed to never visit those other planets in person…

How many intelligent technological species are there out there that are doomed to stay on their planet because at some point evolution incorporated the one constant- gravity- into some life-critical metabolic function? What sort of space program would they have, knowing they would always be prisoners of their one planet? Is life on Earth unique in this respect?

Perhaps a biologist could easily punch holes in this argument- I am certainly NOT well educated on cellular biology- maybe there is an obvious reason life seems to evolve to be able to adapt to a complete lack of gravity, but it is not obvious to me…