[randomImmigrant]

Note: terrestrial chemistry is no different from chemistry that can occur anywhere, given the right molecules and conditions, and even then it’s a matter of degree.

Nitrogen being replaced by helium would actually be fine but for the niggling issue that we need nitrates. There are no heliates (?) to compensate. The name doesn’t even make sense… helium is the sole gas to have an ium end like metals- chemically it’s that meaningless what you call it as an ion…it shines elsewhere though.

For biology, it’s a necessary condition that the environment react with it and it reacts to the environment. Over time the two become deeply intertwined through the process of evolution.

It’s hard to see how that kind of evolution will occur if a lot of the environment is nonreactive.

Survival may be plausible though. There’s been some research showing some bacteria can survive in high helium environments. That’s a far cry from proving something like a bacterium can evolve in a helium environment that’s non-reactive though.

[ant6n]

Why would it be necessary for life to depend on breathing atmosphere? The atmosphere could just help in keeping the temperature even and provide some nice pressure, maybe that’s favorable over vacuum.

[randomImmigrant]

Simple. Energetics. Gaseous exchange has many energetic benefits that you give up if you have a mostly inert atmosphere.

If all your reactive chemistry comes from water/minerals, you’re also a heck of a lot more tethered, as both these cycle slower than the atmosphere.

A dynamic atmosphere is a huge component of the energetic efficiency of life on earth. And while it’s totally plausible other chemistries can match this, what’s implausible is for no chemistry to match it.

And when it comes to complex life on earth, it literally didn’t happen till oxygen percentages got sufficiently high in the atmosphere, a two billion year long process set off by Cyanobacteria. You can’t get big bodies if you can’t get this kind of energy.