What would happen if our atmosphere had something else instead of nitrogen?

Inert gases like helium and argon are used in place of nitrogen in scuba tanks. The nitrogen in the air is not available directly for the nitrogen cycle (what little is 'fixed' only make up only 5-8% of the nitrogen in the cycle), so its absence should not affect life that much. Nitrogen was still necessary in Earth's early atmosphere to form ammonia and amino acids, the basic building blocks of DNA and life, but there could have been less, or the nitrogen could have lost for some reason, or some reason why the atmosphere is deficient in nitrogen. Definitely life would still be possible if a large portion of the atmosphere were currently say argon and oxygen, not really much different than Earth life now. Too much CO2 could cause health problems or death to humans, even with 20% O2. Of course, with 80% CO2, the atmosphere would have an extreme greenhouse effect and would get like Venus and life could not exist.

Helium is very light, and most of it escaped from the early Earth, so a heavier, similar gas like argon, which makes up 1% of the atmosphere would be a more likely good candidate to replace nitrogen.
Dwi A (08-44)