Following up the ideas given in this question, we introduce a new gas to the mix of air.
Meta: Assume an Earth-like planet. The proposed change is a pre-development change (meaning it's there since the forming this planet).
As most known gases will do their best to anesthetize air-breathing beings and because this is about creating new worlds; we will introduce a new gas with no nasty side effects, basically Nitrogen 2.0 - and we'll call it inventigas.
Inventigas has a weight of approx. 20 kg/m^3 at sea level and is inert at normal temperatures (basically a heavier version of nitrogen).
Now that we have this new gas, we replace as much of the air's nitrogen with it as we can (say about 90% of it).
Regarding the development of life: What effects would the proposed change have on flora/fauna in comparison to Earth?