The rise of free atmospheric oxygen and oceanic dissolved oxygen levels were almost certainly a requirement for the development of complex multicellular life. Using oxygen as the electron acceptor in respiration means far more energy can be extracted from reduced fuel molecules (sugars, fats, amino acids, methane, hydrocarbons, ammonia, etc.) than by using species like oxidized sulfur and iron, CO2, or nitrate (NO3) for that role.

This is really where the mitochondria come into play, as they allowed their host to utilize oxygen for this purpose. This can be seen by looking at modern eukaryotes that have reverted back to an anaerobic lifestyle:

> "In lineages of eukaryotes adapted to low oxygen conditions, mitochondria have been drastically reduced, functionally altered and, in one case, completely lost."

"The Origin and Diversification of Mitochondria (2017)"

https://www.cell.com/current-biology/pdf/S0960-9822(17)31179...

Hence, looking for the signature of free oxygen in the atmosphere of exoplanets orbiting distant stars is considered to be a fairly good indicator of the possibility of complex multicellular life of some sort, and at least of an active photosynthetic microbial ecosystem.

(Incidentally, the historical divisions withing academic university departments led to evolutionary biology generally ignoring the importance of early Earth's geochemistry in the evolution of life, as they saw evolution as a kind of cellular/organismal process divorced from the physical surroundings - the latter being the province of the geology department. The renewed interest in exobiology and origin-of-life research has tended to bridge this gap.)