Rock layering, more specifically BIFs (Banded Iron Formations), are layers of rock, comprised of ferrous iron (green/tan bands) and ferric iron (red bands), which is basically rust from the ferrous iron interacting with oxygen in the atmosphere in the Pre-Cambrian oceans.
Early earth's atmosphere had extremely little to no free oxygen (there was some in H2O2 and CO2, but it is not free and cannot be metabolized or react with much of anything).
The red bands show oxygen (free oxygen, O2), a lot of oxygen, reacting with ferrous iron and precipitating out of the water to rest at the bottom, becoming another red band. Where did it come from? It came from photosynthesis: cyanobacteria. That is the only source that could produce that much oxygen.
In a nutshell, the more correct answer is banded iron formations, rock layering. I say more correct because rock layering is signature of cyanobacteria (first life forms) presence, and cyanobacteria cannot exactly be directly found, only signs that it was around.
