? For all we know the first big load of oxygen came in on a comet. Big time oxygen production didn't begin until plants colonized land.
Huh? There's O2 on comets ?! More to the point, without plants to renew it, the oxygen will quickly react and disappear.
Big time oxygen production didn't begin until plants colonized land.
Demonstrably false. I will give two reasons, there are in fact more:
1) The so-called Cambrian explosion. The arthropods, worms, chordates, etc found in the Burgess shale and suchlike depend on oxygen, so it had to be present before the "explosion". There were no land plants at that time.
It is believed that land plants evolved directly from a freshwater (versus a marine or salt-water) green algal ancestor sometime in the mid-Ordovician, between 450 to 470 million years ago (MYA) (Graham, 1993, p 234-235). The earliest known seedless vascular plant (genus: Cooksonia) existed in the mid-Silurian, probably around 420 MYA and eventually went exinct by the early Devonian (about 390 MYA) (ref. 3-4).source
2) Banded iron formations. These are deposits of iron ore that occurred when the atmospheric oxygen reached a particular level. They are from 1.8 to 2.5 billion years old, probably before there were any eukaryotes.