So where did all this oxygen come from, and all
of a sudden? See the Kasting figure again.
The generally accepted best idea is that photosynthesizing
life arose around 2.5 billion years ago. These organisms
took the CO2 in the atmosphere (Kasting figure) and incoming
sunlight and produced oxygen as a waste product. Thus, atmospheric
oxygen content increased.
This does not necessarily imply multicellular organisms.
Remember that there are bacteria and archaea which can
photosynthesize, in addition to plants, which have
organelles to carry out photosynthesis. In fact, it is
probably the ancestors of those organelles which,
as individual organisms, produced the rise in
atmospheric oxygen.
Also note that an oxygen-rich environment would have
been poisonous to some creatures 2 billion years ago
just like a CO2 atmosphere would be poisonous to us
today. Clearly, the rise in atmospheric oxygen would
have forced every organism on the planet to either
adapt -- and use oxygen to its advantage -- or die
trying. Today, plants carry out both respiration (of
oxygen) and photosynthesis (of CO2). Respiration
is a more efficient way to release energy, allowing
plants to grow better and faster and stronger
[like the Six Million Dollar Man]
than if they were stuck using only photosynthesis.