# Was the great oxygenation unavoidable? [closed]

The Background

2.45 billion years ago, oxygen producing photosynthetic-bacteria emerged and changed the very course of life, killing off many of the anaerobic organisms that had already been established and radically altering the earth's atmosphere itself. Known as the Great Oxygenation event, this event opened the way for fast, oxygen-breathing creatures to rise.

The question

Is there a chance that the great oxygenation event might have never happened? Or was it an evolutionary inevitability that photosynthetic oxygen producing organisms would rise and dominate the world?

## closed as too broad by Mołot, James K, Slarty, dot_Sp0T, VylixNov 6 '17 at 1:46

Please edit the question to limit it to a specific problem with enough detail to identify an adequate answer. Avoid asking multiple distinct questions at once. See the How to Ask page for help clarifying this question. If this question can be reworded to fit the rules in the help center, please edit the question.

• Asking the questions that scientists would also like to know the answers to. – Phiteros Nov 5 '17 at 18:58
• The problem is that we only have one sample. So it is (roughly speaking) 100% ± 100% that it was unavoidable, as far as our statistics go. – Mołot Nov 5 '17 at 19:12
• Throw a coin. Heads - Yes. Tails - No. This is the best answer you're likely going to get. – dot_Sp0T Nov 5 '17 at 22:29
• Well, our own Earth managed to avoid it for about 2 billion years. Maybe a more skilled Earth would have been able to avoid it for a longer time. – AlexP Nov 7 '17 at 2:11

Absolutely. There are many ways of getting energy from the environment, and Earth has settled mostly on photosynthesis as the best way to do it. Two of the other, plausible mechanisms are chemosynthesis and retinal, of the Purple Earth hypothesis.

For example, the overall reaction of chemosynthesis looks like this:

$H_2S + CO_2 => H_2O + sugar + S_{(s)}$

No oxygen production necessary!

Photosynthesis dominates on Earth because it's so easy to obtain the required materials- you take in sunlight and CO$_2$, along with some trace elements, and you've got sugar. If you want a world where that doesn't happen, then all you need to do is make those materials harder to obtain. In an atmosphere that has negligible levels of CO$_2$, photosynthesis wouldn't be a viable strategy and organisms would have to turn to another source of energy.

As P. Chapman points out, whatever you use is going to produce a waste product. However, oxygen is an especially violent waste product. Its most common form, triplet oxygen, is a diradical- that's what makes it especially reactive with pretty much everything. That's what was so especially catastrophic in the Great Oxygenation event- it was essentially the equivalent of dumping lighter fluid on every other organism out there. With something like chemosynthesis, teh solid sulfur produced is pretty much nonreactive and would easily be sequestered, allowing chemosynthetic and anaerobic organisms to live together happily ever after.

I'll humbly link to my answer here for some other ideas about mechanisms for energy. Although all of those possibilities produce oxygen, it'd actually be easier for the reaction if it didn't. I had to use some questionable species like peroxides to make O$_2$ at the end, and if you just leave off those last steps you'll avoid the oxygen production entirely.

TL;DR: It's quite possible to avoid the Great Oxygenation event. Just make photosynthesis not worth it, either by limiting the CO$_2$ in the atmosphere or by decreasing the solar input to a point where it isn't worth taking advantage of.

## Maybe avoidable, but difficult on earth

OK it's easily avoidable if you don't have life evolve but I bet that's not what you want. So quick review on why you get water to oxygen. To turn CO2 into sugars which you need to build most life you need extra electrons. On earth it tuned out to be useful to pull then off water and produce O2 as a by-product. To avoid it you would need another abundant source of electrons.

The first problem in coming up with another source is you had an limited number of chemical you can use. Without getting into redox potentials and energy from sunlight and why that's important I cannot say much but there are a limited number of compounds available. but any source of electron is going to have a waste product that is going to change the environment.

The second problem is that oxygen for those organisms that managed to adapt oxygen became a great boon in breaking down sugars and producing energy. So no oxygen greater challenge for complex life.

So my quick answer is if you don't have life or abundant complex life you can avoid it. If you have abundant complex life and you don't have oxygen as your compound you would probably get something else as the compound you have to deal with.

• +1 for the insight that "any source of electron is going to have a waste product that is going to change the environment". I hadn't considered that. Also appreciated the reciprocal nature of CO2<->O2. – Green Nov 5 '17 at 19:26