I’ve been doing some research for another class on a very interesting topic: what would happen if oxogenic photosynthesis never evolved? We know life can exist in an anoxic atmosphere, but just how complex can it get? My research is still incomplete, but I’ll detail my findings here.
Believe it or not, eukaryotes can and do survive in anoxic conditions. Foraminifera, a type of amoeba-like protist, respire by performing substrate-level phosphorylation with malate, which can be derived from phosphoenolpyruvate, the second-last chemical step in glycolysis. Other eukaryotes survive by oxidizing sulphur or reducing nitrates. That pales in comparison to the number of anaerobic pathways exhibited by prokaryotes, however. And since eukaryotic mitochondria are just endosymbiotic bacteria, that means there’s technically a huge variety in alternate pathways.
There are issues, though. Oxygen is used for respiration simply because it’s one of the most powerful oxidizers out there, and because it’s available in huge quantities, thanks to plants. Although I haven’t looked up relative abundances yet, chances are that nitrates and sulphides would be harder to come by. Besides, those pathways just don’t have the oxidizing power to render glucose all the way down to carbon dioxide. These types of anaerobes spit out lactate, succinate, molecular hydrogen, ethanol and all kinds of other products, representing a significant waste of free energy.
Now, the main question is this: how complex can life get under these constraints? It turns out that it can do some surprising things. All you need to do is look at the deep ocean, where oxygen is literally rarer than gold.
That’s Riftia pachyptila, a type of deep-ocean tube worm, pictured here next to a hydrothermal vent. While cost-prohibitive to study (seeing at they hang out on the ocean floor and all), a few specimens have been retrieved, and the results are surprising: they use next to no oxygen. They get by almost entirely by oxidizing sulphur, possibly by themselves, but more likely with the help of symbiotic bacteria. The real kicker: they’re big, reaching up to 1.5 metres in length.
So not only can life get by on chemicals other than oxygen, they can reach pretty significant sizes. Without oxygen, multicellular life probably would have evolved, but with some key differences.
First, I highly doubt they’d be anything but aquatic. Most of the waste chemicals from anaerobic metabolism are liquid at room temperature, which makes their removal from the body problematic without very specialized features, or a constant flow of water to simply wash them away. Second, chemicals like suphides and nitrates would be a significant limiting factor. If multicellular life managed to get onto land, it would either have to utilize an extreme dependance on nitrogen-fixing bacteria or limit their range to volcanic areas.
So there we go. Hope you guys find that as interesting as I do.
R. pachyptila references (must be accessed through JSTOR for some reason)
Felbreck, Horst. 2014. Chemoautotrophic Potential of the Hydrothermal Vent Tube Worm, Riftia pachyptila (Vestimentifera). Science 213(4505):336-338.
Jones, Meredith L. 2014. Riftia pachyptila: Observations on the Worm from the Galapagos Rift. Science 213(4505):333-336.