Over the last couple weeks, I've been working on a research essay for one of my easy elective courses: something us Canadians call a "bird course." On a whim, I decided to write about one of the topics I didn't really know much about, because where's the fun in researching something you already know? That topic ended up being one of the most interesting things I've ever researched, and what I found out was cool enough that I can't resist telling anyone who'll listen about it.
The topic: What directions might life on Earth have taken if oxygen had never appeared in the atmosphere?
Most of us already know where free oxygen comes from: plants, or more accurately, photosynthesizing organisms. Things capable of making their own food, that take the ever-controversial gas carbon dioxide and use the energy from sunlight to convert it into glucose and oxygen. Most of us (I hope) also know that oxygen's the gas we breathe to keep us alive. Fewer (those that took biology in Grade 11 and 12) know what oxygen's used for: an electron acceptor at the end of a complex chemical chain that is responsible for generating most of a cell's energy. It's so essential for life as we know it that astronomers and exobiologists actively search for it in the emissions spectra of other planets.
But not all life on Earth needs it. Plenty of bacteria and the like not only get by without it, but actually find it toxic. We usually refer to these organisms as "anaerobes." While some developed and then lost an aerobic (oxygen-utilizing) metabolic pathway, some never had it to begin with. Those are the closest thing we have to the first life on earth. In fact, when oxidative photosynthesis first appeared, most of these organisms died off as the first cyanobacteria filled the oceans and atmosphere with toxic oxygen. So that's all fine and good. Now, what would have happened if said photosynthesis never appeared?
We already know that bacteria would do just fine. A good portion of the more primitive ones get by on glycolysis, an inefficient chemical process common to all cells that's the first step in glucose metabolism, and one that produces a minuscule amount of energy. Others, more advanced, get by on chemeautotrophy — production of energy from chemical compounds in the environment. The most common of these use ammonia, nitrates and sulphur. Others use a modified version of photosynthesis that doesn't produce any oxygen at all. Instead of water, they use hydrogen sulphide, producing sulphur dioxides, molecular sulphur and hydrogen gas. Eukaryotes, more complex organisms that include protists, fungi, plants and animals are a different story. Their energy needs are significantly higher, and their metabolic pathways considerably more complex. They can't get by without oxygen. Or can they?
The best way to find out if something might exist is to first find out if it does. And, as it turns out, anaerobic eukaryotes do exist. Meet the foraminifera, a type of amoeba-like protist. They're mostly aquatic, and are found just about everywhere in the ocean, even at the bottom of the Mariana Trench. These little guys are remarkable for their ability to produce energy without any kind of electron acceptor, a metabolic pathway referred to as substrate-level phosphorylation. Basically, they just use a bunch of hyper-specialized enzymes to produce energy. Granted, it's much less efficient. Instead of carbon dioxide, they puff out a cocktail of intermediate chemicals that include lactate, malate, ethanol, acetate and molecular hydrogen — all chemicals that represent wasted energy. It's still enough for them to get by, though, and in a world without oxygen, critters like these would be common, as would strains of bacteria that feed on their waste products.
So just how complex can life get without oxygen? Eukaryotes can exist, but what about multicellular life? Are creatures like Halo's methane-breathing Grunts and Mass Effect's ammonia-breathing volus possible? The answer is: maybe. Methane, being a hydrocarbon, represents a decent source of free energy, and there are pathways that reduce ammonia. Would they appear on Earth, though? Again, maybe. It depends on a lot of things. There are bacteria that produce methane, others that produce ammonia, and still more that live off both. But prediction what could happen is next to impossible to guess. It's simply a situation that's too complex to model with any kind of accuracy.
So instead of speculating further, I'll turn again to something that exists today. Meet Riftia pachyptila, a deep-sea tube worm that lives near hydrothermal vents. This critter is poorly understood, and still under study. My information, in fact, comes from an article published a month or two ago, recently enough that the Wikipedia page is now out of date. Anyway, it states that the worm is almost entirely anaerobic, which it would have to be, considering the depths it lives at. It's also unusual in that it doesn't appear to have a gut, or any kind of feeding apparatus. It instead appears to rely on symbiotic bacteria, which oxidize hydrogen sulphide into elemental sulphur. In a way, it's entirely chemoautotrophic. More importantly, it's a multicellular organism that doesn't need oxygen.
Now, since I have a real-life example, I'm comfortable to speculate. If life continued to develop along these lines, I'd reasonably predict sulphur metabolism becoming dominant among eukaryotes. The sulphur-oxidizing bacteria would become the equivalent of aerobic mitochondria, and multicellular life would flourish.
The problem: oxygen is about ten times more common on Earth than sulphur. So biomass would just about automatically be cut by 90%, simply due to limiting factors. It would also be unlikely to be terrestrial, since most of the by-products of sulphur metabolism are in liquid form. Finally, life would most likely be concentrated in areas of volcanic activity, given that volcanoes are the planet's best source of sulphur.
That's just my take on it.