Why do we assume that water and oxygen are essential for all life?

I was watching the Science Channel last night and the shows were focusing on the search for extraterrestrial life. It reminded me of a question that I’ve had for a while.

Why do we assume that water is a prerequisite for all life? And why do we assume that oxygen is a by-product? Why couldn’t beings on other planets in other parts of the universe live off of other substances? Water and oxygen work for life on Earth, but does that mean that all life is dependent on them? Why couldn’t a being on another planet “breathe” ammonia and exhale nitrogen, for example?

oxygen is an electron receptor in biochemical reactions. sulfur could also do that.

Anaerobic respiration is done without oxygen.

Basically, there is only one broad category of chemistry that we know works for life (arguably including extremophiles), so we look for that one. Guessing at other possible forms of life based on different chemistries, and then looking for signs of them is just taking shots in the dark.

Besides, things like water and oxygen are relatively easy to detect. And it’s hard to think of a way that an oxygen-rich atmosphere could have arisen sans-Earth-like-life.

I don’t think we assume all life requires water and oxygen - just life that resembles our own. We even have a few examples on earth that don’t use or produce oxygen.

The problem is we have only one example of a complex biosphere of lifeforms, and until we find another, our best and only basis for determining the suitability of a planet for sustaining life is Earth.

And, of course, there’s always SETI. Since the EM spectrum is agnostic on the biochemistry of the ones making transmissions, we can search the skies for technologically advanced extraterrestrials without worrying about the physical composition of their worlds.

The universe is really, really big. We need to narrow down the search somehow. We have exactly one data point for how life works - Earth. If there’s other life out there that works in the same way, then we know what to look for.

If we expand the search, we don’t really know what we’d be looking for that would indicate the presence of life. No one is saying that life MUST involve water and oxygen just like our planet; it’s just that that’s the best bet for doing a search, because we know it’s happened once. Everything else would be speculative.

Because so far all the life we know about requires water. There are speculations about life that might use some other liquid but at present that’s all science fiction, emphasis on fiction.

Actually, not even all life on Earth is dependent on oxygen. There are at least three multicellular species on Earth that don’t use oxygen. There are even some types of Earth life that find oxygen a deadly poison.

So, while we know that not all life requires oxygen, so far as we know all life still requires water.

However, scientists and some of the rest of us would be thrilled to pieces to find life that doesn’t require water, not the least reason being that it would almost certainly be found somewhere other than Earth. In order words, alien life.

Thanks for that, very interesting link.

I thought plants breath CO2, would that be a contradiction to the idea that oxygen is essential for life?

I think that when scientists say that all life needs water and oxygen what they are likely saying is that model has worked so far in all cases. This is a guess though.

Can someone tell me if it is true that carbon is supposed to be essential to life, according to scientists, due to it being the only element to hold DNA or DNA-like structures?

there is oxygen in CO2.

even if you want to limit it to elemental oxygen, O2, plants use that to in their respiration processes.

Plants do not ‘breathe’ CO2. They use oxygen for respiration, the same as we do. (that is to say, they use oxygen as the electron acceptor when they extract energy from organic compounds). Plants which are not photosynthesizing actively (e.g. Young seedlings, plants under extreme stress, parasitic plants, plants in the dark) are net consumers of oxygen and will die if they don’t have it. Although in some cases some plants can use fermentation processes to break down organic compounds without using oxygen.

Most plants are net producers of oxygen because they have a complementary process going on, photosynthesis, which produces oxygen as a waste product. this is not breathing though, it isn’t analogous to anything that most animals do. it’s a process by which organic molecules are synthesized from raw materials.

Carbon, Oxygen and Hydrogen do represent a bit of a ‘sweet spot’ in chemistry - they’re very common elements - there are lots of ways to combine them and lots of interactive possibilities available when you do that.

Other hypothetical chemistries aren’t impossible (especially perhaps outside of our temperature and pressure parameters), but I think we’re not wrong to be looking for life somewhat like our own.

People have theorized about alternative biochemistry. But until we find or engineer critters under those conditions it’s conjecture.

Whoops. Good point.

I didn’t know, thanks for sharing.

Another plus for water is this: whatever biology you dream up will rely on biochemistry. Even with some exotic (to us, at least) non-oxygen reactions going on to support life, a liquid solvent is nearly essential. Solids don’t provide much chance for molecules to move around, and gases leave the molecules spread pretty far apart - hard to make a reaction go if the molecules can’t get together efficiently.

If a liquid solvent is needed, water is an obvious choice, since it is a small, very stable molecule made up of abundant elements, and it can dissolve a wide range of chemicals. Now, Earth is in a sweet spot, temperature-wise, for liquid water to abound, so maybe some other environment would favor some other liquid as the life-o-chemical solvent, but water seems to be the simplest choice to look for.

In fact, there was a point on Earth where all life was anaerobic. Those lifeforms paved the way for the Oxygen Revolution, generating free oxygen in the atmosphere. Another good bet for life-bearing planets following Earth’s development might be to look for free water, Earth like temperatures, tectonic activity generating massive amounts of carbon dioxide in the atmosphere.

It looks like there’s some confusion here by what is meant by “oxygen.” The OP seems to be asking about free molecular oxygen, O[sub]2[/sub], and free H[sub]2[/sub]O. Free oxygen we know for a fact is not necessary for life even as we know it, and in fact is toxic to pretty much everything. Some lifeforms have figured out ways of dealing with the toxicity, so mammals are largely hypoxic inside but they carry oxygen around in iron cages (hemoglobin). Probably most microbial life, and thus most life overall, is at best unable to grow in the presence of, and at worst actively and viciously killed by, free oxygen. So in searching for extraterrestrial life, an oxygen atmosphere isn’t seen as a prerequisite for life, but as an extremely strong sign of it. Plus we’d want an oxygen atmosphere there already if we ever wanted to move to that planet.

Compounded oxygen is a bit different; basically chemistry has a large but limited toolbox. I suppose it’s not outright impossible to have some life with an all-metal chemistry, but otherwise you’re going to have nonmetals involved, and the second row of the periodic table has the most abundant and reactive nonmetals: carbon, nitrogen, and oxygen. Oxygen especially is utterly ubiquitous as soon as you go to any environment that isn’t mostly hydrogen and helium. Even if you had some hypothetical silicon-based life, the Earth’s crust has nearly twice as much oxygen as silicon in it by weight, so there’s going to be some oxygen chemistry going on there no matter what.

Water’s a little bit trickier, but as tallcoldone and others have pointed out, it does a bunch of things that we think are necessary for life, and it’s utterly ubiquitous in the universe. If your biochemistry involves both hydrogen and oxygen, water is almost certainly going to be produced at some step. It’s conceivable that life can exist without liquid water, and there would be detectable evidence of life without it (for instance, last I heard the excess of molecular hydrogen in Titan’s atmosphere is “consistent with” the presence of life there, which is to say not even as strong as “evidence for” but at least it leaves the door open a crack) but in general we wouldn’t know if we’d found “life Jim but not as we know it” even if we were looking for it. For the time being, it makes more sense to look for life as we know it rather than speculate about exotic forms of life without having any evidence that it’s possible to begin with.