Before photosynthesis, who were the primary producers?

Perhaps it’s unknowable, but there must at least be some scientifically plausible theories floating around. So, there must have been a time when life existed on Earth, but photosynthesis had not yet evolved. At that time, what was the basis of the food chain?

Undersea vents, like the “black smoker” communities in the deep ocean today, don’t sound like a good explanation to me (an admitted layperson). It seems unlikely that delicate biochemical processes could have evolved under such hot conditions.

I haven’t had much luck researching online, so I turn to the big brains at TSD.

I’d still guess it was something like the primary producers* in deep sea hydrothermal vents. Per wiki, that’s chemosynthetic bacteria.

To begin with, there are zones near the vents that aren’t as hot. Also to begin with chemosynthesis could occur other places because there were no higher life forms to compete for the components. And no corrosive oxygen to interfere.

  • anything at the bottom of the food chain that is eaten but doesn’t eat anything else that was alive.

OK never mind about the hot conditions. Things were hot back then. But I think the question is valid nonetheless.

Yeah that sounds right to me too. But what were they eating? I guess I’m wondering if the early Earth was rich in high-energy compounds that chemoautotrophs could exploit, or would the first organisms have come to be near a hydrothermal vent?

And yes I understand that the real answer must be “no one knows”, but again maybe there are some guesses out there more educated than mine. This question has long fascinated me.

Here’s an abstract from a paper on the topic, Niches of the pre-photosynthetic biosphere and geologic preservation of Earth’s earliest ecology

So chemotrophs were first. Do we know whether consumers evolved before or after phototrophs were already on the scene? If there were no consumers until after there were phototrophs, I wouldn’t really consider there to be a pre-photosynthesis “food chain”.

Almost certainly not. Plain old heterotrophs were first.

However life originated, it’s a safe bet that it didn’t spring into existence a fully formed cellular organisms. There would probably have been vast pools of organic molecules available, the very stuff from which life derived int he first place. Whether this took the form of a slick floating on tho ocean surface or sediment in the lake and ocean basins or something else entirely, we have no idea. However we can be fairly certain that it existed, simply because the odds of life appearing from a small pool of precursors seem way to long.

So the first lifeform would have lived quite happily for a while, possibly as much as a billion years, just eating the primordial ooze form which it arose. As the supply of ooze started to become exhausted, then the selective pressure for other lifestyles really kicked in, and life diversified to exploit other niches.

Long, long after. Phototrophism seems to be a relatively recent evolutionary adaptation. It certainly requires some very complex biochemical machinery to make it work. In contrast, every cell, almost by definition, already contains the enzymes required to digest itself. So consumers, at least in the form of scavengers, would have been around since day two, quite literally. The role of dedicated scavenger was probably one of the first niches that life adapted to fill. After all, there is no more complete or conveniently packaged source of both nutrients and energy than the corpse of a near relative.

But even leaving aside scavenging, you don;t need phootropism to have a flourishing community of consumers. Chemotrophs are quite capable fo supporting such a community.

Why not? IWhy does it matter if the producer obtains energy by oxidising Iron or reducing Carbon? There is a point at which there were no organic compounds, then there is a point at which they are assembled, then a chain of organisms feeding upon those compounds. How is that not a food chain?

Do you have a cite for this? I wasn’t aware it was a settled matter.

It’s my understanding that the evidence is that we did; the proteins of life in general appear to be adaptations of the specialized proteins needed to survive in such conditions, rather than such communities having unique protein adaptations they evolved after diverging from everything else.

What part of “almost certainly” do you not grok?

Eh? “Almost certainly” implies a high degree of confidence, higher than I assumed possible for our knowledge of early Earth. What might be plainly obvious to you is not to some of us. I don’t think it’s entirely unreasonable to ask for references in this case.

I went and looked it up and find a few sources repeating the same thing, including this paper from 1983, but if you have better info, I’d still like to read it.

I’ve never seen any source suggest anything else. It’s pretty much an inescapable conclusion. So in terms of references: any tertiary biology textbook that you care to grab.

Huh - according to what I read on wikipedia, phototrophism arose quite early:

Fossils of what are thought to be filamentous photosynthetic organisms have been dated at 3.4 billion years old.

Not photosynthesis as we know it today - they used various compounds as electron donors, but not water, so early photosynthesis did not generate oxygen. But still, they made their living at least partially by “eating light”.

As to what they were eating, it wasn’t eating so much as bringing in chemicals and combining them in a way that released energy to drive the chemical reactions that built them up out of carbon dioxide and hydrogen (mostly). The explanation here shows what they were using for energy (bolding mine).

Chemoautotrophs still exist and can be found in soils and sediments. In addition to the above, they can use ammonia and certain forms of iron as an energy source. I don’t know if those paths were established early on, but it’s possible.

Also, per wiki,

So smoker vents are a good source of hydrogen and sulfur, but ancient chemotrophs wouldn’t necessarily have needed to be up in the heat. At least not until the competition had lowered the availability of those things elsewhere.