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I was thinking about the Great Oxidation Event - how, when they evolved and starting dumping a ton of oxygen into the atmosphere through photosynthesis, they accidentally killed off almost all life on Earth, which up to that point was reliant on a reducing atmosphere. Thinking about sulfate-reducing life got me thinking about sulfate-reducing humans, and then "what if there was an entire race of sulfate-reducing humans, with all their history, that died off in the Great Oxidation Event and then humanity had to re-evolve from scratch?"

For the sake of argument let's assume this Ur-hominid species were basically humans, just with different biochemistry - they looked like humans, they talked like humans, they hunted what humans do and built what humans do; they just breathe hydrogen or methane instead of oxygen. They have the misfortune, though, to live at the same time that photosynthesis is getting popular. Cyanobacteria start dumping huge amounts of oxygen into the atmosphere and start killing almost all life on the planet, and the "humans", not technologically advanced to build a bunker capable of allowing them to survive - or even to be aware of what's happening - are not spared, and are killed off like almost everything else. Life must start over almost from scratch from the few unicellular organisms adapted to the presence of oxygen.

(To be clear, I'm aware the Great Oxidation Event didn't happen in, like, a year. We might imagine that at some point human life expectancy starts just continuously slowly dropping millenium on millenium in spite of advancements in e.g. medicine and food production, until the oxygen concentration gets so bad that adolescent humans die of chronic oxygen poisoning faster than they can reach sexual maturity.)

It's interesting to think of how modern humans, in the current oxidizing atmosphere, would interpret the ruins left behind by the unfortunate methane-breathing ones, ruins literally nobody remembers the significance or builders of anymore because they've been dead for a billion years. Maybe that they're the remnants of the heroic age when men talked with the gods, or that, since tectonics have spread the ruins across the whole world, that it would be fodder for some great "wewuzzing" about how everyone on the planet descends from this or that ethnic group.

...and then I remember that if there's literally billions of years separating these two species of "human", would there even be any ruins for the second species to make up stories about? What evidence would even survive that long? Surely even their grandest stone structures would be eroded away into oblivion by wind and glaciers, or crushed up and smooshed back into mountains by the relentless engine of plate tectonics. Any organic matter - wood, wine residue, skin oil from fingerprints, etc. - would surely have decomposed and folded back into the carbon cycle. So what does that leave?

I've heard it said that even if you buried a car and waited a billion years, there's no amount of time you could wait that would make it go back to looking liking a normal metal ore deposit - the plastic and the fact that the metal is confined to a small patch instead of an entire mountain range would give it away. So, cars, maybe? I don't like how modern that is. That gives the world too much of a post-apocalyptic vibe I don't really like - the presence of ancient technology way more advanced than what the modern humans themselves have at the time, like a Chalcolithic hunter-gatherer coming across a 2.5-billion year old car, would clue them into "hey, wait a minute" earlier than I would like for story purposes. I like the idea of something lower-tech because I think it's more interesting to consider the stories Copper/Bronze Age people would make up about the ruins, than what they could discover from them. I want to ruins to tell them "there were cultures so unimaginably ancient that you have no idea who they were", not "hey, ever heard of the internal combustion engine?"

So, here at last is my question:

What's the lowest-tech evidence of human habitation that could plausibly survive the elements for several billion years? (Not necessarily intact, but recognizably human)

Or could this whole scenario never happen because humans could never evolve that far to begin with without depending on oxygen-producing photosynthesizers?

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  • $\begingroup$ -1 for lack of research. Hematite Tubes nearly 4 billion years old are the oldest evidence of life on Earth. Evidence of your Ur-hominids (e.g. a fossilized bone) would be the lowest technology preservable (e.g., no tech at all, but the body itself). ... and that's the best scientific proof that anything can survive that long I can come up with. Does it matter that this is scientifically possible? People are still arguing about the last 20,000 years. Do you believe proving billions will matter? $\endgroup$
    – JBH
    Apr 23, 2023 at 2:01
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    $\begingroup$ My issue with this question is that it suggests that humans, or any kind of intelligent life, could exist completely out of context with the rest of evolution. We can look at dinosaur bones and know roughly how intelligent they were. We can see the progression of brain size over the past three million years for hominids. The first clue would have to be a fossil history of anything similar around that time. Humans, specifically, no. Anything even vaguely similar to humans? Absolutely, even after two billion years. $\endgroup$ Apr 23, 2023 at 3:12
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    $\begingroup$ (1) The Great Oxidation event began two and half billion years ago and ended two billion years ago. The transformation of the atmosphere took half a billion years. (For comparison, the K-T Extinction was only 66 million years ago.) (2) Humans have hard teeth, and humans have bones. We have 500 million years old fossils of soft-bodies animals. We are very highly confident that there was no such thing as a world full of large animals with hard body parts two billion years ago. $\endgroup$
    – AlexP
    Apr 23, 2023 at 7:26
  • $\begingroup$ Apart from the other issues, the atmosphere would have run out of reducing substances long before oxygen started building up, the major sinks being minerals dissolved in the oceans. There wouldn't have been much to begin with, Earth being too hot and small and the sun producing too much UV for hydrogen and methane to be major components of the atmosphere. These "humans" would have to eat olivine or something to generate hydrogen, and breathe CO2 or consume some other reactant for their chemosynthetic metabolism. $\endgroup$ Apr 23, 2023 at 18:30

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Billions of years is tough. There's not a lot that's going to make it. A car will have leached into the water supply, buildings totally vanished, there's not a lot that can survive. Even a lot of the crust will be subducted into the mantle. However, some candidates:

  1. A layer of rock with weird isotopic balance. With some clever physics, even though almost no background radiation would exist, the different isotopic makeup of the rock, spread over the world, could be attributed to a nuclear event

  2. giant space trash - I guess something in orbit might survive, though I'm not sure how long high energy particles from the sun would take to ablate the surface away, or if tiny amounts of drag would have deorbited it.

  3. Attempts to stop or live on past the disaster. A Human level civilization might take the step of, say, dropping a giant, extremely resilient, obelisk on the moon. It might attempt to colonize another planet, which is not experiencing this. It might set off a mammoth amount of nuclear weapons to blot out the sun, and slow the pesky photosynthesizing organisms

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If this had happened, and the "sulfate-reducing humans" had evolved naturally, then fossilised bones of sr-humans, and their evolutionary predecessors, would still be around. They would be much rarer than fossils from after the Great Oxidation Event, but they would be found in places where the crust is old enough. They would have started to be discovered during the growth of palaeontology in the nineteenth century, and would have made the development of that science and the study of evolution somewhat more complicated.

Overall, this would have slowed the development of the modern evolutionary synthesis (because the pre-oxidation fossils would inevitably have been claimed by creationists as relics from before the Flood), but might well have accelerated the discovery of plate tectonics.

However, that's assuming that the sr-humans would die out. They had at least 100,000,000 years to adapt to the changing atmosphere, which is time for a lot of evolution. That, after all, must be how life survived the Great Oxidation Event. Over that time, they likely could have found ways to prevent the change. Given how easily we're unintentionally changing the Earth's climate, a way might well have been found to prevent oxidation.

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