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Comment by gameswithgo | Hacker News Reader

>If, in the final 7,000 years of their reign, dinosaurs became hyperintelligent, built a civilization, started asteroid mining, and did so for centuries before forgetting to carry the one on an orbital calculation, thereby sending that famous valedictory six-mile space rock hurtling senselessly toward the Earth themselves—it would be virtually impossible to tell.

I consider this claim to be complete nonsense. Change my mind. There are 4 billion year old rocks still on the earth to be found. 7,000 years of civilization is not going to be completely erased in a few million.

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Most likely it is nonsense. The carbon and nitrogen composition of sedimentary layers will show anomalies in case of intelligent civilizations.

https://arxiv.org/pdf/1804.03748.pdf

The article you link directly contradicts your claim.

I think we'd be able to find a previous civilization of our current technological advancement by finding a lot of veins of mineral resources mined out, except for the trailings around the edges. I've never heard even a whisper of such a mineral formation being found, and with the amount of money such mineral industrial research accounts for, I suspect by now we would have found such a thing if it exists, especially if it existed in quantity.

There may be no structures or artifacts that survived 65 millions years, but there's plenty of holes that would have, in natural structures that to all appearances are older than 65 million years, should have been about as available then as they are now, and are obviously undisturbed.

What if they used different minerals because before they mined them out, different minerals were present? To what extent is our dependence on iron, copper, and bronze a historical accident because that's what was available to our ancestors, and to what extent might we have used different metals if easily-extractable ores for them were present?

Despite being the third most common element in the earth's crust, aluminum cost more than gold and was rarely used for industrial purposes before the Hall-Heroult process for smelting it from alumina and the Bayer process for smelting it from Bauxite.

Rare-earths display a similar profile: they are abundant in the earth's crust, but not in any form that we can easily mine and extract. What if a past dinosaur civilization mined out all the easy rare-earth deposits building iPhones and batteries?

"What if they used different minerals because before they mined them out, different minerals were present?"

I'd point out I didn't say that all the minerals were gone, but that the good veins were gone, surrounded by the stuff they left behind. I say that because that's how we do it now, and we do it for economic reasons that any civilization will be subject to. This is a particular pattern that I'm not aware of anyone ever observing.

I am also not aware of anyone even proposing anomalies in element distribution of things like rare earths. Rare earths aren't as rare as their name suggests as I'm sure you know, but as you go up the periodic table, you do naturally get less of them in the crust for several reasons (less created in the supernovas that created the heavy elements, and more of it sinking in the earth during the early phase). It's possible that our understanding of elemental distribution is a "just so" story created by what turns out to be artificial manipulation of the contents in the past, but given that our understanding of the processes that led to the current distribution includes a lot of astronomical observations, it'd be one heck of a "just so" story at this point. I am not aware of even a hint of an observation that requires an intelligent civilization on Earth in the past.

I'd say if you really want to have some fun, hypothesize a dinosaur civilization that attained some sort of singularity, and on the way out chose to leave behind machines that restored Earth to its "natural" state over the course of a few centuries or something, thus wiping out all traces of their presence to make way for the next civilization. It's not impossible to imagine a human singularity ending that way. Come up with some semi-compelling reason why all or at least most civilizations would tend to do that and you've got a fun premise for a sci-fi story universe. Perhaps we're not merely the second, but the dozenth. Heck, maybe we're not even the first primate.

(Another bonus "thing we clearly don't see": At the current rate of human advancement, in another couple hundred to couple thousand years, even putting aside really high-end nanotech, our primary contribution to the history of planet Earth may not be the sudden extinction of a lot of life forms, but the sudden proliferation of a lot of life forms, along with visible cross-transfer of genetic characteristics that couldn't possibly have naturally occurred. Even if the intelligent civilization completely keeled over dead, or Transcended, or whatever, the mark on the planet's genetic history would be visible quite possibly right up to the point the Sun sterilizes the Earth. We aren't there yet, but we are darned close. As much fun as a lot of people seem to derive from wringing their hands and posturing about how terrible humanity is, on a geological time scale, we may not be living in an Athropocene Extinction, but instead in the gestational phase of the Anthropocene Explosion.)

Of all of the above, the existence of holes seem to be the only compelling traces.

I mention it because it's the earliest one I can think of in our technological development. If we continue advancing, we will eventually get to that genetic diversification bit, but it seems that happens after you get the point where you can wipe out your own civilization with nukes, so that one is much less reliable.

(The Singularity paragraph is just for fun. Almost by definition, there is no reasonable way to ever collect evidence for it.)

In addition to the Schmidt/Frank paper (already linked to by namirez in another comment), there's a cute suggestion that the best place to look for dinosaur civ remnants should be on the moon, Mars, and the asteroids, because their spacecrafts should decay less quickly than their Earth settlements. :)

https://arxiv.org/pdf/1704.07263.pdf

By some back of the napkin calculations I did once, I found that a couple billion years is enough for the tectonic plates to have completely recycled themselves into the mantle, erasing any structures built on them.

Correct, the stratigraphic record is 3.5 billion years old. At least if this (https://xkcd.com/1194/) xkcd is to be truested.

What percentage of 4 billion year old rocks are still on earth? What percentage of rocks currently on the earth bear traces of humanity?

When I've gone on Geology 101 field trips, once or twice the instructor was like "And this rock was formed over a billion years ago." It was invariably followed by "Deep within the earth's crust, several miles below the surface." Rocks that are on the surface get weathered and eroded; quartz and feldspar on exposed granite become successive layers of sandstone.

What about if dinosaurs got to the point of early hunter-gatherers - establishing communities, language, basic tools - but were then wiped out?

It's different from the case he cites, but tracks the general point that there are limits to what we can learn about prehistory / "deep time"

Think of it this way: distance in time is like distance in space. It's just another dimension (with the catch that apparently we can't actually move backward). The dinosaurs are as far from us as Alpha Centauri, maybe further.

This is a reasonable way of looking at time for certain purposes, and the conversion factor is c. So, 65 million years "ago" is also 65 million light-years "away" in the -t direction.

If we can find their fossilized skeletons, can't we find their stone tools? Maybe scattered around their ritual burials?

There are plenty of rocks from the late ("upper") Cretaceous, with a boundary containing elevated iridium. Suppose some microscopically thin boundary, microscopically lower, had a slightly different carbon isotope mixture. Who would notice, even looking? Would the two be even distinguishable, or hopelessly jumbled? Indeed, is anyone looking now? Would there be any reason to conclude that the difference was not also caused by the meteorite?

Outside of a blip in CO2 emissions and soil nitrogen, and a dramatic decrease in biodiversity, we will leave almost no trace if we disappeared tonight.

https://youtu.be/8xDK2LgSeyk

If nothing else, telltale traces of our nuclear industries will be clearly identifiable. There was a natural nuclear reactor in what is now Gabon, and the fission products it left behind (e.g. Rutherfordium) are how we know it was there.

For a while, until it all decays. But the total amount of material involved is minuscule. You would be looking for parts per trillion, if indeed you were bothering to look at all. And what would draw your attention to look?

There are fossils of hundreds of millions of years old, and microfossils of microorganisms over 3 billion years old. Why couldn't there be fossils of technology?

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