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So imagine this scenario:

Sapience (which I define as Human-like intelligence or better) is a relatively common occurence once a life-form is on a specific brain-arms-race path like Hominins have been for a few million years. A certain set of mutations can occur that make a dramatic difference in overall capability, so the jumps in capacity are large.
Now various types of hominins (say some version of Neanderthals) suddenly develop a technological civilization. The smarter they are compared to us, the faster the relative rise of their civilization. Instead of 15,000 years from agriculture to space age, it only takes them 1,500 years. After a brief early space age, they either wipe themselves out with MAD-type weapons, or more likely decide to leave the "Cradle" as an ecological preserve of sorts as they expand out in space (or transcend space altogether). Tens of thousands of years later, a different branch of hominins attains sapience, and the cycle repeats.

My question is 3-fold:

  1. Name the one most important factor that would make this scenario implausible (say, hominins before this or that era were physically unable to speak), or if you prefer, more plausible (for instance the larger brains of Neanderthals).

  2. Do you think (m)any XXI-th century level artefacts could be preserved well enough to be detectable from 30-50k ago (especially given the dramatic changes in sea-shore levels in the past 50k)

  3. What would the one most important result of this type of 'pulsar' intelligence be in terms of pulse-driven changes across different intelligence bursts? I'm thinking for instance that the next post-human pulse for instance would have a lot less coal and oil. Perhaps they trigger (the end?) of an ice-age, or genetically modify their successor race before they leave.

Feel free to answer 1,2,3 or any combination thereof.

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  • $\begingroup$ Note: Taxonomists now refer to hominins not hominids. smithsonianmag.com/science-nature/… $\endgroup$ – Ian Lewis Jan 15 '15 at 9:59
  • $\begingroup$ How much do you want the hominins (See Ian Lewis' comments), and how much the waves? If you are willing to drop the human-like bit, you may expand your possibilities for future waves vastly! If you insists on huminins, you need a still quiet "earth-like" earth, and for a very human-like civilisations, you really need to solve the fossil-fuel problem somehow. $\endgroup$ – Layna Jan 15 '15 at 11:20
  • $\begingroup$ These cycles remember me of The Mote on God's Eye (Niven/Pournell). There an entire civilization suffers the Cycles once and again. $\endgroup$ – Envite Jan 15 '15 at 11:42
  • $\begingroup$ @Layna, If you have an answer that would meet all the requirements listed above but do away with near-human (sub?)species, I'm more than willing to listen! $\endgroup$ – Serban Tanasa Jan 15 '15 at 20:00
  • $\begingroup$ Look at the moties in The Mote in God's Eye and its sequels. $\endgroup$ – JDługosz Jun 7 '16 at 7:22
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Name the one most important factor that would make this scenario implausible (say, hominids before this or that era were physically unable to speak), or if you prefer, more plausible (for instance the larger brains of Neanderthals).

Scientifically, the most implausible thing about this is contained in this statement: "Tens of thousands of years later, a different branch of hominids attains sapience, and the cycle repeats." The problem is that if you look at the Earth and the human race today, you will notice that there are no other hominids left.

That's almost certainly because we out-competed them (whether intentional or not). And there's no reason to think that whatever hominid won the arms-race the first time wouldn't have done the same thing, leaving no remaining hominids for us or any other "pulse" to evolve from. However, that's a scientific detail that the general public is almost entirely unaware of, so from a story/fiction standpoint, it might not be a big problem.

Do you think (m)any XXI-th century level artefacts could be preserved well enough to be detectable from 30-50k ago (especially given the dramatic changes in sea-shore levels in the past 50k)

Sure, just not the ones from the shores. If you look at humans today, we have virtually covered the planet to such an extent that I would say that the only way that a succeeding intelligent species would not be aware of us is if we went to great effort and expense to cover ourselves up from them. Really, we would have to scour the whole planet to have a good chance of preventing them from discovering our history.

One possible way around this might be to invoke some specialized technological disaster like a nanobot war that disassembled/disintegrated all humans and their technology. Personally, I don't think that's feasible, but lots of people do think that it is.

What would the one most important result of this type of 'pulsar' intelligence be in terms of pulse-driven changes across different intelligence bursts? I'm thinking for instance that the next post-human pulse for instance would have a lot less coal and oil. Perhaps they trigger (the end?) of an ice-age, or genetically modify their successor race before they leave.

Well, the energy thing is huge (and one of the best ways that we can be certain that their wasn't another intelligent species only 30-50,000 years ago). Discovering so much free energy lying around, easily accessible and exploitable has been a remarkable stroke of good fortune for us, akin to having our whole species win the Lotto. And now that we've used up so much of the easy/cheap stuff, any species becoming sapient after us will have a much tougher time of it.

Other non-trivial persistent changes that we are making:

  • Massive Climate Change:; melting most of the glaciers & ice caps. Though this could recover on its own in a few thousand years.

  • Space Debris: The low-orbit satellites will degrade and fall back to earth eventually. But the stuff in high-orbit (includes all geo-synchronous satellites) will likely still be there. Also, the Apollo landings and the stuff we left on Mars, could be discovered.

  • Radioactive Materials: It's not just oil and coal, we're using up most of the easily usable radioactive ores as well.

  • Species Extinction: We didn't stop with the Hominids, we are still killing off species at a faster rate than anytime in the last 60 million years.

  • Mineral Redistribution: Most of the worlds surface gold, silver, diamonds, etc. are no longer where nature put them. Instead they're either spread out across the whole world (like the rings and other jewelry so many people where), or else concentrated in huge concrete and steel vaults (like Fort Knox). Neither will make any sense to future geologists.

  • Sooooo Much Garbage!: We make a lot of it. Really an incredible amount. We try to bury, sink and hide it, and much of it will decay, but still, incredible barge loads of it will survive. And our successors will find these stunning huge landfills and dumps of unbelievable amounts of plastic, toxic chemicals and other slowly rotting stuff.

Really, I could go on and on here ...

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  • $\begingroup$ Great Reply! Thanks! But how would we be able to tell? Say there were immense amounts of easily accessed uranium lying about, heck, even natural reactors, and the previous set used it, leaving less valuable coal and oil in the ground. How would we know that didn't happen? We would just see lots of lead lying about. $\endgroup$ – Serban Tanasa Jan 14 '15 at 22:33
  • $\begingroup$ and a departing race could re-spawn the destroyed hominid pool, leaving just an odd gap in the fossil record... $\endgroup$ – Serban Tanasa Jan 14 '15 at 22:39
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    $\begingroup$ Really, hiding the fact that a race like us existed only 50K years ago would take technology, resources and effort that would exceed any other project that the human race has ever attempted by several orders of magnitude. $\endgroup$ – RBarryYoung Jan 14 '15 at 22:45
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    $\begingroup$ Major cities are another big one. Even if all the surface buildings collapse, a construct like Manhattan would be blatantly obvious to a geologist. Being so many times larger and made of tougher materials (concrete, steel) than the ancient cities that we think of as having disappeared over thousands of years. $\endgroup$ – Leushenko Jan 15 '15 at 10:41
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    $\begingroup$ Or at the small end of the scale, jewelry. Barring an incident that melts it down, the wedding band I am wearing right now will be recognizable as artificial in origin for many thousands of years, and unless looted, will endure until the sun swells to a red giant and swallows the Earth a few billion years from now. $\endgroup$ – EvilSnack Oct 16 '16 at 22:30
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You wrote

I'm thinking for instance that the next post-human pulse for instance would have a lot less coal and oil.

That's it. You hit the nail on the head.

Each re-generation is inheriting a completely different planet than the last. Some things that would change:

  • Fossil Fuels: As you said, they're going to be used up pretty quickly. I doubt that it's going to be easy for the cycle to continue just as before when less energy is available.
  • Radioactivity: If each generation wipes themselves out with nuclear weapons, there's going to be radiation around for the next. Over time, the planet will become extremely radioactive.
  • Evolution: Other species will evolve normally, and gradually change.

There are other factors that will change.

This would all be okay. The problem is, this means that the civilizations will really develop in different worlds. Not all will be helpful for this particular species. For example, humans couldn't have survived for long during the Cretaceous Period. Eventually, the next-in-line will be wiped out before they can attain intelligence.

I think that's the "one most important factor."

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1) What would keep the intelligent hominids from expanding? Excluding circumstances like a primitive people who is very good adapted to an extreme climate (v.g., Eskimos), our history always shows the most advanced groups trying to expand their territory. Think how little time it took Europeans to colonize over most of the world.

2) Plastic, glass and ceramics are really hard to decompose. Specially if some of it ends under ice or buried. And, about how easily they could be found, here they talk about some fossils with a size between 2 and 0.05 milimeters

3) Resource depletion (not only oil, but also metals and radiactive elements) would be an issue. That said, given that they are more intelligent than the average, maybe would not be that bad; they would focus more in biological sciences (including genetic engineering) to get the materials they need. That said, they would not leave much behind in the realm of altered beasts, because expansion of these lifeforms would be unlikely:

  • because, as "designed" animals or plants, they would probably not be very adapted to the wild (just as current day cows and pigs would not fare well alone).

  • and probably because, being biologist they would have realized the risks involved in introducing a new species if the previous point was proved wrong.

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1.

The most important factor would probably be the sheer number of factors making it implausible. There is biological continuity within hominids, Neanderthals and modern humans for example apparently cross-bred, and anomalous brain capacity would show in Africans being noticeably dumber than people from elsewhere. A global civilization would have transported domesticated species from their native continents. An industrial civilization would have left behind waste dumps and mine shafts, both obviously unnatural in an enduring way.

2.

Already mentioned waste dumps and mine shafts. Gold, glass, and ceramics protected from mechanical destruction could endure a very long time. Even if they are parts of highly advanced technology, such as optical fibers or gold alloy contacts. Same goes for gems and stones cut to unusual shapes. Though most materials that could survive long enough would be difficult to date and a single specimen would be ignored as modern contamination.

3.

Consequences are hard to predict. A mote in the god's eye is a novel with a cyclical civilization. As I recall the main difference was that each cycle built upon the remains of the previous ones and spent also resources preserving its science and culture to its successors. So the cultural outlook would be different.

A cyclical civilization where the cycles do not have continuity might be different. Or not. They'd still figure out they had predecessors and that they probably would have successors. Maybe they would be driven to find out where the precursors went. Precursors are very common in scifi.

There were also issues with resource depletion, but most resources are either renewable (solar energy, ecosystems) or reusable (metals), so this really only restricts the use of fossilized fuels.

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  • $\begingroup$ I like the reasons. A lot. But what if they were less war-like, strongly preferred a certain type of climate and so did not expand globally, and most traces of their existence are now submerged under the 200 m sea level rise of the past 30k years? $\endgroup$ – Serban Tanasa Jan 14 '15 at 19:36
  • $\begingroup$ @SerbanTanasa Higher technology is presumed to require advanced infrastructure and generally progress is related to level of specialization which is related to the volume of available resources. So a civilization driven to progress would be driven to expand its resource base. And often expanding your resources is what motivates progress. There are stories of civilizations that avoid this, but the explanations basically amount to "magic" or hand-waving. You can get away with it, if you wish. Others have before, but it is not realistic enough to really discuss in general, just in specific. $\endgroup$ – Ville Niemi Jan 14 '15 at 20:36
  • $\begingroup$ @SerbanTanasa I can certainly understand the attraction the sea level rise gives to this idea. After all the Bible specifically gives the start of civilization, Eden, a location that is currently at the bottom of the Persian Gulf, aka Gulf of Aden and created in huge flood relatively recently. And says it was destroyed in a huge flood... And the lands in the southern Arabia people used to get there from Africa are also submerged. So there is definite appeal to lost civilizations whose remains are hidden from us. But the tech level was probably closer to the Sumerians. $\endgroup$ – Ville Niemi Jan 14 '15 at 20:42
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There are a lot of good counter-arguments here, but plotwise it might work having the waves separated by ice ages.

  • Each ice age wipes out a lot of the evidence for the previous civilization, except in some tropical regions, which will be lush rainforests during the interglacials.

  • The ice-free regions are a reservoir of hominids, from which new civilizations emerge. The end of the ice-age may see land-bridges which will be submerged later, making for easy migration to other continents. The early farmers are then cut off from the cradle when the water rises

  • A civilization may find remains of the previous civilization when exploring the rainforest.

  • You can construct an evolutionary mechanism which drive the hominids into intelligence at each ice age. The standard example is a receding forest which makes the hominids move out into open grassland, with the effect that has on posture and free hands.

  • Ice ages expose new mineral resources, scour away pollution and replenish farmland, though fossil fuels would presumably not be available.

Story-wise, I think it's beautiful to imagine the planet's quiet solitude during an ice age. The waves of ice ages and waves of hominins also parallell each other nicely. There's a story in there about a hominin wave which tries to rebel against the natural order by stopping the coming ice age.

Or perhaps more elegantly, that the technological development of each hominin wave is in fact behind the ice ages. This makes for a new question, but possible mechanisms include sequestration of carbon in some carbon-based technology (they build houses out of graphene, massiv structures out of artificial diamonds, etc) or shielding of the sun's rays when the civilization starts employing solar panels in space to cover their energy needs.

You can also skip evolution completely, by postulating that the cradle of hominins is a small continent or archipelago where fully sentient stone-age people can live indefinitely, but where there are no opportunities for a civilization to arise. With some exceptions, complex human civilizations have evolved in fertile areas along rivers, where large-scale agriculture was possible. As an example, humans arrived in Australia in 40.000-80.000 BC, and while having a rich culture, never developed what we think of as a classical civilization.

With the hominids emigrating into space, the ice age wrecks the civilization of whoever choose to stay behind. In the only place left to live, the cradle of their civilization, they revert to a stone-age culture. And then the cycle repeats.

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As an addition to SJuan76's answer, whose train of thought took the same direction as mine:

Consider the environments in which animals and at least bacteria thrive: deep seas are actually easy conditions compared to Extremophiles (wiki-link) live. So, a post-human species which has to focus on biological engineering rather than any fossil-fuel based workings could adapt those for all kinds of needs.

When considering such a base for a new civilisation, also consider that they may be more suited to think in large time-spans! Going to Space may be harder for them, BUT they may in turn have the long view to seriously, and very patiently, attempt anyway.

Now, for the question of hominin or not: WHY hominin? A deep sea species may survive a cataclysmic change on the surface, and happen to be the next dominant species. Think: Squids! They ALREADY seem to have a good sense for problem-solving, and if they reach deep sea levels, smaller groups have VERY cheap energy via volcanism (Black/White smokers).

Another possibility is birds. Some species are tool-users and problem-solvers, and even seem to have a cultural background in how they build their tools (I lost the reference here, sorry... it was on TV recently)

Lastly, consider hives developing into something smarter. I admit I have no idea how realistic that would be (seeing an already human-style intelligent species evolve into civilisation is a lot easier than figuring out a hive civilisation...), but in regard to that, one could perhaps even think about a hive that manages to get off planet by some means because the species is driven by slow destruction of "their" planet:global warming/cooling, major tectonic changes, and similar things. Such a hive may actually be totally unaware of past civilisations as in 2!

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