Let's say that humans actually originate from another planet. An extinction level event happened on their home planet, and a handful of them escaped, wandering through space looking for a planet that they could live on. About 100,000 years ago, they landed here and built cities in central Africa. Soon after, a plague wiped out most of the population. Those that remained survived in small nomadic bands. Very slowly, they rediscovered agriculture and developed improved technology once again, which lead them to today.

Would there be any evidence that modern humans could look back on, that would "prove" such a society existed? At what point in history would scientists determine this? Would we simply never know? Assume the past civilization had technology at least on par with 2024, with a population of about 5 million.

  • $\begingroup$ Comments have been moved to chat; please do not continue the discussion here. Before posting a comment below this one, please review the purposes of comments. Comments that do not request clarification or suggest improvements usually belong as an answer, on Worldbuilding Meta, or in Worldbuilding Chat. Comments continuing discussion may be removed. $\endgroup$
    – L.Dutch
    Commented Jun 16 at 3:35

8 Answers 8


We would definitely know it, for a simple reason: incongruency of the fossils.

Humans evolved around 2.5 million years ago, while anatomically modern (which is not to say identical to us!) humans evolved 300 000 years ago. Around 40 000 years ago, Neanderthals roamed the Earth.

If somehow, 100 000 years ago a whoopin' 5 million entirely modern humans appeared on Earth, and promptly died en masse, the fossil record would make no sense. There would be a sudden, inexplicable mass of modern human fossils dwarfing all other hominid fossils in numbers. Worse still, it would sit in the middle of the cradle of humanity, with Space Human, Early Human, and Habilis corpses side by side. It would make paleontologists immediately raise all kinds of noise, as this is blatantly impossible without a divine intervention, time travel or your space-human scenario.

Gets worse actually.

A civilization with 2024-ish level of technology would create concrete, steel, glass, ceramics, and plastics in horrific quantities. They would mine rock, dig foundations, tile fields, and regulate rivers. They would clear forests, kill millions of animals and raise others as livestock. They would plant crops. All of this would leave obvious evidence for archaeologists to find.

An archaeologist randomly finding a shard of pottery in a 100 000 year old soil layer would chalk that up to weird coincidence. An archaeologist finding tons of glass, concrete, ceramics, metals, etc. over miles upon miles of an artificially leveled 100k year-old layer of soil would promptly orgasm, faint, then wake up, get dangerously drunk, then call the guys who made the Ancient Aliens TV show. (Trust me, I'm an archaeologist by trade, this would happen). Right after, everybody and their uncle would be digging through African soil to unearth the Prehistoric Civilization.

It gets worse, yet.

IF there was an space human civilization that came to Earth, was hit by a plague and then wandered off from their cities to escape it, they would bring the same plague to every corner of Africa, thus wiping out the rest of Africans. If the real Africans were immune to this plague, then this means the Alien Humans and Earth humans have different immunities: which means alien diseases would wipe Africans out instead. One way or another, humanity is boned, or at the very least, Africans of all species are.

As weird as it sounds, it gets even worse than that.

Assuming hominids were not wiped out by Space diseases, or the plague that destroyed the Space Human civilization, what happens now? The fundamental question is, can Space Humans boink Early Humans? If so, we kinda get a non-time travel version of the Grandfather Paradox: we would get completely modern human genetics in our ancestral tree 100 000 years ahead of time. This essentially wipes out homo habilis of the map, prevents Neanderthals and Denisovians from even existing, while inexplicably giving us alien copies of their genes much deeper in our past. This way, actual modern humans of 2024 would be completely different from the ones from normal timeline, because we would be completely genetically modern 100 000 years ahead of time, and then evolve from that. You can either have completely modern Space Humans, or completely Space-ish Modern Humans, but not both, because that would require the evolution to either stop or be circular.

So, these are the problems you would need to solve, or handwave, for the setting to make sense.

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    $\begingroup$ 5 million humans might sound like a lot, but is less than the population of many major cities, and as an archeologist you would know how rare actual fossils are. Your answer raises good points, but I think you have overestimated the global impact of a population that may have actually been confined to a relatively small geographical area. $\endgroup$ Commented Jun 13 at 17:27
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    $\begingroup$ 5 million humans isn't much today but 70k years ago we may have been down to just over a thousand npr.org/sections/krulwich/2012/10/22/163397584/… $\endgroup$
    – SPavel
    Commented Jun 13 at 17:56
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    $\begingroup$ Also, @MichaelHall - given that we have found hominid fossils from around the bottleneck SPavel mentioned, not finding any remains from five million humans when there were supposedly no humans seems wildly unlikely. $\endgroup$
    – jdunlop
    Commented Jun 13 at 19:57
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    $\begingroup$ @MichaelHall: The bigger problem is more of their distribution - 5 million humans in the space of a single person's travelling distance in fossils would be pretty unheard of - even in the medieval era in the 14th Century right before the Bubonic Plague, big cities in Europe had ~100K people in that city, max. By medieval era tech, this is...50 Venices' worth of cities i(Or 25 Medieval London's worth of cities) n Central Africa, and some probably aren't going to be port cities. $\endgroup$ Commented Jun 13 at 21:40
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    $\begingroup$ Not that it helps the overall picture a whole lot, but the Space Diseases element at least could be explained away by that a civilization advanced enough to ply the stars also were advanced enough to eliminate all pathogens (at least within the relatively small and highly controlled confines of their colony ship) such that they simply did not bring any Space Diseases with them. $\endgroup$
    – Gene
    Commented Jun 14 at 6:38


  1. We use the exact, same DNA code as every other critter on this Earth, except for some minor variants
  2. We can measure the divergence between Homo sapiens and the other Hominidae

If the hypothesis of your story were correct, you would need to explain:

  1. The apparent universality of the genetic code: e.g. is it necessary that GAA and GAG both specify glutamic acid?
  2. Why do we (H. sapiens) fit so neatly into the Tree of Life?

(I see that @going-durden has already touched on this: could the aliens have bonked early Homininae, and produced viable offspring?)

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    $\begingroup$ Exactly this. Modern science doesn't know every detail, but people underestimate how thoroughly we understand the interconnected origins of cellular life forms. Even if you did have non-terrestrial humanoids whose DNA somehow "worked the same way," their genomic history would be completely different. In other words, everything that evolved from our precursors shares some genetic clutter from those forms (and their viruses); "same but different" humans from space would essentially have a completely different genetic signature from other primates. $\endgroup$ Commented Jun 14 at 15:31
  • $\begingroup$ Yeah, there's nothing universal about the sequences that identify individual genes, the ribosomes that interpret the genetic code and assemble proteins, or about the function of those proteins. The lower level functionality is so similar among all life on Earth simply because any major changes are likely to be lethal, not because it has to be that way. Even if convergent evolution made an alien superficially identical, they'd be glaringly obvious at the genetic/biochemical level. $\endgroup$ Commented Jun 15 at 13:44
  • $\begingroup$ @ChristopherJamesHuff You are probably right: the code has a rather arbitrary look to it. I'd still like to see a lifeform from Enceladus or somewhere, though: 1 is a rather small sample size. $\endgroup$ Commented Jun 15 at 19:13

Yes, if we are talking about modern technology levels 100,000 years ago.

If we're talking any archaeological evidence at all, then we have found evidence of tool use over 3 million years ago. The Paleolithic Period lasted 3.3 Million to ~11,000 years ago. In that time, we have evidence of multiple species of hominid, enough to grasp our evolutionary history as a species, and have evidence of rock art, tools, jewelry, early textiles, and early domestication.

The only reason more hasn't survived is because early humans didn't build things to last that long. The Pyramids of Giza have survived 5000 years, and I'd wager that after another 95,000, even if they were rubble, we'd still know something was there. And they were made of simple stone and mortar.

Now we have concrete and steel, knowledge of electricity, industrialisation, and more. And, most notably, synthetic chemicals, which would be impossible to attribute to natural processes. While plastic may degrade over 1000 years, we still mine and burn through billions of tons of oil and petrochemicals a year. That leaves a geological mark on the very rock itself. The level of global industrialization that we see today is actually quite incredible. If these proto-humans came from space, it's possible they have even more durable materials and we do.

Even if the only surviving humans needed to become nomadic, they would still have knowledge of the old civilisation around them, and that history would potentially be passed down over the millennia, perhaps becoming little more than myth and legend, but enough to make modern scientists curious.

As a fun bit of reference: It is thought that Indigenous Australians reached the mainland roughly 50-60,000 years ago, while the land-bridge between Australia and Indonesia made short rafting journeys possible. 10,000 years ago, the Pleistocene ended, and the sea levels rose. This is responsible for the large eastern shelf of Australia that contains the Great Barrier Reef. The Aboriginal settlements along the eastern coast have stories detailing how this occurred, and it mirrors and confirms the geological theories behind the Reef's formation.

All in all, 100,000 years, or even a million years, would likely still have traces of modern civilisation. It would likely take a very large catastrophe to erase or overshadow all remnants of human history, and that would still take millions if not tens of millions of years. A big asteroid ought to do it (but, then again, we still know about the dinosaurs).

Also for a bit of bonus biology, if humans arrived 100,000 years ago from another planet, our biology would have definite signs of alternate origin, even if we had integrated and were fully compatible with Earth's biosphere, fitting homo sapiens into a taxonomic family would become a headache and that would also likely give the game up.

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    $\begingroup$ The bit about the aboriginal Australians raises another issue: humans will spread into new territories even if they have to set out across the sea in a crude raft to reach them. If refugees from some catastrophe were able to reach Earth, why hadn't it already been reached by actual colonists? What about all the other planets and systems they colonized? $\endgroup$ Commented Jun 13 at 11:50
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    $\begingroup$ @ChristopherJamesHuff Wellll.... Perhaps the humans who colonized Earth were the very fringes of this diaspora. Perhaps the civilization that spawned them was not as adventurous as terrestial humans. Perhaps many things. I'd give the OP that one. $\endgroup$
    – Jay
    Commented Jun 13 at 15:43
  • $\begingroup$ The Pyramids have been looted, not just of grave-goods, but also the original white limestone cladding material. This highlights another problem: the biggest damage will come from people. Once civilization breaks down, people will loot anything that isn't nailed down. I imagine Kalashnikovs will be valuable, but what happens when the hoarded ammo runs out? I imagine the metal will be useful... $\endgroup$ Commented Jun 14 at 1:29
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    $\begingroup$ Assuming materials are more durable simply because they were produced by modern tech is a pretty bad assumption. Modern concrete is less durable than Roman concrete, especially when exposed to water regularly (we only recently rediscovered how they made it); modern concrete lasts just decades, some Roman concrete structures have lasted over two thousand years. Pyramids are famously the most durable thing humanity has ever built; nothing built today is as durable as large stone blocks piled closely on top of each other. $\endgroup$ Commented Jun 14 at 15:54
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    $\begingroup$ And the Egyptian pyramids benefit from being in an area with little to no plant life to break apart the rocks, and little to no water to erode them. Similar pyramids built in the Yucatan jungle were only recently discovered thanks to satellite imagery and LIDAR, and they've fallen apart and been overtaken by jungle in less than 2000 years; 100K years of deterioration of less durable construction in the African jungle would be far more thorough in obliterating traces. $\endgroup$ Commented Jun 14 at 15:57

I have serious doubts on the feasibility of

the past civilization had technology at least on par with 2024, with a population of about 5 million.

That apart, a technology on the same level as our would leave a clear trace in the geological records. Just to name a few fingerprints that we are leaving as of today:

  • by burning fossil fuels and by exploding nukes in the atmosphere we are altering the ratio of $C^{14}/C^{12}$ in the atmosphere and in all the living organisms taking carbon from the atmosphere. This fingerprint will fade in few thousands years, though.
  • by mining nuclear fuel and dumping nuclear waste we are altering the isotopic ratio in rocks
  • by smelting ores we are altering the distribution of those ores and their impurities in the resulting waste material. For example by purifying copper ore we take away the copper and concentrate arsenic in the waste.
  • by building dams we are altering the deposition patterns of big rivers, reducing the amount of silt reaching the oceans and depositing there.
  • by using catalytic converters in our cars, we are increasing the amount of rare earths in the dust deposit produced by our cities

each of these fingerprints alone might have other plausible explanation, but taken all together they would strongly point toward some sort of civilization.

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    $\begingroup$ RE C-14 and other isotope issues: This all assumes that you know what the "right" ratio is. But how would you know that if the only sample you have to look at, Earth, was contaminated? If before the aliens arrived the C14/C12 ratio in the general environment was, say, 200 to 1, and today it is about 100 to 1, how would you know that the "normal" ratio was 200 to 1? Every experiment you did would say its 100 to 1, ergo, that's normal. You would, of course, find samples with higher ratios, but this would just lead you to mis-date them. Indeed a classic problem with radiometric dating is that ... $\endgroup$
    – Jay
    Commented Jun 13 at 15:49
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    $\begingroup$ ... you must assume an initial ratio between the isotopes, when no one was there millions of years ago to measure it. $\endgroup$
    – Jay
    Commented Jun 13 at 15:50
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    $\begingroup$ @Jay we have millions of samples trapped in ice going back long before that time. $\endgroup$
    – John
    Commented Jun 13 at 20:56
  • $\begingroup$ "Over the last glacial period, atmospheric 14C/12C ranges from values similar to modern values to values 1.70 times higher (42,000 to 39,000 years ago). " (science.org/doi/10.1126/science.aau0747) It's not like there's a constant baseline going back millions of years. $\endgroup$
    – Jay
    Commented Jun 15 at 1:50
  • $\begingroup$ Two words: plastics and ceramics $\endgroup$
    – SJuan76
    Commented Jun 15 at 12:10

Here is a frame challenge.

You write:

Let's say that humans actually originate from another planet. An extinction level event happened on their home planet, and a handful of them escaped, wandering through space looking for a planet that they could live on. About 100,000 years ago, they landed here and built cities in central Africa. Soon after, a plague wiped out most of the population. Those that remained survived in small nomadic bands. Very slowly, they rediscovered agriculture and developed improved technology once again, which lead them to today.

And that really annoys me.

The idea that a species would become advanced enough for interstellar travel but wouldn't explore the stars to advance science and to find planets habitable for them and settle them, but would remain restricted to their original homeworld until disaster theatened it, has always annoyed me.

A long time ago when I was young, I sometimes read old "golden age" science fiction stories and novels much older than I was, stories written 80, 90, and 100, years age. And some of those had a plot where some alien species became as advanced as Earth, and the natives of that planet continued to live on that one planet while their technology continued to advance for tens of thousands, or hundreds of thousands, or even millions, of years.

And they would have continued to live on their orginal planet, and only on it, forever, until some disaster threatened their world. Perhaps their star was dying, or perhaps their star was going to explode. Maybe a giant asteroid or a rogue planet from outer space was on a collision course with their home world.

And then, and only then, when it is almost certainly too late, do they scramble to invent and build interstellar space ships to take all - or more often a tiny fraction of - their population on voyages to find another habitable planet to settle on and save their species from extinction. And naturally the first habitable planet they discover is Earth, and so they try to conquer Earth from humans.

And if they had been exploring interstellar space for countless thousands of years and discovered many habitable planets and established colonies on them their species would not be in danger of extinction and they they would have the lesser problem of building enough space ships to take their entire population and distribute them amoung the colony planets. And after many thousands of years of interstellar colonization, they could have thousands of colony planets each with approximately the same population as the home planet, and so distributing their population among all the colonies would not strain the resources of any colony.

I think that a lot of other people who read "golden age" science fiction stories felt the same way about those plots. Thus there have been suggestions that humans need to move many people off of Earth as soon as possible, to artificial space habitats or Mars, to insure against the extinction of humanity if something happens to Earth.

So I would suggest an alternative story. 100,000 years ago, an advanced world was exploring the stars and establising colonies in other star systems. One of those was Earth. But then something destroyed civilization on the home planet and all the colonies. Maybe none of the colonies had enough people or machines to sustain civilization without contact with the home planet, and so once civilization fell on the home planet, it fell on each of the colonies.

But some of the colonists survived as hunter-gatherers on Earth, and maybe on some of the other colonies or the home world. And eventually the humans on Earth built a new civilization.

So that is a better plot than the idea that the home planet had not be exploring and colonizing space for centuries or millennia before the disaster to the homeworld.

Of course, you would then have the problem of all the other objections which other people have made to your story idea.

Maybe you should decide where you want your story to be on the sliding scale of science fiction hardness.


If you care about getting a higher score on the scale you should try to think of ways to answer those objections.

Maybe there wasn't a large civilization on Earth. Maybe people came to Earth to live the lives of their hunter-gatherer ancestors for a period, as a break from civilized life. and so there was little civilized infrastructure bulit on Earth for the "vacationers". Bu tof course somebody could still discover the ruins of the spaceport, for example.

Perhaps something like the events in Heinlein's Tunnel in the Sky (1955) happened. I note that it has been suggested that the Green Children of Woolpit were being teleported between star systems when an accident sent them to Earth instead.

You might want to make the date of the stranding on Earth happen much earlier, to account for stone tools and skeletons of older date. Or you might want to put the date of the stranding much later, at the time of a prehistoric bottleneck in the prehuman or human population.

According to a 1999 model, a severe population bottleneck, or more specifically a full-fledged speciation, occurred among a group of Australopithecina as they transitioned into the species known as Homo erectus two million years ago. It is believed that additional bottlenecks must have occurred since Homo erectus started walking the Earth, but current archaeological, paleontological, and genetic data are inadequate to give much reliable information about such conjectured bottlenecks.[8] Nonetheless, a 2023 genetic analysis discerned such a human ancestor population bottleneck of a possible 100,000 to 1000 individuals "around 930,000 and 813,000 years ago [which] lasted for about 117,000 years and brought human ancestors close to extinction."[9][10]

A 2005 study from Rutgers University theorized that the pre-1492 native populations of the Americas are the descendants of only 70 individuals who crossed the land bridge between Asia and North America.[11]

The Neolithic Y-chromosome bottleneck refers to a period around 5000 BC where the diversity in the male y-chromosome dropped precipitously, to a level equivalent to reproduction occurring with a ratio between men and women of 1:17.[12] Discovered in 2015[13] the research suggests that the reason for the bottleneck was not a reduction in the number of males, but a drastic decrease in the percentage of males with reproductive success.

The controversial Toba catastrophe theory, presented in the late 1990s to early 2000s, suggested that a bottleneck of the human population occurred approximately 75,000 years ago, proposing that the human population was reduced to perhaps 10,000–30,000 individuals[14] when the Toba supervolcano in Indonesia erupted and triggered a major environmental change. Parallel bottlenecks were proposed to exist among chimpanzees, gorillas, rhesus macaques, orangutans and tigers.[15] The hypothesis was based on geological evidence of sudden climate change and on coalescence evidence of some genes (including mitochondrial DNA, Y-chromosome DNA and some nuclear genes)[16] and the relatively low level of genetic variation in humans.[14]

However, subsequent research, especially in the 2010s, appeared to refute both the climate argument and the genetic argument. Recent research shows the extent of climate change was much smaller than believed by proponents of the theory.[17]

In 2000, a Molecular Biology and Evolution paper suggested a transplanting model or a 'long bottleneck' to account for the limited genetic variation, rather than a catastrophic environmental change.[8] This would be consistent with suggestions that in sub-Saharan Africa numbers could have dropped at times as low as 2,000, for perhaps as long as 100,000 years, before numbers began to expand again in the Late Stone Age.[18]


So maybe all humans descend from a few hundred or a few thousand individuals stranded on Earth when their home planet's civilization fell, and there were no humans on Earth before the population "bottleneck" you select.

As for the morphological and genetic similarities between humans and other animals on Earth, maybe super advanced aliens have been transplanting life forms between Earth and many other planets for billions of years. So instead of all those planets having plants and animals which evolved separately on each planet, they would have the same plants and animals. And those transplanted animals might include humans. Maybe prehuman and human groups and individuals have been transferred between Earth and other panets many times.

So it is possible that there humans on Earth before those human colonists from another world were stranded on Earth, and that the stranded colonists intermarried with the humans already on Earth. Possibly the stranded colonists were Neanderthals or Denisovians.


We would find a genetic bottleneck https://en.wikipedia.org/wiki/Population_bottleneck as the human population dropped by a colony ship would be rather small and limited in diversity.

And 10000 years would be short enough, so that the past would appear in the myths and tales told from father to son.

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    $\begingroup$ Here in Aotearoa New Zealand, the Maori people have traditions about their ancestors migrating here from somewhere called Hawaiki. If you trace those stories back to the presumed origin of our Maori people, you will find legends in other islands about Hawaiki. It seems that all Polynesian people share this story: they all came from somewhere else... $\endgroup$ Commented Jun 13 at 22:02
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    $\begingroup$ @SimonCrase Well, they all did, right? And not all that long ago. $\endgroup$ Commented Jun 14 at 4:31
  • $\begingroup$ The OP said 100,000 years, not 10,000. Even assuming oral traditions could last 10,000 years is pushing things. We don't have a single verifiable story from an oral tradition that predates the written word, the best we have are stories like "The lake in the Great Dismal Swamp (formed 6K-10K years ago) was made by the Firebird", and a working theory that maybe it was formed by an underground peat fire or meteor, not nearly enough to go on. On top of that, humanity had multiple population bottlenecks over the last 100K yeary, far narrower than 5M people. $\endgroup$ Commented Jun 14 at 16:03
  • $\begingroup$ @ShadowRanger " We don't have a single verifiable story ...": maybe you don't, but there are verifiable stories. David Lewis was able to recreate long distance Polynesian voyages using navigators who could interpret the ancient accounts. Admittedly we are talking about stories that are closer to 1,000 years than 10,000. $\endgroup$ Commented Jun 14 at 20:28

If you want a secret society of human like aliens living amongst us - it's well with the realms of sufficiently believable for a piece of fiction. However you'd need a few things to make it work.

First the aliens beamed (ala star trek) down to earth or otherwise got down with minimal equipment that could later be discovered*, and the ship was some how damaged so they couldn't return. Maybe it has crashed into Jupiter or the Sun as otherwise there would be evidence of it that we could probably notice soon if not already.

Second assume most of the survivors weren't expecting to bootstrap a civilization - most modern people couldn't really tell you how to make a cell-phone or computer or smelt metal. Without high tech equipment hunter/gatherer would be the only real option, and if much of it was lost deep in the ocean during the crash (or wasn't brought down because it was an emergency beam) there'd be little evidence for us to find.

Third, even if the aliens and humans were both "seeded" from the same original life stock billions of years ago, they'd almost certainly be unable to interbreed - it's not entirely impossible that they could look close enough to humans to pass (see Carcinisation ) but highly unlikely they could do more than have recreational sex.

This could plausibly leave you with a secret society of aliens living amongst humans, most likely they'd have forgotten the true purpose of many of their "artifacts" though their religion might have outward facing simulacra of them (Teffilin** for example) with only 1 or 2 "real" versions kept in a vault by the leaders.

*If the "landing craft" sunk deep in the ocean, and whatever life-raft they had also washed out to sea both could easily have sunk deep in the ocean where chances of finding them are slim to nil.

**Just to be clear I'm not trying to put down any religious practices, there are many other things in many religions that are 'weird' for non-members which could theoretically be descended from a "sufficiently advanced technology" but now is basically just a cargo cult imitation.


Scientifically we would be able to tell, but in world building terms that’s irrelevant.

It was only 50k years, but MUTINEERS' MOON by David Weber deals with this in a single paragraph:

"Correct," Dahak said unemotionaly. "Following the First Imperium's fall , one of its unidentified non-human successor imperia re-seeded many worlds the Achuultani struck. Earth was one such planet. So also was Mycos, the true homeworld of the human race and the capital of the Second Imperium until its destruction some seventy-one thousand years ago. The same ancestral fauna were used to re-seed all Earth-type planets. Earth's Neanderthals were thus not ancestors of your own race but rather very distant cousins. They did not, I regret to say, farewell against Dahak's crew and its descendants."

In reality this makes no sense at all, we can trace our lineage back to before the dinosaurs. At the very least it would need to be closer to the Known Space where the Thrint basically seeded all planets with their algae, although even that would be insufficient.

But again, for world building purposes it doesn’t need to be realistic, it needs to be enough so that you can get on with the rest of your story. In fact the more details you give, the more likely you are to seem unrealistic. Don’t make the reader evaluate how reasonable it is, assert it so and then move on.


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