This idea comes up quite frequently in science fiction, but there has not been a question about it yet on Worldbuilding.SE. Is it possible that intelligent life was brought to Earth by an alien civilization?

Would it be possible for these precursors to seed intelligent life here on Earth without leaving a trace, either archeologically or scientifically? Or put another way, how long ago would this have had to happen for us to not find evidence of it? By intelligent life, I mean that either the aliens directly created the intelligent life, or put in place a system designed to cause intelligent life to form.

  • $\begingroup$ Two separate questions...seed with life (can almost be considered long long term terraforming) vs seed with intelligent life. What are we answering here? $\endgroup$
    – Twelfth
    Commented Oct 30, 2014 at 21:10
  • $\begingroup$ Lets say intelligent life. If it is just seeding with life, that life must eventually become intelligent life (by design). $\endgroup$
    – bazola
    Commented Oct 30, 2014 at 21:37
  • $\begingroup$ Heh, I had an answer for non-intelligent life (an asteroid impacting a planet with life and sending pieces of it everywhere could seed life on thousands of planets...sorta spore style, unintentional in this example, but could be intentional. Didn't 'bugs' in starship troopers through spore asteroids for seeding?). Intelligent life is a much different matter $\endgroup$
    – Twelfth
    Commented Oct 30, 2014 at 21:40
  • $\begingroup$ I'm going to throw my vote in here and suggest that as it stands this is off topic. I think it's more philosophical than world builder based. I would like to see the question re-targeted to say "Could aliens seed life on a planet without leaving traces?" rather than "Could this have happened on Earth?" but without that change I think it's a discussion about our pre pre pre history... $\endgroup$
    – Liath
    Commented Oct 31, 2014 at 11:21
  • $\begingroup$ I wouldn't mind a title change. What is the off topic reason here? I don't think it is off topic at all personally. Many science fiction worlds include the concept of precursors seeding life on Earth. $\endgroup$
    – bazola
    Commented Oct 31, 2014 at 11:28

5 Answers 5


What you can do if you want no traces is to seed planet by microbes, and patiently wait few billion years as the complexity of the life increases. You come over every few dozen million years to see how life is doing and if it is evolving right.

If you get into some unexpected problems like gigantic dinosaurs ruling the planet so (supposedly smarter) mammals are surviving in small niches, you can orchestrate meteor bombardment to force rapid climatic changes, hard to survive for species overspecialized to be big and ugly. Smarter species will survive. Such bombardment leaves no traces of extra-terrestrial intelligence involvement.

Ice ages are another way to introduce changes, which give advantage to species better able to adapt. No traces on the planet if you can park your planet-shading spaceships somewhere away, like in Oort cloud (or different star system).

Why to do it? maybe for fun, or for PhD level terraforming degree? To see what other kinds of intelligence can evolve?

If you read about forming the Moon by impacting old Earth, in just right time (for Earth life), with rock of just right size and just right material in just right angle, you may start wondering... :-)

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    $\begingroup$ Read "Calculating God" by Robert J. Sawyer. Alien and human paleontologists team up to figure out why each of their worlds has an essentially identical schedule of mass extinctions. $\endgroup$ Commented Oct 16, 2015 at 14:25
  • $\begingroup$ I already did, and that was one of the inspiration for my answer :-) $\endgroup$ Commented Oct 16, 2015 at 14:56
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    $\begingroup$ for PhD level terraforming degree haha, nice one. :) $\endgroup$ Commented Jul 22, 2016 at 18:59

Seeding a planet with microbial life is easy. In fact, that's one of the standing theories for how life on Earth originated, a bit of space debris from some other life-bearing planet came near the Earth and some microbes migrated over and spread. We probably have the technology required to do so to other planets right now, though finding planets with appropriate conditions for their survival is harder. (Not to mention how absurdly long it would take for anything interesting to happen as a result.) Just put some hardy microbial life in a well-shielded, life-supported rocket and point it at a likely planet, more or less.

Planting intelligent life is much harder, at least if you don't want them to find out about it in short order. The problem here is the same problem faced by creationists in the real world; humans fit absolutely perfectly into the geological history. We know precisely where our species came from, where the species that our species came from came from and so on very nearly all the way to the base of the animal family tree. The only way aliens could have planted intelligent life is if they were capable of somehow falsifying our planet's entire geological record while also transporting every species the planet needs here or if they were capable of such fine and subtle genetic engineering that they could make humans fit in perfectly with the existing life with no telltale deviations. It would be easier to say that they came here, found the apes and "uplifted" some of them to sentience, though again you'd run into the problem of telltale deviations. It is believed that part of the reason humans have such large brains is because we possess a defective gene which, in other apes, causes the development of large and powerful jaw muscles and that lacking it gave our brains room to expand. The aliens could have been the instigators of that. That being said, the shear amount of observational data and computational power needed to determine the effect of that minor mutation would have been enormous.

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    $\begingroup$ There's one point where an alien seeding might have escaped notice. The Cambrian radiation, when all sorts of complex life forms that had never been seen before evolved and the rate of evolution greatly accelerated. Or, when native Earth life got eaten by the new arrivals? It Was About 600m Years ago, though. $\endgroup$
    – nigel222
    Commented Jan 12, 2016 at 21:06

If you want to place intelligent life on a planet with an existing biosphere, perhaps the most feasible way would be to manipulate the evolution (or genome, for a more direct approach) of an existing species on that world. We know what animals we're related to, we've pieced together a family tree, and we know roughly when and where we've evolved. If we had come out of nowhere we'd have noticed a good while ago.

However, if they had picked a species of ape and manipulated it into us, depending on just how it was done, I'm not sure we could tell. There doesn't seem to be a terribly good reason for why putting apes on savanna would make them hugely intelligent. It looks like a freak occurrence. Intelligence on our level has not happened once before in the past several hundred million years the Earth has possessed complex life.

I don't intend to say that there's any reason to believe aliens were involved in our evolution in reality. There's nothing infeasible about our evolutionary history, and you can posit a lot of possible mechanisms for why we turned out the way we did. But my impression is it's probably a highly complex perfect storm of selection pressures, features specific to the species we evolved from, etc. We can suggest a great number of mechanisms by which it could have happened, but it's difficult to find out the actual truth so that we could point to a particular thing and say 'that's why we became intelligent'. As long as those factors are shrouded in mystery, it should be easy enough to justify alien influence for a story.


To answer the question directly; I don't think it is possible "intelligence" was "brought" by anything; as an earlier answer details, the evidence is overwhelming that intelligence arose slowly over hundreds of millions of years. Our neurons are not functionally different than the million found in a fly or cockroach; we just have a hundred thousand times as many, far more complexly organized, and even then not very much differently than neurons in great apes. Mice and Corvids are "intelligent" and creative problem solvers.

A working definition of intelligence will help us: Not all scientists agree, but a pretty good working definition for AI researchers (including me) is that intelligence is the ability to form models based on observations that let us predict most likely states in the future or past. The more accurate and far reaching these predictions are, the greater the intelligence. Animals with less intelligence can be trapped by animals with greater intelligence; the fish doesn't realize what the hook shape will do in its mouth until it is too late; the human fashioning the hook shape does so with a model of how fish bite and behave, and a model of how the hook (like an arrow) can enter flesh easily in one direction but be difficult to pull out in the other.

In the "past" direction; we have forensics, or something like astronomy or geology: Intelligence, in the form of working models, lets us narrow down what must have happened, to either a singular event (e.g. a neutron star exploded) or at least a small set of possible explanations (e.g. Adam either killed himself or his wife murdered him and made it look like suicide).

Either way; consider it an ability to model interactions, predict the outcome of various interventions, etc. Then super-intelligent aliens may have supervised evolution and intentionally caused mutations they could predict would be key to developing future intelligence. That isn't "bringing" intelligence, more like "creating" it, or the conditions for it to arise.

The difference between causing seeds to be scattered on fertile ground, versus planting already sprouted seedlings.

To our knowledge there is some chaos theory that suggests very far reaching intelligence is not possible; there are too many confounding factors. So likely, if guided evolution is how we came to be sufficiently intelligent to create a civilization (however one might define that, but you know what I mean), then it could be "without a trace", because we have no idea where mutations come from. We think they are chance; but for a super-intelligent race, they may have chosen to mutate genes in ways that were specifically plausible but hadn't happened yet. Or they may have, opportunistically again, arranged certain "accidental deaths" to prune the tree into a shape they wanted.

I'd call that topiary: Some bushes sprout in many directions; but if you want the bush to look like a duck, you can choose to clip branching sprouts that won't fit into the overall pattern you see in your mind's eye. Darwin's unnatural selection model for livestock did exactly this: The mutations in the livestock were natural, but the human livestock managers terminated undesirable mutations, or at least did not let them mate, while desirable random mutations were mated extensively. Aliens supervising human evolution could have done something similar; with greater foresight into the ramifications of each mutation, both physically and socially.


Having trouble understanding your question. Define "intelligent life". Since the experts can't define what intelligence is, I'm doubting you'll have any greater success. (And I'm avoiding altogether the definition of "life"...) Intelligence isn't a "yes/no" property. Tool use dates back (in our genetic lineage) to somewhere between 2½ and 4 million years. So, sure, little green men could have come down and tinkered with our genes. Probably could still be doing it, although the vector(s) they'd be using would have to be more sophisticated. I don't understand why you think such interference/modification would necessarily leave traces? It wouldn't have to.


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