Ancient Aliens???

NASA recently released a free ebook called Archaeology, Anthropology and Interstellar Communication only to remove it because of an errant reference to the possibility that Earth was visited by aliens in ancient times.

Picture credit:

Picture credit: Aboriginal Art

So what’s the deal?

Could aliens have visited Earth in the remote past?

Theoretically, yes, but the problem is there’s no evidence for this idea.

We’ve all seen pictures of cave paintings depicting people pointing at stars or stick figures of seemingly alien-esque creatures on cliff walls, but is there any merit in this concept? I’d argue the answer is no.

If you look at any scientific theory you’ll find there are always multiple lines of independent evidence converging on a conclusion.

Evolution, for example, isn’t proven by fossils. Evolution is demonstrated by a number of different disciplines from molecular genetics to the history of agriculture and the artificial selection of plants and animals. Fossils provide us with a wonderful means of understanding extinct life, but they’re secondary evidence for evolution. You can look at geology and biology and arrive at the same conclusion about the sequence in which life arose on Earth. Every independent line of investigation we examine leads us to the same conclusion: life evolved naturally on Earth over billions of years.

When it comes to ancient aliens, there’s a dearth of evidence. All we have are some poorly interpreted cave paintings to speculate about, and speculation isn’t science.

Picture credit: Prometheus. Here's an ancient alien depicted as an ancient priest or something.

Picture credit: The movie Prometheus even went so far as to depict an ancient alien as an ancient priest just to make sure no one missed the point

So if Earth was visited by aliens in ancient times, what would we expect to see?

Here are a few ideas.


Think about the impact of the Spanish on Aztec society, or the arrival of Europeans everywhere from Africa to India, America to Australia. None of these societies remained intact. They all suffered from the intrusion of terrestrial aliens, and sometimes in entirely unforeseeable ways, like the introduction of European diseases. There were cultural changes, linguistic changes, habitat changes, lifestyle changes, religious changes.

The arrival of an advanced alien civilization at any point in the last 100,000 years would have had a massive impact on human society, and that would be measurable today in the artifacts and habitats these people left behind.

Picture credit:

Picture credit: Aliens vs Predators

Disruption need not be negative. There are plenty of examples of positive disruption, like when the iPhone or the iPad entered the computer market. Traditional desktop computers and even laptops found themselves outmoded and sales dropped. In the same way, some have suggested that the pyramids and Stonehenge were built with the assistance of ET, but there’s a flaw in this argument. There’s no sign of construction techniques or tool markings that are inconsistent with the low level of technology available in that day.

We don’t see any negative, destructive disruption, like the conquistadors arriving in South America. We also don’t see any positive, constructive disruption, like the use of a diamond-toothed saw or some super fangled laser cutting these stones. We see technology consistent with those societies producing these marvelous monuments.

Physical Presence

Although an alien building on Earth could erode over time, there  are plenty of archaeological remains that have lasted for thousands of years (the pyramids, the sphinx, etc) and some that have lasted tens of thousands of years (burial pits, fire pits, stone axes, etc), but we don’t find anything out of place. There are no huge, perfectly circular indentations from the landing pads of a massive UFO weighing thousands of tons, and yet there are plenty of indentations from prehistoric village life.

Picture credit: Knowth Ireland

Picture credit: Knowth Ireland is estimated at almost five thousand years old

There’s sooo much to explore on Earth, so many different ecosystems and an astonishing degree of biodiversity. We still haven’t scratched the surface of the sheer number of species that call Earth home. With some estimates sitting at a hundred million different species, ET would have his hands full exploring Earth. He’d be here for a while, and just as our aircraft leave tell-tale signs of jet fuel contaminant trapped in the ice of Antarctica for the next few million years, ET would invariably leave some trace signature that would be distinguishable from natural processes.

Some of our most interesting animals are found miles below the surface of the planet. If ET was conducting a biological survey (a rather likely scenario), he’d surely explore those depths, leaving a clear sign of his presence: tunnels.

When Europeans first discovered the exotic flora and fauna of Australia, they didn’t stop with a single visit. They colonized. In the same way, it is reasonable to assume any visitor from the stars wouldn’t be satisfied with a single visit to Earth. Repeat visits, by aliens with different motives and intentions, would leave more and more evidence.

And life expands. If ET was here for any length of time, he’d have baby ETs. Sounds silly, I know, but it’s a legitimate point. From the perspective of a physicist, life is expensive. Life is outlandishly extravagant and often wasteful (the sunlight-to-biomass efficiency of photosynthesis for a typical plant is less than 1%). Life requires mineral resources and energy, and harvesting these for even a temporary colony would leave tell-tale signs, like a nice big quarry in the middle of the desert or something.

And our extraterrestrial visitors have no reason to leave. From observing the history of life on Earth and the marvel of evolution, we know the value of observations made over time. Rather than simply taking a snapshot of life on Earth, any advanced civilization capable of reaching Earth would want to document how life continues to evolve on this tiny planet.

Homo sapiens have only been around for a million or so years, just a fleeting moment in evolutionary time. Civilization is a blip compared to the grand history of 3.8 billion years in which life has flourished on this planet. ET wouldn’t bother hiding from a pack of Johnny-come-lately upstarts. He’d be too busy looking at the big picture.

Celestial Presence

As we’re dealing with the possibility of intelligent aliens discovering life on Earth, we should consider their response to such a discovery.

Life in our solar system is rare. Life in the universe seems incredibly rare. Any advanced space-faring race that discovered Earth is likely to have stuck around for a while. You don’t cross thousands of light years for a day trip. So how would an alien species from the stars conduct a biodiversity study of Earth? If our intrepid aliens were following some kind of Prime Directive, they’d set up shop on the lunar surface as it’s perfect for the observation of Earth.

Picture credit: 2001 A Space Odessey

Picture credit: 2001 A Space Odyssey

Arthur C. Clarke struck on this with 2001: A Space Odyssey, and the presence of an alien monolith on the Moon. The Moon is an ideal spot to conduct a scientific investigation of Earth. It’s close and yet it is completely isolated. There’s an abundance of mineral resources, plenty of solar power, etc. Compared to a trip between stars, it’s next door. It’s an ideal staging point for ET.

From our perspective, the Moon is a pristine, untouched environment, and one that preserves any interruption for hundreds of millions of years. If ET had set up shop at any point in the last billion or so years, we’d know.

Another point is that distant orbits are particularly long-lived (the Moon’s been in orbit for 4.5 billion years). Any alien satellite providing communication or surveying the planet could still be sitting in a geostationary orbit or at a Lagrange point. That’s where we’d put them, but there’s nothing there.

The absence of evidence isn’t evidence of absence, but it should give us pause to think. The challenge of not leaving “footprints” is absurdly difficult, and not something an alien species would go out of their way to do, particularly if they were observing from the lifeless platform of the Moon.

Earth Probably Isn’t Friendly to ET

We think Earth is pretty swell, but for an alien adapted to a different atmosphere content, at different atmospheric pressure and different gravity, Earth could seem as hostile as Mars or Titan does to us. Getting around in such an environment would take some kind of spacesuit, making exploration cumbersome and possibly error prone.

Earth has nasty weather, volcanoes, tornadoes, hurricanes, earthquakes, sandstorms. If ET was around for a few decades, something that is entirely plausible given the distance involved in getting here and the work involved in exploring the planet, there’s a good chance he would run into environmental problems and perhaps crash or lose equipment or damage his ship, etc. ET would have plenty of opportunity to leave signs we could later detect.

Have you ever gone for a walk in a virgin forest? Even in an “untouched” forest, we walk on paths carved into the terrain. Our idea of getting out in nature is generally quite tame, and not very natural at all. In the same way as we shape nature to suit ourselves, we would expect to find similar imprints from an extraterrestrial visitor. Perhaps they’d build a visitor’s center at the entrance to their wilderness experience or something, they’d leave some kind of imprint.

Shadow Biosphere

One of the very real problems we face in exploring our own solar system is the microbial world. For all our efforts to the contrary, we can’t help but take Earth microbes to Mars.

In the same way that Europeans inadvertently brought smallpox to the New World, aliens would bring their microbiome to Earth and would invariably take some of our microbiome with them. Don’t underestimate the tenacity of microbes, H.G. Wells certainly didn’t in War of the Worlds.

Any alien life taking hold here would show up on our radar as a significant departure from the phylogenetic tree of life.

The movie Prometheus played on this idea, with “engineer” aliens altering the DNA of life on Earth and advancing evolution. If this had happened, we’d see evidence of such tampering in the genetic code that defines Homo Sapiens. Instead, we see humanity as just one tiny branch on the evolutionary tree of life.

In the image below, we’re near the (arbitrary) border between pink (representing eukaryotes) and purple (representing bacteria).

Picture credit: Wikipedia

Picture credit: Interactive Tree of Life

With millions of species in the tree of life, even this marvelous image covers just a fraction of the relationships between species. How good are your eyes when you squint? From right-to-left at that junction, we have Pan troglodytes (chimps), Homo sapiens (us) and rattus norvegicus (you guessed it, rats).


As fascinating as it is to consider the possibility that Earth has been or is being visited by extraterrestrials, there is no supporting evidence, and certainly no convergence of consistent evidence, but don’t despair, organisations like SETI won’t rest until they find intelligent life that originated beyond Earth.

Picture credit: SETI

Picture credit: SETI


2 thoughts on “Ancient Aliens???

  1. Let’s face it. if aliens were here they wouldn’t leave. We got plenty resources they can pick with drones and we are one as well. I like the thought that sentient beings could be harvested for, well, reasons. The same way we don’t hesitate to step in ants and eat pork, etc., I doubt an advanced alien race with interstellar tech would see us as nothing more than termites with intricate constructions… at least today.

    100k years ago we were in caves, we would be simple animals to them, maybe their pork since we evolved as omnivores. If they can cross the stars, their traces would exist for, well, ever (to cross a universe of micro-projectiles, radiation and who knows what, you gotta build stuff to last and that would extend to their entire civilization).

    The excuse of being visited in the past is good on fiction stories and can be likened to the religious explanation for the origins of the universe. A distraction and easy way to take control of the masses with a vague and empty promise of selfish future grandeur. I don’t want the easy way out, I want to find facts and truths. I want to know what WE did in the past that could be useful in the future.

    Maybe they did came, but I doubt they even landed. Or maybe they just veni, vide and egressus. Who knows?…

    • Yeah, the but-they’re-so-good-at-hiding concept holds no water. They have no reason to hide tell-tale signs, so we’re grasping at straws trying to crowbar them into ancient cave paintings. Good fiction, bad science 🙂

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