If you own a tv, you have probably seen the wild haired host of The History Channel's hit show, Ancient Aliens. This man is Giorgio Tsoukalos. Like Erick von Daniken before him, Mr. Tsoukalos argues that aliens helped mankind progress culturally and technologically in the past. For example, aliens could have helped ancient Egyptians in their construction of Stonehenge. Despite being laughed off by professional historians, anthropologists, Mr. Tsoukalos' "ancient astronaut hypothesis (from now on, the AAH)" is taken seriously by large hunks of the public.
To his credit, I think Mr. Tsoukalos' case is a lot more plausible than many other fringe beliefs like astrology and homeopathy. This is because the aliens he posits are entirely physical beings whose existence would not prove scientific naturalism is incorrect. Another reason why it cannot be ruled out a priori is because astrobiologists and the men and women at SETI believe that there is more than likely other intelligent life in the cosmos. This is something that other believers in odd (but natural) beings like bigfoot cannot claim.
Despite being more plausible than a lot of other bunk out there, the AAH still seems to be bunk. Like other fringe beliefs, the proponent of this idea are not formally trained in history, anthropology, or archaeology. Von Dankien was a motel operator. Mr. Tsoukalos has a degree in sports communication. Their theories have also been vehemently rejected by the community of scholars. While these signs do not prove that the AAH is bunk, it should cause us to raise our red flags. My concerns, however, are not historical (those criticisms are out there if you are interested in them). Instead they stem from two major pitfalls that all variation of the AAH I have seen share.
The first pitfall is that it commits the "alien of the gaps" fallacy. Much like the god of the gaps fallacy, the AAH is nothing more than finding a gap in our understanding and plugging it with a super powerful being. Don't know how the pyramids were built? Aliens did it. Have no idea how the heads were moved at Easter Island? Aliens levitated them. Like all other arguments from ignorance, the alien of the gaps fallacy takes the form of "I don't know how x happened. Therefore, it was y."
Unfortunately for the alien proponent, not knowing how something happened is not evidence for any hypothesis. It is merely a statement of ignorance. To justify the leap to "aliens did it," Mr. Tsoukalos is going to need positive evidence. After he has this, he is going to have to show that the AAH adequately explains it. This demand is very similar to what many philosophers of science, like Gregory Dawes and Maarten Boudry, expect from intelligent design advocates. If the AAH is the best explanation, then it should possess the following traits (source) :
- Testability: better explanations render specific predictions that can be falsified or corroborated.
- Scope (aka “comprehensiveness” or “consilience”): better explanations explain more types of phenomena.
- Precision: better explanations explain phenomena with greater precision.
- Simplicity: better explanations make use of fewer claims, especially fewer as yet unsupported claims (“lack of ad-hoc-ness”).
- Mechanism: better explanations provide more information about underlying mechanisms.
- Unification: better explanations unify apparently disparate phenomena (also sometimes called “consilience”).
- Predictive novelty: better explanations don’t just “retrodict” what we already know, but predict things we observe only after they are predicted.
- Analogy (aka “fit with background knowledge”): better explanations generally fit with what we already know with some certainty.
- Past explanatory success: better explanations fit within a tradition or trend with past explanatory success (e.g. astronomy, not astrology).
The second flaw in the hypothesis can be understood through a challenge. I submit to Mr. Tsoukalos that "there is an alien in my garage" and dare him to disprove this statement. If he were to walk to my garage and open the door, he would point of there is no alien. I would respond by saying that "the alien is invisible." Next he would try to feel it. I would allege that the alien is incorporeal. Finally, he would say "lets try to detect its body heat." I would state that the alien has on a shield that emits no heat.
Much like the alien in my garage, the AAH can always add ad hoc (auxiliary) assumptions to avoid being proved wrong. For example, a retired construction worker named Wally Wallington has single-handedly built a Stonehenge replica in his backyard (link here). What makes this interesting is that Mr. Wallington used nothing but very simple tools that would have been available at the time of the Celts. This shows that one can move and erect very heavy stones without the assistance of ancient aliens.
Do proponents of the AAH take this as evidence that their hypothesis is wrong? Of course not. Like my garage dwelling alien, the AAH can be insulated through tacking on auxiliary assumptions. The aliens proponent will respond to Mr. Wallington's Stonehenge by saying "OK, well how about this other giant thing?" or "well, the aliens helped them do the step before that." The problem with this strategy is that prevents the AAH from being testable in any meaningful way. Since this sensitivity to testing is one of the core characteristics of science, the AAH falls into the same pit of bunk as creationism and Freudian psychology.
The AAH proponent may respond to my charges by saying "that does not conclusively disprove that there was no ancient aliens. It is still possible." I agree with this statement. Despite being a bad explanation and that suffers from being untestable, it is still possible that there were ancient aliens. This response, however, misses the point. Science does not seek to know that is merely possible. This is because everything that is not logically impossible is possible. This includes the possibility that hyper-dimensional beings are living inside your butt and that there is an actual, undetectable alien living in my garage.
Instead, science wants to know what ideas and explanations are the most probable. This is how science came to embrace its most important ideas, like evolution, the periodic table, and thermodynamics, and reject things like astrology and Tarot cards. It is also how any cogent form of inductive reasoning works and how we judge events at our college and jobs. If the AAH wants to be taken seriously, it must conform to these standards because they are the most successful ones we have.
If you would like to know more about the AAH and many other ideas, I highly recommend Guy P Harrison's 50 Popular Believes that People Think are True. This book is very fun to read and contains lots of recommendations about further resources.