Pop quiz! Let’s test your scientific acuity!
Which of the following concept aliens do YOU think is the most plausible?
Now, finally, how about this one:
All right, the answer will follow later…
You probably noticed a lot them were humanoids. Humanoids are sentient creatures that resemble our own species, Homo sapiens, in most respects. In science fiction, they’re a very popular way of depicting intelligent aliens from outer space. But how likely is it that evolution in biospheres far away from Earth’s would end up with basically the same results? Let’s attempt to find out!
Humanoid aliens are the staple of contemporary science fiction. But why? Why would we expect sentient creatures evolved on another world to look just like us? Well, for starters, the only ones that we know the existence of are humans. It is very hard for us to imagine beings at our own level, that are not in some or most ways similar to ourselves.
However, already as early as 1695, renowned Dutch physicist Christiaan Huygens reasoned that rational beings existing on other worlds may have some things in common with us like having hands, feet and an upright stature, but that that is where the similarities would end. In many other aspects, he stated they could be very different indeed like for example having fur or feathers or even an exoskeleton, as unappealing as that may be to us humans.
For there is such an infinite possible variety of Figures to be imagined, that both the Oeconomy of their whole Bodies, and every part of them;
Christiaan Huygens, 1695
may be quite distinct and different from ours.
Yet, the idea of aliens being just like humans persisted into modern days. Novelist Edgar Rice Burroughs, especially known for creating Tarzan, envisioned Martians along those lines in his 1917 novel “A Princess of Mars”. With it, he laid the foundation for science fiction in literature as well as movies and TV series. But there is also a more down-to earth reason, pun not intended: For early TV series like Star Trek it was fundamentally about budget and lack of technology: It’s a lot easier to make actors look alien with some minor cosmetic tricks than to create elaborate suits and special effects.
But perhaps most importantly, humanoid aliens allow the audience to relate to the characters. That is also why science fiction invariably features humans as main protagonists. Like for instance Luke Skywalker, even though he and his kind cannot possibly be human. Star Wars is set “A long time ago, in a galaxy far, far away…” and we know for a fact that human beings evolved naturally on our own planet. So the “humans” in Star Wars, or whatever passes for humans, must be entirely unrelated and… coincidentally be indistinguishable from humans! In the many billions of galaxies in the known universe, what are the odds of that happening?
How likely is the genesis of an alien race
almost genetically identical to Earthly humans?
How likely is the genesis of an alien race almost genetically identical to Earthly humans? By looking at our DNA, could we simply calculate the odds? Well, the number of coding genes in the human genome is about 20,000. The letters of genes are formed by nucleotides, of which there are 4 natural varieties. In humans, the coding part of the genome contains about 3.2 billion base pairs in total. So, The likelihood of creating an identical sequence randomly is 1 out of 4 to the power of 3.2 billion! This is an insanely high number that is practically infinite. Now, the number of stars in the universe is estimated at 10^19, so even if we assume one habitable planet for each star, and a million trillion random sequences being generated each year over a period of 15 billion years, the age of the universe, we would never even come close. So is it just impossible? Well, in reality, this tells us very little about the actual odds of evolving a humanoid being.
Let me explain: An entire genome isn’t generated randomly, but has evolved as a result of natural selection. And expecting the exact same DNA sequence can be very misleading indeed. DNA holds information about proteins which in turn determines what cells do. Proteins are made out of chains of amino acids each encoded by 3 successive nucleotides in so-called codons. Now, there are many codon synonyms and there are also many non-functional sections of proteins where it doesn’t matter what exact amino-acids are used. So a lot of genetic differences would still lead to the exact same proteins being produced. Generally speaking, DNA can differ to up to about 34% and still give the same results. But we can relax the required DNA sequence many, many times further still! Many proteins have specific aspects just to be able to interface with other proteins in a so-called gene regulatory network. So the only thing that really matters are the protein end products as well as the timing of their production. The intermediate proteins can be very different themselves.
But even the exact same proteins can in principle be encoded in entirely different ways. The translation of a gene to a protein is mediated by a molecule called tRNA, which determines what combination of base-pairs becomes what amino acid in the protein chain. This can be represented in a translation table and appears to have been arrived at arbitrarily during the earliest stages of the evolution of life. In other words: The translation table for Earth’s life was frozen at a primeval stage allowing for only minor deviations between different lineages. Life on other planets could have their own distinct translation tables, so if you would put their DNA in our cells they’d not give the same results at all!
In addition, a lot of similarities at the molecular and biochemical level could already be taken care of by convergent evolution. There may only be a limited set of molecules that does something useful for life, or there could also be many alternatives that do the same. In the latter case, you would not even need the exact same biochemistry to end up with something looking very much alike! Just like how operating systems may look very similar even when running on very different hardware.
So we don’t get a lot out of analysing our DNA. In any case, any possible humanoids that are genetically similar to our own species somehow are a subset of a much larger group that are not. And since on Earth, there is only one group of species that is humanoid, that could mean that there is an even larger group of sentient aliens that are not humanoid. Or does it? What if the humanoid body plan really is the only sensible way to make a technology-using species? Would sentient aliens be expected to have arms and hands like us? Would they then naturally stand upright? Would they even likely have a head too? Let’s look at those questions by breaking down the basic human body plan and trace down the evolutionary origin of each element. This will be the first of an entire video series covering human anatomy to assess how unique or common all of its different aspects may be. That way we can perhaps get some idea of how likely it is that it may evolve independently in some other branch of life somewhere else in our galaxy or beyond.
So how about that Quiz I started out with? Well, it was a bit of a trick question. In reality each of these aliens is equally plausible. That is because we are comparing specific manifestations each with their own unique evolutionary history. Now, comparing broad categories like “humanoid”, “reptilian” or “insectoid” is a different case. But without a survey or catalog of different life-forms in the universe, we have no statistics to base the odds on. And without statistics, we can only make educated guesses.
Still: If one were to ask how likely the humanoid category of alien beings is, then I think the answer is: Out of all the conceivable options for sentient life, probably not very common. But what do you think? We may learn more in my upcoming videos, so stay tuned!