Science fiction is full of humans interbreeding with intelligent aliens from all over the galaxy.
In the original Star Trek series, Captain Kirk mates with a wide variety of aliens, although he never fathers any children with them that we know of. Spock is half Human, half Vulcan. There are several characters in Star Trek: The Next Generation who are the result of alien-human mixing, as well. Worf is 100% Klingon, but his wife, K’Ehleyr, is half Klingon and half Human. Their son, Alexander, is therefore one quarter Human and three quarters Klingon. Deanna Troi is half Human, half Betazoid. The list goes on and is certainly not limited to Star Trek.
Star Trek’s Spock is half Human and half Vulcan. Image from wikipedia.org
I hope I’m not ruining anyone’s fantasy by pointing out up front that this is not even remotely plausible.
Imagine a human trying to produce offspring with an insect or a tree. Once you’ve stopped laughing, imagine trying to do the same with a bacterium, which is as distantly related from humans as you can get on Earth. Now consider that reproducing with a life form from another planet is far less likely to work out than trying to reproduce with a bacterium from Earth.
For two different species to be reproductively compatible, they cannot have very much evolutionary distance between them. Zebras (Equus zebra), horses (Equus ferus) and donkeys (Equus africanus) can all reproduce with one another. Lions (Panthera leo) and tigers (Panthera tigris) can sometimes produce offspring. The common chimpanzee (Pan troglodytes) and the bonobo (Pan paniscus) can produce offspring. (As is typical of cross-species hybrids, the offspring of these pairs are not fertile themselves.) I hope you noticed a commonality among all of these matches: they are all in the same genus. Lions and tigers are both in the same genus (Panthera) and they can sometimes reproduce together, but neither the lion nor the tiger can reproduce with a chimpanzee or a zebra because they are not closely related. This is not a hard and fast rule, though. Many species that are in the same genus cannot reproduce together, and occasionally species that are not in the same genus can reproduce together.
The liger is a hybrid between a tiger (Panthera tigris) and a lion (Panthera leo). Image from wikipedia.org
Life on Earth began somewhere between 3 and 4 billion years ago. Since then, life has evolved and diversified to produce the wide range of species that we see today. Another planet with life would have had its own origin and its own evolution to produce whatever diversity that planet has in whatever amount of time it took. An intelligent species on another planet would be unable to reproduce with most of the other species on its planet, just as most species on Earth cannot reproduce with one another. Humans are not able to reproduce with any other species on our planet. Even chimpanzees and bonobos, which are our closest evolutionary relatives, cannot produce offspring with humans (though it is not for a lack of trying). We share almost 99% of our DNA with chimpanzees, and are physically capable of mating, and yet we cannot reproduce with them. So what are the chances of being able to reproduce with a species that evolved from a completely separate origin of life? In this particular case, the likelihood is roughly equal to the percent of our DNA that we share: 0%.
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With the remake of Teenage Mutant Ninja Turtles coming up, I thought I’d talk about the way mutations are depicted in movies vs how they are in real life. Again, this is not to diminish anyone’s appreciation for these movies, but to understand where the science stops and the fiction begins. Many movies, especially superhero movies, describe some sort of mutagenic event as the cause for the hero’s superhuman abilities. In the original Teenage Mutant Ninja Turtles, the four turtles and one rat were given human size and intelligence through exposure to “radioactive ooze.” It also seems to have removed two of their fingers — turtles, like humans, have five fingers rather than the three they are depicted with in the movies.
Real turtles have five fingers and five toes. Teenage mutant ninja turtles only have three. Image from collecltions.countway.harvard.edu
In the X-Men franchise, “mutants” are caused by inheriting mutant alleles. In The Incredible Hulk, one of Dr. Bruce Banner’s experiments goes wrong and he is accidentally exposed to gamma radiation. In The Fantastic Four, the astronauts are exposed to cosmic rays. In Daredevil, Matt Murdock is exposed to radioactive waste as a child.
The Incredible Hulk gained his abilities through unspecified mutations. Image from screenrant.com
In Spiderman, Peter Parker is bitten by an irradiated spider, not exposed to radiation himself. Presumably this resulted in the transfer of some of the spider’s genes into Peter Parker’s genome, making this an event of “horizontal gene transfer,” rather than a typical mutation. I plan to discuss this at some point in the future.
Now, everyone knows that exposure to radiation or toxic waste will not give anyone super powers. But what do mutations look like in real life? In short, a mutation is any time there is a change to the sequence of the DNA of an individual or cell.
An organism’s genome can be compared to a book. The book is broken down into chapters, called chromosomes, and made up of letters called “nucleotides” (A, G, T, and C). Genes can be compared to sentences, and are made up of three-letter words called “codons.” Codons are always three letters.
In English, the three-letter word “cat” refers to a group of predatory mammals with long claws. In a sentence, “I keep two cats as pets.” In the language of DNA, the three-letter word “CAT” means the amino acid “valine.” A sentence of three letter words makes up the sequence of amino acids that build a protein. The function of a protein is created by the exact sequence of amino acids that make it up, just as the meaning of a sentence in English is created by the exact sequence of words that make it up. The DNA sentence “TACCATAAACGGGTGACT” means “Methionine, valine, phenylalanine, histidine, alanine, [stop].” The codon “ACT” is one of three “stop codons,” which act like periods in English. It means the sequence of amino acids has come to an end. Proteins are usually thousands of amino acids long. In most written languages, minor changes in the words or punctuation can drastically change the meaning of the sentence. A change of one letter can change “I keep two cats as pets” to “I keep two bats as pets.” Similarly, changing one letter of a genetic code can change the meaning of a sentence. Changing “TTC” to “TTA” will change phenylalanine into leucine. These two amino acids have different properties, and will result in slightly different proteins being made. This protein might work basically the same, despite the minor difference. It also might work very poorly. There is also a small chance that the different protein will work better. The first two options are overwhelmingly the most likely of the three possibilities. A change in punctuation can change “let’s eat, grandma” to “let’s eat grandma.” The same is true in genetics. If a stop codon gets put into a gene too early, part of the protein will not be built. This may result in a complete failure of the protein to do its job. It is very easy for a mutation to cause a stop codon very early in the sequence. CAG codes for glutamine; change the C to a T and it becomes the stop codon TAG. AGA codes for arginine; change the first A to a T and it becomes the stop codon TGA. Having one protein that is missing or incorrectly built may not seem like a big deal — after all, the human body contains tens of thousands of different proteins. But the human body is a marvelously complex machine that requires the coordination of many different pieces. Taking one piece out of the human body is like taking one piece out of a Swiss watch. There are many genetic conditions that are caused by single mutations, such as Marfan syndrome, achondroplasia and paroxysmal nocturnal hemoglobinuria. But mutations are not always bad. Blue eyes, for example, are a relatively new trait that first originated in eastern Europe only a few thousand years ago. There is no major advantage or disadvantage to survival in people with blue eyes. Beneficial mutations are what natural selection operates on to produces complex traits over millions of years.
Mutations can be caused by exposure to radiation, certain toxic substances or just random chance. This is something that the movies get right.
Mutations that are inherited will be present in every cell of a person’s body. Everyone began as a single cell. If that first cell contained a mutation, then every cell that comes from that cell will also contain that mutation. Mutations that are acquired later in life will only be present in a few cells. If a person is exposed to radiation, the radiation will cause random mutations in whatever cells it interacts with. It may only cause mutations in a couple of cells, or it may cause mutation in many cells. Each mutation is random, so the likelihood of two different cells having the same mutation is vanishingly small. It is even vanishingly smaller for the same mutation to occur in more than two cells. For every additional cell and for every additional mutation, the likelihood of the same thing set of mutations happening gets astronomically smaller. For a person to acquire superhuman abilities through exposure to radiation, two things would need to happen. First, a complex mutation would have to occur. A single mutation is very unlikely to produce the type of complex, new trait that superheroes have. Second, the same suite of mutations would either have to occur in the entire body, or at the very least in the organ system that the trait operates on.
Taken together, this things only become less likely. A collection of random mutations that builds a complex, beneficial trait AND occurs simultaneously in billions of cells? Consider the following scenario for comparison: you are transcribing notes you took by hand into a computer file. Through a series of accidental typographical errors, your economics notes turn into The Rime of the Ancient Mariner. This is analogous to a series of random mutations producing a complex, beneficial trait in a single cell. Now imagine that everyone in your economics class accidentally and independently wrote The Rime of the Ancient Mariner when trying to copy their notes. This is analogous to Bruce Banner’s exposure to gamma radiation turning him into the Incredible Hulk. Now imagine that not only did everyone in your economics class accidentally write The Rime of the Ancient Mariner, but everyone in your history class accidentally writing The Song of Hiawatha, everyone in your math class accidentally writing Where the Sidewalk Ends, and everyone in your biology class accidentally writing Beowolf. This is approximately the likelihood of Reed Richards, Susan Storm, Johnny Storm, and Ben Grimm (The Fantastic Four) all being transformed into superheroes through exposure to cosmic rays.
The Fantastic Four all got their powers through exposure to cosmic rays. Image from static.comicvine.com
The original Teenage Mutant Ninja Turtles, which came out in 1990, was one of my favorite movies growing up, and I am cautiously optimistic about the remake, which comes out on August 8. I encourage everyone to enjoy some superhero movies this summer, but don’t be fooled by bad science.
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ChristopherEppig 