Humans vs. X-Men: Are they two different species?



Or are we one big, fighting family?

Since their debut in 1963, the X-Men have sworn to protect a world that hates and fears them, but here at AiPT! we’ve got nothing but love for Marvel’s mighty mutants! To celebrate the long-awaited return of Uncanny X-Men, AiPT! brings you UNCANNY X-MONTH: 30 days of original X-Men content. Hope you survive the experience…AiPT! Science is going all-in for Uncanny X-Month, with the most detailed look at X-Men biology anywhere, EVER. Today, comparative genomics expert and taxonomist Rob DeSalle asks, if mutants really are the “next step in human evolution,” does that make them their own species?

We humans have a particularly strong predilection for giving things names, and the writers of X-Men comics are no different. These names are sometimes obvious, and sometimes cunningly imaginative.

In the real word, geneticists have been incredibly tongue-in-cheek in the naming of mutants that occur in their favorite organisms. Scientists who work with the small fruit fly Drosophila are outrageous in that respect, with my favorite of many examples being a mutant called “Swiss Cheese,” in which the flies’ brains are riddled with holes. Another favorite is called Cheap Date, a mutation that causes flies to be susceptible to very low concentrations of alcohol.

Naming Marvel’s X-Men mutations is easy then, as a tongue-in-cheek or very descriptive approach can be taken (you can decide which category “Lupine” falls under). But one thing the comics have done that’s piqued the interest of several scientists is the naming of potential species. This is a bit harder than just naming individual mutations (there are even rules, called the International Code for Zoological Nomenclature).

Image credit: Marvel Comics

The name game

My goal here is to discuss whether or not Marvel mutants genuinely deserve recognition as a new species. The most common definition of speciation is when genetic crosstalk (via reproduction) ceases between two population groups. John Rennie, editor emeritus of Scientific American, has written about this very subject with respect to X-Men:

X-Men mutants are not innately a new species, just another variant of Homo sapiens. They cannot become a new species (or more than one) unless geophysical or other circumstances create an irrevocable barrier to their breeding with the rest of humanity.

But I think Rennie was only referring to the split between our species (Homo sapiens) and the recently derived X-Men mutants, Homo superior. His inference about the lack of a species boundary is correct, as there are many examples of H. sapiens interbreeding with mutants. These demonstrate the lack of reproductive isolation between our species and so-called Homo superior.

To be more explicit about this point of reproductive isolation, let’s look at a specific example in science, involving our species and our closest ever relative, Homo neanderthalensis. Many readers will have learned from commercial DNA tests that anywhere from 1 to 4 % of their genome is of Neanderthal ancestry. What does this mean with respect to the species status of H. neanderthalensis? Does this clear sign of interbreeding between our species and Neanderthals mean that H. neanderthalensis is not a good species?

Moat paleoanthropologists claim that Neanderthals are a good species with the valid name of H. neanderthalensis because they differ so much in anatomical appearance with our species. The combination of clear anatomical diagnostics of Neanderthals and the low level of genetic leakage are key here in maintaining that H. neanderthalensis is indeed a different species than H. sapiens.

Neanderthal Museum, Germany (Wikimedia Commons)

Literature search

So, with this concept of what a species is, let’s look at the taxonomy that has developed around the X-Men. The first attempt by comics writers to taxonomize their subjects that I could find comes from the 1980’s Days of Future Past (Uncanny X-Men #141). In this story, three major groups or classes of humans are described –  “H Class,” or basic humans that are devoid of mutations; “A Class,” or “anomalous” humans who have mutant potential at the genetic level; and “M Class,” or those mutants who express the multitude of X-Men phenotypes.

This way of classifying things isn’t really taxonomy, but rather a way of hypothesizing that these A, H and M groups might be species. A species needs a binomial name to be valid. The first name is a genus name and the second is its species epithet.

Things got very complicated in 1989’s Uncanny X-Men Annual #13, as the very early history of mutants on Earth was described. To the best of my ability, I have tried to reconstruct that history with a phylogenetic tree (see figure below where M stands for an engineered mutation and the circle represents the engineering of Homo erectus), starting with the common ancestor of mutants and several real genus Homo species – H. habilis, H. erectus, H. antecessor, H. neanderthalensis and us, H. sapiens.

Potential species that were spun off as a result of “experimentation” by Celestials and others are H. descendus (Deviants), H. immortalis (Eternals), H. superior, H. mermanus (Atlanteans), and Inhomo supremus (Inhumans). There are several real genus Homo species that are left off the figure, such as H. ergaste and Denisova (despite being genomically very different, Denisovans also mated with Neanderthals and our own species, and don’t currently have a binomial name).

If they mated

So, the question becomes are H. descendus, H. immortalis, H. mermanus, H. superior and Inhomo supremus good taxonomic names? Are they “good species”? We already know that H. sapiens interbreeds regularly with H. superior and produces viable, presumably fertile offspring. Atlanteans (H. mermanus) can mate with H. sapiens and mutants (H. superior), but their offspring are generally infertile, with some exceptions.

Inhumans (Inhomo supremus) can mate with mutants, and their offspring sometimes reproduce. Eternals (H. immortalis) can interbreed with H. sapiens, and their children are almost always normal and fertile. Deviants (H. descendus) do interbreed with other genus Homo species, but their offspring die before reproducing.

The following diagram, where red arrows indicate interbreeding and viable offspring, summarizes this information, which I admit might be incomplete. Of course we’ve only sequenced the genome of one of these species, H. sapiens, so we really can’t use DNA to do our taxonomic work.

It appears that the only good species here are H. sapiens and H. descendus (the Deviants). A taxonomist would revise the species status of the genus Homo by “lumping” (the actual term they use) superior, immortalis, supremis and mermanus into a single species under H. sapiens, since that name has what taxonomists call priority (it was first coined in the 10th edition of Carl Linneaus’ Systema Naturae in 1757, well before X-Men comic books were published).

Upon formally lumping, the names superior, immortalis, supremis, and mermanus would never again be allowed to describe a species in the genus Homo, though some astute readers might ask, “Well why don’t we revise so that the lumped species names are subspecies epithets?” Others might wonder why we don’t we call them “races.” Those readers … should probably see this article.