Most of you undoubtedly know me as an autism researcher. But my work has brought me into close contact with the field of Evolutionary Biology as I’ve studied the evolution of autism and developmental genes and other aspects of autism paleogenomics (aka, ancient genetics). As such, sometimes I find myself crossing the line into Human Evolution, since studying autism is a great way to learn about the human condition.

Very recently, I coauthored a paper with my good friend, Miriam Konkel, in Nature reviewing a recent and very fascinating study by a group of scientists who discovered a particular type of genetic mutation that occurred around 25 million years ago, which likely played a major role in the loss of the tail in ancient early apes and which some believe has potential implications for our own human lineage. The mutation occurred inside an intron (a region of the gene that’s initially expressed but then spliced out before it’s translated into protein) and this intron lies within the Brachyury gene (also known as TBXT in humans). But this isn’t any old mutation. The variation in question is an Alu element, which is a mobile element that’s unique to primates. On the whole, almost half of your genome is made up of various types of mobile elements. Why are they called “mobile”? Because they have the potential to make copies of themselves and insert into other places in the genome. Thankfully, most mobile elements in the human genome aren’t active (i.e., they’re extinct), so they aren’t creating new copies and pasting themselves into other parts of your DNA. But in spite of this inactivity, their remnants litter our genomes and provide scientists a fascinating and long history of their evolutions.

Truth be told, I shouldn’t use the term, “litter,” because these mobile elements have the potential to be exapted or borrowed by our genomes for other uses, like regulating the expression of genes. And that’s exactly what this Alu element in the Brachyury gene is doing in apes and ourselves. Once the gene transcript has been produced and before all the introns have been spliced (cut) out, this Alu element pairs up and binds to another Alu element in the reverse orientation in the next intron down, forms something called a stem-loop structure in the gene product, which ultimately tells the splicing machinery to cut out exon 6, which is sandwiched between these two introns and would otherwise normally code for part of the protein. So, long story short, the finished protein product has a section missing, which the scientists had shown dramatically shortens tail length when exon 6 is snipped out of the mouse Brachyury gene just like we see in humans and apes.

What is the Brachyury gene doing in mice, apes, and ourselves? This gene codes for a product that’s vital for development of a structure in early development called the notochord. The notochord, in turn, is an important signaling center, not only for the developing central nervous system (CNS) that’s closely adjacent to it, but it also tells the vertebral column (i.e., your spine) how and where to develop. Without Brachyury, the notochord doesn’t form properly and therefore neither do the CNS and spine. While we can’t say for certain, presumably there’s something about that exon 6 in TBXT that’s very important for tail development in particular. Without it, Brachyury isn’t doing it’s job and the tail (and the nervous tissue that would be included in it) doesn’t form.

The authors of the paper– as well as other scientists before them– proposed that tail-lessness was important and necessary for our own human mode of erect, bipedal walking. The researchers have even gone so far as to say, “We’re now walking on two feet. And we evolved a big brain and wield technology, all from just a selfish element jumping into the intron of a gene” [1].

As enthusiastic as I am about the results of this new study, this is where I have a problem. Why? Because this study did not test the question: “Is tail-lessness necessary for human bipedal motion?” It has addressed the question: “What is the genetic mechanism underlying tail loss in early apes?” Granted, it’s typical for scientists to explore big picture ideas in the Discussion sections of their papers. However, I suspect one of the main reasons this article has made such a media splash is its implications for human evolution. After all, we are a navel-gazing species, aren’t we? We love stories about ourselves.

Interestingly, I’m not the only scientist to have these reservations. According to another report on Nature discussing the new study, Gabrielle Russo, a biological anthropologist at Stonybrook University, has similar doubts, stressing that “early apes moved on four legs like tree-dwelling monkeys, and … bipedality evolved millions of years later” [2], suggesting that the loss of the tail wasn’t necessarily under positive selection.

Taking a Tip from Gould

Stephen Jay Gould was an evolutionary biologist and prolific writer who, on occasion, loved to be a scientific shit-disturber. While most of his contemporaries were busy imagining all the adaptive purposes biological traits could have in both living and extinct organisms, he threw a spanner– or should we say “spandrel”?– into the works. In one of his more famous papers, The Spandrels of San Marco and the Panglossian Paradigm, he and his coauthor, Richard Lewontin, made an elegant argument that not all traits are necessarily adaptive and some may be a byproduct of selection on another accompanying trait. Gould used the metaphor of a spandrel, which is an architectural feature in some churches that is often highly ornate but is really just a byproduct of the arches below, to illustrate this point. To Gould, some traits were simply spandrels and not under direct selection. Therefore, we should use caution when assuming every trait serves an adaptive purpose. This concept has been expanded since and sometimes includes all potentially neutral traits, regardless of whether they accompany another trait under selection.

The adaptationist perspective is culturally built into our societies. After all, we love a good yarn. Rudyard Kipling’s short stories, such as “How the elephant got its trunk” or “How the leopard got its spots,” illustrate these ideas through hyperbole.

To the best of our knowledge, the Alu insertion event that Xia et al. (2024) studied occurred approximately 25 million years ago, somewhere around the base of the ape lineage. One would expect that tail shortening, and perhaps even entire loss of the tail, would have been immediate like we see in the mouse model developed for the study. One could make an argument that suspensory behavior (suspending or hanging below branches) seen in modern apes benefited from the loss of the tail– and, indeed, if one has ever watched a gibbon swing swiftly and elegantly from branch-to-branch with a pendulum-like motion, one could see how tail-lessness could potentially support that locomotory style, as its presence would presumably create drag.

Gibbon suspensory locomotion. Borrowed from: http://www.gibbons.de/main/introduction/chapter_english05.html

However, we know that suspensory behavior has evolved multiple times within the primates and is also seen in modern atelids like the various howler and spider monkeys [3]. And it probably comes as no surprise that these other suspensory primates do indeed have tails– and in some cases, like the spider monkeys, are very dependent on them! So, having a tail doesn’t necessarily prevent suspensory behaviors in primates. Therefore, it’s difficult to make the argument that ape locomotion depended on its loss, suggesting that tail-loss in early apes may not have been adaptive. Or if it was, it’s not as simple as suspensory locomotion.

But what about human locomotion, which was of course the major appeal of the Nature paper? Was it necessary that our earliest human ancestors– more than 20 million years later– not have tails in order to walk bipedally?

Although most of the research on this subject is coming from the field of engineering, current data suggests that human locomotion and balance would in fact benefit from the addition of a tail, not its loss [4, 5]. If we look at our cousins, the capuchin monkeys, a subset of whom regularly carry heavy loads bipedally, they are only able to carry such heavy loads thanks to the aid of their tails [6]. Why does carrying heavy loads matter? Well, it’s one of the more popular theories as to why we humans may have become bipedal in the first place [7].

And so if tail-loss in early apes didn’t prove of benefit, why in the world was it retained? Well, much like Gould argued, biological features that are non-adaptive can indeed be retained, and in this instance I propose that speciation more likely occurred thanks to physical isolation of a small population of monkeys (a process known as allopatry) that experienced this Brachyury mutation leading to tail loss, which, thanks to a genetic bottleneck common in these types of situations, allowed the Alu mutation to eventually become fixed in that small group [8]. Interestingly, mutation rates scale inversely with population size [9].

While the paleontological record may currently be too sparse to test this hypothesis, there are a few facts we know of that are a bit of a smoking gun. First off, allopatry often occurs during times of climatic upheaval, which has the potential to disrupt a once-contiguous population [10]. Spirodonov and Eldredge (2024), in their recent paper, talk about different levels of environmental “fragmentation” that, paired with allopatry, can promote speciation (or extinction). Relevant to ape evolution, we know that rifting began to occur around 25 million years ago in these parts of East Africa, which roughly matches ape divergence from other monkeys [11]. This tectonic activity led to significant volcanism, lake development, and the reorganization of river networks. These major changes may have led to significant environmental fragmentation and alterations in resource availability for primates living in these areas, including the early apes, leading to changes in population structure and, ultimately, isolation [12].

Despite this compelling circumstantial evidence, many readers feeling the pull of the adaptationist perspective may still have a hard time accepting that an occurrence such as tail-loss, with the combined detriment of spinal malformations, could possibly become fixed in a population as a result of genetic drift (aka, chance). And to that, I give you the modern example of the Manx cat.

Manx cats developed from a small founder population of domestic cats isolated on the Isle of Man. Much like apes and humans, these cats have a bobbed tail and are prone to congenital spinal cord defects, urinary and fecal incontinence, and disturbances to locomotion [13]. Like we human counterparts, many bobbed tail cats the world over have mutations in the Brachyury gene. Ninety-five percent of Manx cats have only a single copy of the mutated gene (heterozygous) because inheriting two copies (homozygous) is usually deadly.

Of particular note, due to the obvious geographical isolation of Manx cats on the Isle of Man, as well as the variety of health problems to which these cats are prone, fixation of the mutant Brachyury gene in this small population is not considered adaptive but rather a “founder effect.”

In much the same way, I would suggest that the Alu insertion and probable tail loss in early apes is the result of genetic drift in a small, relatively isolated population, which happened to eventually evolve into what we now know as apes. While the absence of a tail is one of the apes’ defining features, it is quite possible it was just a case of sheer bad luck– as mundane and ordinary as that may seem!