Kendra Pierre-Louis: For Scientific American’s Science Quickly, I’m Kendra Pierre-Louis, in for Rachel Feltman.
Most of us, if we think about chins at all, do so rarely. But it turns out that chins are an evolutionary clue. Among our primate kin, chins are a distinctly human trait, which raises a question: Why do we have them?
Research that was published in January presents a strong potential answer. To dig into the murky origins of the human chin, we spoke to one of the study’s co-authors, Lauren Schroeder, a paleoanthropologist at the University of Toronto Mississauga.
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Pierre-Louis: Thank you for joining us today, Lauren.
Lauren Schroeder: Thank you so much for having me.
Pierre-Louis: You recently wrote a paper looking into the evolution of the human chin. This is maybe a very silly question, but, like, what is a chin?
Schroeder: Yeah, so a chin is just a bony sort of protuberance at the lower part of the jaw; in sort of scientific terms we call this a mental protuberance. But it’s basically where the jaw comes together, you have this bone that is sort of sticking out a little bit.
We are the only species to have one.
Pierre-Louis: Mm-hmm.
Schroeder: Even our closest ancestor, Neanderthals, did not have a chin. So it is unique [Laughs] to our species.
Pierre-Louis: You do raise an interesting—is it “The Three Little Pigs” where they say, “Not by the hair of my chinny chin chin”?
Schroeder: [Laughs.]
Pierre-Louis: Apparently, that is fake! Pigs don’t have chins. We’ve all been lied to as children.
Schroeder: [Laughs.]
Pierre-Louis: Before we get into sort of the theories that your paper posits, what are some of the theories about, like, why humans have [a] chin?
Schroeder: Yeah, so because the chin is unique to humans, it has been a question that has been sort of asked and tried to be answered over a very long time in the literature, and many different proposals exist, things like helping with chewing or reinforcing the jaw in some way, so sort of a structural buffering of jaw forces. There’s also been proposals about the chin being—or playing a role in speech and language. Even shaped by sexual selection, this is also another proposal that has been put out there. Basically, you know, if you have a large chin, that makes you sexier. [Laughs.]
Pierre-Louis: [Laughs.]
Schroeder: I don’t know quite what to say. There’s also been this other proposal that basically posits that the chin is a byproduct of natural selection or evolution acting on other structures in the jaw and the cranium. And so the paper that we wrote was trying to sort of answer these questions.
Pierre-Louis: Yeah, and I feel like sort of in the culture we only think about chins when they’re, like, very large, sort of like Jay Leno—I don’t know if you remember who that is.
Schroeder: Yeah. [Laughs.] Yes.
Pierre-Louis: So it’s really interesting that scientists are really fascinated by the chin when I feel like most people aren’t. And I guess the question I have for you is: What got you into this research?
Schroeder: My research is sort of focused on looking at morphology, or the way we look as a whole, and how, if evolution was acting over time, how that evolution has sort of structured our morphology. So we sort of understand morphology as not, you know, separate parts but as a whole that works together.
And so looking at the chin within this sort of integrated structure is what really got me interested because this is sort of what I do—so how morphology is integrated and how evolution affects that whole structure.
Pierre-Louis: So before we get to what your paper concludes, can you tell me sort of what you did in your paper, the steps that led you to this conclusion, if you will?
Schroeder: Yeah, sure, so we sort of tested three broad possibilities. So whether the chin has been under direct selection—whether there’s an adaptive meaning to the chin, whether there’s an adaptive function to the chin. And so we tested that one hypothesis versus two others: one, that the chin is a byproduct of natural selection on other structures in the jaw and the cranium, and then the other is that the chin is sort of just a random outcome of what is called genetic drift.
Genetic drift is basically an evolutionary process that describes how evolution can also happen not just via natural selection but also through random changes in genetic traits over time. This would be what is called neutral evolution or stochastic evolution, so it’s evolution happening randomly.
You know, let’s say you have a population of bugs [Laughs], and you have green bugs and you have purple bugs. And let’s say they’re all one species, green and purple bugs, and they all live in one place. If, let’s say, someone walks along with, you know, a giant foot and steps on [that] population of bugs, okay, but just randomly, what they do is kill a bunch of the green bugs, what that means is that the purple bugs are going to be more numerous in this population. And so further down the line, in terms of reproduction, the purple color is going to carry on and the green bug color is going to be lost. That is sort of an example of genetic drift.
So we basically tested these three hypotheses using primates, lots of different primate species, including humans. We took measurements on all of these different individuals, so lots of primates and humans, and then using an evolutionary tree we basically looked to see the jaw and the cranium, how those structures were evolving through time across the primate lineage, basically.
And what we found was that, in the human lineage, we do see strong evidence that direct selection was affecting evolution of the cranium and the jaw. But when we looked specifically at the chin traits, we found that the signature of strong directional selection was not there. And so our sort of conclusion was that the chin looks to be a byproduct of evolution happening in other spaces or other regions in the cranium and the jaw.
Pierre-Louis: So in reading your paper the analogy that—I don’t know, this is maybe a silly analogy—but the analogy that I thought is that if you’re, like, a college student, you rarely, you know, set out to end your evening eating Taco Bell, but it is a byproduct of you going out and, like, drinking a lot of alcohol, so, like. [Laughs.]
Schroeder: [Laughs.] Yeah, exactly. Yeah.
Pierre-Louis: But you did select for the alcohol; the Taco Bell was sort of a secondary characteristic that popped up. [Laughs.]
The other question I had: What were the direct pressures sort of, like, in the skull and in the cranium that you observed versus the indirect pressures that, you know, led to our, our chins?
Schroeder: So we see the strongest directional-selection signatures tied to dental reduction. So we know that, you know, through time humans have evolved smaller teeth, smaller jaws. And so as the sort of jaws have gotten smaller, and especially the tooth-bearing part of the jaw, which is called the alveolar region, as that has gotten smaller, that’s what we think has caused the sort of byproduct forces in the lower part of the jaw, which then led to the chin.
But in addition to, you know, smaller teeth and smaller jaws, we also have a flatter, less-projecting face with this—we also see as a result of direct natural selection. And then, just in general, more gracile-looking jaws, so less robust jaws, we see that sort of transition over time.
Pierre-Louis: Do we know what the selective pressures are that, like, kind of made our faces change and our jaws change?
Schroeder: Yeah, so over time this is going to be related to diet, cooking food. It’s also going to sort of be related to the structure of our cranium as we started walking upright. All of these things have influenced the sort of selective pressures on, on our cranium. There’s also the fact that our brains became much larger. [Laughs.]
So all of these things together, we see definite signatures of direct selection, like I said, but then we don’t necessarily see the same signatures in the lower part of the jaw.
Pierre-Louis: The title of your paper is “Is the human chin a spandrel?” Can you tell us what a “spandrel” is?
Schroeder: So a spandrel is—actually, the concept of it in evolutionary biology comes from two guys whose names are Stephen J. Gould and Richard Lewontin. They argue that not every trait is an adaptation and some traits arise as a byproduct of other evolutionary forces on other structures.
The spandrel analogy is actually from the architecture field. And so basically, when you build arches you automatically sort of create the—these, like, triangular spaces between them. Those are spandrels. And why this analogy is, is such a good analogy is that no one designed those spaces for their own sake, but they [are] just a byproduct of having two arches come together. So evolution can, like we showed, happen in the same way.
Pierre-Louis: And that’s really beautiful.
Schroeder: In recent years there’s been this sort of push to not sort of try to explain every single trait as, you know, having some kind of functional importance or functional significance. And so testing these sorts of ideas about morphological integrations of how the cranium and mandible, cranium and jaw, are integrated, that sort of sits within this more recent push in the literature.
And we are not the first to come up with the idea that the chin is a byproduct of natural selection. This is something that has been hypothesized before.
Pierre-Louis: Are there other traits that researchers increasingly think might be byproducts as opposed to direct selection?
Schroeder: A study I was reading last week was looking at the sort of integration between the lower limb and the upper limb, and it showed that, you know, sometimes when you have direct selection on the lower limb, where we can really test that, we see that the upper limb is sort of just going along for the ride. And so we do see a lot of these sort of byproduct signatures when we look closely at different measurements.
Pierre-Louis: That’s really interesting ’cause I feel like it’s a much more nuanced understanding of human evolution than I think many of us, you know, got in our high-school or—bio classes or, like, Bio 101 in college.
Does this research kind of satisfy your curiosity about chins, or are you gonna keep looking?
Schroeder: I, I think that the evidence that we show in this paper really supports that the chin is really a byproduct of evolution. I’m not gonna say that the door is closed [Laughs] on chin research forever, but I think I will move on to, to other traits for sure. [Laughs.]
Pierre-Louis: That’s all for today! Tune in on Friday, when SciAm’s associate books editor, Bri Kane, chats with Alexis Hall, the author of Hell’s Heart. The novel is a queer sci-fi space opera, or to borrow from the book’s own tagline, a “sapphic Moby Dick in space.”
Science Quickly is produced by me, Kendra Pierre-Louis, along with Fonda Mwangi, Sushmita Pathak and Jeff DelViscio. This episode was edited by Alex Sugiura. Shayna Posses and Aaron Shattuck fact-check our show. Our theme music was composed by Dominic Smith. Subscribe to Scientific American for more up-to-date and in-depth science news.
For Scientific American, this is Kendra Pierre-Louis. Have a great week!