Every time you nod along to your favorite tune or chit chat with a friend, an intricate system is at work making it possible. Your ears are biological marvels. There’s a lot going on in there. “It’s a very complex structure,” Anne Le Maître, an evolutionary biologist, biological anthropologist, and post-doctoral researcher at the Konrad Lorenz Institute in Austria, tells Popular Science.
That complexity is the result of intense selection pressure to hear better over millions of years. Except, possibly, for one part of them: the lobes. Amid all the finely tuned parts, there’s these bits that don’t quite add up. Here’s how scientists make sense of our sensory organs.
The story of our ears
Ears capture sound from the outside world with cartilaginous structures that protrude from your head, funnel it through a canal to a membranous drum, then to a series of teeny tiny middle ear bones and the snail-shaped labyrinth called the cochlea, which transmits nerve pulses all the way to your brain. Mammals, Le Maître notes, have particularly involved ears–with three middle ear bones instead of just the one that reptiles and birds have. Plus, we have large external ear structures (the pinnae), which other vertebrates lack. So how did we get such elaborate ears? Through evolution, of course.
Over millennia, bits of jaw bone in our not-quite-mammalian ancestors migrated and detached, forming two of the middle ear bones as well as the bone that supports the eardrum, Le Maître explains. Fossils found in China and elsewhere show the starts and stops of this evolutionary process throughout the Cretaceous in long-dead mammaliaformes, the evolutionary precursors to modern day mammals. “You see different intermediate forms [between different species and fossils]… but there’s a trend towards the mammal form,” Le Maître says. With these special, sound-conducting bones and our unique, extra-long, coiled cochlea mammals are able to hear a wider range of frequencies than most other vertebrates, she adds.
Our outer ears, conspicuous flaps of cartilage and skin, are also unique to mammals and play an additional helpful role, amplifying sounds and enabling us and our relatives to locate sound, says Mark Coleman, an associate professor of anatomy at Western Atlantic University School of Medicine in The Bahamas. Coleman has studied primate and mammalian auditory systems, comparing how different animals’ ears are tuned and how that relates to structure.
“Not every trait that evolved is adaptive.”
Varied species have ears specialized to pick up on different types of sounds, he says. Kangaroo rats, for instance, have extra large middle ears that allow them to detect especially low frequency sounds for their size and avoid predators like rattlesnakes. Human ears are similar to those of chimpanzees, but small differences mean chimps’ auditory system picks up on high and low frequencies the best, while human hearing is most sensitive to frequencies in the middle range–between about 1,000 and 4,000 Hertz, Coleman says.
And animals with similar habitats often land on the same sort of ear, says Le Maître. Subterranean species tend to have middle ear bones that look remarkably similar to one another, regardless of how closely related they are–same with aquatic mammals. “There’s convergent adaptation across mammals,” she says.
Even the ridges on our pinnae have a specifically evolved purpose. The peaks and dips of our ear topography filter and pinpoint sounds even more precisely. Nocturnal hunters like bats and tarsiers have especially bumpy outer ears that enable them to snag insects in the dark, Coleman notes. Though human ears are comparatively simple, our brains still have to adjust and re-learn how to identify the source of a sound when our outer ears are altered.
Which all brings us to something of a mystery.
The evolutionary exception: Ear lobes
These dangly bits of soft, non-cartilaginous tissue appear relatively recently in the animal record–they’re only really present in humans, chimpanzees, and gorillas, Coleman says. So far, biologists haven’t identified any clear purpose for them. “I think their function is to have a safe place to put an earring,” he jokes.
Ears have lots of blood vessels in them, and so it is theoretically possible that lobes play a role in temperature regulation, similar to how elephants’ huge ears help them stay cool, he says. Though he and Le Maître both note that theory is little more than a guess. Alternately, some scientists, like zoologist Desmond Morris, have previously proposed the idea that earlobes developed as an erogenous zone to facilitate pair bonding, but there’s little direct evidence that this sort of sexual selection has molded our ears.
Unlike most other parts of the ear, lobes vary quite a lot between people. You may have learned in a highschool science class that you either inherited attached or detached lobes from a single set of allele passed down from your parents. And though it turns out that vast oversimplification isn’t actually true, our lobes do come in different shapes and types, as determined by genetics. That level of variability suggests that lobes are under far less pressure to fit a particular form and purpose than something like the well-honed middle ear bones, says Le Maître.
Instead, our lobes may simply be evidence that evolution isn’t a perfect design process. “Not every trait that evolved is adaptive,” says Bridget Alex, a paleoanthropologist at Harvard University. There are physical and biological constraints to what features can emerge. Randomness is involved through genetic drift–where a certain set of traits becomes dominant in a group by chance.
And then there’s so-called evolutionary “spandrels”–a term thought up by famed paleontologist Stephen Jay Gould, which references the triangular space between a cathedral’s arch and ceiling. Those triangles aren’t necessarily an intentional part of the blueprint, but rather a byproduct of the desired architectural feature, the arch. Similarly, some parts of our bodies might be the accidental byproduct of others, Alex explains. Perhaps lobes formed because of a cartilage shift elsewhere in our ears that maximized our hearing acuity and inadvertently left some flesh hanging around.
Earlobes aren’t the only indicators of evolution’s sometimes shoddy and ongoing work. We also have vestigial muscles in our ears, left over from mammalian ancestors that could pivot and point their pinnae like cats can. Despite still having these scraps of muscles, most humans can’t move their ears at all (similar to how our tailbones no longer connect to tails). Other useless mysteries also remain. “No one’s really sure why the human chin evolved [either],” says Alex. “Is it adaptive, a byproduct, an accident, sexual selection?” You may stroke your chin (or your lobes) pondering the question, but that probably won’t make the answer any clearer.
This story is part of Popular Science’s Ask Us Anything series, where we answer your most outlandish, mind-burning questions, from the ordinary to the off-the-wall. Have something you’ve always wanted to know? Ask us. And sign up for our newsletter to learn something new about our weird world every day.
Source : Popular Science