A 'Useless' Organ You Might Find On Your Ear Once Had A Unique Purpose

No two human ears are exactly alike. Like our fingerprints, they bear enough variances to make each of us unique. Most of these differences are subtle, slight variations in the shape and size of the ear, but there's one feature that definitely stands out. Some people have a small bump at the inside fold of their upper ear, as if they were on track to become an elf, but only got one step past the starting line. This bump is known as Darwin's tubercle or Darwin's point, and it poses a frustrating mystery to biologists.

The feature is named after the father of evolutionary theory, Charles Darwin, who described it in his book, "The Descent of Man," however, the feature was only brought to Darwin's attention by a sculptor named Thomas Woolner, and Darwin himself referred to it as a "Woolnerian tip." Woolner theorized that the point could be a vestigial feature, a type of anatomical feature that is a remnant of the evolutionary past.

Darwin agreed with Woolner's theory, and used the tubercle to support his theory of human evolution. Darwin noted that several types of monkeys, most notably macaques, have slightly pointed ears, and the presence of a similar point in some humans supports our link to this primate lineage. As our family tree split, some primates retained pointy ears, while others, like humans, saw the feature shrink or vanish entirely. At some point, our ancient ancestors had a purpose for Darwin's tubercle, but biologists aren't entirely certain what it was.

When the primate line first began, pointy ears would have likely been the norm. The earliest proto-primates were small, rodent-like animals akin to modern day squirrels and shrews, and the earliest true primates were similar to today's lemurs. Over time, the primate lineage divided into different branches, and while some groups, like macaques, marmosets, and squirrel monkeys still have pointed ears today, the feature died out amongst the apes, like chimpanzees, gorillas, and humans. What could account for this diversion?

The pointed ear shape seen in many other primate species is thought to help funnel sound into the ear canal. The Darwin's tubercle seen in modern humans is typically folded downwards as part of the helix (the fold along the upper part of the outer ear), but the pointed ear tips on primates stick outwards, creating a funnel-like structure. This would amplify volume beyond what human ears can detect, but although that may sound advantageous, there's a very good reason that the feature died out in our lineage.

Human hearing is actually much more precise than that of monkeys. They may be able to amplify sounds better than us, but their ear canals are smaller and they don't hear details very well. Humans are able to hear a wider range of sounds, including very subtle ones, and this is critical for something we do that our earlier primate ancestors did not: talk to each other.

Biologists aren't just in debate over what function Darwin's tubercle was once related to; they're also in debate over how it's inherited today. It isn't even clear what percentage of the global population has the trait, although a 2016 study published in Dermatology and Therapy notes that it has been documented in 40% of adults in India, 10.5% of adults in Spain, and 58% of school-aged children in Sweden. This disparity seems to raise more questions than answers. There's also a good deal of variance within groups that have Darwin's tubercle. Some people have it equally prominent in both ears while others have asymmetrical ear tips.

The human ear begins in gestation as six lumps of tissue called the hillocks of His, which grow and then fuse together. Darwin's tubercle forms at the point where the fourth and fifth hillocks merge. It has long been thought that the reason some people's ears form like this comes down to a single gene with two alleles, the dominant one of which creates the protrusion. However, concrete evidence for this theory is lacking. In fact, about a quarter of people whose parents lack Darwin's tubercles end up with the trait anyways, and even in pairs of otherwise identical twins, it is not uncommon for one twin to have the point and the other to not. That leaves us without a firm grasp (just yet) on why this vestigial trait has lingered so long in a select few of us.