Our ears allow us to pick up a wide range of sounds. From high-pitched squeaks to low long-distance calls. The secret to their capability lies in the tiniest bones in our body. These are the three bones of our middle ear.
All mammals, including people, have these three bones that sit inside the skull. And what they do is they form a little lever system that turns the vibration of air into sound that our nervous system can perceive. We, like all mammals, have three of these bones in our middle ear, but reptiles only have one.
These two extra bones, the malleus and anchus, enable us to amplify sounds, which is why our hearing is so much more sensitive than most reptiles. This presents us with a simple yet extraordinary puzzle. How did our acute sense of hearing using three ear bones, emerged from our reptilian ancestors who used only one.
For nearly two centuries, scientists like Karen Sears have been fascinated by this mystery. She's one of my former students studying evolution by observing the development of these possums. So what we see in the opossums is from that time they're born through their adults, they almost go through 300 million years of evolution in terms of their ears. By looking at each stage of development, Karen has evidence of how our ear bones evolved.
So what's really surprising is if you look at these early opossum embryos, what you see is essentially a reptilian style ear with one ear bone. Those two extra middle ear bones that are going to be in the ear in the adult, they are part of the jaw. And so the jaw joint between the skull and the jaw really looks like that of a reptile. Then after they're born and while they're growing up, those little bones that are going to be the mammalian middle ear disconnect from the jaw.
They get relatively smaller and move up to their final position to become a mammal-like ear. So if you have modern reptiles with one ear bone, you have modern mammals with three ear bones. I think what the embryos do is they almost provide a link between those two. So how do we know that the bones we use to hear with came from the bones reptiles used to eat with?
Well, this amazing transition we see in development from jaw bones to ear bones, we also see in the fossil record. And we see it fossils from here. in South Africa.
And if we look at some of these primitive mammal-like reptiles, we see something in the lower jaw that's very different from us, because what it has is one bone here which carries the teeth, and then there's a bunch of other bones that are sort of moderately sized that sit at the back of the jaw. jaw, they form the jaw joint itself. As you go forward in time, what you start to see are creatures like this, which have a larger bone that holds all the teeth, and ever smaller bones at the back of the jaw that form the jaw joint.
When we get to animals that lived around 200 million years ago, more recent in the fossil record, what you find is that they've formed a completely new jaw joint. That's because the bone that holds the teeth had grown so large, it made contact with the skull. So over millions of years, bones that formed the old jaw joint began to shrink, and eventually became redundant. But instead of being lost altogether, these old bones were repurposed to take up a new role in the ear.