This is the story of three little bones.They’re called the ossicles (literally little bones in Latin), and they’re in your ear. Unless, that is, you happen to be a reptile reading this, in which case you’ll only have one in each ear with the other two (or rather four) distributed symmetrically in your jaw. It’s a curious bit of evolutionary history, and nicely illustrates an aspect of evolution which called exaptation, the re-purposing of an existing structure. The presence of the ossicles in the reptilian jaw is one of the factors which allows some reptiles (particularly snakes) to swallow things significantly bigger than their own heads (think of a python swallowing a deer, and you’ll get the idea).In mammals on the other hand, these ossicles migrated the short distance from the tip where the jaw meets the skull into the zone known as the middle ear, where they joined the single bone of the reptilian ear which connects the ear-drum to the cochlea. By better articulating the connection between these two latter structures, this triad of bones significantly boosted the sensitivity of mammals’ entire hearing apparatus.
As a result, mammals lost the ability to open their jaws as much as reptiles, but in exchange, did gain the ability to do many other things: better hearing, the ability to chew and talk at the same time (bad table manners aside), the ability to listen to dinner-time conversation whilst eating, and the ability to chew properly. Indeed, it is very possible that without this evolutionary adaptation, mammals (humans, dolphins, etc.), would never have developed a sophisticated sound-based language.
The middle ear also accounts for the fact that mammals have not only the sharpest hearing on the planet, but also the greatest diversity of listening styles (for want of a better term): bats and dolphins make use of high- to ultra-high- pitched sonar with which they hunt, whilst elephants, giraffes and whales use extremely low-frequency infra-sounds with which they communicate over long distances.
The mammalian ear is also exceptionally good at hearing very quiet sounds, because the three ossicles act as a system of levers, augmenting the small movements of the ear-drum oscillating in response to a sound wave, and transmitting these movements to the cochlea. This lends support to the well established thesis that early mammals were at one time small, nocturnal insect-eaters (even today, most mammals, especially the hunters, are nocturnal), which adopted this strategy as a way of staying out of the clutches of the dinosaurs.
The story gets more interesting yet, and reveals evolution in action. In growing mammals, as the embryo matures, the ossicles which are initially attached to the jaw bone, break loose and migrate backwards, from their original (i.e. reptilian) position in the jaw, and into the middle ear. Our suspicions about the origins of these bones were, in fact, first raised in embryological studies. Among modern mammals, the ossicles are one of the last structures to mature and to migrate to their proper location, and even after they so do, they may (in certain cases) still retain a few lingering strands of cartilage connecting them to the jaw in an earlier stage of embryonic development.
Why does this happen? For the longest time, we did not know. However, it now appears that it was due to the growing mammalian brain over evolutionary time. As space became limited, the developing ossicles were pushed backwards into their position in the middle ear. We know this from a study conducted on opossum embryos. In opossums, the ossicles stop growing after about three weeks, but their brains continue to grow for another nine weeks. You can literally see the ossicles being forced backwards by the growing brain. It appears that the evolution of a more specialised brain in mammals was one of the driving forces behind this bit of evolutionary change.
Believe it or not, the fossil record is good enough through this period that we even know roughly when this change took place. About 195 million years ago, Hadrocodium, a miniscule mammal, or one of its close ancestors was one of the first animals to develop this new auditory structure. However, either it was lost at some point, or else it evolved again separately in another group of mammals, because about 70 million years later (i.e. 123 million years ago), Maotherium asiaticus (a mammal found in what is now China), has an ear structure that is half mammalian, half reptilian, and 125 million years ago we have Yanocondon, whose ossicles have separated from the jaw and are to be found in the ear, but are still connected to the jaw via some ossified cartilage. Now, whether this is because of some sort of evolutionary back-sliding or not is still unclear, but based on the fossil record it appears that this three-part structure evolved at least twice in the fossil record, presumably because it was such a good idea – by which I mean: because it conferred such an adaptive advantage to certain animals, to the point where every one of the five and a half thousand known mammal species today, be they placental, pouched, or monotremes, have three ossicles in each ear.
The fossil record of this period is in fact very well known, and we can tell that this particular piece of evolutionary history was neither steady, nor linear, and it appears, as one would expect from a non-teleological process like evolution, that the change went back and forth, including many throwbacks, and changing in fits and starts.
This particular exaptation occurred simultaneously with certain other refinements chief amongst which were: the expansion of the brain, which I have already mentioned, and an elaboration of the dentition. Mammals have the most varied dentition of any class of animals on the planet, as a quick look in the mirror before brushing your teeth will confirm: you have matched sets of four types of teeth, incisors, canines, pre-molars, and molars, each of which is designed to do something different, and all of which (orthodontic issues aside) come together in joyous occlusion. This is very different from say reptiles, or fish which tend to have their mouths filled with one simple, conical, or modified conical tooth-type. This dental change means that we (mammals) can chew our food, rather than just rip off chunks and swallow it whole, and may also have to do with how some members of our class were able to extract enough energy from our food (by chewing it first, rather than just swallowing it whole) to support a large brain with all that having such a structure implies.
So, all in all, in my view, we have a lot to thank those three little bones for.