Middle ear

What is it?

The middle ear is the area just inside the ear drum.  Here, vibrations in the air are turned into ripples in the ear’s fluids. 

In humans and all other mammals, the middle ear contains 3 bones: the incus (or anvil), the malleus (or hammer) and the stapes (or stirrup). 

Which species have one?

This type of middle ear is unique to mammals.

The middle ears of other types of animals look and work differently to mammals' middle ears, but they do share some similar features.  Birds, reptiles and amphibians have a stapes-bone like we do, which helps them to hear.  They also have an ear drum, but this probably evolved separately to the mammal ear drum.

Evolution of the middle ear

The middle ear of mammals evolved from the jaw bones of their ancestors, which were similar to reptiles' jaws.

Reptiles' jaws have many bones in them that are missing in mammals.  This is easiest to see in our lower jaw.  Humans and all mammals have only one jaw bone that holds the teeth, while reptiles' jaws have this bone plus 5 or more other bones behind it.  Some of these 'missing' bones now form part of our middle ear.

Alternative ways of hearing

Before this unique way of hearing developed, our ancestors probably learned what was happening around them by feeling vibrations in the ground, through the jawbone.

Advantages of the middle ear

You cannot feel for vibrations at the same time as eating, moving, or doing anything else. The evolution of the middle ear meant that mammals could move and listen at the same time.

Reptiles, amphibians and birds share this advantage as they also have a middle ear that allows them to hear at the same time as moving, although it is different from the middle ear of mammals.

However, the mammal middle ear has another advantage.  Its evolution is linked to the evolution of the mammal jaw bone.  In the ancestors of mammals, the jaw muscles, which control how strongly an animal can bite or chew, evolved so that they were attached directly to the bone that holds the teeth.  This makes it stronger for biting and chewing food. It may be particularly important for mammals to have a strong jaw as they are warm-blooded and need a lot of food to maintain their body temperature.

As the other bones in the jaw were no longer needed to support the biting muscles, they could evolve into the middle ear bones.

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