You Have Reptile Bones in Your Ears! — HHMI BioInteractive Video

Summary

This video script explores the evolutionary mystery of how mammals developed their acute sense of hearing with three ear bones, contrasting with reptiles that have one. Scientists, like Karen Sears, study the development of opossums to trace the transition from a reptilian-style ear to a mammalian one. The script reveals that the malleus and incus, which amplify sound in mammals, were originally part of the jaw in reptiles. Fossil evidence from South Africa supports this transformation, showing a shift from a jaw joint to an ear structure as the teeth-bearing bone grew larger and the original jaw bones shrank, repurposing for hearing.

Takeaways

• 👂 Our ears are capable of detecting a broad spectrum of sounds, from high-pitched squeaks to low, long-distance calls.
• 🦴 The secret to this auditory ability lies in the smallest bones in our body, specifically the three bones of our middle ear.
• 🐾 All mammals, including humans, possess these three middle ear bones, which form a lever system to convert air vibrations into perceivable sound.
• 🦎 Reptiles, in contrast, only have one such bone, which is why their hearing is not as sensitive as that of mammals.
• 🔍 Scientists have been intrigued by the evolutionary mystery of how mammals developed their acute sense of hearing from reptilian ancestors.
• 🦘 The study of opossums by Karen Sears provides insights into the evolution of ear bones, showing a developmental transition from a reptilian-style ear to a mammalian one.
• 🐣 Early opossum embryos exhibit a reptilian-style ear with one bone, which is part of the jaw, indicating a connection between jaw and ear bones.
• 🦴 As opossums grow, the bones that were part of the jaw detach and move to form the mammalian middle ear, demonstrating a physical transformation.
• 🔬 The transition from jaw bones to ear bones is also evident in the fossil record, showing a gradual change in the structure of the jaw and ear bones over millions of years.
• 🦷 Primitive mammal-like reptiles had a different jaw structure with a single bone carrying teeth and other bones forming the jaw joint, which later evolved.
• 🌐 Around 200 million years ago, the jaw joint of certain animals changed completely as the tooth-bearing bone grew larger, leading to the repurposing of the old jaw bones into ear bones.

Q & A

What is the main function of the three bones in the middle ear of mammals?

The three bones in the middle ear of mammals, known as the malleus, incus, and stapes, form a lever system that turns the vibration of air into sound that the nervous system can perceive.

How do the extra ear bones in mammals enhance their hearing compared to reptiles?

The malleus and incus, the two extra bones in mammals, amplify sounds, making their hearing much more sensitive than that of most reptiles, which only have one ear bone.

What is the evolutionary puzzle that the script discusses regarding the ear bones of mammals?

The script discusses the puzzle of how the acute sense of hearing in mammals, which uses three ear bones, evolved from reptilian ancestors who used only one ear bone.

Who is Karen Sears and what is her contribution to understanding the evolution of ear bones?

Karen Sears is a scientist and a former student of the narrator, who studies evolution by observing the development of opossums' ears, providing evidence of how our ear bones evolved.

What does the development of opossum embryos reveal about the evolution of ear bones?

The development of opossum embryos reveals that they initially have a reptilian-style ear with one ear bone, which later in their growth disconnects from the jaw and moves to become part of the mammalian middle ear.

How do the jaw bones of early opossum embryos relate to the ear bones of adult opossums?

In early opossum embryos, the bones that will become the mammalian middle ear are part of the jaw. As they grow, these bones disconnect from the jaw and move to their final position in the ear.

What evidence from the fossil record supports the transition of jaw bones to ear bones in the evolution of mammals?

Fossils from South Africa show a transition in primitive mammal-like reptiles from having a larger bone holding teeth and smaller bones forming the jaw joint, to a new jaw joint formed as the bone holding teeth grew larger, leading to the repurposing of the old jaw joint bones as ear bones.

What is the significance of the change in the jaw structure of primitive mammal-like reptiles over time?

The change in the jaw structure signifies the evolutionary process where the bones that originally formed the jaw joint shrank and were eventually repurposed as the malleus and incus in the mammalian middle ear.

How did the script illustrate the link between modern reptiles and mammals in terms of ear structure?

The script illustrated the link by showing the developmental transition from having one ear bone in reptiles to having three ear bones in mammals, as seen in the development of opossum embryos and supported by the fossil record.

What role did the old jaw joint bones play in the evolution of the mammalian ear?

The old jaw joint bones, which became redundant due to the growth of the tooth-holding bone, were repurposed to take on a new role in the ear, forming the malleus and incus.

How does the script suggest that the study of opossum ear development can shed light on the evolution of mammalian hearing?

The script suggests that by observing the stages of ear development in opossums, from a single ear bone to a complete mammalian middle ear, we can gain insights into the evolutionary process that led to the advanced hearing capabilities of mammals.

Outlines

00:00

👂 Evolution of Mammalian Ear Bones

This paragraph delves into the evolutionary process that led to the development of the three ear bones in mammals, which are absent in reptiles. The narrative explains that these bones, known as the malleus and incus, enhance our hearing sensitivity. The paragraph introduces the concept of how these bones evolved from the jaw structure of our reptilian ancestors, which initially had only one ear bone. It highlights the work of scientist Karen Sears, who studies the ear development in opossums to understand this evolutionary leap. The summary of embryonic development in opossums shows a transition from a reptilian-style ear to a mammalian one, illustrating the transformation of jaw bones into ear bones.

Keywords

💡Ears

Ears are the organs responsible for detecting sound waves and transmitting them to the brain. In the context of the video, ears are highlighted for their ability to pick up a wide range of sounds, from high-pitched squeaks to low, long-distance calls, emphasizing the evolutionary advantage of having a sensitive auditory system.

💡Middle Ear Bones

The middle ear bones, also known as the ossicles, are the smallest bones in the human body and include the malleus, incus, and stapes. These bones form a lever system that amplifies sound vibrations from the air, making them crucial for our acute sense of hearing. The video discusses how the evolution of these bones from jaw elements to ear components is a key aspect of the development of advanced hearing in mammals.

💡Mammals

Mammals are a class of warm-blooded vertebrates characterized by the presence of mammary glands and hair. The video script emphasizes that all mammals, including humans, possess three middle ear bones, which differentiate them from reptiles and contribute to their heightened sensitivity to sound.

💡Reptiles

Reptiles are a group of cold-blooded vertebrates that includes lizards, snakes, turtles, and crocodiles. In the video, reptiles are mentioned as having only one ear bone, which contrasts with the three found in mammals, illustrating the evolutionary progression from a less sensitive to a more sensitive auditory system.

💡Evolution

Evolution is the process by which species change over time through genetic variation and natural selection. The video script explores the evolutionary development of the mammalian ear, showing how structures initially used for a different purpose (the jaw) were adapted for hearing.

💡Karen Sears

Karen Sears is a scientist mentioned in the script who studies the evolution of the ear bones by observing the development of opossums. Her research provides insights into how the transition from a reptilian-style ear to a mammalian one occurred, highlighting the importance of studying developmental biology in understanding evolutionary processes.

💡Opossums

Opossums are marsupials native to the Americas, and they serve as a model organism in the video for studying the evolution of ear bones. The script describes how opossums' ear development from birth to adulthood mirrors 300 million years of ear evolution, providing a living example of the transition from reptilian to mammalian ear structures.

💡Embryos

Embryos are the early developmental stage of multicellular organisms. The video script reveals that studying early opossum embryos provides evidence of the reptilian-style ear with one ear bone, showing the developmental transition from jaw to ear bones, which is a critical piece of evidence for understanding the evolution of hearing in mammals.

💡Fossil Record

The fossil record is the collection of all preserved remains of past life found within rock layers or as free fossils. The video mentions the fossil record as a source of evidence for the evolutionary transition of ear bones, showing how the jaw bones of primitive mammal-like reptiles eventually became the ear bones of modern mammals.

💡Jaw Joint

The jaw joint is the point where the jawbone articulates with the skull, allowing for the movement necessary for eating and speaking. The video script explains how the bones that originally formed the jaw joint in reptiles shrank and were repurposed as ear bones in mammals, illustrating a fascinating example of evolutionary adaptation.

💡Reptilian Style Ear

A reptilian style ear refers to the ear structure found in reptiles, characterized by a single ear bone. The video script uses this term to describe the initial ear structure in early opossum embryos, which then evolves into the mammalian ear with three bones, demonstrating the evolutionary process from a simpler to a more complex auditory system.

Highlights

Ears allow us to perceive a wide range of sounds, from high-pitched squeaks to low, long-distance calls.

The secret to our hearing capability lies in the three smallest bones in our body, located in the middle ear.

All mammals, including humans, have these three bones that form a lever system to turn air vibrations into perceivable sound.

Reptiles only have one ear bone compared to the three in mammals, which is why our hearing is more sensitive.

The malleus and incus bones in mammals amplify sounds, enhancing our hearing sensitivity.

The evolutionary puzzle is how our acute sense of hearing with three ear bones emerged from reptilian ancestors with only one.

Karen Sears, a scientist and former student, studies the evolution of ear bones by observing possums.

Opossums' ear development from birth to adulthood represents 300 million years of ear evolution.

Early opossum embryos have a reptilian-style ear with one ear bone, which is part of the jaw.

As opossums grow, the jaw bones that were part of the reptilian ear disconnect and move to become mammalian middle ear bones.

The transition from jaw bones to ear bones in opossum development mirrors the evolutionary process seen in the fossil record.

Fossils from South Africa show the evolutionary transition of jaw bones to ear bones in primitive mammal-like reptiles.

Primitive mammal-like reptiles had a different jaw structure with one bone carrying teeth and others forming the jaw joint.

Over time, the bone holding teeth grew larger, making contact with the skull and leading to the formation of a new jaw joint.

The old jaw joint bones shrank and were eventually repurposed as the middle ear bones in mammals.

The embryonic development and fossil evidence provide a link between modern reptiles with one ear bone and mammals with three.

The evolutionary process demonstrates how bones that were once used for eating in reptiles were adapted for hearing in mammals.

Transcripts

>> Our ears allow us to pick up a wide range of sounds.

From high-pitched squeaks to low, long distance calls.

>> Okay.

>> 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 incus,

enable us to amplify sounds, which is why our hearing

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,

emerge 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.

[ Music ]

>> So what we see in the opossums is from the time

that they're born through their adult, 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.

[ Music ]

>> 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, they're 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 they're going to be the mammalian middle ear,

disconnect from the jaw as 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 that 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 in 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.

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 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.