(bright orchestral music)
Every day we use organs
which are hundreds of millions of years old,
but we know nothing of their history.
Nowadays, new scientific imaging technology
offers us a sudden new view of our anatomy.
A unique occasion to explore our body,
to retrace its history,
and understand what relates us to other species.
We are truly a time machine.
Each of our body's organs appeared and evolved
on a timeline that science can now reestablish.
We had genitalia before we had ears,
and eyes before we had limbs.
The genealogy of our organs
shows that we share a long history
with the other inhabitants of our planet.
Our body holds the memory of the odyssey,
which has taken us from there to here.
All around the world, scientists are revealing
little by little the incredible mechanisms
that have fashioned our organs.
The first great chapters of our evolution
take us back to a distant aquatic past.
It was in the water that the organs were born,
which today allow us to see, touch, and reproduce.
This fabulous journey within our body,
takes us back through the history of species,
and shows that we are a puzzle of evolution.
This body which we inhabit every day
is so familiar to us,
that we don't pay attention to it anymore.
However, looking in the mirror in the morning,
we could well get interested
in the construction plan of our body.
We could admire the perfect symmetry of our eyes and ears,
appreciate the mobility of our limbs,
or our vertical posture.
But this didn't happen all at once.
A dive into the interior of our body
will allow us to understand how we are made.
The organization of our nerves,
our skeletal structure,
the branching out of our blood circulation system,
are some examples which bear witness
to a very old anatomical structure.
It was put in place at the time
that we were still aquatic creatures.
It relates us to species
that sometimes seem very far from our own.
If we go back to the biggest groups of animals,
they are the bilateria,
which are animals with bilateral symmetry.
The organs on the right of the animal
are the same in the mirror
as those which are found on the left of the animal.
This bilateral symmetry is understood
to be the result of living beings
that moved in an environment
where they propelled themselves from behind,
and their sensory organs were placed forward.
Welcome to the bilateria.
The symmetrical axis, which runs from the mouth to the anus,
makes us distant heirs of little marine creatures,
like anthioxus.
This axis is arranged around a nerve
which can be distinguished on the dorsal part of the animal.
It prefigures formation of the future spinal cord,
and vertebral column.
It's from this median nerve,
that the structure of the animal is arranged.
This fundamental plan, 600 million years old,
is the result of a series of genes
which determine the segmentation of the body.
Our DNA is like a memory card, which keeps a record of this.
(upbeat jazz music)
Through evolution, the median dorsal cord became rigid,
giving birth to a vertebral column.
All of humanity shares vertebra with 52,000 other species,
and the vertebral column was formed
around 500 million years ago.
Going back to the origin of the vertebrates
isn't that difficult, just go to a fish shop.
The story of our origins can be explored
by buying a fish.
Fish illustrate the history of evolution.
The fact that vertebrates were born
in an aquatic environment,
and that half of all vertebrates are bone fish.
There's the form that vertebrates must originally have had.
At the same time as the vertebral column,
another piece of our anatomic puzzle was formed.
500 million years ago, the skull appeared.
Then another component
of our bone structure was added.
All these fish here have a lower jaw and an upper jaw,
and this jaw goes back 420 million years.
So we inherited from fish our vertebral column,
our skull, and our jaw.
Again, in the water, at the same moment,
the great adventure of our eye began.
It's one of the most ancient organs of our body.
From its aquatic origin,
the eye hasn't stopped diversifying,
adapting, and improving itself.
Our camera-type eye is as good
as the most developed technologies,
linking 3-D vision, color distinction,
and depth perception.
We can imagine that such a complex organ
must have evolved over a very long period,
but the origins of our eye,
have long remained a mystery.
It's only recently that two scientists,
one English, the other Australian,
have brought together exceptional clues,
allowing us to advance in the history of our eye.
They've managed to unlock the secrets of unique fossils.
Gavin Young was interested by placoderms,
a prehistoric species of fish with shells, now extinct.
Sarah Gabbott concentrated on the lamprey,
a vertebrate fish, without jaws.
These fossils bear witness to key moments
in the evolution of our eye.
Placoderms, if you like,
are more evolved, they're closer to us,
they're closer to fish that we see in the oceans today.
Placoderms went extinct.
Lampreys didn't go extinct,
so we can still study them as living animals today.
It's status as a living fossil
makes this strange fish interesting.
It's at the end of winter, or in spring,
when the lamprey moves up river to lay its eggs,
that it's easiest to find.
While the lamprey interests fishermen
from the Bordeaux region,
as a gastronomic specialty,
it's also of great interest to science.
Lampreys are known as jawless fishes,
and they're extremely primitive fish.
They're really interesting because they sit
at a pivotal point in the evolution of vertebrates,
so if we go more primitive than these creatures,
we have the amphioxus and the sea squares.
And then, if we move more towards us, if you like,
then we have the fish, the bony fish.
So they're the most primitive, living vertebrates,
and that's why they're so important.
Luckily, we didn't inherit
the alien-like sucker, which serves as their mouth.
On the other hand,
lamprey eyes have very similar characteristics to our own.
The real question is to know if their ancestors,
which lived 300 million years ago,
already had such as elaborate eye.
Until now, scientists were confronted by a total enigma,
with the evolutionary history of the eye.
Indeed, although the bony structures of bodies
fossilize well,
soft organs, such as the eyes,
decompose in a few days, leaving no trace.
He social record has really been very silent
on this whole area of evolution,
until I made this discovery of these eyes
in these 300 million year old fossils,
and realized that actually the eye can become preserved,
and it can be exquisite in its nature,
and it can tell us a lot more than we previously thought.
I realized there was eye preserved,
because there's a little sort of circular black blob,
and I thought that was it,
we could just tell, it's an eye.
But when I looked at it under the scanning
electron microscope,
and turned the magnification up really high,
I realized that the actual structure,
the cellular structure at the back of the eye,
the retina, was preserved,
and this was amazing to me.
So a 300 million year old eye, it's still there.
An electronic microscope allows
both the observation
of the internal structure of the sample,
and exploration of all the details,
enlarging much more than an optical microscope.
This is how Sarah could reveal that a fish
which lived 300 million years ago,
could already see much like we do.
This is the fossil fish eye
magnified to about 5000 times,
and what we're looking at is the retina,
and all of these structures here,
and the way that they're aligned,
tell us that this particular fossil
could form a very clear visual image.
The lamprey fossil shows
that a complex eye already existed 300 million years ago.
The placoderm fossils, studied by Gavin Young,
lets us go further in this inquiry
into the history of our eye.
In fact, for the first time,
an entire ocular globe, 400 million years old,
can be observed.
This is unique because the inside
has been ossified, so the inside,
which is normally cartilage is now bone,
and it's preserved,
and the braincase is also preserved,
so we can actually work out how that ancient eye capsule
articulated against the braincase.
For the first time, we can actually use the computer
to do the digital dissection
on something that's been dead for 400 million years.
Here's the eye capsule to be scanned.
It's extremely fragile, and it's unique so,
this is a one-off chance to see its internal structure.
Well done, just careful.
I've never seen inside this before,
can you turn it?
That's the optic nerve, hole for the optic nerve,
going into the eyeball there.
That's superb.
With his team,
Gavin dissects the eye virtually.
So that should be a muscle scar,
that big depression in there.
Yeah.
What about this?
Well that's something coming from the brain.
Maybe the optic artery coming out and entering the eyeball.
To avoid manipulating the fossils,
as fragile as they are precious,
Gavin Young takes a 3-D impression of the ocular globe
and the cranium.
This reproduction in resin, enlarged many times,
shows the slightest anatomical detail.
Some particularly interesting indications appear.
Once you study everything in detail,
you find that some things are different
to every living species of jawed vertebrate,
that they resemble the jawless vertebrates.
That is it retains a primitive character.
The connection to the braincase,
the position of the eye muscle,
the opening of the optic nerve,
these are things that you don't see
in any other high vertebrates, so therefore,
it's an intermediate stage that shows
how complexity is built up over long periods of time.
We are able to compare
this very early placoderm eyeball,
with the modern jawless lamprey,
and it suggests that there are similarities,
which are very primitive,
so they'd go back to the branching point
of the jawless vertebrates and the jawed vertebrates,
so they probably goes back 450 or 500 million years,
to establish that basic pattern.
The fossil record is becoming really important,
in plugging the gaps between the living animals,
and helping us to understand the secrets of evolution.
If we go before the lamprey,
the fossil that I've been looking at,
we go to anthioxus,
and they have just a kind of single pigment cell,
that tells that animal which way is up and down,
which is light and dark.
And then, we evolve another component of the eye.
Say you get a few more of these cells,
so you start to form a retina,
and then gradually you get a cup shape,
and then you get a lens, and so on and so forth.
A flashback to 500 million years ago,
shows that our eye was indeed formed in successive phases,
starting from a primitive organ.
Returning to our prenatal life,
also allows us to understand the genesis of our eye.
The unique images made by Professor Bradley Smith
reveal its secrets.
We see in this human embryo the development of the retina,
which is this dark area, that we see in the eye.
Those are the cells that actually detect the light
as it comes through the eye.
The retina is in fact an outgrowth of the brain.
Nervous tissue from the brain travels along
what forms the optic nerve,
and eventually creates the retina.
This is something that's shared by all vertebrates.
Sight is a key advantage
in the logic of evolution.
It is the ability to situate oneself in space,
to have direction, to move about.
Vision also allows the detection of prey,
detecting danger,
selecting food, identifying members of your own species,
and interacting with them.
Often, sight is associated with action.
This is where limbs are a major advantage for mobility,
and interaction with the environment.
The origin of our limbs takes us back 400 million years.
At that time, life on our planet was exclusively aquatic.
This was the moment when boned fish,
which make up the most of today's species,
diverged from cartilaginous fish,
such as sharks and rays.
These conserved anatomically primitive characteristics,
especially a series of flaps of skin,
which protected their gills.
These flaps were stiffened by arcs of cartilage,
which could be related to our limbs,
an astonishing hypothesis,
tested by Professor Andrew Gillis.
To undertake his research on the subject,
Andrew regularly sets up at the marine biology laboratory
of Woods Hole, near Boston.
On this Atlantic Coast, specimens are right at hand.
And every morning Andrew
can expect a delivery of fresh fish.
As the shark is a cartilaginous fish
that's difficult to handle,
Andrew studies the ray, more common, and easy to capture.
The anatomical features
that we are particularly interested in,
are the features of the gills in cartilaginous fishes.
And so one of the things
that's really interesting about their gills,
is that they have little cartilaginous rods
that extend off of their gills.
And these features are particularly interesting
because it was proposed over 150 years ago,
that they could represent sort of
an evolutionary precursor to limbs,
so just as we have a limb that extends off of the shoulder,
the gills of cartilaginous fishes
have little rods called branchial rays
that extend off the gill cartilage at the side of the head.
(dramatic music)
To observe this anatomical peculiarity,
Andrew Gillis needs to identify
the skeletal structure of rays.
That's why he programs x-rays
for the freshly delivered fish.
A light anesthetic bath is needed
so that the subject doesn't tremble during the x-ray.
While the animal's skeletal structure appears clearly,
the gill arches don't stand out with the detail expected.
You've got the whole gill region for sure,
but we just can't really see,
I think the gill elements may just be too small,
and too lightly mineralized to see with this imaging method.
To be sure not to miss any clue,
Andrew also asks for a ventral image of the rays.
The gill arches appear more clearly,
but the relationship of the structure
with our skeleton appears distant.
The link is perhaps at a less-obvious level.
It is in ray embryos that Andrew will seek
the origin of our limbs.
He samples specimens delivered by the morning boat.
Embryonic development has classically been viewed
as a really key source of information,
in understanding the evolutionary relationships
among organisms, and also among structures
within an organism.
And so it's generally thought that structures
that share an evolutionary history,
will share common developmental programs.
Andrew opens the protective coating
of the embryos at different stages of their development.
Then he can observe how the gill arches form
through the microscope.
Much like the development of limbs in mammals,
the gills of cartilaginous fishes
begin to form as a little bud
that forms off the side of the head,
and it very much parallels the way
that the limbs form in animals like ourselves.
The similarity isn't by chance,
but is due to the action of the genes
we share with the rays.
The genes that drive development of the human embryo,
and other vertebrate embryos, are the same genes.
They're homeobox genes,
and these hox genes, as they're called,
create segmentation in the animal,
and essentially a limb is just a specialized segment
of the vertebrates' segmented body.
These particular genes are activated,
such that a limb develops
out from the spinal cord at that level of development.
So it's the same genetic program
that triggers the development of our limbs,
and the formation of gill arches in rays.
You can think of how evolution solves problems,
not necessarily by inventing new ways
to do things every single time,
but perhaps by taking genetic programs
that work perfectly well, and reusing them again,
maybe with some modification.
And the similarities between the gills
of cartilaginous fishes,
and the limbs of other jaw vertebrates,
could be an example of this reuse or recycling
of other ancient genetic programming.
Our work is focused really on the role of a particular gene,
called sonic hedgehog,
in the development of the gills of cartilaginous fishes.
And this gene is known to be very important
for the development of limbs,
where it plays two very distinct roles.
Initially it's involved in telling the limb
which way is front and which way is back,
or which end will form a thumb,
and which end will form a pinkie.
And it's also involved in controlling
the number of elements that form at the end of the limbs,
or the number of digits.
And we've been looking at how the sonic hedgehog gene
functions during the development of gills,
and branchial rays in cartilaginous fishes,
and we found that it functions actually
in a very similar way.
To prove this,
Andrew experimented with blocking this genetic program
in the ray embryos.
Here we can see skeleton of a skate gill,
and in this normal gill,
we can see that they have a set of these
finger-like rods sticking off the gill skeleton.
Then when we treat the embryos with a drug,
that blocks the activity of the sonic hedgehog gene,
what we can see is a really striking reduction
in the number of these rods.
So it's kind of like the equivalent
of losing several fingers from your hand.
This effect of the sonic hedgehog gene
controlling the number of skeletal elements,
and its symmetry,
is something that seems to be shared
between the sharks and skates, and the humans.
So our two legs and two arms
are the fruit of a genetic program
that had already succeeded in fish.
It orders the development of pairs of appendages
along our bodies.
Another substantial surprise, this program
is also what is at the origin of our most intimate organs.
Our external genitalia were originally a sketch
for a third pair of limbs.
This dramatic turn of evolution
happened around 360 million years ago.
Professor John Long has launched this unusual inquiry.
An enigma buried in the rocks,
about the first great sexual revolution
in the history of species.
A mystery longtime guarded by our distant aquatic cousins,
now fossilized.
The placoderms, whose path we have already crossed
to understand the evolution of our eye,
are at the heart of John's research into the origins of sex.
The fossils of these strange fish
are buried in geological layers
that are 350 to 400 million years old.
John searches for the slightest indication
that would tell him, here there is a fossilized placoderm.
John is interested by placoderms,
because they hold the key to the origins of our sexuality.
Placoderms had the first external genital organs,
male reproductive, and female genital plates,
for any vertebrates.
In other words, they're the first kind of vertebrate animal
to have distinct male and female types.
John has been able to arrive
at such a conclusion,
thanks to the unhoped for discovery of a unique specimen.
I was collecting fossil fish
up at a site in northwestern Australia,
and in 2005, we found a well-preserved placoderm fish,
and when we prepared the fossil out of the rock,
using weak acetic acids to dissolve the rock away,
we found something truly extraordinary.
We found an embryo,
and that embryo was the oldest known embryo
of any vertebrate ever discovered.
Basically we discovered the fish
that was the oldest mother on the planet,
and that had an embryo inside it,
an embryo connected by an umbilical cord.
So from studying the bones of this particular fossil,
we were able to make a reconstruction
of what it looked like when it was alive.
And from this, we were able to make a model,
and then from the model, make a 3-D animated movie,
showing the very first act of a mother fish
giving birth to its young.
John Long quickly deduced
that this happy event, fixed in time,
was preceded by sexual contact between a male and a female.
So he became interested
in the intimate anatomy of placoderms,
so as to understand what happened at that time.
It was then that another fossil came out of the rocks,
and gave up to him the secrets of love
amongst the placoderms.
It still took the alert eye and experience of John,
to interpret certain anatomical details.
When you spend your whole life studying one group,
like placoderms, you become very intimate
with their anatomy and their structure,
and sometimes you can find things
that other people have missed.
Here we have a fossil, called microrackeus from Scotland.
It lived about 395 million years ago,
and the area I'm studying today,
is this part here, which are actually
the male reproductive organs or claspers.
And the specimen is very important,
because we're able to then make a three-dimensional
reconstruction, based on these specimens.
So this is the male here,
with the claspers, and this is the female,
which has these smaller genital plates in position.
These arms on these placoderms
were a mystery for over 100 years.
Everybody thought, what are they for?
What do they do with these tiny, puny little arms,
but now we realize that because the claspers here,
would have been very difficult for a male to maneuver
into the female area for mating,
so that the little arms were interlocking
to give them purchase,
and you'll see this on the movie we made,
the animated film of them mating.
(sexy jazz music)
These fish invented the first sexual position,
which was side-by-side, like dancing.
Intercourse was born
more than 300 million years ago,
and has since been adopted with success by many species.
The coupling of sharks in a lateral position
is reminiscent of that of the placoderms.
For this reason, John thought that the structure
of the reproductive organs was comparable,
but the placoderms held other surprises.
When we first discovered the sexual organs of placoderms,
we thought they were like sharks.
Sharks have claspers, which are male reproductive organs,
which develop off the pelvic fin,
or part of the hind limbs.
The placoderms were different.
We only recently discovered
they developed their paired sexual organs,
exactly like limbs, as a second pair of legs.
So in other words, we vertebrates today
have arms and legs, we have four limbs.
Placoderms originally had six limbs,
and the hind limbs were actually the reproductive organs.
The unlikely heritage of placoderms
reveals an essential notion of evolution, plasticity.
This is what Professor Patrick Tschopp is studying,
starting from an easily observed model, a chicken egg.
My main interest is how morphological
diversity is generated,
but you cannot study this as a global question,
so you pick yourself some very like examples
that allow you to tackle this question.
Patrick is particularly interested
in how the buds of members form in chicken embryos.
So here we have a chicken embryo
at three days of development,
and you can see already the first structures
of the developing chicken, with the two limb buds.
So this one is the fore limb, will turn into the wings.
This is the hind limb,
will turn into the legs of the chicken.
The notion of plasticity
is completely illustrated by this.
The same animal will develop two pairs of limbs
that are totally different, wings and feet.
But at an early stage of development,
the buds of limbs are not yet differentiated.
At this point of development,
the limb bud looks very similar in all vertebrates
that are able to walk on land,
but only later on you will get
like the species-specific differences,
like for example that this will turn into a wing,
for us it will be an arm.
One of the so-far major observations
was that there is a tremendous amount of plasticity.
So meaning, even if you change the skeletal pattern,
muscles and nerves which need to correctly
attach to the individual bone or skeletal elements,
they seem to be quite flexible in doing so.
It was while observing what happened
at a microscopic level,
that Patrick discovered the key
to this surprising plasticity.
This is a communication between two tissue types,
it's the neuronal tissue trying to find its way,
and it's being guided, as far as we understand it right now,
by the surrounding tissue that releases signaling molecules,
and that's what makes them find their way
to make contact with the muscle.
We find this plasticity
in the formation of genital organs in human babies.
The embryo has a structure known as a cloaca.
Cloaca is a structure or an organ
that's common to many vertebrates.
It is an organ that eventually becomes
both the urethra, in a human for example, and the rectum,
but at this early stage of development,
the organs are combined into one single organ.
In the embryo, the mechanisms
of intercellular communications come into play,
which are crucial for the development of different organs.
You have certain sender populations
that send a signal to another cell population.
You become this type of organ,
but the sender does not care so much
what the recipient is.
And as long as the recipient can respond to it,
then there is this flexibility,
and so in that example of the genitalia,
the descender is the cloaca,
can send signal to either the tail bud,
or to the limb bud, and recruit cells.
The position of the cloaca in the body,
varies according to the species.
The cells which intervene
in the formation of genital organs,
are therefore different.
In lizards and snakes,
it seems that this cloaca has shifted relatively,
to a more anterior position,
so that means more towards the head,
and is actually lying next to the limb bud,
and that the signals from the cloaca
then seem to recruit actually cells
from the limb bud, for the outgrowth of the genitalia.
Whereas, for example, in mice,
this inductive structure, this cloaca,
is more posterior, more towards the tail,
and therefore also starts recruiting cells from the tail,
rather than the limb bud.
But this is just due to proximity.
It is in this way that the cells,
which were originally programmed to form a tail or feet,
finally give birth to a genital bud.
This cellular mechanism flexibility
lets us understand better how the third pair of limbs,
or for placoderms, could evolve towards the formation
of external genital organs.
As it once was famously said by Francois (muffled).
(speaking in foreign language)
So really, because it is so flexible,
that's why nature, or evolution,
can actually tinker around and change little things,
because it is quite flexible,
and can adapt to small changes.
Our body has the marks
of the successive handiwork of evolution,
and we wouldn't be the same
if we'd been designed by engineers.
Let's imagine the fantastic scenario,
where a handful of enlightened researchers
study this question.
How can we create an ideal body
and optimize its performance?
In the secret location,
they would think about possible improvements
to be made to the functioning of our organs.
They would reason according to logical
and efficient criteria.
Surely they would redesign our anatomy.
They'd be looking for solutions to correct incoherences,
or rationalized mechanisms.
A human body has clearly not been designed
by an engineer.
Why?
Because there are systems,
really their efficiency is questionable.
For example, we have a pump inside the thoracic cage,
which is the heart,
and well the exit, the main aorta,
is just a sort of pump,
there where the pressure is at its maximum,
which is angled at 180 degrees.
You can't say that's optimal.
So things work, but we'll never know in biology
if the way that we're made up,
if such and such an organ could work better
than it works now.
If our body doesn't work as badly as all that,
its functions aren't always optimal.
Some organs recount the sometimes tortuous genesis,
which led them to their current form.
An example of the handiwork of evolution,
is a specific nerve, called the phrenic nerve,
which allows the diaphragm to contract.
The diaphragm is the muscle beneath the thoracic cage,
and which allows the thoracic cage to open,
and thus to open the lungs,
so as to ventilate and to breathe.
So it's an important muscle,
but well, the diaphragm's motor nerve
is not connected to the spinal cord
in the segment right in front of it,
the shortest distance, I'd like to say,
but it's connected around the third cervical vertebra.
To simplify things,
it would doubtless have been more logical
to connect this nerve to the spinal cord closer,
and to spare it this long, useless journey.
If things are like this, there's a good reason for it.
380 million years ago,
this muscle block we have beneath the thoracic cage,
was close to the gills, behind and under the head,
in the first tetrapods.
In animals with fleshy fins
which were still in the water at the time.
During the next 200 million years of evolution of tetrapods,
four-limbed organisms,
this block of muscle kept on moving back,
right to the rear of the thoracic cage,
where it is now in mammals.
And the course of the phrenic nerve
is a historical route which follows the muscle,
which it has always commanded.
Since it's a long journey,
from time to time this nerve folds,
and well, that's when you have hiccups.
(hiccuping)
To sum that up, hiccups remind us
that in a very distant past, we were fish.
Perhaps it's a vague memory, buried in our brain,
which leads us to drink water
when we have those damned hiccups.
This little discomfort of our daily life
bears witness to the long history, which links us to fish.
Amongst our distant aquatic cousins,
surely the placoderms left us the richest inheritance.
Placoderms were the first vertebrates
that began the body plan
that would carry on to the rest of evolution.
In other words, the first creatures
to have paired limbs at the front, and paired hind legs,
and jaws and teeth, and a skull that was fairly advanced.
For a long time, the placoderms
were associated with another species,
which is still living, sharks.
We used to think that placoderms and sharks
were very closely related,
but the latest theories are starting to show
that placoderms are a separate group,
and that they were ancestral
to both sharks and the bony fishes,
which include the tetrapods, the lineage leading to us.
So placoderms are our deep deep ancestors
of all living vertebrates.
Having been the dominant species in the oceans
for 70 million years, the placoderms disappeared.
Their heritage is now written in stone.
Fossils are the grand narrative
of the evolution of life on the planet.
Every fossil is a life once lived.
It's a story that tells us
about the great diversity of life.
It is sure that just before
the placoderms disappeared,
another major event marked the history or our evolution,
the conquest of the Earth.
Is the terrifying Bocourt's skink,
that was thought be extinct,
a distant copy of the first terrestrial tetrapods?
It's in the southeast of Australia,
in the rocks of the Grampians National Park,
that John Long hopes to find traces
of the first animals that had ventured beyond the water.
(birds singing)
(dramatic orchestral music)
These environments here
represent ancients seas and ancient rivers,
and they were full of life.
Ancient fishes for example, many different kinds.
But also, probably the first land animals,
the tetrapods, left their footprints in these rocks,
about 390 million years ago.
Some of our aquatic ancestors,
which tried the first expeditions out of the water,
perhaps looked like this strange animal.
The periophthalmus, which lives
in the muddy mangroves of tropical zones,
has the look of a chimera.
Half fish, half frog, it lives both in and out of the water.
It's multi-use fins are its feet,
and though it has lungs,
it also breathes through its skin, like atraceans.
Now here's an interesting mix of adaptations,
which could reflect an intermediate phase
between life in the water, and life on land.
(bright orchestral music)
But it was later, on solid land,
that other crucial adaptations took place,
which punctuated the history of our body.
Recent research shines a new light
on some of the great stages of our terrestrial evolution.
The maxillary bones,
which allowed our reptile ancestors to chew,
have progressively formed
the miniature bonelets of our middle ear,
thanks to which we can hear.
This incredible mutation
can still be observed in marsupials.
Our brain is not that different from that of the great apes,
even if it is larger.
But most astonishingly,
we don't have the largest brain of our line.
40,000 years ago,
neanderthal had a larger brain than we do.
And finally, our hand, so skillful
and adapted to precise manipulation,
is a lot more primitive than we think.
But all that is another chapter
of the secret history of our body,
a history which made us the humans that we are today.
(upbeat jazz music)