The gamechanging amniotic egg April Tucker
Would you believe that walruses,
rattlesnakes,
and parakeets
all once lived in the same house?
Let’s go back about 350 million years.
Look around.
Steamy swamps and rain forests
of horsetails and ferns
cover the region.
Amphibians are the dominant
land vertebrates.
They range in size from newts
to crocodiles.
And all require water
to do their egg laying.
If they don’t go to the water,
their shell-less, jelly-like
eggs will dry out.
Because of this hazard,
they spend most of their time
living in or near fresh water.
That is, until a breakthrough
in evolution changes everything:
the amniotic egg.
The amniotic egg is shelled,
waterproof,
and can be laid on dry land.
It is produced by the amniotes,
a new group of animals named
after their revolutionary egg.
The first amniote is a tetrapod,
a four-legged animal,
resembling a small lizard.
While some amphibians
can walk around on land
and bury their eggs in wet
soil or highly humid areas,
nothing before the amniotes
has the ability
to lay its eggs on completely dry land.
Because of this evolved egg,
the amniotes are the first animals
with the ability to live
a fully terrestrial life.
But, despite their move inland,
the amniotes have not abandoned
their pond-dwelling upbringing.
In fact, the amniotic egg
brings the pond with them
by enclosing the aquatic
environment within its shell.
This is achieved by four main upgrades
that are unique to amniotic eggs.
Let’s take a closer look.
The first development is the most obvious:
the egg’s protective shell.
It’s tough but flexible,
and has a leathery surface,
still seen in reptile eggs today.
The shell protects
the eggs from predators,
bacteria,
damage,
and drying out.
But, unlike the walls of a fish tank,
the shell of the amniotic egg is porous,
allowing oxygen to pass through
so that the growing amniote
inside doesn’t suffocate.
The next two developments
are two separate membranes
that work together like a pair of lungs.
They bring oxygen into the embryo
while removing carbon dioxide.
The first is the chorion,
which is the protective layer
that oxygen passes through
after entering the shell’s tiny pores.
You may recognize the chorion
as the thin skin you peel
away on a hard boiled egg.
Think of this waterproof membrane
as the in and out doors of the egg.
It’s the entrance for oxygen
and exit for carbon dioxide.
The membrane working with the chorion
is the allantois.
If the chorion is the doors,
then the allantois is essentially
the lobby of the building.
It directs the oxygen and carbon dioxide
while simultaneously storing
unneeded waste from the embryo.
The chorion and the allantois
make sure the embryo
has everything it needs
and gets rid of anything it doesn’t.
The last and perhaps
the most important development
is the amnion, the membrane
for which the egg is named.
The amnion is also contained
within the chroion
and holds the fluid
in which the embryo floats.
Because it has left the watery
world of the amphibians,
the amnion is necessary for preventing
the embryo from drying out.
It is the transportable pond
that allows the amniote
to lay the egg on dry land.
Its fluid also protects the embryo
from any collisions or rough landings,
like a shock absorber on your bike or car.
Together, the shell
and these four membranes
create a safe, watery environment
for the embryo to grow and develop.
The new amniote offspring will continue
the process of vertebrate evolution
as it explores new land
away from the water.
They will spend the next million years
splitting into two distinct groups:
the synapsids and sauropsids.
Synapsida is the group of animals
that contain mammals,
while sauropsida
is the group that contains
reptiles,
birds,
and dinosaurs.
These two amniotic groups
collectively contain
the walruses,
rattlesnakes,
and parakeets we know today.
Like a family reunion,
with relatives of every shape and size,
coming together from different
corners of the Earth,
these animals can all call one place home:
the amniotic egg.