[♩INTRO]
If you’ve heard of Chlamydia, it’s probably as the bacterium responsible
for a certain sexually transmitted infection.
It belongs to a broad group of bacteria called the Chlamydiae,
members of which are responsible for illnesses from pneumonia
to eye infections.
They don’t do much to endear themselves to us, in other words.
Except… we may have them to thank for life as we know it.
In 2020, scientists made a discovery that reveals clues to how complex life
might have emerged.
And this story has it all, from chlamydia to deep sea exploration to Norse gods.
Many scientists think that both the simplest forms of life and more complex
organisms originated around deep sea hydrothermal vents.
These underwater volcanic formations are
rich in energy and minerals ideal for supporting life.
They host tons of microbes, and have probably been doing so for a long time.
Some researchers believe they could be where the first microorganisms
evolved around 3.8 billion years ago.
These original single-celled organisms were prokaryotes
simple cells that don’t have a nucleus.
They were the ancestors of the two types of prokaryotes alive today,
bacteria and archaea.
Then, about 1.8 billion years ago, prokaryotic cells
gave rise to a more complex type of cell: eukaryotes.
Eukaryotic cells have a nucleus,
and make up all of the complex organisms we know today, including us.
And one of the biggest open questions in biology is:
How did complex eukaryotic cells
evolve from their simpler prokaryotic ancestors?
Scientists think it started when two types of prokaryotes partnered up
to share nutrients.
But that partnership got kind of weird
when an archaeal cell swallowed a bacterium.
Specifically, an alphaproteobacterium.
And what’s even stranger is that these cells remained partners
and formed a mutually beneficial relationship.
That’s thought to be the origin of mitochondria
the structures that allow eukaryotic cells to produce energy from food.
They convert energy from sugars to ATP,
the molecule that cells use as their energy currency,
using up oxygen in the process.
That process is known as cellular respiration.
But there’s a mystery.
The first eukaryotes probably didn’t use oxygen
because there wasn’t much of it available 2 billion years ago.
Now stay with us for a second, because we’re getting to the chlamydia part.
In the absence of oxygen, eukaryotes would not have had access
to the oxygen-based respiration we know today.
So, many scientists think that instead of oxygen,
the first eukaryotes may have had a system that revolved around hydrogen.
Specifically, molecular hydrogen, or H2.
And that’s not unheard of today.
You can find organisms who use hydrogen
at the bottom of the ocean and inside cow stomachs.
But different microbes have different ways of using, or metabolizing, hydrogen.
So scientists aren’t exactly sure how the original eukaryotes did it.
But since the first eukaryotes likely metabolized hydrogen,
scientists think that one of the two original partners
archaea or alphaproteobacteria
had already evolved the ability to produce it.
And the other would have consumed hydrogen,
leading to a beautiful and lasting friendship.
But they had no clue which of the two organisms brought the hydrogen
to the relationship.
And then recently, researchers discovered that
it might actually have come from a totally different organism.
It turns out, there were three microbes in this marriage.
And that’s where chlamydia comes in.
In 2008, scientists discovered a field of hydrothermal vents
with mineral formations that looked like a fantasy castle.
So they named it Loki’s Castle, after the Norse trickster god.
Later, another group of scientists were scouring sediments near Loki’s Castle,
searching for microbes around the vents.
And they found an archaean they named Lokiarchaeum, or Loki for short.
Later discoveries would play up the theme
the group also includes Thorarchaeota, Odinarchaeota and Heimdallarchaeota.
But we still didn’t know how those ancient archaea
and bacteria were using hydrogen.
So they rummaged around Loki’s Castle, and they found... chlamydia.
But not the kind you’re thinking.
Up until 2020, scientists thought all members of Chlamydiae were parasites
that could only thrive by taking up residence inside eukaryotic cells.
And they often make the host organism sick in the process
like the Chlamydia we know and don’t love.
But the chlamydia bacteria at Loki’s Castle are probably not parasitic.
When the scientists examined the deep sea sediments,
they found abundant chlamydia but very few eukaryotes.
So the researchers don’t think the chlamydia
need to mooch off eukaryotes to thrive.
In fact, these deep sea chlamydia are so distinct and so much nicer
than other chlamydia that the researchers put them
in their own taxonomic order—the Anoxychlamydiales.
And as the name suggests, they don’t need oxygen.
Instead, their metabolism uses, you guessed it, hydrogen.
Here’s the best part, though:
The genes these chlamydia use to metabolize hydrogen
most closely resemble the ones used by eukaryotes.
So the researchers think eukaryotes actually nabbed their
early respiratory genes from chlamydia.
In other words, our mitochondria would still be descended from
an alphaproteobacterium, not a chlamydian.
And our archaean ancestor would still be a relative of Asgard archaea
but it would have had some help.
See, many microorganisms can swap DNA with other microbes
through a process called horizontal gene transfer.
So here’s what may have happened a few billion years ago.
An ancestor of Asgard archaea and an ancient chlamydia bacterium
walked into a bar.
Er, a vent.
They met up, and the chlamydia passed some of its hydrogen respiration tools
on to the archaean.
The archaea then brought that skill to eukaryotes,
allowing them to thrive in oxygen-deficient oceans
and eventually to evolve into us.
Without those hydrogen-processing genes,
eukaryotes as we know them wouldn’t exist.
So… without chlamydia, we might never have evolved.
So I can’t believe I’m saying this, but… thank goodness for Chlamydiae.
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[♩OUTRO]