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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 Achaean 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 Achaean 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 Achaean.

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. Thank you for watching this episode of SciShow! If you liked it, you might be  interested in becoming a channel member.

The money you pledge each  month as a channel member helps us keep making fun science  videos like this one right here, and helps us keep them free and accessible. So thank you to everyone who  already is a channel member and if you want to get in on that action, check out the Join button right below the video. [♩OUTRO].