Around 10,000 years ago, somewhere in Africa, a microscopic parasite made a huge leap.

With a little help from a mosquito, it left its animal host - probably a gorilla - and found its way to a new host: us.

And this wasn’t unusual - pathogens jumping from animals to people is a tale as old as time.

But this time was different.

This parasite would go on to be the deadliest in the history of humanity.

Its scientific name is Plasmodium falciparum, but you probably know it as the cause of malaria.

Now, this species isn’t the only parasite that causes malaria in people - at least five different Plasmodium species have made the jump from great apes and other primates into us...

But it’s the most lethal member of the group - responsible for the vast majority of worldwide malaria deaths.

And where these single-celled parasites got their start - before plaguing our primate relatives, I mean - is pretty...weird.

Because, they didn’t start out as parasites at all.

Over 500 million years ago, the ancestors of Plasmodium swam the ancient seas as harmless algae.

In the mid-1990s, researchers studying Plasmodium found the first clear clue to its bizarre origin.

Inside each parasite were the remains of structures called plastids.

One kind of plastid that you might’ve heard of are chloroplasts, which are found in algae and plants.

These kinds of structures have their own DNA, which is separate from the rest of the parasite’s nuclear DNA.

And the researchers found that the DNA of the plastids in Plasmodium matched the DNA of the plastids in algae and plants pretty closely.

And this made no sense, because algae and plants use their chloroplasts for photosynthesis.

So why would a structure used for photosynthesis exist in a parasite that lives in the dark -literally inside of the bodies of animals- and never sees the light of day?

Now, the researchers could tell that those plastids weren’t capable of harvesting sunlight anymore, but they clearly had been at some point in the past.

It turned out that they had lost the genes for photosynthesis.

And this was another clue in the mystery of Plasmodium’s origin - a hallmark of its branch of the tree of life, a group scientists called Apicomplexa.

Almost all of the members of this group are parasites, but they still carry this leftover plastid, called the apicoplast.

It’s an evolutionary hangover from their photosynthetic past.

This discovery raised a bunch of new questions, like what did the ancestor of Plasmodium and the other Apicomplexans look like?

What was its lifestyle?

Where did it get the plastid?

And why did it give up a wholesome photosynthetic life to become a brutal parasite of land animals?

Well, the fossil evidence is pretty limited.

We are talking about a single-celled organism, after all.

But in 2005, a paper was published on a piece of amber from the Dominican Republic.

The amber contained a mosquito from probably 15 to 20 million years ago with a few small spore-like structures on its abdomen.

These were identified as different parts of the lifecycle of Plasmodium, which told us that it’s been around and been transmitted by mosquitoes for at least that long.

But that fossil was much too young to tell us anything about the deeper origins of the apicomplexans.

What we really needed was a living example of a close relative to the Apicomplexans -- one that had kept its photosynthetic, marine lifestyle.

You know, one that hadn't turned evil.

And, in 2008, that's exactly what was described from the waters of Sydney harbor: a photosynthetic single-celled organism very closely related to the Apicomplexans, called Chromera velia It bridged the gap between completely independent, photosynthetic algae and parasitic apicomplexans.

It had the best of both worlds - it could harvest sunlight with its plastid, but it also benefited from a symbiotic relationship with corals.

Not only did Chromera give us a glimpse into what the ancestor of apicomplexans might have been like, it also offered clues as to where the apicoplast came from in the first place.

A key piece of evidence is that Chromera’s plastid is very similar - structurally and genetically - to the apicomplexans’, which suggests that both groups inherited it from a common ancestor.

And their similarities meant that this common ancestor got its plastid in the first place by engulfing a red algae in a process called secondary endosymbiosis.

Now primary endosymbiosis is when a complex cell called a eukaryote engulfs a simpler cell called a prokaryote, and just keeps it instead of destroying it.

It’s how our ancient ancestors gained their mitochondria, and how plants gained their chloroplasts.

And secondary endosymbiosis is when that first eukaryote is engulfed and incorporated by a second eukaryote - and still keeps the structures from the first simple cell.

It’s basically a cellular Turducken.

In this case, over 1 billion years ago, a simple photosynthetic bacterium was engulfed by a eukaryote which gained the ability to photosynthesize as a result.

This was the primary endosymbiosis.

And this event gave rise to many of the photosynthetic lineages we know today, including the red algae, green algae, and land plants.

Then, probably a few hundred million years later, a red algae was engulfed by the common ancestor of apicomplexans and Chromera.

This allowed it to steal the photosynthetic plastid from the red algae.

This was the secondary endosymbiosis.

Ok, so that’s where the plastid came from - but what happened next for the apicomplexans?

How did they split off, become parasitic, and lose their photosynthetic abilities?

Well, that transition is still mysterious.

But researchers think that after hundreds of millions of years of being partially photosynthetic and probably living in symbiotic relationships with early animals in the oceans like Chromera does with corals today, the apicomplexans turned to the dark side.

They became what are called obligate parasites - organisms that can’t live independently and rely fully on exploiting the bodies of their hosts for survival.

This transition is thought to have first occurred around 500 million years ago, and may have happened independently more than once in the apicomplexans.

As the animal kingdom radiated, so did the apicomplexans.

And as animals made their way onto land, they brought the parasites with them.

So the relationship between these two groups is pretty ancient, and may at least partially explain why so many animals are parasitized by apicomplexans today - including us.

Over 6000 species of these parasites have been described to date - with many, many times that number probably undiscovered.

Apicomplexans infect all kinds of animals, from snails to horses and pretty much everything in between.

And Plasmodium isn’t even the only one that infects us - there are at least three other groups that do, too.

While their ancient transition to parasitism led to them losing their ability to photosynthesize, the vast majority of them still have their plastid.

And it actually still makes other key compounds that they need, like cellular building blocks and resources they can’t scavenge from their host.

It’s yet another example of how the process of evolution works with what it’s got - adapting and reusing parts an organism already has to keep up with its changing environment.

And it’s only thanks to that that we were able to stumble upon the real story of the malaria parasites’ origins as algae.

Because if the apicomplexans had simply lost their plastids, we might never have guessed that they started out as harmless marine organisms that relied on the sun for energy.

But, lucky for us, the plastids are still there -- an ancient family heirloom passed down from generation to generation with a history stretching back over a billion years.

And while the discovery of their plastids means that we’re finally starting to know our enemy, they already know us intimately -- having been with us since the dawn of animals, first as friends, and then as foes.