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Since the beginning of life on Earth, there's been parasites. Wherever there's life, there's other life looking for a free ride. Join Stephan Chin for a new episode of SciShow and take a look at six of the world's oldest parasites.

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[ intro ].

Where there's life, there's other life that's looking for a free ride. A parasite is an organism that lives on or inside another organism, and it gets its nutrition at the expense of that host.

It may not always be pretty, but parasitism is an extremely successful life strategy. Among animals, for example, around 50% of living species are considered parasitic. And this isn't a new technique.

Many of the parasitic strategies we see today have been employed for millions of years, and when we're lucky, we can find evidence of them in the fossil and archaeological record. So here are six superlatively old parasites -- among the oldest of their kind that we know about so far. Now we humans like to think we're pretty special, but we're still animals, and we have our fair share of parasites.

Among the worst are schistosomes, a group of flatworms that cause the disease schistosomiasis. According to the US CDC, schistosomiasis is second only to malaria as the most devastating parasitic disease in humans, with more than 200 million people infected worldwide. These little worms can cause rashes, chills, anemia, organ damage, and even cancer.

And they also have the distinction of being some of the most ancient human parasites on record. A 2014 study reported evidence of a human infected with schistosomes in a Syrian archaeological site dating to at least 6000 years ago. The evidence didn't come from the worms themselves, but from their eggs.

A close look at the pelvic region of one human skeleton revealed that the sediment contained tiny objects, only 130 micrometers long, that turned out to be schistosome eggs. See, for these worms, our bodies are a nice place to make babies -- they lay their eggs inside our guts, and our bodily functions carry the eggs off into the world. And it's actually our bodies' reaction to the eggs that causes all those nasty symptoms.

The fact that eggs were found around the pelvis of this ancient individual, but not near any other body parts, is a good sign that this poor person had a gut infection. This is the oldest known example of this kind of parasite in humans , and it raises the question of how this human ended up infected. Before infecting us, the worms spend an earlier part of their lives inside water snails.

Humans commonly pick up the parasites while wading in freshwater. So maybe this infected person wandered into the wrong stream, but there could actually be a more interesting explanation. At the same ancient settlement, there are signs of crops that wouldn't normally survive in the local dry climate, which leads scientists to suspect the farmers were employing an early form of irrigation.

So, it could be that by redirecting water to their crops, the locals accidentally provided a pathway for one of our oldest parasitic foes. In other words, this ancient infection could be a clue to human advancements in agriculture! Now, if you wanted to check for intestinal parasites and you didn't have access to an intestine, the next best option is poop.

And it was inside of some ancient poop -- a type of fossil called a coprolite -- that a 2019 study identified the oldest known parasite DNA. The coprolite in question came from a rock shelter in Argentina and was dated to around 17,000 years old. Fossils and artifacts show that this site was home to lots of mammals over the years, like ground sloths, horses; even humans later on.

But based on the size and shape of this particular poop, researchers suspected it came from a big cat. And while they examined it, they noticed lots of tiny eggs inside that looked like they belonged to nematodes, or roundworms. To confirm their identifications, they extracted ancient DNA, which revealed that they were right on both counts: the poop came from a puma, and the eggs belonged to the nematode species Toxascaris leonina.

Today, this is a species that infects the guts of cats and dogs, both wild and domestic, all around the world. And as of that study's publication, it's the oldest DNA evidence of a parasite, but its age also reveals something about the parasite's history with its hosts. It's been widely thought that, in the Americas, these parasites jumped from domestic animals to wild species.

But this infected cat is too old to have met humans in this part of the world -- or to have exchanged worms with their fluffy companions. So it seems these parasites have been plaguing wild cats since before our species got involved -- meaning that there's more to learn about the history of these worms. Now, while we and our feline friends have to worry about being infested with eggs that wreak havoc inside our intestines, some insects have more… dramatic concerns.

Certain wasps lay their eggs inside other insects. After the eggs hatch, the newborn larva feeds on the host for a while before bursting out of its body like the xenomorphs from the Alien movies. These wasps are called parasitoids.

They're not true parasites, since only the young are dependent on a host, which their life cycle usually kills. Even so, it's a winning strategy. It's estimated that between 10 and 20% of all insects are parasitoid wasps.

And as you might expect, this somewhat terrifying strategy has been around for a while, going back at least 35 million years. This lengthy history was revealed in a 2018 study that examined fossilized fly pupae from southern France. A pupa is the cocoon-like phase these flies went through between being larvae and adults.

And the researchers found over 1500 that had been preserved in this stage of life. But what was most intriguing -- and kind of disturbing -- is that when they used X-ray imaging to peek inside the pupae, they found that 55 of them were occupied by parasitoid wasps that had hatched and developed inside. Most of these wasps were already adults, and they seem to have been waiting for the right time to emerge from their hosts.

Some parasitoids today will sync up their adult emergence with their host species, so that there's plenty around for the next generation. But these are the oldest known fossils of parasitoid wasps inside their hosts, and among them, the researchers identified four brand new species. Two of these species were grouped into a new genus very appropriately named Xenomorphia.

Finding such a diversity of chest-bursting wasps in one fossil site is a good clue that the parasitoid lifestyle has been a lucrative and successful strategy for a long time. Now worms and insects are pretty common parasites, but animals also have to be wary of infections by fungi. And if you thought chest-bursting wasps were disturbing, might I remind you that there are parasitic fungi that mind control their victims before erupting from their skulls.

Some modern day fungi, like Ophiocordyceps, get inside the bodies of insects like ants and manipulate the bugs to crawl high up on vegetation. From this elevated position, the fungus reproduces by sprouting long, spore-covered stalks from the insect's head or body. It might be the most horrifying parasitic strategy on this list, and these fungi have been at it since the age of dinosaurs.

A 2008 study reported an insect preserved inside a piece of Burmese amber from the Cretaceous Period, about 100 million years old. Emerging from the head of the insect were two long fungal stalks, each longer than the insect itself. Researchers could even examine the spores preserved on the stalks.

Amazingly, this fungal infection was fossilized in the act. And at the time of publication, it's the oldest record of animal parasitism by a fungus. The researchers identified the fungus as a close relative of Ophiocordyceps, and so named it Palaeoophiocordyceps.

And by comparing this ancient species to its cousins, they were even able to reconstruct the evolutionary pathway of this group. Using a method called ancestral state reconstruction, which compares closely-related species to estimate what traits their ancestors had, the team concluded that this group of fungi started off as plant parasites. And then at some point in their history, they made the jump to animals and became mind-controlling, head-bursting terrors.

But some even older parasites are actually a little more familiar to our own human experience: tapeworms! Tapeworms are an infamous group of flatworms that get inside our intestines and set up shop stealing our nutrients and laying eggs. And some have been so prolific in this life strategy that they can grow as long as 25 meters.

But we're not the only ones targeted by tapeworms. We share this burden with lots of other animals around the planet -- and throughout time. The oldest evidence of a tapeworm infection in a vertebrate comes from the Permian Period, around 270 million years ago.

And once again, the proof is in the poop -- this time, a spiral-shaped coprolite from an ancient shark. Because sharks have spiral-shaped intestines and thus spiral-shaped poop. Inside one coprolite from Brazil, described in 2013, researchers identified a cluster of 93 eggs, each around 150 micrometers long.

The size and shape of the eggs, plus the way they were grouped together in a long segment, is just right for tapeworms. At least one egg even contained a partially preserved larva inside. So this was a shark infested with at least one tapeworm, but this fossil also provided evidence of what else the worms went after.

Like many parasites, tapeworm life cycles often include several hosts, such as insects, frogs, or fish, and the worms usually pass from prey to predator when their host is eaten by the next host in line. And inside this shark poop, alongside the tapeworm eggs, were bones and scales of fish. So it seems that this shark may have picked up some parasites during dinner -- and left us with some incredible insights into the life cycle of some very old tapeworms.

Now, throughout this list, we've been working our way back through time, so let's wrap it up with the most superlative parasite of all -- the oldest animal parasites ever found. A 2020 study identified parasitic worms from the Early Cambrian Period of South China, around 510 million years ago. These parasites were part of the one of the very earliest animal-dominated ecosystems on Earth.

But these aren't intestinal worms -- there's no poop involved this time. Rather, they're tubeworms that spent their lives in sturdy tubes attached to the outside of brachiopod shells. Brachiopods are two-shelled animals similar to clams, and they're known from this fossil site by the thousands.

In this study, the researchers found that about 200 of them had worms stuck on them. Now, hitching a ride on another animal doesn't necessarily make you a parasite, and we've found older hitchhiker fossils before. But the authors of this study are pretty sure that these worms were true parasites, and their main clue comes from the condition of the brachiopods.

The brachiopods with worms on them were significantly smaller, a clue that they weren't as well-fed, perhaps because those worms were stealing their food. See, brachiopods feed by sucking in water and filtering out tiny morsels, and all these worm tubes were angled with their opening stretched out toward the brachiopods' intake area. The worms may have been relying on their hosts to create a current, and then reaching out and intercepting food on the way to the brachiopod's mouth.

This is a strategy called kleptoparasitism, which relies on stealing resources from a host. And these are, so far, the oldest known fossil parasites, and an amazing indication that parasitism has been a winning strategy for about as long as there have been animal ecosystems. Parasites may often be small and undignified, but they have a huge impact.

They cause disease, impact the health of other organisms, and make up a major portion of the diversity of life. So, the more opportunities we have to study ancient parasites, the better we'll understand the intricacies of life on Earth. And if you want to help us help you understand life on Earth -- and all kinds of other things -- consider becoming a channel member.

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