NARRATOR: In 2015, PhD student Ilya Bobrovsky launched a project that would turn the world of paleontology on its head. But first he'd have to convince his advisor that his idea wasn't crazy. Bobrovsky wanted to investigate dickinsonia, a mysterious organism that existed more than 500 million years ago. It's one of the most famous fossils from the Ediacaran Period. And it's also one of the trickiest to classify. Scientists had no idea what type of organisms the life forms of the Ediacaran were-- lichen, colonies of bacteria, fungi, or something else? Bobrovsky's advisors said that the identity of these mysterious organisms was the holy grail of paleontology. That's because the Ediacaran was the period that directly preceded the Cambrian, a time of enormous diversity and the radiation of complex life known as the "Cambrian explosion." So understanding the nature of life before the Cambrian could help put later multicellular life into a clearer evolutionary context. Were the bizarre creatures of the Cambrian the first true animals? Or did something come earlier, during the Ediacaran? Bobrovsky had a wild idea for how to uncover the answers. He would take a helicopter to the remote White Sea in Russia, known to be a source for well-preserved Ediacaran fossils. There, he'd repel down cliffs a hundred meters high and dig out fossils of dickinsonia some 558 million years old from the surrounding sandstone. But that wasn't all. Bobrovsky would then crush up these priceless fossils and feed them into highly sensitive mass spectrometry machines to study their composition. His results would revolutionize our understanding of early complex life, and would finally settle at least part of the debate about the nature of living things during the Ediacaran Period. It turns out the key to solving the puzzle of Precambrian life was a tiny bit of fossilized fat.

(Describer) Title: Eons

The Ediacaran Period began around 635 million years ago, though the strange fossils that the period is known for didn't really start appearing until 60 million years later. It marked the first time in Earth's history that complex organisms appeared, setting the stage for the even greater biological blowout of the Cambrian explosion. One of the strange organisms discovered from this time was the blobby, up to 1.4 meter long thing named dickinsonia. The first specimens of Dickinsonia costata were collected in 1946 from a location near the eastern edge of Lake Torrens in South Australia. And, for decades, scientists argued over how to interpret these weird fossils. And they came up with a ton of hypotheses. For example, some said the wavy lines around the margin of the fossil were tentacles. Later, those tentacles were reinterpreted as marks left by the body as it contracted after it died. And others thought the fossil was some kind of worm that undulated along the sea floor. But regardless of whether it was actually a worm or not, it was likely the first animal that moved on its own. Another group of researchers argued that dickinsonia belonged to the Lichen family. Others still thought they were giant protists, organisms that aren't animal, plant, or fungi, like amoebas. But, compared to an amoeba, dickinsonia would have been massive. Paleontologists couldn't even decide whether the fossils showed bilateral symmetry or not. That's when a body plan looks the same on both sides of a central axis. If you cut a human like me in half, right down the middle, for example, you'd have an eye on both sides, an arm and a leg on both sides, and so on. But with the dickinsonia, scientists weren't sure that the two sides of the body were actually symmetrical. And since the fossils offered so few clues about the organisms, researchers weren't even sure how it got nutrients. And they still aren't. Some think it must have had some kind of mouth and gut, while others think it digested things externally, using, well, the sole of its foot. And they thought this because they also found fossils of microbial mats with weird marks on them, marks that maybe looked like dickinsonia had been eating the microbes. The whole thing was a big mystery. Now, normally when paleontologists interpret fossils, they rely on two things-- morphology and taphonomy. Morphology is the physical form of the creature. And taphonomy is the process by which it fossilized. And both of these were really complicated for dickinsonia. Since Ediacaran organisms were mostly soft-bodied, it's been tricky to figure out how they got preserved in loose sediment to begin with. Like, how did dickinsonia manage to leave its mark on the fossil record? Scientists hypothesize that some of the squishy organisms might have been preserved by a so-called "death mask." OK, picture this. Dickinsonia is scooting along the ocean above a microbial mat. And these mats have also been found in the fossil record. But then the organism gets buried by sediments during a storm. Sandwiched between the sediments and the bacteria from the microbial mats, it starts to decay. In that low-oxygen environment, iron from the sediment reacts with hydrogen sulfide produced by decomposers and creates a hardened mineral shell over the carcass of dickinsonia. That shell forms as a cast over one side of the body of the organism-- a death mask that remains in the fossil record. But scientists weren't sure whether the mask of dickinsonia was the cast of the outer body being imprinted onto the sediments or some kind of more rigid internal structure. To try to sort through these competing hypotheses, Bobrovsky decided to take a different approach. Instead of looking at the body shape, he studied the ancient molecules inside and immediately around the fossil because the cells of different organisms are filled with their own molecular signatures. Even though things like DNA can't be preserved for millions of years, other molecules can-- molecules like steroids. OK. OK. I know what you're thinking. I'm not talking about illegal bodybuilder supplements-- although those things are part of a whole class of organic compounds that are more properly known as steroids. In nature, steroids are found everywhere in plants, animals, and fungi, like in hormones, but also in things like the cell membranes of plants and even some vitamins. And then there's cholesterol, which appears in basically all animal cells as lipids, also known as fats. So looking for traces of steroids can help scientists decipher what type of organism they're dealing with. Stigmasteroids, for example, are found in green algae. And cholesteroids are produced by animals. By comparing the proportions of steroid molecules from within the fossil to those around it, Bobrovsky hoped to finally identify dickinsonia. Was it a protist, an animal, or a plant? Deposits above and below dickinsonia had a mixture of about 10% cholesteroids and about 75% stigmasteroids. That meant that most of the organic material in those sediments probably came from green algae. But when he studied molecules from the organic matter of the largest dickinsonia a fossil itself, he found 93% cholesteroids. Such high levels of cholesteroids meant that dickinsonia must have been an animal. But Bobrovsky and his colleagues didn't stop there. Another mystery of the Ediacaran is how such a variety of organisms came to be. Like, what kinds of environmental factors could have promoted so much diversity? And Bobrovsky hypothesized that it had to do with food availability. So, in an experiment that was published in 2020, he and his team tested this idea by measuring levels of other molecules found in ocean sediments from the Ediacaran. This time, instead of steroids, the team measured the ratio of hopanes over steranes. Hopanes are the chemical byproduct of aerobic bacteria left behind in the fossil record. So, for example, cyanobacteria are one source of hopanes. Steranes on the other hand, are left behind by algae and other multicellular organisms. Comparing the ratio of the two biomarkers in sediments let scientists estimate what proportion of the ancient environment was filled with bacteria versus algae in more complex organisms. And it turned out, around the beginning of the Ediacaran, 635 million years ago, there was a huge drop in the ratio of hopanes to steranes. This meant that the environment transition from being filled with bacteria to hosting lots and lots of algae. By studying those sediments, Bobrovsky concluded that the Ediacaran organisms lived in an environment almost like our modern one, at least when it comes to the ratio of bacteria to green algae. This rise of algae from 650 million to 635 million years ago totally changed the world's ecosystems and provided new food sources for new life forms, like dickinsonia. And this wasn't the only blob roaming the oceans looking for food. There was kimberella, which has recently been described as something like a mollusk, and spriggina, which looks sort of like a cross between a trilobite and a worm. And scientists are still working on the identity of all the other weird organisms from this period, whether they're plants, or bacterial colonies, or something else entirely. There's still a lot we don't know about dickinsonia and its Ediacaran friends. One big question that has yet to be answered is, what happened to them? Why, at the dawn of the Cambrian Period, did they just seem to vanish? Some scientists suggest they were all driven to extinction by the more mobile Cambrian predators, or maybe they were victims of some geochemical catastrophe, or some other environmental change to which they couldn't adapt. Or maybe all of the above. It will take a lot more work to piece together the weird puzzle of the Ediacaran Period. But thanks to the pioneering research by Bobrovsky and others, we can finally say dickinsonia belongs to the animal kingdom. And the molecular method that they've developed could also provide ways to tell us even more about the period. Bobrovsky thinks it could even be used to tell what Ediacaran organisms ate for their last meal, if something resembling a gut could be found in the fossil record. Even today, despite all that we've learned, the history of the earliest animals is still full of unanswered questions. But they're still out there. Sometimes when the morphology of a critter is just too weird, you've got to think outside the body. And, in this case, it was looking for molecules of fossilized fat. It just goes to show, sometimes you have to go small to get the big picture.

(Describer) Titles: Eons Kallie Moore: Content Consultant, Host PBS Accessibility provided by the US Department of Education.

[MUSIC PLAYING]