In 2015, Ph.D student Ilya Bobrovskiy 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. Bobrovskiy 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 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. Bobrovskiy’s advisor 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? Bobrovskiy 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 of well-preserved Ediacaran fossils. There, he’d rappel 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: Bobrovskiy 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. 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 the weird fossils - and they came up with a ton of hypotheses. For example, some said that wavy lines around
the margin of the fossils 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 seafloor. 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’ve
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, and an arm and a leg on both
sides, and so on. But with Dickinsonia, scientists weren’t
sure that the two sides of the body were actually symmetrical. And since fossils offered so few clues about
this organism, 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 hypothesized that some of the squishy
organisms might have been preserved by a so-called “Death Mask.” Picture this: Dickinsonia is scooting along
in the ocean above a microbial mat; 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 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 a 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, Bobrovskiy decided to take a different approach. Instead of looking at 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 I know what you’re thinking! I’m not talking about illegal body-builder
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, Bobrovskiy hoped to finally identify it: Was
it a protist, a plant, or an animal? Deposits above and below Dickinsonia had a
mixture of about 10 percent cholesteroids and about 75 percent stigmasteroids. That meant 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 fossil itself, he found 93% cholesteroids. Such high levels of cholesteroids meant Dickinsonia
must have been an animal! But Bobrovskiy and his colleagues didn’t
stop there. Another mystery of the Ediacaran Period is
how such a variety of organisms came to be, like what kinds of environmental factors could
have promoted so much diversity. And Bobrovskiy hypothesized 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 lets scientists estimate what proportion of the ancient environment was
filled with bacteria versus algae and more complex organisms. And it turned out, around the beginning of
the Ediacaran period, 635 million years ago, there was a huge drop in the ratio of hopanes
to steranes. This meant that the environment transitioned
from being filled with bacteria, to hosting lots and lots of algae. By studying those sediments, Bobrovskiy 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 identifying
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 the victims of some geochemical
catastrophe, or some other environmental change to which they couldn't adapt. Or maybe, all of the above? It’ll take a lot more work to piece together
the weird puzzle of the Ediacaran Period. But thanks to the pioneering research by Bobrovskiy
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. Bobrovskiy thinks it could even be used to
tell what Ediacaran organisms ate for their last meal, if something resembling a gut can
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, that was looking
for molecules of fossilized fat. It just goes to show, sometimes you have to
go small to get the big picture. Metazoan high fives to this month’s Eontologists:
Patrick Seifert, Jake Hart, Jon Davison Ng, Sean Dennis, and Steve! Become an Eonite at patreon.com/eons for access
to exclusive content like the Eonites Only Podcast hosted by me. Hey did you know that life during the Ediacaran
represents a major biological diversification event? Well if not, check out our episode, “The
Other Explosion You Should Know About” to learn more. Also thank you for joining me in the Konstantin
Haase studio. Be sure to subscribe at youtube.com/eons for
more adventures in deep time.
