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.