Dickinsonia Was Named an Animal by a Molecule

Chapters

1. 0:00 A film on a tidal mat
2. 0:23 150 years, no consensus
3. 0:51 No mouth, no gut, no limb
4. 1:20 Protist, lichen, fungus, or animal?
5. 1:43 A molecule instead of a shape
6. 2:06 Cholesterol, ergosterol, stigmasterol
7. 2:36 Reading the fossil's own film
8. 3:03 The Lyamtsa specimen
9. 3:26 93 percent against 11
10. 3:48 What no alga does
11. 4:11 The lichen objection
12. 4:31 Glomeromycota's own molecule
13. 4:55 Where 558 million years falls
14. 5:16 Seven years later
15. 5:35 The sewage marker
16. 5:59 A question with no enzyme
17. 6:23 Elevated in one body plan only
18. 6:43 Not spread through the ecosystem
19. 7:03 Digesting the mat below
20. 7:26 Eating without a mouth
21. 7:47 Measured, and argued
22. 8:11 The meal is still there
23. 8:34 Next: an older claim, and a takedown

Transcript

A film on a tidal mat 0:00

Five hundred and fifty-eight million years ago, on a tidal flat on the coast of what is now the White Sea, a film of microbes covered the sand like wet leather. Something large and ribbed lay pressed against it. For a hundred and fifty years, nobody could agree what kind of living thing that was. Not from its shape. From a molecule still inside it.

150 years, no consensus 0:23

This is Dickinsonia: a flat, oval, ribbed impression, up to a metre and a half across, with no mouth, no gut, and no limb anyone has ever found. It has been called a giant single-celled organism, a lichen, a fungus, and a kingdom of its own. By the end, you will know which molecule settled that argument in 2018, and which one reopened it in 2025 -- without changing the answer.

No mouth, no gut, no limb 0:51

Dickinsonia is known from South Australia and from the White Sea coast of Russia, in rock about five hundred and fifty-eight to five hundred and sixty million years old. The body is symmetric along a midline, built from repeated ridged units that get larger toward one end. No mouth. No gut. No eyes. No legs. Just an outline and a rib pattern, pressed into sand.

Protist, lichen, fungus, or animal? 1:20

That is not much to build a kingdom on. Since the nineteenth century, Dickinsonia and its relatives have been read as a giant single-celled protist, a lichen, a fungus, and as members of a proposed extinct kingdom called Vendobionta, related to nothing alive today. Shape alone could not settle it. Shape was all anyone had.

A molecule instead of a shape 1:43

In 2018, Ilya Bobrovskiy and colleagues tried a different kind of evidence: not the outline, but the chemistry still locked inside the fossil film. Different groups of organisms build their cell membranes from different steroid molecules. If that chemistry survives half a billion years, it can fingerprint a kingdom that a flat impression cannot.

Cholesterol, ergosterol, stigmasterol 2:06

Animals build their membranes mainly from cholesterol, a steroid with twenty-seven carbon atoms. Fungi mostly use ergosterol, with twenty-eight. Algae and plants lean on stigmasterol and its relatives, with twenty-nine. After half a billion years, the original molecules are gone, but their carbon skeletons survive as steranes -- cholesteroids, ergosteroids, stigmasteroids -- still countable by carbon number.

Reading the fossil's own film 2:36

Getting that signal out is careful chemistry, not excavation. A film of organic matter from the fossil's surface is scraped away, dissolved in solvent, and separated by gas chromatography. A mass spectrometer then reads the fragments and sorts them by carbon number. The output is a ratio: how much of the film is C27, how much is C28, how much is C29.

The Lyamtsa specimen 3:03

On one specimen from Lyamtsa, on the White Sea coast, the largest Dickinsonia Bobrovskiy's team analysed, the result was unambiguous. Ninety-three percent of the steroid molecules in the fossil film were cholesteroids, the twenty-seven-carbon, animal kind. In the sediment surrounding it, the same class made up only about eleven percent.

93 percent against 11 3:26

That gap is the whole argument. A living microbial mat, made of algae, bacteria and fungi, should leave a mix weighted toward stigmasteroids and ergosteroids -- which is exactly what the surrounding sediment shows. Dickinsonia's own tissue does not match its own environment. It matches an animal instead.

What no alga does 3:48

No known alga, fungus or single-celled protist concentrates cholesterol the way Dickinsonia's fossil film does. That is what let Bobrovskiy's team call it: not a giant protist, not a fungus, not a lichen. An animal, identified by a ratio of carbon skeletons, not by an eye or a limb it never had.

The lichen objection 4:11

The claim did not go unanswered. Later in 2018, geologist Gregory Retallack published a public response arguing for the lichen reading after all: he pointed out that Glomeromycota, a group of fungi that form lichens, also do not make ergosterol -- so their absence would not rule a lichen out.

Glomeromycota's own molecule 4:31

Bobrovskiy and colleagues replied with the chemistry Retallack's argument needed. Glomeromycota do produce a steroid of their own, twenty-four-ethylcholesterol -- but that is a twenty-nine-carbon stigmasteroid, not cholesterol. And the stigmasteroids found near Dickinsonia traced back to the microbial mat underneath it, not to any algal partner living inside the animal.

Where 558 million years falls 4:55

The lichen hypothesis needed a fungus that made cholesterol, or close to it. Glomeromycota does not. The identification held: cholesteroid-dominated tissue, sitting on a stigmasteroid-dominated mat, is the chemistry of an animal resting on algae and microbes, not a lichen growing on rock.

Seven years later 5:16

That settled what Dickinsonia was. It did not settle everything the same molecules could say. In 2025, a second team went back to the steroid data with a different question -- not about identity, but about a related compound almost nobody had asked about: coprostane.

The sewage marker 5:35

Coprostanol is a steroid that gut bacteria make by chemically reducing cholesterol as it passes through an animal's digestive tract. Today it is the standard marker labs use to detect sewage: find coprostanol in a river, and something with a gut has been digesting cholesterol nearby. Coprostane is its geologically stable, fossilised form.

A question with no enzyme 5:59

Chelsea Mulligan and David Gold, at UC Davis, asked whether Ediacaran fossils carry that signal, and what it would mean if they did. There is a complication: no enzyme for making coprostane has been found in the microbiome of any living invertebrate. Whatever produced it in these half-billion-year-old rocks, it was not an ordinary animal gut doing what a mammal gut does.

Elevated in one body plan only 6:23

They measured coprostane across multiple Ediacaran fossils from the same beds, and one group stood out. Dickinsonia and its close relatives carried far more coprostane than any other fossil measured alongside them -- at levels comparable to sewage-polluted modern seawater, or to modern vertebrate faeces.

Not spread through the ecosystem 6:43

Every other Ediacaran fossil in the comparison sat far below that line. Only the dickinsoniomorphs concentrated coprostane the way a gut, or something acting like one, would. The compound was not evenly spread through the ecosystem. It was concentrated in one body plan.

Digesting the mat below 7:03

Since no known invertebrate enzyme explains that directly, Mulligan and Gold propose a different mechanism. Dickinsonia was digesting the microbial mat it rested on, and concentrating the coprostanol already present in that mat within its own tissue, rather than manufacturing it fresh in a gut. The molecule would then mark feeding, not membrane chemistry alone.

Eating without a mouth 7:26

That does not overturn the 2018 identification. It sharpens what it was evidence of. A body with no mouth in the fossil record, no gut cavity anyone can see, may still have been digesting food across part of its underside -- external digestion, on a surface pressed flat against a meal it never had to swallow.

Measured, and argued 7:47

So: what can the rock actually say? The steroid class is measured, and it held against a direct rival hypothesis, tested with real chemistry, not just asserted. The feeding mechanism is argued, from a comparison across fossils, with a caveat its own authors state plainly. No enzyme has confirmed how the molecule got there.

The meal is still there 8:11

The story most people repeat stops at 2018: a molecule proved Dickinsonia was an animal. The rock kept talking after that headline. Seven years later, the same class of molecule, read differently, suggests this mouthless body was not just alive. It was eating, and the meal is still there to measure.

Next: an older claim, and a takedown 8:34

Next time, another molecule, and an older claim: rock from Oman, up to six hundred and fifty million years old, holding a steroid read since 2009 as the oldest evidence of a sponge. One of the scientists who confirmed Dickinsonia also helped tear that claim apart. We go back to see what is left of it.

Description and sources

Dickinsonia: a flat, ribbed, segmented impression up to 1.4 m across, White Sea coast (Russia) and South Australia, ~558-560 Ma. Read since the 19th century as a giant protist, a lichen, a fungus, or a member of an extinct kingdom (Vendobionta). Bobrovskiy et al. (2018, Science): on the largest Lyamtsa specimen, 93% of steroid molecules were cholesteroids (C27, the animal class), against 10.6-11.9% in the surrounding sediment. Retallack (2018 eLetter) proposed a lichenised Glomeromycota alternative; Bobrovskiy et al. replied that Glomeromycota's own steroid, 24-ethylcholesterol, is a C29 stigmasteroid, not cholesterol, and that the stigmasteroids near the fossil trace to the microbial mat below, not an algal symbiont. In 2025, Mulligan & Gold (Geobiology) reassessed coprostane, the fossil form of a steroid gut bacteria make from cholesterol -- today the standard marker for sewage. No known invertebrate microbiome enzyme produces it, yet Dickinsonia and its relatives carry far more coprostane than any other fossil in the same beds, at levels the authors compare to sewage-polluted water or modern vertebrate faeces. Their proposed mechanism: digestion of the microbial mat, concentrating coprostanol already present in it -- feeding, not just membrane chemistry.

PRINT-READY, FROM THIS CHANNEL

The Deep Time Record, Vol. 01 — the cores, the curves and the clocks

https://therepository.gumroad.com/l/deep-time-record

PRIMARY SOURCES

• Bobrovskiy, Hope, Ivantsov, Nettersheim, Hallmann & Brocks 2018. Science 361:1246. https://doi.org/10.1126/science.aat7228
• Retallack 2018, eLetter response. Science. https://www.science.org/content/article/how-do-you-know-fossil-animal (via Science eLetters, Ancient steroids establish the Ediacaran fossil Dickinsonia as one of the earliest animals)
• Bobrovskiy et al. 2018, reply eLetter. Science.
• Mulligan & Gold 2025. Geobiology 23:e70029. https://doi.org/10.1111/gbi.70029
• Matthews, Liu, Yang, McIlroy, Levell & Condon 2021. GSA Bull. 133:612. https://doi.org/10.1130/B35646.1
• Zhou et al. 2017. Geol. Mag. https://doi.org/10.1017/S0016756816001175
• Eden, Manica & Mitchell 2022. Palaeontology.

ARCHIVE IMAGES — author, date, archive and licence.

• Susan H. Butts — 2021 | Wikimedia Commons | CC0
• James St. John — 2006-08-16 21:18 | Wikimedia Commons | CC BY 2.0
• Ruthven — 2015-01-24 | Wikimedia Commons | CC0
• Rebecca Eden, Andrea Manica, Emily G. Mitchell — 2022-05-17 | Wikimedia Commons | CC BY 4.0
• National Marine Sanctuaries — 2016-08-29 13:14 | Wikimedia Commons | Public domain

The 1 generated plates were made for this channel with FLUX.1-schnell. No person is depicted in any of them.

Deep Time Archives rebuilds the pre-human Earth from the physical record — outcrops, drill cores, golden spikes, catalogued specimens. Every episode starts from an object you can go and check, and the sources are listed above.

About the images. The Dickinsonia photographs are real fossil specimens, credited on screen and captioned by locality: a specimen at the Yale Peabody Museum (D. minima, public domain, Susan H. Butts), two from South Australia (D. costata, James St. John and the Muséum de Toulouse) -- not the exact Lyamtsa, White Sea specimen Bobrovskiy's team analysed, which has no image released under an open licence. The Ediacaran community reconstruction is from Eden, Manica & Mitchell (2022). The deep-sea sponge in the closing seconds is a modern glass sponge (Rossellidae; NOAA Ocean Exploration), shown as a preview of the next episode, not the ancient fossil. The tidal flat is an AI-generated reconstruction; no organism is generated. This video is labelled as containing altered or synthetic content. The diagrams are drawn in code from the published numbers.

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