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Analytical Chemistry

Sitting There For Five Hundred Million Years

This paper is really a tour de force of analytical chemistry, because it does something that I didn’t think was possible. The team is looking at a rather ancient creature, Dickinsonia. In fact, you could argue that it’s the ancient creature, since it’s one of the Ediacaran organisms that are part of the first explosion of complex living forms in the entire fossil record. This is well before the (already ancient) Cambrian explosion, the one that produced the strange Burgess Shale fauna. The Ediacaran community (PDF), at first glance, appears to be about as close as we’re going to get for a while to extraterrestrial life, because, as far as can be told, it was totally wiped out at some point for reasons that remain controversial. None of the Ediacarans have left any obvious, or even non-obvious descendants; it’s as if a giant reset button was hit on macroscopic life. Did the Cambrian organisms arise and displace (or even just eat) the Ediacaran ones? Did ocean conditions change too much or too quickly for them? All of the above?

You may have noticed that I’m tiptoeing around the words “animal” and “plant”, and that’s because no one has really been sure if those describe Ediacaran organisms or not, which is a convincing demonstration of how odd they are. Dickinsonia fossils are found in a number of places around the world (the Flinders range in Australia, near Arkhangelsk in Russia, in Ukraine, etc.), and they look pretty much like the one shown at right. They have bilateral symmetry – well, sort of, because those segments alternate, so technically it’s a glide symmetry. And they have one end rounder than another, and if you’d like to call one of those the head and one the tail, be my guest.

They appear to have lived on the bottom of the seas of the time, feeding on microbial mats, although they certainly don’t have mouths (way too early for anything as advanced as a mouth to be showing up, apparently). The best guess is that they might have anchored themselves to the bottom with some sort of sticky gunk and secreted enzymes to absorb bacterial food through their ventral surfaces (they’re often found with microbial mat impressions around them). But who knows? They appear to have competed with each other – you never really find them overlaying each other in a particular fossil, and some of those fossils really look like one Dickinsonia is trying not to be too close to another one. They range in size from a few inches to several feet across, and if you’d like to assign the larger ones to a different species (Dickinsonia rex), you might be right. Or not. Who knows?

The belief is that they were a sort of thick jellyfish consistency, a flat inflated bag (or collection of bags), which brings up a very reasonable question: how come they left such excellent fossils? Something was clearly different about Ediacaran conditions – maybe more silica in the water, maybe different sorts of bacteria that allowed fossilization to proceed in a different manner, who knows. But we have surprisingly nice fossils of Ediacarans, the better to confuse us.

This new paper seems to have settled the “Is it an animal?” question. Analysis of the fossils themselves show cholesterol-like steroids associated with them, and these are only found in animals (as opposed to plants, protozoans, or fungi). There are always a lot of stigmasteroids in these fossil layers, an ancient biochemical pathway associated with green algae. The next most common class are ergosteroids (generally from fungi), and the least common are the cholesteroid steroids. But in the organic matter of the Dickinsonia fossils you switch to over 90% cholesteroids. In the zone immediately above and below the organisms, the sterane distribution is also skewed, in a way that suggests decay bacteria at work on the steroids of the dead Dickinsonia themselves. The same team had reported, earlier this year, a similar analysis on another Ediacaran organism (Beltanelliformisthat established it as large round colonies of cyanobacteria, so the “molecular fossil” approach is really working out.

The idea of these compounds surviving for this long in analyzable, interpretable condition is quite striking. It’s been increasingly clear that more recent fossils (dinosaur bones, etc.) can harbor bacterial communities of their own, modern organisms moving in to consume this (ancient but still useful) food. That complicates things, if decomposition is still an active process after seventy million years, but in this case, there seems no doubt about the animal interpretation. If you want to claim intact proteins or nucleotides, though, that’s going to be a very different question. Those things are a lot more fragile and a lot more subject to bacterial confusion.

What this new paper tells us about the Ediacarans is that my remark above about “close to extraterrestrial” is completely wrong. These things are animals, biochemically like today’s animals in their steroid content, and are not some weirdo branch of large lichens or something (a theory that’s been hard to disprove until now!) The Ediacaran creatures, as odd as they look, are indeed our multicellular macroscopic forerunners and not a start at large organisms that’s off in its own world as compared to the Cambrian explosion. There are connections between the two, and the organisms in each can be classified as plants, animals, fungi, protozoans, cyanobacteria and so on. These categories have been with us for a very long time indeed.

18 comments on “Sitting There For Five Hundred Million Years”

  1. anonymous says:

    Amazing! I just wish I could access the full paper. I’ve been reading a couple books recently on mass extinctions and it has gotten me pretty excited about geology and paleontology. Derek, do you have any recommendations for good geology/paleontology books for the non-specialist?

    1. Chris Dicus says:

      I am currently reading The Story of Life in 25 Fossils by Donald Prothero. I do recommend it, but not as highly as I recommend Prothero’s Evolution: What the Fossils Say and Why It Matters. He also has a new book that I haven’t yet read called The Story of the Earth in 25 Rocks.

      So, basically anything by Donald Prothero.

  2. anon says:

    I can recommend “the dating game” by Cherry Lewis particularly if you’re a scientist yourself. It’s an excellent story of the British scientist Arthur Holmes who laid a lot of the ground work for getting an accurate age of the earth. Lots of geology, space science and analytical chemistry involved.

    1. anonymous1 says:

      Thanks for the rec, I’ll check it out!

  3. Question says:

    Isn’t it hard to believe that a small organic molecule has survived 500 million years?

    I’ve never analyzed dinosaur bones or anything but in my first job as a chemist, I was working in process research working on two different steroids. I had to monitor the bulk stability of those drugs at different temperature and humidity. (Very tedious.). They were pretty stable but I can tell you that they would not have lasted 500 million years at any temperature or humidity. They would lose a tenth of a percent of purity every year or so. How is it these small organic molecules survived that long?

    1. Steroids for days says:

      Sealed in rock, most likely.

    2. Steve says:

      these were completely saturated cholestanes etc – no double bonds or ketones etc to be degraded. Admittedly lost of tertiary C-H bonds for oxidation, but as these were dispersed in solids there is no autoxidation chain reactions possible.

      1. Anonymous says:

        Atmospheric oxygen was just starting to creep up from ~3% starting around 650 million years ago and is estimated to have been ~10% around 580 Mya (wikipedia; current oxygen = 21%). With less O2 around, maybe the dead critters didn’t oxidize as much as the minerals and sediments were piling on.

        Weird hydrocarbons (with enantiopure chiral centers, hence presumed to be naturally derived) were isolated from (contemporary) microbes living in volcanoes in the 1980s. I can’t put my hand on the refs. Steroids derive naturally from linear HCs (squalene C30) (but not exclusively). From the table in the paper, there is very little C30 steroid, so additional metabolism must have been in place already to burn them down to C27s and C28s.

        Very interesting stuff.

  4. loupgarous says:

    It is an impressive finding – establishing that biochemically, at lest some Ediacarans are more similar to animals than other forms of life.

    I know that, from an analytical chemisty standpoint, your comment

    “If you want to claim intact proteins or nucleotides, though, that’s going to be a very different question. Those things are a lot more fragile and a lot more subject to bacterial confusion.”

    is spot-on.

    But what else could make cholesteroid steroids but organelles such as the endoplasmic reticulum and mitochondria, which are structurally dependent on peptides?

    I don’t insist on nucleotide-mediated activity as the source of those cholesteroid steroids, but if we later find something else could made them, that would be an even more dramatic find, wouldn’t it?

  5. Mach4 says:

    Beautiful work! Its like looking at a long-lost relative here.

    Just to add a little popular culture- a species of trilobite was named
    Mackenziurus deedeei ‘Dee Dee Ramone’ – after Dee Dee Ramone. Three other trilobites, Mackenzius joeyi, Mackenzius johnnyi and Mackenzius ceejayi, were discovered at the same time, and named after other Ramones by taxonomists Adrian and Edgecombe.

    Hey Ho! Let’s Go!……..Extinct!

    1. cynical1 says:

      ‘I Want to Be Predated’

      1. Well played. A Ramones reference AND a pun in one of my favorite words. If I had a hat I would remove it.

      2. Vader says:

        How do you date a 500-million-year-old fossil?

        I’d suggest taking it to a rock concert.

  6. Anonymous Researcher snaw says:

    The louse Strigiphilus garylarsoni was named in honor of cartoonist Gary Larson, whose Far Side cartoons were ENORMOUSLY popular among biologists. Stephen Jay Gould even wrote an introductory essay for one of Larson’s books discussing why he and his colleagues loved those cartoons. I can testify that in the late 1980s and early 1990s University Biology buildings had MANY of those cartoons on office doors.

  7. Here’s a theory (mine) of what may have triggered the Cambrian Explosion:
    https://jetpress.org/v20/phoenix.htm

    TLDR: Precambrian animals had cells that could differentiate, but didn’t know how to commit to roles. Epigenetics (of the DNA/histone functionalization type) on-command in response to chemical signaling enabled much more intricate and stable organ structures, and possibly enabled synaptic learning.

    Huge advantages in intricate organs made of stem cells – at the cost of greatly reduced reproductive options (e.g. jellyfish can reproduce by polyp defection). It’s just intricate and trade-off-y enough to make it very hard but not impossible to evolve – hence the long delay followed by sudden “explosion” as animals invented brains, circulatory systems, and modern organs.

  8. Torbjörn Larsson says:

    I think the careful differential analysis against the sandstone background close the case. It is extremely rewarding after papers that have put Dickinsonia as stem lineages to rangeomorphs [“Cambrian petalonamid Stromatoveris phylogenetically links Ediacaran biota to later animals”, https://onlinelibrary.wiley.com/doi/abs/10.1111/pala.12393 ] and part of a complex ecology [“High ecological complexity in benthic Ediacaran communities”, https://www.nature.com/articles/s41559-018-0663-7 ].

    Speaking of “it was totally wiped out at some point for reasons that remain controversial.” For reasons of preservation (soft vs hard tissue for sure; perhaps size growth https://en.wikipedia.org/wiki/Small_shelly_fauna as well) and mass extinction (perhaps due to the supercontinent Pannotia breaking up) the later “Cambrian explosion” now seems much like “business as usual” diversification to me.

  9. milkshake says:

    if one tries to find “alien” creatures on earth, lipids are a good starting points. Archaea and especially extremophile archaea have some bizarre things in their membranes – ladderanes and ether-lipid dimers. There are pentacyclic cycloprenoids in shale that no known microbe forms… And if you look at archaea genome, their DNA but it codes 26 proteinogenic aminoacids – they don’t use that many stop codons…

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