Since a paper would be years in coming, I got permission to repost a conversation (I love this guy, but makes Sheldon Cooper look like a social butterfly)
His initials aren’t HDT, but he didn’t like any of my other nicknames and he’s a big Thoreau fan.
HDT : Hello Will. Good questions. I’m obviously forced to do a bit of speculating, but I believe on the merits your theory(?) is likely just as viable as the current one. However, having done some analysis I think there’s a greater point that’s being missed.
William : I hear that a lot from you, you know
HDT : And you keep coming back for more. .g.
William : Okay, so. . . I’m hooked. Tell me more!
HDT : What do you know about mitochondria?
William : The little power plant cell that all animals have in them, right? The chimera?
HDT : And plants. Mitochondria corresponds with muticellular life. There are a couple of exceptions, like loricifera, but they could have lost them or evolved on their own path. There also were some large multicelled organisms that existed before eukaryotes, but for simplicity’s sake, anything that’s got specialized multicellular cells that we’ll be talking about is a eukaryote.
William : With you so far, except the whole ‘why are we talking about mitochondria’ thing.
HDT : .g. Don’t worry, it always comes together. At some point an organism managed to co-opt a prokaryote that became our mitochondra. Like you said, it’s a power plant, and it gave a huge competitive advantage. This would, without making too many assumptions, likely mean that all eukaryotes are evolved from a single chimera, at the most a small number that are genetically similar enough to function simlarly. What does this indicate?
William : That. . . holy shit, do you mean that was basically a big reset button for genetic diversity?
HDT : Exactly. So, likely one “Eve” about two million years ago. Still a long time before you hit the Cambrian. I bring it up as something to keep in mind down the road. When an organism has a combination of traits evolve that gives it a tremendous advantage, that actually can greatly reduce genetic diversity rather than increase it.
You’ve clearly done some digging on the Cambrian explosion. I’ll add as a point of emphasis that sea creatures tend to have a large number of parts that wouldn’t fossilize well. When they develop any sort of hard exoskeleton that’s a huge advantage, and we do have some softer fossils, but they’re disproportionately rare.
You mentioned suspension feeding sea urchins, and I think they’re an excellent example of this. If not for more modern examples we’d probably think that fossil sea urchins used their spines to move (for some it’s part of their locomotion). We’d likely never surmise that they could grab particles of food out of the water with those tube feet and then pass them to their mouths.
With me so far? Ready to talk symmetry?
William : Yup.
HDT : You mention pentaradial symmetry, but we probably need to start with cells. At some point some cooperative organism found a tube to be a very efficient shape. Cells can fold themselves into two-layer sheets and each cell can get sufficient food, but once you get past that you need some sort of vascular system to feed each cell in the organism. A tube was a likely initial step.
A tube has no symmetry, but the cells themselves have to arrange themselves in a pattern so they can zip together properly. These cells are going to be arranged in a mathematical pattern, so there will inevitably be a multiple of 2, 3, 5, 7 and so on.
Five isn’t an uncommon number when it comes to radial symmetry. But let’s take a closer look at the sea pig. It is an echinoderm, yet it has two rows of feet and apparently two rows where tentacles of some sort connect on the back, true?
William : Oh. Oh! Yes, I totally missed that. Thanks!
HDT : Anytime. .g.
So, let’s think about why. Why would there only be 2/4 axes when there clearly should be five?
William : Because . . . it’s choosing not to express anything along that fifth row, just like it’s only expressing the a couple of the . . . thingies . . . along the top?
HDT : Exactly. So why did the symmetry break down? What was it a response to?
William : Gravity!
HDT : Primarily, yes. I think it would be better to look at it as conforming to a flat surface, but I do think you’re right in that gravity is the primary factor. There’s also cost. Expensive useless things tend to get selected out.
Going back to symmetry, cells, and tubes. There are a LOT of cells in each tube, the symmetry you see is just the ratio of how many cells express the feature vs. how many don’t. If they line up you have symmetry and if they don’t you have a spiral.
I should add that nature has a lot of work-arounds, you can see in the coleopleurus exquisitus you linked, there is also the zig-zag.
William : Cool.
HDT : Yes. Now let’s combine everything. You have the Cambrian explosion, which is a huge burst of morphological diversity. This indicates some recent change or adaptation that gave a new form of multicelluar life a sudden advantage. We don’t know much about their DNA, but most of their evolutionary building blocks came from a far simpler sort of life. It’s easier to use existing code differently or ignore it than it is to create something new or delete it.
Put all that together, and it’s likely not viable to apply our modern understanding of speciation to the bulk of Cambrian life. They could easily have been genetically more similar than a bumblebee is to a honeybee. Even that may be disingenuous, as Cambrian life didn’t have hundreds of millions of years of muticellular life behind them.
William : So. . . they’re all cousins removed the same thing?
HDT : Wouldn’t that be interesting? I doubt hallucigena could have bred with a trilobite or anything. .g.
However genetically they could all be using very similar building blocks to express new features, so their variances in morphology would be, I suspect, disproportionate to the variance in their DNA. That at least when viewed through the lens of modern genetics.
So a better question would be ‘Is attempting to classify hallucgenia as a lobopod a bit aggressive’ rather than ‘Is hallucigenia an echinoderm’
I spoke with a friend, by the way. He was pretty enthusiastic about the sea cucumber angle, and enjoyed the ‘soft’ pun. Did you know some of them can essentially liquefy themselves to escape into cracks? Fascinating creatures.
William : Believe it or not, yes. They kind of started me down this road.
HDT : Before I forget, one more thing. I’m wondering if it’s safe to assume that halucigena isn’t upside down now.
William : You mean, it walked on the spines? Oh, wait, were those even really spines?
HDT : Well, they were structures. I definitely think you’re on to something with the radial disks, or maybe just a firmer more heavily supported structure. All the creatures discussed here use hydrostatic skeletons, so that makes sense. I suspect something similar in a couple of the lobopods that you mentioned as well.
No, what I’m thinking is that if you look at the urchin you can see quite a few where they use their tube feet heavily or locomotion, or a combination of spines and feet, and they use their spines to help them sift through the sediment.
Meanwhile, hallucigenia were soft bodied creatures, so we don’t even know how much of what we see of them is a natural posture.
William : Yeah, most critters aren’t terribly polite unless they’re big on the shell, I’m with you there. What about the top? What’s their defense?
HDT : Not everything needs spines. .g. Chemical defenses aren’t exactly a new invention. Just because we can’t see it doesn’t mean it wasn’t there.
William : Hey! And what would they have been useful against? Wasn’t anomalocaris supposed to be the big predator back then? And that one with the trunk with claws at the end?
HDT : Good thought. I suspect the food chain may contain quite a few creatures we don’t have fossils for. Perhaps cnidarians were the top predator for them. Hard to say.
William : Maybe they held up little hydrozoans to protect themselves! Like boxer crabs!
HDT : I can’t actually discount that theory, but you’re getting weird now. Not implausible, but peculiar. .g.
That’s the meat. I love when people really think around questions and this guy can’t help it
(Edit: fixed a couple of phraseology bits with permission from ‘HDT’)