Orthogenesis Refuted
Spores of Myxidium oviforme.
Egil Karlsbakk/Havforskningsinstituttet (CC BY-SA 4.0)
The Myxozoa are a group of parasitic, amoeba-like microbes ubiquitous in across the world, with over 2,000 known species and perhaps fifteen times that number yet unknown to science1. They have a two-phase life cycle during which they alternate between a vertebrate , such as a fish or frog, and an annelid , such as an earthworm1. To transfer from one type of host to the other, they produce spores which float through the water until they contact a suitable host. When that happens, the spore fires a set of harpoons into the host's flesh, injecting the parasite1.
As you can probably tell from the above picture, myxozoans are very simple and unsophisticated organisms, unable to do much more than absorb nutrients and reproduce. In fact, some do not even possess 2 (the powerhouse of the cell!), another sign of their primitiveness and oh I can't do this anymore.
All of that last paragraph was a lie. Myxozoans are not primitive, single-celled who haven't evolved since the dawn of time. In fact, they're not even protozoa at all1.
So what are they then? Fungi? Plants? Bacteria?
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Okay, I'll stop messing around. Myxozoans are...
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Wait for it...
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ANIMALS!
Sphaeroeca, one of the closest protozoan relatives of animals alive today. The first animal likely looked something like this.
Dhzanette (Public Domain)
Yes! Animals! Like a fish or an octopus or a sea anemone is an animal! Honest-to-goodness animals!
If you're anything like me, you might be asking, "Wait, that's an animal?" "How is that an animal?" "Aren't animals supposed to have things like, I don't know, mouths and muscles and brains and stuff?" "Has science finally gone too far?"
Okay, let's back up a bit. There are good reasons why myxozoans are animals, but we'll have to do some history to understand why.
So it's uncontroversial that things like birds, frogs and squid are animals. They all move around, they all eat other living or once-living things, they react to the environment, they reproduce using egg and sperm cells and so forth. But things that we might not immediately think of when we think 'animal', like corals, jellyfish and sea sponges, share these characteristics. And most importantly, all animals from sponges to humans are descendants of a single common ancestor. This common ancestor was a species of protozoa which lived some 650 million years ago3 that decided to give multicellularity a spin. That worked out pretty well, and now its descendants can soar the skies, swim the seas, and have petty arguments on the Internet.
Buddenbrockia plumatellae, the jellyfish which decided to be a skin disease, then decided that wasn’t good enough and became a worm.
Gruhl and Okamura (CC BY 4.0)
So this is the reason myxozoans are animals: even though they don't have mouths or muscles or brains, even though they don't move around, reproduce using eggs and sperm or sense their environment, they descend from the same common ancestor that humans, sponges, octopuses and all other animals did.
Now that that's established, you might be wondering what myxozoans' closest relatives are. And once again they surprise us. They are not cousins of few-celled micro-animals like water bears or simplistic beasts like sponges. Instead, their closest relatives are...
...(drumroll, please)...
Jellyfish1! Yes, you've got that right—jellyfish, with their stinging tentacles and pulsing bells and weird life cycles! And in hindsight, it's not all that unexpected: the harpoons that myxozoans use to inject themselves into their hosts are actually the same ones jellyfish use to inject their prey with venom1, just modified for a different if equally sinister purpose. While there is no shortage of parasitic jellies, it is still unprecedented that one lineage would give up such salient animal features like muscles, nerves and even the ability to breathe oxygen and instead take life advice from an amoeba. But it has clearly worked out well for them, for they outnumber their jellyfish cousins three-to-one and have conquered every ecosytem on Earth, including land—there are myxozoa living in shrews and earthworms which never touch water4—while no jellyfish has survived more than a few minutes out of the sea.
And myxozoans, ever determined to outdo everyone else, didn't even stop at turning from jellyfish into skin diseases.
So there's one myxozoan, Buddenbrockia plumatellae, which decided it wanted bigger things in life than being a skin disease. Unfortunately for Buddenbrockia, it's a myxozoan and its ancestors decided they were too good for things like 'muscles' and 'nerves'. But such issues were no big deal for little Buddenbrockia, which simply reevolved all that stuff5. When Buddenbrockia lands in its favourite host, the Plumatella, the amoeba divides and divides until a multicellular worm is formed, which squirms free of the host's flesh and goes off to spread its spores5. This is only one of two times the worm has ever evolved—the other being the venerated ancestor of the bilaterian animals. Talk about overachieving!
And all this is what I love so much about these little guys. They saw our ideas of how evolution produces all this wonderful complexity and progress, went 'bet', then proceeded to throw away all of their organs and conquered the world anyways. Then, when we accepted their simplicity, they went 'bet' again and straight-up reinvented the worm. They are an in-your-face reminder that evolution does not make strong, fast, smart 'perfect' lifeforms. It makes ones that survive and reproduce at all cost, even if that cost is 'all of your organs'. And their streak of success is not going away anytime soon—they're thriving as the world warms and humans accidentally introduce them to foreign habitats6.
References
1 Atkinson, S. D., Bartholomew, J. L., & Lotan, T. (2018). Myxozoans: Ancient metazoan parasites find a home in phylum Cnidaria. Zoology, 129, 66-68. https://doi.org/10.1016/j.zool.2018.06.005
2 Yahalomi, D., Atkinson, S. D., Neuhof, M., Chang, E. S., Phillipe, H., Cartwright, P., Bartholomew, J. L., & Huchon, D. (2020). A cnidarian parasite of salmon (Myxozoa: Henneguya) lacks a mitochondrial genome. Proceedings of the National Academy of Sciences of the United States of America, 117(10), 5358-5363. https://doi.org/10.1073/pnas.1909907117
3 Gold, D. A., Grabenstatter, J., de Mendoza, A., Riesgo, A., Ruiz-Trillo, I., & Summons, R. E. (2016). Sterol and genomic analyses validate the sponge biomarker hypothesis. Proceedings of the National Academy of Sciences of the United States of America, 113(10), 2684-2689. https://doi.org/10.1073/pnas.1512614113
4 Prunescu, C.-C., Prunescu, P., Pucek, Z., & Lom, J. (2007). The first finding of myxosporean development from plasmodia to spores in terrestrial mammals: Soricimyxum fegati gen. et sp. n. (Myxozoa) from Sorex araneus (Soricomorpha). Folia Parasitologica, 54(3), 159-164. https://doi.org/10.14411/fp.2007.022
5 Jiménez-Guri, E., Philippe, H., Okamura, B., & Holland, P. W. H. (2007). Buddenbrockia is a cnidarian worm. Science, 317(8), 116-118. https://doi.org/10.1126/science.1142024
6 🔒Hoffman, G. L. (1990). Myxobolus cerebralis, a worldwide cause of salmonid whirling disease. Journal of Aquatic Animal Health, 2(1), 30–37. https://doi.org/10.1577/1548-8667(1990)002%3C0030:MCAWCO%3E2.3.CO;2
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