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Fossil Friday: Say Hello to Our Microscopic Granddaddy?

December 13, 2024, 6:55 AM
Image credit: PaleoEquii, CC BY 4.0 , via Wikimedia Commons.

This Fossil Friday features the reconstruction of the microfossil species Saccorhytus coronarius, which was first described by Han et al. (2017) from the Earliest Cambrian (510-520 mya) of South China. Even though these millimetric fossils are nothing but teeny-tiny, spiky, and wrinkly sacs, the scientists concluded that they represent the earliest fossil record of deuterostomes, which is the group of animals that includes not just hemichordates like acorn worms, echinoderms like starfish and sea urchins, and simple chordates like tunicates and lancelets, but also all vertebrates, like us.

The BBC reported that “scientists find ‘oldest human ancestor‘” (Gosh 2017), and that is indeed what the official press release by the University of Cambridge (2017) boldly claimed. Other legacy media chimed in with this sensational theme, from The New York Times (Wade 2017) and LA Times (Khan 2017) to The Guardian (Davis 2017), which funnily titled “A huge mouth and no anus — this could be our earliest known ancestor.” A (meanwhile deleted) blog article at ResearchGate (Lindermann 2017) explained that “newly discovered fossils of a tiny sea creature are the earliest known step in humans’ evolutionary history” and invited readers to meet your earliest known ancestor.” So, should be really say hello to our microscopic granddaddy?

Well, Not So Fast

Two years ago, a new study by Liu et al. (2022) analyzed several hundred more fossil specimens of this tiny organism, of which 31 were very well preserved as phosphatized three-dimensional fossils (also see the preprint by Liu et al. 2020). The organism had an apical opening interpreted as a mouth, but lacked an anus. The scientists concluded from their analysis that “Saccorhytus is an early ecdysozoan and not the earliest deuterostome.” This means nothing less than moving Saccorhytus closer to roundworms and bugs than to humans. The previous attribution of Saccorhytus was exclusively based on the interpretation of some round structures as pharyngeal openings and thus as a putative synapomorphy with deuterostomes. The new study could convincingly demonstrate that these round structures are not openings at all but the closed bases of spines. This is a valid point and arguably shows that the attribution of Saccorhytus to early deuterostomes should be rejected.

Of course, this dethroning of our alleged earliest ancestor made much less headlines, and the press releases just reported that “new evidence proves Saccorhytus is ecdysozoan” (Chinese Academy of Sciences 2022, also see News Staff 2022), while the University of Bristol (2022) even proclaimed that “scientists [are] relieved to discover ‘curious’ creature with no anus is not earliest human ancestor”. We can certainly all relate to this great relief [sarcasm disclaimer], so that at least the popular science journal New Scientist reported that this “sac with a mouth and no anus wasn’t our earliest ancestor after all” (Wong 2022). However, all those ordinary lay people, who do not regularly check the specialized science outlets were left in the dark. Apparently, a debunking of evolutionary fairy tales is not of that much interest for the science editors of the New York Times and the Guardian. Better not to confuse the public and make them think too much.

So-Called Synapomorphies

But what about the new attribution to early ecdysozoans, which is the group of animals that includes nemathelmith worms and (pan)arthropods? Any convincing attribution to a particular group in evolutionary biology requires the identification of so-called synapomorphies, which are supposed evolutionary novelties that are unique and diagnostic for this group. Not just any kind of similarity will do the job. Did the new study show any such synapomorphies between Saccorhytus and Ecdysozoa? No, not a single one. The article did not even bother to mention the term synapomorphy, and there are none identified in the supplementary material either. The attribution to ecdysozoans is purely based on a Bayesian computer analysis of a data matrix of many different animals and anatomical characters, and the computer algorithm produced a tree topology that resolved Saccorhytus in a polytomy with arthropods and nemathelminth worms within Ecdysozoa.

Why? Only the computer algorithm knows. We cannot point to any specific feature that identifies Saccorhytus as an ecdysozoan. Just trust the machine. But surely the authors at least discussed some diagnostic features that would align Saccorhytus with ecdysozoans, even if it would only be some general similarities and not specifically synapomorphies. No, again the authors did not even mention the term “diagnostic characters.” Apparently, they also just had blind faith in the black box result of the computer analysis. This should be considered as outrageous and unacceptable in real science, but not so in evolutionary biology. The BBC again proudly reports “mystery of half-billion year old creature with no anus solved” (Gill 2022). Strange that BBC forgot to mention in their earlier report (Gosh 2017) about the discovery of our alleged oldest ancestor, that Saccorhytus should still be considered as a mystery. Looks like those evolutionary mysteries remain and survive their alleged solutions.

An Enigmatic Organism

By the way: Shu & Han (2020) had already proposed that Saccorhytus could be a scalidophoran ecdysozoan, but Saccorhytus lacks the spiny pharyngeal apparatus of an introvert, which is a synapomorphy of the more inclusive group Cycloneuralia within Ecdysozoa. This clearly excludes Saccorhytus from crown group cycloneuralians including scalidophorans. About the absence of the anus, which is probably a synapomorphy of all bilaterian animals (compare Hejnol & Martín-Durán 2015), the authors say in the supplementary information that “this character is phylogenetically uninformative because it has occurred independently in several bilaterian groups.” This is of course nonsense: the fact that the most primitive bilaterian animals (i.e., Xenacoelomorpha) lack an anus, and a few highly derived bilaterian worms (e.g., most flatworms, some ribbon worms, as well as gnathostomulid worms) have a digestive system but secondarily lost the anus, does not make the lack of an anus in an enigmatic Early Cambrian microscopic organism “uninformative.” It is primarily evidence against a position within Nephrozoa (all bilaterian animals apart from xenacoelomorphan “flatworms”), unless we have positive evidence that the organism is closely related to one of the groups with secondarily reduced anus or shows otherwise positive evidence for a secondary reduction of this feature. Are there any other ecdysozoans known that lack an anus? No, only in a few larval stages of some derived species.

So, what is Saccorhytus? It is a spiny microfossil with an apical opening. An enigmatic organism, which could be a primitive bilaterian animal as supposed as possible alternative interpretation by Shu & Han (2020), or it could be a larva of some marine invertebrate of unknown relationship, or maybe it could even have belonged to an unknown and strange extinct group of skeletal protists. Could it also be some early ecdysozan? Sure, it could be, but there is no tangible evidence for that. What we do know is that it is definitely not our earliest ancestor. Another overhyped missing link bites the dust. I am quite confident that there will be many more such cases to discuss in future Fossil Friday articles.

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