The ancestor of deuterostomes? A new report.

A new paper in Nature by Jian Han et al. (reference and free link below; one of the coauthors is Simon Conway Morris, of Burgess Shale fame) describes the earliest known deuterostome: that superphylum of animals in which the blastopore (the first opening into the central part of the embryo) becomes the anus, and the second the mouth. (“Deuterostome” means “mouth second”).  This is in contrast to the group of protostomes, in nearly all of which (there are exceptions) the first opening becomes the mouth, and the anus develops later at the other end.

Deuterostomes include all chordates (including vertebrates like us), echinoderms, and “hemichordates” (acorn worms and graptolites). Protostomes include everything else, including segmented and unsegmented worms, molluscs, insects, rotifers, and so on. Here’s a diagram showing the developmental difference, and a second diagram showing the big divisions of life.

dpcomparison2

deuterostome

 

The first deuterostomes, previously dated at 510-520 million years ago, have now been supplanted by Han et al.’s finding of a tiny creature (about 1.2 mm across) in central China, with the sediments dated to 540 million years ago. Although it has a huge mouth and no anus, it still shows features suggesting it was an early deuterostome, one that lived among the sand grains of the sea floor. The authors named it Saccorhytus coronarius, part of a new group called the Saccorhytida.

But first, some journalistic errors. If this creature was an early deuterostome, it would be one of the first creatures from the lineage that led to humans (and other chordates as well as echinoderms) after that lineage split from the lineage leading to protostomes. But that does not make it “humans’ oldest known ancestor,” as is blatantly (and erroneously) indicated by the title below.

screen-shot-2017-02-01-at-7-52-28-am

That title is bogus! For all living and extinct species, including humans, had an ancestor that was much older than 540 million years—the “last universal common ancestor” (LUCA) of all creatures, which probably lived a bit over 4 billion years ago. The title above (click screenshot to go to link) was in fact from a press release by St John’s College in Cambridge, where Conway Morris works. How could they get it so wrong?

At any rate, that error seems to have been picked up by several other journalists. Here are two examples:

screen-shot-2017-02-01-at-7-46-23-amand this:

screen-shot-2017-02-01-at-7-47-31-am

I like this one from LiveScience; it’s not only a bit more accurate, but funny. Actually, it’s not completely true, for this species didn’t have to be a human ancestor itself, for it could have gone extinct without descendants, like some of the early robust hominins. All its deuterostome ancestry shows is that it evolved after the split between the protostomes and the deuterostomes.

screen-shot-2017-02-01-at-7-49-38-am

But on the paper, which I’ll summarize briefly. Han et al. report finding 45 of these creatures, with the reconstruction in color a couple of photos down. Here’s a scan of the anterior (front) part of the fossil itself, showing its big gob:

deuterostome-fossil

The most prominent feature of these fossils is the large mouth opening, clearly seen above, which is surrounded by several rows of papillae that may represent sensory organs. Each side of the body also bears four cones (8 total; you can see four above the mouth in the photo above). Han et al. suppose that the cones could have been used to expel water and waste, though they’re not sure.

These creatures were tiny, as I said, and were examined by both electron microscopy and CT scanning.  Here are some photos; note the scale bars (a μm, or one micron, is one millionth of a meter, or one one-thousandth of a millimeter):

nature21072-f1

(from paper): a–c, Holotype XX45-20. a, Right side. The mouth (M) arched dorsally along the anterior–posterior axis. b, Chevron pattern (Ch) on the inner surface of the integument. c, A spine (Sp) close to the mouth. d–f, XX45-56. d, Left side. e, Detail of the dorsally arched and folded mouth with radial folds (Rf) and oral protrusions (Op) in d. f, Circular pores (Cp) on the dorsal, right side. g–i, XX48-64 with limited compression. g, Ventral view, showing body cones (Bc) bilaterally arranged around the anterior, including the mouth. Two circular pores are adjacent to the first body cones (Bc1) and a small circular pore is on the mid-ventral line of the body. h, Oral protrusions lacking distal ends and appearing as a circle of pores. i, Left view reconstructed by microcomputerized tomography data. Lbc1–Lbc4, left body cones; Nr, nodular rugae; Rbc1–Rbc4, right body cones; Rf, radial folds; Sc, sub-layer of cuticle; arrowed AP, anterior–posterior axis.

But if these are deuterostomes, with the mouth forming after the anus, where’s the anus on these things? They don’t have one! The authors explain its absence this way:

Early deuterostomes have a through gut, so the apparent absence of an anus in Saccorhytus could be secondary, as in brachiopods and ophiuroids. [That is, its ancestors could have had an anus but lost it.] It remains possible, however, that this feature was inherited from more primitive bilaterians, possibly linked to the acoels and xenoturbellids.

But if there’s no anus, how can they call this an early deuterostome? It turns out that the small creature has other features that link it with the deuterostomes, as shown in the phylogeny below derived from several characters (the bootstrap support isn’t all that high). In particular, they show that Saccorhytus bears resemblances to “vestulocystids,” or early echinoderms, which are clearly deuterostomes.  These features, also studied by Conway Morris and his colleagues, include truncated cones on the body, a convoluted anterior part of the body, and “well developed radial ribs.”

Here’s the phylogeny showing the new species (in red) falling out with the deuterostomes:

nature21072-f3

(from paper): a, Lateral, hind and ventral views. b, The most parsimonious tree (tree length, 96; consistency index, 0.6771; retention index, 0.8394; rescaled consistency index, 0.5683) arising from a matrix of 25 taxa and 61 characteristics. The values at nodes indicate bootstrap support greater than 50% (see Supplementary Information for details).

Here’s a figure from a 2004 Nature paper showing the truncated cones (A and C) in some vestulocystids:

nature02648-f1-2

Finally, where the fossil was found and the appearance of the sediments (it must have been hard to spot these!):

nature21072-sf1

Caption from paper: Geographical location of horizon and its petrography. (836 KB) a, Locality map of the Zhangjiagou section, Xixiang, Shaanxi Province, China. In addition to Saccorhytus, the phosphatic limestone of Bed 2 of the Kuanchuanpu Formation in the Zhangjiagou section (see ref. 16) contains numerous small shelly fossils. b, c, Petrographic sections (plane-polarized light) of Bed 2 showing the phosphatic bioclastic grains, carbonate matrix and cements.

The authors conclude that our earliest deuterostome ancestosr might well have been tiny and lived among sand grains of the sea floor; they were “meiofaunal”, meaning small bottom-dwelling animals that inhabit the sediments. They also conclude that since these species could have been very tiny, we may have missed even earlier appearances of deuterostomes in the fossil record. Finally, they conclude that respiration occurred through the body surface, and the presence of pharyngeal arches (structures bearing gill slits), which are present in all modern deuterostomes at some developmental stage, could have evolved later.

h/t: Gregory
_______________

Han, J., S. C. Morris, Q. Ou, D. Shu, and H. Huang. 2017. Meiofaunal deuterostomes from the basal Cambrian of Shaanxi (China). Nature advance online publication. doi:10.1038/nature21072

58 Comments

  1. jaxkayaker
    Posted February 1, 2017 at 10:40 am | Permalink

    Unless something has changed, both deuterostomes and protostomes are in the clade Bilateria.

    Very interesting find. Hopefully more fossils will be found to give a fuller picture of the evolution of the lineage.

  2. Posted February 1, 2017 at 10:44 am | Permalink

    Oh no more to read : )

  3. Mark Sturtevant
    Posted February 1, 2017 at 10:50 am | Permalink

    Very interesting. I wonder if those wrinkles around the mouth represent bands of cilia, which are features seen in some echinoderm larvae.

    • loren russell
      Posted February 1, 2017 at 6:19 pm | Permalink

      Probably no external cilia — the authors interpret the integument of Saccorhytus and its relatives as being a chitin-like cuticle.

  4. Posted February 1, 2017 at 10:56 am | Permalink

    Jerry, correct me if I’m worng.

    As I understand it, there’re some serious caveats that must be given when using the words “human” (or even “deuterostome”) and “ancestor” with respect to this or any other fossil.

    And most of them could be summarized by noting that this is 99.999% guaranteed to be, at best, an aunt / uncle / cousin rather than a grandparent.

    Of course, that’s even more overwhelmingly true for any given individual fossil. Dig up some random human’s grave and it wouldn’t be all that surprising to discover that his or her own line eventually dwindled out — even if that person was part of a noteworthy or even still-extant family. For microbes, that scenario is greatly exacerbated.

    But it also applies to populations and even entire species. We have lots of hominid fossils that belong to long-extinct species that we know are not human ancestors but are, superficially, indistinguishable for even knowledgeable laymen from suspected-ancestor-species fossils.

    So, just we as can be reasonably confident that our hominid ancestors were very much like the non-ancestor fossils we found, we can perhaps also be confident to some extent or another that our deuterostome ancestors were very much like the fossil in this paper…but we can also have overwhelming, nigh-on-unshakeable confidence that these fossils are not our actual ancestors. Even if none of us could pick our actual ancestors from these out of a lineup…

    …and all that’s a run-up to the fact that our analysis of the 0.5 GYA fossil record is in its infancy….

    Cheers,

    b&

    • Posted February 1, 2017 at 11:39 am | Permalink

      I was going to make the same point, but you said it sooner and better.

    • Mark Sturtevant
      Posted February 1, 2017 at 12:56 pm | Permalink

      You are (as always, I think), absolutely right. But of course saying that this is an earliest known ancestor is just a way to appeal to the general public and to get their interest. In the most cynical sense it is also done to get them to click on the link.
      That it is not likely in a direct line to our ancestry is also indicated in the tree above, where it is clearly drawn as a dead-end. That scenario is far, far, more likely.

  5. BobTerrace
    Posted February 1, 2017 at 10:59 am | Permalink

    Subscribe and have read this science post.

  6. Simon Hayward
    Posted February 1, 2017 at 11:02 am | Permalink

    Slightly off topic, but some I understand that echinoderms – brittle stars for example – have a blind gut – so no anus – they poop orally I guess (lovely thought). Presumably that’s a later modification to the generic deuterostome game plan. Don’t know anyone who does echinoderm embryology to ask, so just throwing it out there.

    • Simon Hayward
      Posted February 1, 2017 at 11:44 am | Permalink

      Let me clarify that question, I just reread its general incoherence. It was more along the line of; is the lack of anus seen here and in brittle stars something common to primitive deuterostome lineages retained in some derivatives with the anus starring aa a feature that subsequently developed in most lines, or it it just something that comes along and is subsequently lost in some groups?

    • Mark Sturtevant
      Posted February 1, 2017 at 12:59 pm | Permalink

      Starfishes have an anus. It is on the ‘dorsal’ surface, facing up. So the underside is called the oral surface, since that is where their mouth is. Their ‘aboral’ surface faces up, with an anus.
      In a way, a starfish is standing on its head, with it rear facing up.
      I expect (but do not know) that brittlestars also have an anus.

      • Posted February 1, 2017 at 1:09 pm | Permalink

        Starfish have an anus, but I think that in adults it is not functional.

        • Dave
          Posted February 1, 2017 at 1:44 pm | Permalink

          No, Simon is right, the anus is absent in brittlestars, which use the mouth opening for traffic in both directions. I’m not sure of the exact embryological mechanism but it’s generally believed that the lack of an anus is secondary, since all other living echinoderms have one.

          • Simon Hayward
            Posted February 1, 2017 at 2:19 pm | Permalink

            Thanks Dave, that makes sense. I deal in mice and humans, if there is no anus it’s a problem! However not always one that gets noted, for example when the group making the knockout are interested in limb development and don’t look at the back end of the gut. Depending on who you talk to the descriptor might be “this mouse has no legs” vs “this mouse with no hind gut happens to be torpedo shaped”

  7. darrelle
    Posted February 1, 2017 at 11:03 am | Permalink

    It is amazing that anyone can make heads or tails (mouths or anuses?) out of the fossils shown.

    • Billy Bl.
      Posted February 1, 2017 at 12:52 pm | Permalink

      That was my reaction when I read BBC’s report. I could understand such a confident claim if they had had a living specimen to examine, but a 540-year-old fossil?

  8. TJR
    Posted February 1, 2017 at 11:04 am | Permalink

    So we are all, quite literally, arse first.

    That explains a lot.

    • BobTerrace
      Posted February 1, 2017 at 11:28 am | Permalink

      Safer to go down the water slide that way. Also, quarterbacks go that way to be safer.

  9. Kevin
    Posted February 1, 2017 at 11:10 am | Permalink

    Wow. Pooping always reminds me that I am an inefficient entropy increaser. This little guy would never get such reminders.

    • Posted February 1, 2017 at 3:12 pm | Permalink

      Think that you are releasing substances to the environment in a plant-usable form.

  10. DaveP
    Posted February 1, 2017 at 11:19 am | Permalink

    No anus? Must be Kim Jong-un’s ancestor.

  11. Mike Cracraft
    Posted February 1, 2017 at 11:40 am | Permalink

    How did these critters reproduce ?

    • Mark Sturtevant
      Posted February 1, 2017 at 1:06 pm | Permalink

      Most likely by hermaphrodite sex. I say this because simple deuterostomes do that today, generally.

    • Simon Hayward
      Posted February 1, 2017 at 2:28 pm | Permalink

      Really low lights and a background soundtrack of crashing waves probably did the trick 🙂

  12. Posted February 1, 2017 at 11:49 am | Permalink

    A wee one, for sure.

    China (geographically anyway) has certainly provided a lot of interesting new developments recently.

    What makes the opening a *mouth*, as opposed to the general opening that I (at least) was taught was found in (say) hydrae?

  13. Heather Hastie
    Posted February 1, 2017 at 11:50 am | Permalink

    I have nothing to say. This is way beyond my ability to comment. Just noting I read this so Jerry knows I appreciated it.

    • Posted February 1, 2017 at 1:10 pm | Permalink

      That’s the reason I commented, too.

      • Jeff Lewis
        Posted February 1, 2017 at 2:30 pm | Permalink

        Same

        • Dee
          Posted February 1, 2017 at 3:04 pm | Permalink

          Same here. Pretty cool stuff.

    • jeremy pereira
      Posted February 2, 2017 at 6:55 am | Permalink

      ditto

  14. Barney
    Posted February 1, 2017 at 11:50 am | Permalink

    Doesn’t that phylogeny show the new species in with the Vetulocolians and some other deuterostomes, but after they split from Chordates and Ambulacrarians? In that case, it couldn’t be ancestral to us.

    But, if it is meant to be right at the crown of all deuterostomes, is the “earliest/oldest known ancestor” claim more reasonable, because it says known. Is there a known species that was ancestral to both protostomes and deuterostomes (or, going even further back, also ancestral to jellyfish etc.)? We know there must have been one, but do we have a fossil of it?

    • Gregory Kusnick
      Posted February 1, 2017 at 1:43 pm | Permalink

      I was wondering the same thing. There are (I presume) older fossils, but do we know that they’re ancestral in some (approximate) sense?

  15. eric
    Posted February 1, 2017 at 12:09 pm | Permalink

    Interesting science article.

    Though with a spherical body, giant mouth, and 8 cone-like protuberances, for my money they should’ve named it Saccorhytus beholderus. 🙂

  16. Barney
    Posted February 1, 2017 at 12:16 pm | Permalink

    Looking at the artist’s reconstruction, I wonder if it was ancestral to the legendary Black Beast of Aaaaaarrrrggghhhh:

  17. Posted February 1, 2017 at 12:49 pm | Permalink

    I even heard about this on NPR this morning (surprisingly). Cool, thanks for posting this! 🙂

  18. Ken Kukec
    Posted February 1, 2017 at 1:12 pm | Permalink

    So, this deuterostome pooped out of the same large orifice it used to eat? Makes the Donald a big-league instantiation of atavism, I suppose?

  19. GBJames
    Posted February 1, 2017 at 1:22 pm | Permalink

    sub

  20. DickK
    Posted February 1, 2017 at 1:43 pm | Permalink

    Ditto with Heather Hastie. I really like this type of article because I am not a biologist and have few other opportunities to learn from someone who is a biologist. I am a retired statistician, but who cares?

    Please keep it up, Jerry.

  21. grasshopper
    Posted February 1, 2017 at 2:15 pm | Permalink

    From the ABC (Australian Broadcasting Commission) news site today there is a story with the headline “Pores found in ‘platypus of microbiology’ bacterium push boundaries of evolution”.

    The first paragraph got my attention and the second even more so, because it tells us that the theories of evolution are now in doubt.

    A bacterium dubbed the “platypus of microbiology” is even stranger than first thought, with the discovery it contains structures normally only found in more complex cells.

    The find, by an international team led by University of Queensland researcher Emeritus Professor John Fuerst, adds to the debate about how complex cells evolved, and casts doubt on long-held theories of evolution.

    Will you now retract your book, Jerry?

    There is a link to the PLOS article in the news story.

    • Michael Fisher
      Posted February 1, 2017 at 3:04 pm | Permalink

      Are you having a bit of fun Grasshopper?

      Did you peruse the PlosOne research article section entitled “Evolutionary implications of the findings”? It is clear from that the authors are not questioning the foundations of Darwinian evolution!

      In the ABC link you provided it says this: Professor Fuerst said there were two possible interpretations from the discovery. First that the nuclear-cell structures developed in bacteria and eukaryotes completely independently of each other.

      “Evolutionarily it could be that these nuclear pores are a good solution to the problem and maybe there is a reinvention a number of times in nature […] the alternative view is that the bacterium share a common ancestor as far back as 3.8 billion years ago. It is a major question of biology being investigated right now and it is not easy to resolve”

      • grasshopper
        Posted February 1, 2017 at 4:16 pm | Permalink

        Yes, I am having a bit of fun. Glad to see that you actually read the PLOS article, unlike the accredited journo at the ABC.

    • Posted February 1, 2017 at 3:17 pm | Permalink

      The story has been written this way just to bait people like you to click.

  22. rickflick
    Posted February 1, 2017 at 2:26 pm | Permalink

    It’s so common for journalists to title scientific findings so as to entice clickage. Usually it involves SEX or humanities absolute decadence or total saintliness, or something that was certainly, absolutely impossible. Something found for the very first time ever! Something which has scientists feeling stunned and nearly suicidal with amazement.
    The truth, however, is never that earthshaking. But, the truth is interesting in it’s own right. Skip the title, read the rest.

  23. grasshopper
    Posted February 1, 2017 at 3:06 pm | Permalink

    rickflick – a google of the journo’s CV shows the lack of a science background, though that hasn’t stopped the ABC from employing her as a “freelance writer for ABC Science Online”.

    • rickflick
      Posted February 1, 2017 at 5:25 pm | Permalink

      Doesn’t surprise me.

  24. loren russell
    Posted February 1, 2017 at 6:16 pm | Permalink

    A few comments on this beautiful, if unlovely, little PacMan:

    1. [PCCe] “In particular, they show that Saccorhytus bears resemblances to “vestulocystids,” or early echinoderms, which are clearly deuterostomes.”

    The connection to deuterostomes is via a series of mostly recently described genera that Conway Morris, Shu and their colleagues see as stem-deuterostomes. Most of these are from the Chengjiang lagerstatte, but some are known also from Sirius Passet, the Burgess Shale and the Wheeler Formation.
    Among these, the vetulicolians (Banffia and relatives) and Vetulocystis have been enigmatic. All of these are segmented [not myotomes as in undoubted chordates like Pikea], with a forebody and ‘tail*’ [unlike chordates the anus is terminal, so it’s not truly a tail]. Based on the segmentation and presence of a cuticle, Wolcott described Banffia as an annelid, and until recently it was conventionally depicted as an arthropod-like animal that lacked appendages]. A relationship to kinorynchs has also been suggested. Conway Morris is persuasive in arguing that all of these are deuterostomes, but his argument almost entirely based on the presence of pharyngeal pores.

    *Saccorhytus is unusual in lacking this ‘tail.’ Again, despite its earlier provenance, it’s poosible that this is a reduction from a free-swimming form.

    2. PCC(e): “Intriguingly, extant echinoderms lack pharyngeal structures, but fossil records reveal that ancestral forms of echinoderms had gill-like structures.”

    Yes, and this is significant. Note that there are few if any characters linking Vetulocystis and relatives to echinoderms, as opposed to being stem-deuterostomes.

    • Michael Fisher
      Posted February 1, 2017 at 6:23 pm | Permalink

      @Loren

      What does “enigmatic” in this scientific context mean?
      I’m guessing = “can’t be fitted into the evolutionary tree” or “not enough detail to determine where it fits [because partial specimen for example]”

      • loren russell
        Posted February 1, 2017 at 9:47 pm | Permalink

        I used ‘enigmatic’ in an entirely vernacular sense: there are [or have been] 2 or more interesting and mutually-exclusive affinities proposed for the taxon — these [or rather Banffia, which is the only well-known example from my school days] have been thought of as one-off closely related to anything then or now [eg, Gould’s Wonderful Life story]; as annelid relatives, as odd stem-arthropods, and even kinorhynchs. [ A lot of that sort of thing from the Burgess Shale fauna, some of which [like Hallucinigenia, Wiwaxia, and perhaps Nectocaris] has been tidied up very recently. But the story continues to be written, and I’d expect a quite different look at early to mid Cambrian metazoans in the next decade or so.

        I’m watching this purely as a hobby– it’s been about 50 years since I memorized the Libby Hyman series for my written comprehensive exams in invertebrate =zoology.

        • Michael Fisher
          Posted February 2, 2017 at 9:45 am | Permalink

          Thanks Loren

  25. ThyroidPlanet
    Posted February 1, 2017 at 6:49 pm | Permalink

    Sub

  26. Posted February 1, 2017 at 6:57 pm | Permalink

    Nothing to add except for: wow, cool! Thanks, PCC(E) Amazing stuff!

  27. Leigh Jackson
    Posted February 1, 2017 at 8:16 pm | Permalink

    I skipped past this story in the newspaper.
    My bad!

  28. Gary Radice
    Posted February 1, 2017 at 8:29 pm | Permalink

    Love the science stuff. Thanks!

  29. jaxkayaker
    Posted February 2, 2017 at 6:39 am | Permalink

    I’m surprised to learn that “…pharyngeal arches (structures bearing gill slits)… are present in all modern deuterostomes at some developmental stage…”. I learned, and therefore taught, that pharyngeal arches were a synapomorphy of chordates specifically, not present in deuterostomes generally.

  30. Posted February 2, 2017 at 3:27 pm | Permalink

    Reminds me of the Dana Carvey Show: The Cutting Room Floor Remembers: “If I only Had an Ass”.

    This clip, unfortunately, has disappeared from the internet.


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