Controversal paper on origins of caterpillars debunked

You may remember that in September, retired Liverpool professor Donald Williamson published a paper in The Proceedings of the National Academy of Sciences claiming that the caterpillar stage of the butterfly/moth life cycle arose when an ancient lepidopteran mistakenly mated with a velvet wom (onychophoran), producing an inter-taxon hybrid that had an adult butterfly but the larval form of the worm. The whole idea seemed ludicrous for many reasons (I called it “the worst paper of the year“), but of course ultimately Williamson’s view needed a formal appraisal in the scientific literature.

Now, in a new paper in PNAS, Michael W. Hart and Richard K. Grosberg give this appraisal and conclude that, on the basis of data that already exist, Williamson’s idea is full of it. Among their criticisms are these:

1. Contrary to Williamson’s prediction, the genomes of Lepidoptera are generally smaller than those of other orders of insects that lack the larval stage, and are certainly not the large size you’d expect if modern butterflies resulted from hybridization between a purely flying ancestor and a wormy onycophoran.

2. Similarly, contrary to Williamson’s predictions, the genomes of onycophorans are not smaller than those of Lepidoptera or of other “holometabolous” insects that undergo a striking transformation between larval and adult stages.

3. Contrary to Williamson’s prediction, DNA sequencing shows NO close relationship between any lepidopteran genes and any genes in onycophorans.

There is other evidence as well, but the above is sufficient to completely refute Williamson’s claims. I stand by my characterization of his paper as the worst of the year.

You can see Scientific American’s report on the Hart and Grosberg paper here.

Well, that takes care of that. The remaining issue is how did this execrable piece of work get published in the first place? The communicating editor was Lynn Margulis, who is a huge booster of evolutionary processes that involve hybridization, undoubtedly because of her pathbreaking work showing that mitochondria and chloroplasts of eukaryotic cells used to be bacteria. She even wrote a book suggesting that hybridization may be the key to understanding the origin of species. (She’s wrong except for allopolyploidy in plants.) Handling this paper, Margulis appeared to violate journal policy by asking reviewer after reviewer to look at it until she got the positive reviews needed to assure publication. But, as Scientific American reports:

In August, when Williamson’s paper was published, Margulis told Scientific American that she needed “6 or 7” peer reviews to secure the “2 or 3” positive responses needed to present the work for publication. That statement set off a cascade of criticism of PNAS‘s two-tiered submission process and, according to Nature News, led the journal’s editor in chief to write to Margulis demanding “a satisfactory explanation for [her] apparent selective communication of reviews.” Her reply, obtained by Nature News, explained that three researchers had declined for scheduling reasons or lack of expertise, and two were omitted from the official PNAS submission because they lacked formal credentials.

Um. . . “lacked formal credentials”?? Why, then, were these reviewers asked in the first place? Shame on PNAS for publishing Williamson’s paper; it was a complete waste of journal space and required several scientists who are busy with other things to sit down and refute it.

Doesn’t the journal have scientifically-literate people (beyond the editor of each paper) who monitor their submissions for quality?


Fig. 1. Peripatus, an onycophoran but not a direct ancestor of the caterpillar.


Hart, M. W. and R. K. Grosberg. 2009. Caterpillars did not evolve from onychophorans by hybridogenesis. Proc. Nat. Acad. Sci. USA. online. (You can get the PDF file from this link.)


  1. Posted October 29, 2009 at 3:17 pm | Permalink

    More Expelling of bad work? You’d think science has standards, or at least that it should have them.

    But that’s so unkind to rubbish and to muddled thinkers.

    Glen Davidson

  2. Matthew Cobb
    Posted October 29, 2009 at 3:45 pm | Permalink

    Bravo to Hart and Grosberg, but a real shame they had to waste their time on this. However the tone of their article is fantastic – seething, barely contained irritation, coupled with devastating data. We need more of this kind of thing in the scientific literature! Let the emotion show!

  3. newenglandbob
    Posted October 29, 2009 at 4:27 pm | Permalink

    Some people run around with a hammer and the entire world looks to be made of nails. Put down that hammer, Margulis!

  4. MadScientist
    Posted October 29, 2009 at 6:48 pm | Permalink

    I am so disappointed. No crocoducks from the hybridization of a raunchy duck with an unsuspecting crocodile?

  5. SLC
    Posted October 30, 2009 at 5:55 am | Permalink

    The question to be asked here is why does any reputable scientific publication take Prof. Lynn Margulis seriously? It has been quite clear for some time that she, like Linus Pauling before her, has turned into a whackjob. The evidence for this conclusion is as follows.

    1. She has hob nobbed with Holocaust deniers.

    2. She has associated herself with Peter Duesberg and denies the relationship between HIV and AIDS.

    3. She is a 9/11 troofer.

  6. Posted October 30, 2009 at 7:05 am | Permalink

    So much for PNAS’s status as “Post Nature, After Science”…this publication does not help keep this journal among the top-tier science journals.

  7. Posted October 30, 2009 at 9:13 am | Permalink

    an ancient lepidopteran mistakenly mated with a velvet wom

    Ah, the “Spring Break” theory of evolution…

  8. Tulse
    Posted October 30, 2009 at 10:19 am | Permalink

    It is indeed a shame that the original paper got published, but this is an excellent example of the corrective nature of science. People can make claims, however outlandish, and others can then apply objective methodologies to test those claims. Alas, this is a quality not shared by many other human endeavours…

  9. Sven DiMilo
    Posted October 30, 2009 at 12:24 pm | Permalink

    People (read: Margulis) just develop a taste for the feeling of iconoclasm, I think. Having to fight for an unconventional but ultimately correct idea is so ego-satisfying that people start emphasizing the fighting-for-unconventional-ideas part and de-emphasizing the being right part. *shrug*

  10. Josh Slocum
    Posted October 30, 2009 at 3:11 pm | Permalink

    Ah, the “Spring Break” theory of evolution…

    OMG. You win the internet!

    I hope this doesn’t sound too uninformed, but I couldn’t get past the laughter stage when Williamson first came out with this. I’m just a layman, so correct me if I’m wrong, but wasn’t this just absurd on its face? Did it really require such a detailed scientific response?

    I mean, how is this any different than believing in a Jackalope?

  11. Sven DiMilo
    Posted October 30, 2009 at 7:40 pm | Permalink

    I couldn’t believe it when the PNAS article came out, because several months ago I had read a semi-popular treatment of the same ideas in American Scientist, mouth agape. That one was more about trochophore larvae in annelids and mollusks, and weird echinoderm larvae, etc. The onychophoran-origin hypothesis of caterpillars was mentioned in passing, and I thought it was the most laughable thing in the whole laughable article. I seem to remember tadpoles being evidence of fish/salamander hybridization too, but I might be making that one up.

One Trackback/Pingback

  1. […] The latest example of this kind of claim has come, unsurprisingly, in the wake of the article by Donald Williamson on the evolution of caterpillars, and its recent utter and total debunking by Hart & Grosberg, which has been dealt with extensively by Jerry on this blog. […]

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