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Paper Lays to Rest "Vernanimalcula," Supposed Precambrian Ancestor of Bilaterian Animals

Vernanimalcula.jpgWhat was “Vernanimalcula“? Maybe the antagonist in the latest teenage vampire movie? No, Vernanimalcula is a fossil — or anyway some kind of a structure in a rock, which some evolutionary biologists claim was the hallowed Precambrian ancestor of all bilaterian animals. (Bilaterian animals are those with bilateral symmetry, symmetrical on their right and left sides, but having distinct fronts, backs, tops, and bottoms. Humans, frogs, and fish, are all bilaterians.)

In a 2005 paper in Scientific American, University of Southern California paleontologist David Bottjer crowned Vernanimalcula as the “oldest fossil animal with a bilateral body plan yet discovered.” Because Vernanimalcula was dated to tens of millions of years prior to the Cambrian explosion, his article used the fossil to attack the view that the Cambrian explosion was any kind of an “explosion” at all:

Biological complexity of the kind seen in Vernanimalcula implies a period of evolution that transpired long before the 580-million- to 600-million year-old world in which the tiny animal lived. After all, it could not have gained that degree of symmetry and complexity all at once. … Vernanimalcula certainly gives paleontologists new inducements to go out and hunt for fossils of soft-bodied animals. We have a good deal left to learn, but the work so far has given substance to our earlier suspicion that complex animals have a much deeper root in time, suggesting that the Cambrian was less of an explosion and more of a flowering of animal life.

(David J. Bottjer, “The early evolution of animals,” Scientific American, Vol. 293: 42-47 (2005).)

Now, however, a new article in Evolution & Development has taken Bottjer’s arguments about Vernanimalcula to pieces. The authors don’t just question whether Vernanimalcula was a bilaterian ancestor — they’re not even sure it represents a fossil, period.

Titled “A merciful death for the ‘earliest bilaterian,’ Vernanimalcula,” the article unmercifully concludes that “There is no evidential basis for interpreting Vernanimalcula as an animal, let alone a bilaterian.” The scientific paper uses uncommonly strong language to refute the idea that Vernanimalcula was a bilaterian ancestor, calling that interpretation “fallacious.” After a close study, the authors conclude that “the structures key to animal identity are effects of mineralization that do not represent biological tissues” and “it is not possible to derive its anatomical reconstruction on the basis of the available evidence.” As a result, they warn that “The conclusions of evolutionary studies that have relied upon the bilaterian interpretation of Vernanimalcula must be called into question.”

But this isn’t quite the end of the story.

Confirmation Bias
Dr. Bottjer won attention in 2006 when he was featured in the film Flock of Dodos as an example of a hardline evolutionary biologist who wasn’t very good at communicating his adamantly pro-evolution feelings. He gained further notoriety in the California Science Center museum case where he was part of a group of Los Angeles-area evolutionary scientists who sought to pressure the museum into cancelling the contract of a small non-profit group that had tried to rent the museum for a private screening of the pro-ID film Darwin’s Dilemma. In one of the many intolerant e-mails revealed in that case, Bottjer called the film “a creationist movie on the Cambrian explosion” — despite the fact that the film’s arguments are strictly scientific, accept the conventional geological timescale, and are pretty hard to confuse with creationism.

There are some other troubling comments, however, from Bottjer’s 2005 paper in Scientific American, that I haven’t mentioned yet, which reveal more. He wrote:

We had come to Guizhou [China] in 2002 to hunt for microscopic fossils of some of the earliest animals on earth. Specifically, we were hoping to find a bilaterian.

Bottjer later states, “Of course what we were really focused on finding was a bilaterian.” And wouldn’t you know it? — a “bilaterian” is exactly what he and his colleagues found!

Is it possible that there’s some confirmation bias going on here? That’s exactly what the paper in Evolution & Developmentproposes. After recounting those same passages from Bottjer’s paper, the critics of Vernanimalcula offer uncommonly strong words for a scientific paper:

It is likely that the fossils referred to as Vernanimalcula were interpreted as bilaterians because this was, as our epigram betrays, the explicit quarry of its authors. If you know from the beginning not only what you are looking for, but what you are going to find, you will find it, whether or not it exists. As Richard Feynman (1974) famously remarked: “The first principle is that you must not fool yourself — and you are the easiest person to fool. . . . After you’ve not fooled yourself, it’s easy not to fool other scientists.”

Conversely, once you have fooled yourself you will fool other scientists. And so Vernanimalcula has been marshaled in evidential support for the timing of bilaterian evolution and of multifarious bilaterian innovations. The “little spring animal” has taken on a life of its own, a life it never had in the Neoproterozoic. It is our hope that Vernanimalcula will now be laid to a merciful rest, freed from the heavy burden of undue evolutionary significance that has hitherto been heaped upon it.

(Stefan Bengtson, John A. Cunningham, Chongyu Yin, Philip C.J. Donoghue, “A merciful death for the ‘earliest bilaterian,’ Vernanimalcula,” Evolution & Development, Vol. 14(5):421-427 (September / October 2012).)

Rest in peace, Vernanimalcula.

Image: “Vernanimalcula,” Wikipedia.


Casey Luskin

Associate Director, Center for Science and Culture
Casey Luskin is a geologist and an attorney with graduate degrees in science and law, giving him expertise in both the scientific and legal dimensions of the debate over evolution. He earned his PhD in Geology from the University of Johannesburg, and BS and MS degrees in Earth Sciences from the University of California, San Diego, where he studied evolution extensively at both the graduate and undergraduate levels. His law degree is from the University of San Diego, where he focused his studies on First Amendment law, education law, and environmental law.



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