Pharyngula

Complex eyes in the Cambrian

I got a letter from a creationist today, claiming that “Darwinism is falsified,” based on an article in Nature. It’s kind of amazing; this article was just published today, and the metaphorical digital ink on it is barely metaphorically dry, and creationists are already busily mangling it.

It’s a good article describing some recent fossil discoveries, found in a 515 million year old deposit in South Australia. Matthew Cobb has already summarized the paper, so I’ll be brief on the details, but it’s very cool. What was found was a collection of arthropod eye impressions, probably from cast-off molts. No sign of the bodies of these animals was found, suggesting that perhaps they were not fully sclerotized, or as the authors suggest, that disarticulated eyes were more prone to rapid phosphatization than eyes attached to a decaying body. There is no evidence of biomineralization, so these were animals with a very light armor of chitin alone.

What’s wonderful about the eyes is that they are relatively large and contain numerous ommatidia, the individual facets of a compound eye. They have over 3,000 lenses, and there’s also evidence of regional specialization in the eye. These were highly visual animals that were capable of forming a good image of the world around them.

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Complex arthropod eyes from the Early Cambrian. a-d, Three fossils of compound eyes from a large arthropod from the Emu Bay Shale, South Australia (a-c), shown in similar hypothesized orientation to the compound eye of a living predatory arthropod, the robberfly Laphria rufifemorata
(d; anterior view of head). All fossil eyes have large central ommatidial lenses forming a light-sensitive bright zone, b, and a sclerotized pedestal, p. Because the fossil eyes are largely symmetrical about the horizontal axis, it is not possible to determine dorsal and ventral surfaces, and thus whether the eyes are left or right. All fossils are oriented as if they are left eyes (medial is to the left of the figure). In b there is a radial tear (white line) with the top portion of the eye displaced downwards to overlie the main part; extensive wrinkling causes some central lenses (arrow) to be preserved almost perpendicular to the bedding plane.

These eyes are also from the early Cambrian, so they appeared in the early stages of large animal evolution. The closest thing to them in ommatidial number are the sophisticated eyes of many trilobites, but even there, these eyes were early and relatively large.

i-1c548b0b7b6021fe20accaedefa965c7-ommatidianumbers.jpeg
Complexity of the Early Cambrian Emu Bay Shale eyes compared to eyes in other early Palaeozoic taxa. a, b, Number of ommatidia (a) and lens size (b) plotted against stratigraphic age for Cambro-Ordovician arthropods. The Emu Bay Shale eyes have many more ommatidia and much larger individual ommatidia than eyes in all other Cambrian taxa. Trilobites are plotted according to eye type: schizochroal eyes have relatively few, large lenses and are optically unusual compared to typical compound eyes.

Where in this is the refutation of evolution? I don’t know. But I did receive a letter from that Canadian idiot, David Buckna, crowing about it, and linking to his very silly creationist article describing it, in which you’ll find the abstract for the paper with curious random spastic boldfacing added which supposedly highlight the parts of the story that contradict evolutionary theory, words like “complexity” and “Cambrian explosion” and “more complex” and “great evolutionary event”. It’s a bit bizarre and like looking at the obsessive activity of a squirrel gathering nuts.

Here’s the creationist summary of the paper, however.

The Cambrian explosion is affirmed; complexity appears suddenly without transitions; Darwinism is falsified; the inference to the best explanation is intelligent design. Let the world know.

Let’s deal with each of these claims one by one.

  1. The “Cambrian explosion” is a term coined by scientists to describe the rapid (in geological terms) appearance of large, complex animals with hard skeletons over the course of a few million years roughly half a billion years ago. There is no creationist gotcha in pointing out the existence of this geological period; scientists have written whole books on the subject.

  2. The sudden appearance of complexity is no surprise, either. We know that the fundamental mechanisms of eye function evolved long before the Cambrian, from the molecular evidence; what happened here was not that, poof, eyes instantly evolved, but that the evolution of body armor gradually increased from the pre-Cambrian through the Cambrian, making the organization of eyes visible in the fossil record.

    It is also the case that the measure of complexity here is determined by a simple meristic trait, the number of ommatidia. This is not radical. The hard part in the evolution of the compound eye was the development of the signal transduction mechanism, followed by the developmental rules that governed the formation of a regular, repeating structure of the eye. The number of ommatidia is a reflection of the degree of commitment of tissues in the head to eye formation, and is a quantitative difference, not a qualitative one.

    And finally, there’s nothing in the data from this paper that implies sudden origins; there can’t be. If it takes a few hundred thousand years for a complex eye to evolve from a simple light sensing organ, there is no way to determine that one sample of a set of fossils was the product of millions of years of evolution, or one day of magical creation. It’s a logical error and a failure of the imagination to assume that these descriptions are of a population that spontaneously emerged nearly-instantaneously.

  3. “Darwinism” is not falsified. Darwin himself explained in great detail how one should not expect fine-grained fossil series, due to the imperfection of the geological record. Creatonists, read chapter 9 of the Origin; here’s a brief excerpt.

    It should not be forgotten, that at the present day, with perfect specimens for examination, two forms can seldom be connected by intermediate varieties and thus proved to be the same species, until many specimens have been collected from many places; and in the case of fossil species this could rarely be effected by palaeontologists. We shall, perhaps, best perceive the improbability of our being enabled to connect species by numerous, fine, intermediate, fossil links, by asking ourselves whether, for instance, geologists at some future period will be able to prove, that our different breeds of cattle, sheep, horses, and dogs have descended from a single stock or from several aboriginal stocks; or, again, whether certain sea-shells inhabiting the shores of North America, which are ranked by some conchologists as distinct species from their European representatives, and by other conchologists as only varieties, are really varieties or are, as it is called, specifically distinct. This could be effected only by the future geologist discovering in a fossil state numerous intermediate gradations; and such success seems to me improbable in the highest degree.

    Finding a fossil eye with numerous ommatidia a hundred million years after molecular biology tells us that eyes evolved does not in any way falsify the idea of a gradual evolution of the eye.

  4. Given that there is nothing in this story that contradicts the idea of a natural process generating increasing complexity over time, and given that it’s an observation that fits perfectly comfortably within the body of evolutionary theory, there is no reason to leap the utterly unfounded conclusion that an invisible spirit zapped these fossils into existence — an invisible spirit for which there is no evidence. Furthermore, what evidence is in this paper directly contradicts Buckna’s beliefs: he is a young earth creationist, and this is a paper describing organisms that lived 515 million years ago. If you look at the chart I reproduced above, you might also notice that the pattern of complexity (ommatidial numbers) in trilobites shows a trend of increase over 80 million years.

  5. I shall gladly let the world know that David Buckna is an irrational fool who doesn’t know how to read a scientific paper and makes illogical leaps in his arguments.


Lee MSY,
Jago JB,
García-Bellido DC,
Edgecombe GD,
Gehling JG
Paterson JR (2011) Modern optics in exceptionally preserved eyes of Early Cambrian arthropods from Australia. Nature 474: 631-634.