Last night I gave a talk, “The Positive Case for Intelligent Design” (based partly on my article by the same name), at North Dakota State University in Fargo. The event was sponsored by a campus group, the Intelligent Design Fellowship. The talk went very well, as did the Q&A, with some skeptical faculty and students asking very good questions (more on this below). The audience was great and we had a very fun, friendly, lively discussion.

Two issues of note dominated the Q&A: the Type III Secretory System (T3SS) as a supposed evolutionary precursor to the bacterial flagellum, and whether the Cambrian explosion really was an actual event in the history of life.

During my talk I had cited the bacterial flagellum as an example of irreducible complexity. Afterward, multiple people, including a student, asked whether the T3SS could be an evolutionary precursor to the flagellum. I responded that the T3SS has only a small number of flagellar genes, and it doesn’t account for most of the flagellum. Moreover, the genes it shares are from the portion of the flagellum that serves to anchor it in the cell membrane.

If you think of the flagellum like an outboard motor, and the T3SS like a squirt gun, the parts they share are the ones that allow them to be mounted on the bracket of a boat. But the parts that give them their distinct functions — propulsion or injection — are not shared. I said that thinking you can explain the flagellum simply by referring me to the T3SS is like saying if you can account for the origin of the mounting-bracket on the back of you boat, then you’ve explained the origin of the motor too — which obviously makes no sense.

I also explained that phylogenetically, the T3SS makes an unlikely precursor to the flagellum. The T3SS is found in a narrow range of bacteria which makes it seem to be a late innovation, whereas flagella are widespread throughout many bacterial groups, which implies it was an early innovation. Another reason why the T3SS would be a late innovation is the function it performs: it helps bacteria kill and eat eukaryotic microorganisms. Since eukaryotes arose late in the history of life, it stands to reason that the T3SS did as well. So it doesn’t make sense to say the T3SS (or something very much like it) served as a precursor to the flagellum. For details on these arguments, see “Spinning Tales About the Bacterial Flagellum.”

Regarding the Cambrian explosion, an evolutionary biology professor argued that the Cambrian explosion might just represent the origin of hard parts, which fossilize more easily than soft-bodied organisms. I replied that we see lots of soft-bodied fossils in the fossil record of both the Cambrian explosion, and the Precambrian, but we don’t see ancestors to the Cambrian phyla. The Cambrian explosion itself bears fossil representing several soft-bodied worm phyla, and other soft-bodied organisms. Indeed, Simon Conway Morris notes in his book Crucible of Creation that in the Burgess Shale fossil collections which document the Cambrian explosion, “about 95 per cent are either soft-bodied or have thin skeletons.” (p. 140)

Similarly, jellyfish-like fossils are found in Precambrian strata, and they have no hard parts. Clearly the Precambrian strata could preserve complex soft-bodied organisms. But ancestors to the Cambrian phyla are not found.

No wonder Robert L. Carroll said:

The most conspicuous event in metazoan evolution was the dramatic origin of major new structures and body plans documented by the Cambrian explosion. Until 530 million years ago, multicellular animals consisted primarily of simple, soft-bodied forms, most of which have been identified from the fossil record as cnidarians and sponges. Then, within less than 10 million years, almost all of the advanced phyla appeared, including echinoderms, chordates, annelids, brachiopods, molluscs and a host of arthropods. The extreme speed of anatomical change and adaptive radiation during this brief time period requires explanations that go beyond those proposed for the evolution of species within the modern biota.

(Robert L. Carroll, “Towards a new evolutionary synthesis,” Trends in Ecology and Evolution, Vol. 15(1):27-32 (2000) (internal citations removed).)