Is the Blood Clotting Cascade "Irreducibly Complex?"


In his 1996 book, Darwin's Black Box, Michael Behe argued that the vertebrate blood clotting cascade was "Irreducibly Complex." What Prof. Behe means by this is that each and every element of the complex cascade of enzymes and cofactors must be in place for blood clotting to work. Since, according to Behe, an irreducibly complex system cannot be produced by Darwinian natural selection, it must have been produced by something else. It must have been designed.

After the publication of Behe's book, a number of scientists were quick to point out that Behe was mistaken in many of his assertions about the blood clotting cascade. The work of Russell Doolittle and many other scientists has shown quite clearly that the system evolved by a process of gene duplications from serine proteases that once were digestive enzymes. Not surprisingly, Behe asserts that the many criticisms of his work are incorrect. In August, 2000 he placed an essay on the internet:

In defense of the Irreducible Complexity of the Blood Clotting Cascade.
A Response to Russell Doolittle, Ken Miller, and Keith Robison.


 A Response to Behe's Attempted Defense


As I wrote in my 1999 book Finding Darwin's God, Russell Doolittle's pioneering work on protein evolution has indeed shown that the blood clotting cascade could, and indeed was, produced by Darwinian evolution. Dr. Behe's defense of his position to the contrary requires him to explain why my description of the system's evolution (pp. 152-161) is not valid. In his web-published defense he writes that my description is "a just-so story that doesn't deal with any of the difficulties the evolution of such an intricate system would face."

Curiously, Behe all but ignores my description of the evolution of blood clotting in the lobster, and provides no rebuttal to my scenario for its evolution (based, of course, on Doolittle's research work). I described the lobster system in my book for two reasons: (1) It, like the vertebrate system is (by Behe's standards) irreducibly complex, and (2) It is a simpler system whose step-by-step evolution is relatively easy to account for.

Behe quite properly notes that my own description of the evolution of the vertebrate clotting system is rather brief... a decision, alas, of my Editor at Harper-Collins (the book's publisher). Nonetheless, I still have my original draft of this edited-out section, which the reader may wish to consult. (Click here for my ideas on the evolution of the vertebrate clotting system). However, which elements of my description, besides its sketchiness, does he take issue with?

Behe asserts that the targeting of a protease, a digestive enzyme, to the bloodstream is a "potentially deadly situation," and tells the readers of his web document that we can tell how deadly this might be by looking at situations "where regulatory proteins are missing from modern organisms." In other words, Behe wants us to look at what happens when the highly-regulated current versions of clotting proteases are missing their regulatory factors. Despite this bluster, however, Behe has no evidence that the mistargeting of an inactive protease to the bloodstream would cause harm. Indeed, the recent discovery that antifreeze protein genes in fish arose from exactly such a mistargeting of proteases into the bloodstream (Chen, L., DeVries, A. L. & Cheng, C.- H. C. Proc. Natl Acad. Sci. USA 94, 3811­3816 (1997); and Chen, L., DeVries, A. L. & Cheng, C.-H. C. Proc. Natl Acad. Sci. USA 94, 3817­3822 (1997)) suggests that exactly the opposite is true.

Having made unsupported claims about the "danger" of such a mutation, Behe says that it would be difficult to see what "advantage" this would present to the organism. The answer, of course, is that it would provide a slight improvement in the organism's ability to clot blood - and that's the point. The clotting system doesn't have to work full-blast right away. In a primitive vertebrate with a low-pressure circulatory system, a very slight improvement in clotting would be advantageous, and would be favored by natural selection.

Behe then wonders how the circulating protease could become localized at the site of a clot, as if this were an insurmountable difficulty. It's not. As I suggested in my original draft on the evolution of clotting, a well-understood process called exon shuffling could have placed an "EGF domain" onto the protease sequence, and the "problem" that Behe puzzles over is solved in a flash.

Finally, Behe emphasizes that the real problem is not to generate a clot - it is to "regulate" that clot by means of an inhibitor of the protease so that it doesn't become destructive. But that's not a problem for evolution, either. As usual, Behe envisions a clotting protease that is just as powerful as the fully-evolved proteases in modern vertebrates. However, remember that this is the same guy who fretted a moment or two ago that the protease would not be strong enough to clot effectively. He wants to have it both ways. The answer to his objection is just what I wrote in the draft:

" ... a primitive clotting system, adequate for an animal with low blood pressure and minimal blood flow, doesn't have the clotting capacity to present this kind of a threat. But just as soon as the occasional clot becomes large enough to present health risks, natural selection would favor the evolution of systems to keep clot formation in check. And where would these systems come from? From pre-existing proteins, of course, duplicated and modified. The tissues of the body produce a protein known as alpha-1-antitrypsin which binds to the active site of serine proteases found in tissues and keeps them in check. So, just as soon as clotting systems became strong enough, gene duplication would have presented natural selection with a working protease inhibitor that could then evolve into antithrombin, a similar inhibitor that today blocks the action of the primary fibrinogen-cleaving protease, thrombin."

In short, none of the points raised by Behe are adequate to explain why the vertebrate clotting system could not have evolved. Furthermore, as Doolittle's work has shown clearly, the hypothesis of evolution makes testable predictions with respect to the DNA sequences of clotting proteins, and these predictions have turned out to be correct time and time again.

Why has Behe's "Biochemical Challenge to Evolution" met with so little support within the scientific community? I would suggest that the reason is simple. His hypothesis is wrong. The complex biochemical systems of living organisms, including the vertebrate clotting cascade, are fully understandable in terms of Darwinian evolution.

Kenneth R. Miller
Professor of Biology
Brown University
Providence, RI 02912