Evolution of the Immune System

One of the major arguments of Michael Behe in the Dover trial, in which intelligent design creationism was shown up to be the vacuous nonsense that it is, was that the immune system is too complex to have evolved through purely natural processes. I think its reasonable to assume that for someone to make such a wide, sweeping claim they must be up to date with the literature...

Not Behe.

In fact, when presented with a stack of thick textbooks, all of which included chapters on the evolution of the immune system, Behe had to admit that he hadn't read any of them. Not one!

Let's just say that his honour, Judge Jones, was not amused. And, of course, the rest is history.

Well in the January edition of Nature Reviews Immunology, there is an interesting commentary on the very subject that Behe knows nothing about, and yet feels qualified enough to dismiss - evolution of the immune system.

How did our complex immune system evolve?

Max Cooper and Brantley Herrin discuss the evolution of innate and alternative adaptive immune systems for defence purposes and conclude that successful vaccines and other therapeutic manipulations of the immune system will require a composite strategy.

The immune system basically consists of two arms, innate and adaptive immunity (although, as ever, this is an oversimplified account).

Innate immunity works tirelessly to keep you free from infection every second of every day. If you need any evidence of this, think of how quickly an unrefridgerated body can decompose following death. And innate immunity isn't picky; it will protect you against anything it recognises as foreign. If you've heard of interluekin, interferon, TNF, macrophages, neutrophils, Toll-like receptors or complement, you've heard of innate immunity.

In comparison, the adaptive immune system is only called upon when innate immunity fails to eliminate a microscopic invader. But let me tell you, that little bug is in for a whole lot of trouble, because the adaptive immune system will target it specifically and mount a massive immune response to attack it. If innate immunity is like low level police constantly patrolling the streets, adaptive immunity is more like an elite squad of detectives and sharpshooters out to get their man. Common players in adaptive immunity include T cells, B cells and antibodies.

However, the benefits of adaptive immunity are only enjoyed by higher eukaryotes, including me, you and anything with a backbone. Organisms 'below' this in the evolutionary tree make do with innate immunity alone - indicative of the power and efficiency of the innate immune system. We share many innate immune components with mice, chickens, fish, fruit flies and even plants (in fact, in the same issue of Nature Reviews Immunology there is a whole review dedicated to a comparison of our innate immune system with that of the worm). And as one would expect if the immune system evolved, the closer we are to another species in the evolutionary tree, the more sequence similarity we find in the genes encoding these common proteins.

How does Behe explain this?... Well who knows, but my guess would be that he'd trolley out the usual creationist line that similar sequence could simply mean similar designer. Of course, the existence of endogenous retroviruses easily refutes this argument, but that is something for another day (but if you want to know more right now, click here).

As mentioned, the adaptive immune system is found exclusively in vertebrates. Cooper and Herrin discuss recent work that has cast new light on the evolution of adaptive immunity. It turns out that jawless vertebrates (hagfish and lamprey) have an adaptive immune system that functions slightly differently - in fact, the preliminary work on this was published in 2004, you know, before Dover. Specifically, hagfish and lamprey use entirely different types of antigen recognition receptor, meaning they use different proteins than we do to identify foreign pathogens. However, despite this difference, following recognition we all use similar mechanisms to direct our immune response to eliminate the invader.

Now the exact step-by-step evolutionary pathway that created these two different adaptive immune systems isn't known. Of course it isn't. It's completely unreasonable to expect that level of detail to be discovered, and Behe knows this. That's the reason he uses this type of argument - it's classic God-of-the-gaps drivel. However, based on current knowledge the authors sum up the probable evolutionary mechanisms involved:

The evolution of alternative adaptive immune systems was facilitated by two rounds of whole genome duplication, which enabled the original function of a gene to be maintained while allowing evolutionary selection of modifications of additional gene copies for new purposes. The common ancestor of lamprey and hagfish probably emerged between the first and second rounds of genome duplication, as amphioxus and tunicates have single gene copies, lamprey have two gene copies and jawed vertebrates typically have four copies of retained genes.

And of course, as usual with real science, this work leads to more questions:

Such convergent evolution of mechanisms for the generation of diverse antigen receptors after the split in jawless and jawed vertebrate ancestry raises the question of whether the two pathways of lymphocyte differentiation arose in a common vertebrate ancestor.

It's nice to know that people are working on this stuff, as opposed to others who try to stifle science with fanciful stories about mousetraps and magic.