Becoming a boob.
Yes, today I am talking about boobs but, for once, there's actually a purpose to it.
From a biological point of view, the breast is an interesting structure. On the one hand, its existence has contributed mightily to the evolution of Human cognitive abilities. With our huge brains, and proportionately large heads, the ability to nourish physically immature young outside of the womb is of tremendous benefit. However, on the other hand, the development of breasts and the production of milk is metabolically expensive. All that fat and oil that makes milk so nourishing for growing children places a strain on the mother's resources. As a result, the development of breasts for nursing is not without certain drawbacks.
What makes this more complex is the question of how this system evolved in the first place. Consider, if you will: in order for a breast to be a worthwhile investment of metabolic resources, there has to be an immature child to nurse from it. Yet, at the same time, in order for a mutation producing birth at an earlier stage of fetal development to be useful, the mother of that fetus must be able to support it. It seems that what would have to happen would be two or more simultaneous changes in both adult human structure, and infant human structure, in order to make this system useful. It would appear that separate, complimentary mutations must take place at the same time. That seems like a tough order.
If this line of reasoning seems familiar to some of you, it should- it's almost exactly the argument made by intelligent design advocates about irreducible complexity. Put simply, an irreducibly complex system is supposedly one that has three distinct properties. First, it requires several interrelated parts to function. Second, the removal of even one of those parts makes the entire system fail. Third, the sub-parts are not useful for anything themselves in their current forms. As an illustration you might think of a mousetrap: it is a machine made of a spring, a hinge, a metal bar, and a trigger. Remove any of those parts and not only does the mousetrap no longer function- it doesn't do anything useful at all. Intelligent design advocates argue that many biological systems are irreducibly complex and that, by their very nature, irreducibly complex systems cannot have evolved. This is because an incremental addition of functionality would not have been possible as the adaptation only becomes useful when accompanied by a host of other adaptations. They further argue that since these systems cannot have evolved, they must have been designed, and so we have evidence for the intelligent design of life.
With me so far?
Using this understanding, the breast/infant system appears to be irreducibly complex. It does require several interrelated parts. At the most gross level, it requires both an infant that needs nourishment and a biological structure to produce and deliver it from the mother. Second, the removal of either part makes the system collapse. An infant without a breast is a waste of the mother's resources to produce, and a breast without an infant is essentially the same.* Finally, it is difficult to see how this system could have evolved incrementally as the benefit of it is only realized late in the game. The mammalian practice of nursing young would, in this light, seem to be irreducibly complex. So, is every nursing mother an argument for intelligent design?
Well, as you might guess from having read this blog before, the answer is no. One of the huge stumbling blocks that the intrinsically-appealing irreducible complexity argument encounters is that structures that evolved for one function can later be adapted to fulfill an additional, or entirely different, subsidiary function. This addition can later even become the primary role, leaving us with a complicated evolutionary chain that gives the appearance of irreducible complexity. As it happens, the breast is one such case.
A recent post over on Pharyngula explains how in much more detail than I can, but the essence is simple: mother's milk doesn't just provide nutrition, it enhances immune function. This has been known for a while now, and is one of the reasons for the recent shift from formula to breast feeding. We have typically regarded this function as a subsidiary of the nursing function but there is new, and compelling, evidence that this was in error. It now appears that human breasts began not for the nutrition of young, but as part of the immune system.
Humans and many other organisms have what is known as an "innate immune system." You can think of this as a system that constantly produces and releases anti-bacterial compounds. Many of these compounds are excreted onto our skin to act as an initial barrier to infection. In essence, much as bleach kills most bacteria, our bodies produce chemical substances that are hostile to a wide variety of micro-organisms. While this isn't perfect, it helps cut down on more serious infections. Interestingly, the structures producing these substances have long been known to be concentrated in places where the secretions can come into contact with eggs and/or young. It only makes sense: a mother who secreted a little extra of these substances in places where her young could pick them up offered enhanced protection. That's a real advantage! As it also happens, much of the immune function conferred by a mother to a child via milk is through these innate immune substances. So, on a primitive level, you can think of breasts as having their origins in glands that produced an immunoprotective mucus for the benefit of the child.
Isn't that a thrilling idea? Well, as PZ Myers says, "They got better over the years."
This evidence by itself isn't sufficient, but molecular biology supplies another piece of the puzzle. The proteins that are critical to the nutritive function of milk are, in fact, modified versions of the proteins used in the innate immune system. In other words, mother's milk doesn't just contain anti-bacterial substances, it's made out of modified versions of those substances. We have clear molecular evidence that breasts began not as a way to feed babies, but as a way to protect both the parent and the child from infection.
So what does this do to the irreducible complexity argument? Well, in short, it blasts it out of the water. We can trace a chain of gradual evolution from simple innate immune function, to enhanced function to protect young, to added nutritive function, to the birth of less mature offspring, and we never encounter a point where the adaptations required were useless.
On a larger level, however, this is a lesson for us all: evolution is tricky. Unlike a human designer who sets out to produce a particular thing, the blind watchmaker simply modifies and adapts whatever is available, as it is available, with no particular plan in mind. As a result the "design" process becomes complex and nearly-incomprehensible. It also becomes very deceptive, with normal adaptation sometimes appearing as impossibility.
By attempting to fit the work of evolution into the mold of design we are guilty of anthropomorphizing nature. We ascribe to it not only human properties, but human goals and human ways of achieving those goals. Since nature has none, it is hardly surprising that its solutions to problems seem counterintuitive to us. If we remember this, then we stand a good chance of understanding the world.
But if we forget it, then we shall all become boobs.
* Please note that I am speaking in strictly biological terms. I wish to express no other opinions as I find breasts rather aesthetically pleasing.