What evolution can teach us about breastfeeding and natural childbirth

by Alan Cohen

IMG_0109This little guy is my son, Soren, born March 13 (and the reason I haven’t posted for a while). In the photo, hours after birth, he is hooked up to electrodes, a blood pressure cuff, and various paraphernalia of the modern medical establishment, a result of having to undergo CPR to restart his heart and breathing as he emerged from a cesarian. He is doing great now, but it is clear that modern medicine saved his life (and possibly that of my wife), probably 2-3 times over between the long labor, the cesarian, and the resuscitation. (He is doing great now.)

Like many in our generation and social class, especially here in Quebec, we had wanted to have a “natural childbirth,” i.e., to see if we could deliver without any anaesthesia or other interventions of modern medicine. Unlike many of our friends, we had not signed up with a midwife or birthing center: the public hospital here in Sherbrooke has an excellent maternity ward, and nurses are trained to help with natural or medicalized childbirth, or any combination, depending on the wishes of the parents.

Despite my preference for “natural,” I was also acutely aware that childbirth is different than other aspects of nature: in this case, natural implies very high levels of both infant and maternal mortality. In contrast to breastfeeding, where all the evidence points to breast milk being superior to any technologically developed infant formula, “natural” childbirth does not always equate to good childbirth. As is often the case, this is clearer when seen in the light of evolution:

All mammals feed their offspring through lactation/breast milk. It’s what makes them mammals, and why the word “mammal” has its origins in the Latin word Mammae, or breasts. Mammals have been around for ~200 million years (in contrast, dinosaurs have “only” been extinct for 65 million years, or 1/3 of that time). That’s one heck of a lot of evolutionary time to be able to tweak breast milk to make sure it’s got the proper nutritional content for the young. And there is very good evidence that this time has been put to use: breast milk is full of antibodies, hormones, nutrients, and all kinds of good things to help the young grow well and fend off diseases.

Of course, not all mammals have the same nutritional requirements – each species needs a different balance of all these goodies, depending on its lifestyle, how it needs to develop and grow, the pathogens around, and so forth. But 200 million years is long enough that in addition to evolving good nutritional content, mammals have almost certainly also evolved evolvability of breast milk nutritional content: the ability to adjust the content quickly (in evolutionary time) as conditions change.

Evolvability means that there are genes that can be easily tweaked by natural selection to create the proper nutritional composition of the milk. For example, the number of limbs in mammals is not very evolvable at all: all mammals have four limbs, and even if a mammal really needed to evolve six limbs to survive well in its environment, it couldn’t do so because there is not the proper genetic variation available. On the other hand, the immune system is highly evolvable, and is constantly adjusting the balance of different components in order to be able to best fight whatever pathogens happen to be around this generation.

How do we know that mammalian breast milk nutritional content is a highly evolvable trait? To be honest, I’m just guessing, but it’s a highly educated guess. I know that the nutritional content of cow’s milk, goat’s milk, sheep’s milk, and human milk are all quite different, showing that content does evolve quickly (cows, goats, and sheep are all closely related). I know that even within humans, the nutritional content of the colostrum (just after birth) is quite different from that of milk, which starts several days later, and that the composition of breast milk changes over the course the first year of life, as well as even within each individual feeding. All of this points to a system that is quite complex and is fine-tuned by natural selection to be just right for each species given its needs and its environment.

Given all this, and the large number of antibodies and molecules involved, it is apparent that we are still a long way from being able to chemically formulate infant formula that is as good as natural breast milk. 200 million years of evolution of a complex system is superior to human technology. (This is not to say that formula is a disaster or is really unhealthy; many people go overboard with the advantages of breast milk. But from an evolutionary standpoint, the argument for breast milk is clear.)

The same is not true for natural childbirth. Yes, most mammals do have live births. (We should exclude the monotremes – echidnas and platypuses – which lay eggs, as well as marsupials like kangaroos, which do have live births, but at a much earlier developmental stage.) Placental mammals like us still go back 166 million years. So it would seem on the surface that we have been tweaking live births for a very long time. But the difference here is that, while human nutritional needs fall well within the normal range of other mammals species and are not so different from our closest ancestors, the anatomy of live birth in humans is very different from other mammals. Our hip structure changed dramatically when we started to walk upright. Our head size increased dramatically as we evolved larger brains. Developmental stage at birth became younger and younger as we tried to get the smallest head possible through the birth canal – meaning that infants are born at more and more vulnerable developmental stages.

In other words, several simultaneous changes in recent evolution of humans (and our immediate ancestors) produced conflicting pressures on the birth process in humans, and brought us into to heretofore uncharted territory for mammals. Certainly, there has been no time to evolve evolvability of these traits (perhaps 200,000 years since the evolution of modern humans?). More importantly, the system is not yet in equilibrium. Compared to every other mammal species, humans have very, very high levels of maternal mortality during childbirth, as well as very high levels of infant mortality during childbirth. This is all the more striking given that human mortality is one of the lowest of all mammal species at every other point in the life course.

What this means is that natural selection is working very strongly on human childbirth, trying to find a way to balance all of the conflicting selective pressures of birth canal diameter, upright posture, brain size, head size, birth posture, developmental stage at birth, and so forth. It’s not an easy task, meaning that it will take a while, and in the meantime there is lots of death of both mothers and infants. Give us another million years or so and we should solve the problem, but that’s cold comfort to the many families that would experience tragedy along the way.

Enter modern medicine. Modern medicine has had huge success in driving down both maternal and infant mortality rates using techniques such as hormonally induced labor, fetal heart rate monitoring, anaesthesia, and (most importantly) cesarian sections. We’ve succeeded where evolution has (so far) failed. But in the process, we’ve mostly stopped evolution. All the women (such as, perhaps, my wife) and infants (such as my son) that would have died and been weeded out by natural selection are now surviving and reproducing, passing on their genes for an imperfect birth process. I am very glad we have succeeded in circumventing nature’s redness in tooth and claw in this case, but the consequence is that in 1 million years we will still need all this medical technology, because we are not letting natural selection solve the problem for us  in the only way it knows how.

All of which points out a contradiction in the perspective of the hippies, midwives, and other advocates of all things natural (for whom I nonetheless have a great deal of sympathy!): natural includes death. It includes lions eating zebras, and it includes mothers and babies dying during childbirth. This is not to say that all medicalization of childbirth is good, nor that people should not go to midwives and birthing centers. In fact, most midwives are very aware of the risks and are able to get women to a hospital quickly should anything start to go wrong. But perhaps our idealization of nature should be a bit more targeted: breast milk deserves more idealization than childbirth as a masterpiece of nature’s beauty and ingenuity.