Monday, October 10, 2011


The field of life-history evolution, into which the stuff I do roughly fits, spends a lot of time thinking about optimality. What is the optimal age to start reproducing? What is the optimal amount of time to spend foraging each day? What is the optimal body size for a creature in a particular niche? The factor being optimized here is fitness, usually measured by the rate at which a sub-population with a particular trait would increase in size. The trait value that leads to the fastest increase is the optimum. Quite often when we calculate an optimum value and then measure the actual values in the population, most individuals are fairly close to the optimum. But fairly often this does not happen, and then we start talking about "optimality with constraints." By this we mean that there are certain conditions that must be met, and the optimum we are looking for is the best value that is consistent with those conditions. The most commonly considered constraint in my field is the constraint of limited resources. If you have only 100 calories a day of energy to expend, you can't expend it all on growth and maintenance and all on reproduction, your reproduction is constrained by the need for maintenance, and vice versa. So we can calculate, given a set of biologically informed assumptions, what the optimal investment in reproduction is at each age. This kind of constraint makes sense to people, and yields many useful insights. That said, it is often the only type of constraint considered in situations where many other constraints potentially come into play.

One type of constraint that is particularly hard to build theory around is that natural selection can only favor those traits that exist. That is, a trait may be drastically suboptimal, but if all individuals in the population have that trait, and the genes which determine it cannot easily be altered by mutation such that they allow a higher fitness solution, the population will continue being far from optimal.

A classic example of this type of suboptimality is known as the 'obstetric dilemma.' This is the problem that humans have narrow pelvises and big heads, and the head has to pass through the pelvis during birth. In a (now somewhat out of date but still sound for our purposes) summary of one hypothesis of how humans diverged from our chimply relatives, Kristen Hawkes (the anthropologist behind the Grandmother Hypothesis) described (in 2003) the central role this obstetric dilemma played in human evolution thusly:

* Drying environments in the late Tertiary constricted African forests, making capacities to use alternative foods more advantageous among ancestral apes.
* Bipedalism was then favored because it freed hands for tool use, which
increased success at hunting big animals, and this put a premium
on larger brains.
* But the mechanics of bipedal locomotion limited pelvic width, so brain expansion created an ‘‘obstetrical dilemma’’ requiring most brain growth to be postnatal.
Consequently, children with developing brains were immature longer and were more dependent, for a longer time, on maternal care.
* The care requirements interfered with maternal hunting, so mothers relied on
provisioning from hunting mates. This help from fathers allowed mothers to produce more surviving offspring.
* Thus, parents formed lasting bonds and nuclear families became the fundamental
units of cooperation in which a sexual division of labor served familial goals of production and reproduction.

Now according to this story, variations of which are still supported by the scientific evidence,much of the distinctness of human life-history comes through:
1. The need for large brains and small pelvises
2. Which explains why our babies are so undeveloped
3. Which explains we take so long to mature
4. Which is an important part in explaining why we end up with our social system.
5. Which explains why we live so long.

So the optimality of a narrow pelvis, the optimality of a large brain and the need for
that brain to pass through that pelvis ends up being a central fact of human evolution. And why, we may ask, is it optimal for the baby's skull to pass through the mother's pelvis? The apparent answer is that if there is only one possible trait, that trait is the best of all possible traits.

The pattern of vertebrates expelling their young through their pelvis dates back to
before vertebrates actually had pelvises.

Note that this fish has its gonads above and in front of its pelvic fin. That is a common trait among fish, including the lobe-finned fish from which all terrestiral vertebrates are descended. The lobe-finned fishes had bony feet with which they could support themselves on the sea floor, and the bones in their pelvic fins would eventually be modified by evolution into the legs and pelvis.

Now the first terrestrial vertebrates were amphibians, and like most frogs and salamanders, laid small soft eggs, so it was probably no problem for them to continue having the gonads in front and running a tube through the pelvis to the cloaca. This system only became problematic when the eggs got large and hard, as they are in reptiles like turtles. Turtle people like to talk about "pelvic consraint" when they discuss why turtles don't make bigger eggs.

The only non-fish vertebrates to escape the need to run the babies through the pelvis are those that no longer have ana full pelvis, like whales and most snakes. To my knowledge nobody has managed to invent an alternative outlet, so everybody, including us, has to find one way or another to get through the pelvis. In fact, the only alternative is a human invention, the cesarian section.

This obstetric dillema is a very obvious contraint of the 'no alternative' type. Whenever I get a chance to write another longish post, I'll give an example of a constraint where the lack of alternatives is less obvious because it is genetic rather than anatomical.

1 comment:

jte said...

Excellent stuff. More long posts!

Still, given how every so often you find some mutant with a leg growing out of its neck or whatever, it does seem strange that no vertebrate ever introduced a birth canal that was above the pelvis. Hmm, now that I think of it, it occurs to me that such a mutant would be at a disadvantage in mating--for animals that must copulate, having your vagina in the expected location would likely be a benefit, and having it in an unexpected location would likely be a detriment.