Monthly Archives: April 2011

Evolution: this time it’s personal

Usually if I’m thinking about evolution, it’s in a broad academic sense, wondering how the pressures of natural selection have shaped certain characteristics in humans or other species over thousands or millions of years.  Rarely do I stop to think about my own personal relationship with the evolutionary process, or about how my membership in the taxonomic group Homo sapiens predestines me to particular biological challenges.

But now that I am — SPOILER ALERT — in the midst of my first pregnancy, the realities of my evolutionary history are hitting me fast and hard.  I’m suddenly distinctly aware of the physical and physiological difficulties of maximizing my reproductive fitness.  Though it means I am successfully playing the evolutionary game, it turns out that in practice, passing on my genetic material  to future generations is much more of an ordeal than I had anticipated!

Of course, this is not an entirely new revelation for me.  As a student of anthropology I have long been taught that the evolution of bipedalism combined with increases in brain size substantially increased the difficulty of human birth by necessitating a larger head to pass through a narrower pelvic outlet than is seen in chimpanzees and other primates.  In order to enter the world, a human baby must engage in a series of gymnastic maneuvers through the birth canal, and women everywhere — regardless of their knowledge of evolutionary history — know that it’s no exaggeration to say that the process of labor and childbirth is a difficult one.

Image copyright Karen Carr, Smithsonian Institution

Left: chimpanzee infant head size relative to pelvic opening; Right: human infant head size relative to pelvic opening

But the seemingly endless weeks of overwhelming fatigue, constant nausea, headaches, and light-headedness that characterized my first trimester and have now continued into my second have got me thinking about the challenges of reproduction beyond the context of childbirth (still months away), and for the first time I find myself envious of the seemingly easier reproductive pathways that have been favored in other lineages.

For example, my hormone-drenched brain has recently been wondering what’s so great about internal gestation.  After all, it’s not the only viable reproductive strategy.  And with all the discomforts I’m experiencing as a result of the baby growing inside me, I can’t help but fantasize about how much more pleasant things would be if I could emulate a bird and bring forth life simply by sitting on an egg.  Seems ridiculous, right?  Perhaps, but when it comes down to it, some of my favorite activities — reading, crossword puzzles, jigsaw puzzles — are best accomplished while seated, and I’ve gotten  quite good at it over the years.  Moreover, external gestation would go a long way toward evening the sexual division of labor associated with reproduction.  My husband and I, equally capable of egg-sitting, could share incubation duties (as in most species of penguin), and neither of us would need to be subjected to back pain, heartburn, or any of my other aforementioned symptoms.

In a similar vein, I find I have a newly disdainful attitude toward the functionally useless nipples of men.  I am perfectly aware that males have nipples only because of the default human developmental trajectory; selection has had no reason to eliminate them.  Yet I recently found my train of thought wandering along rather critical tracks:
“Come on selection, can’t we step things up a notch?  The equipment is more or less in place — let’s follow the example of the Dayak Fruit Bat* and put it to use!”

And what about sex determination mechanisms?  In humans, sex is dictated by the chromosomal combination that results when sperm randomly meets egg: females have an XX genotype while males are XY.  But in some species, primarily in the reptile class, sex determination is temperature-dependent.  Alligator eggs incubated at or below 86°F produce females, while those incubated at or above 93°F yield males.  In still other cases, both chromosomes and temperature are involved, and “certain incubation temperatures can “reverse” the genotypic sex of an embryo.”

Image copyright Laura Dewan, University of Hawaii

Now, I’m not saying that I want to choose the sex of my baby.  In fact, my husband and I have decided not to find this out pre-delivery.  But I’m sure that some couples, given the choice, would gladly renounce their evolutionary heritage in order to gain this ability.

As for me, I have no doubt that I am oversimplifying.  The seeming perks of other animals’ reproductive pathways must surely be counterbalanced by their own set of drawbacks.  And, as my mother reminded me, I should keep certain broad facts in mind:
1. Memories of my discomfort during pregnancy will fade over time
2. It could always be worse — the road to reproductive success for a female elephant begins with a 22-month gestation!

Kudos to you, Madame Elephant.  Kudos to you.

newborn-elephant

*The Dayak Fruit Bat example may not have been part of the original train of thought recounted here.

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