lots and lots of babies

I am at that age where my Facebook feed constantly offers up pictures of engagement rings, weddings, and lots of babies. Lots and lots of babies.

While some of my fellow single ladies find this to be a tad annoying, I don’t mind so much. It’s nice to see good things happening for good people when they’re ready for said good things to happen. Even if those things are kind of red and wrinkly and a little gross looking. With weird heads.

When you look at pregnancy patterns across Mammalia, you see a wide variety of systems. Rats have a gestational period of just over two weeks, while elephants can take 2 years to develop in utero.

Humans take, on average, 40 weeks to develop before birth. It’s long been thought that this timeline is determined by female pelvis size. Basically, that babies pop out right before their heads grow too big to fit through the birth canal.

Long ago in our evolutionary history, before we stood on two legs, we were quadrapedal rodents running around birthing multiple babies at once. But as the millennia passed and we rose to walk on just two feet, our pelvises had to evolve to support an upright posture. It narrowed so as to balance our distribution of weight. The tradeoff here was females no longer being able to (comfortably) give birth to multiple, or more developed and not-so-helpless offspring.

This interpretation was long held, and even suggested that gestation had to shorten in duration especially for the human lineage as intelligence evolved. With bigger brains came bigger heads, making birth essential earlier in the pregnancy timeline. This too would be a tradeoff- a newborn with only a small percent of its brain development completed makes for a very helpless little nugget of neediness.

New findings suggest that evolving a wider pelvis would not be disadvantageous to females. Walking and running is not compromised, and only 3 extra centimeters would be needed to produce babies with 40% brain development at birth (which is more than we currently average). Though it sounds scary, an extra 3 centimeters is statistically not that far-fetched. Evolutionarily, it’s actually pretty doable.

So why end the pregnancy party so early? Why not let it bake for a little longer, ensuring a more precocial offspring?

Scientists recently took a different approach to understanding gestation length in humans. They compared body size to gestation length in different species of primates. Homo sapiens actually break the mold, gestating longer than primate patterns would predict. That’s quite different than the suggestion of the previous model: that human gestation was shortened to accommodate babies with bigger brains. So females are pushing our bodies to the limits—but if not pelvis width, what are those gestational limits?

Evidence suggests those limits are metabolic. Our bodies are capable of supporting spurts of metabolic activity about 2 times that of resting metabolism. By the third trimester, a woman’s metabolism is already running at twice its normal rate. Those last few months are extremely taxing on a woman’s body. By 40 weeks, the baby is requiring nutrients and energy at a rate that a woman simply cannot satisfy past 40 weeks or so.

The article posted on livescience.com ends on a thoughtful note- suggesting that maybe helpless offspring aren’t so bad after all. And that makes sense; a brain that comes into this world with more than 60% growing capacity has room for learning through experience. There are some advantageous survival behaviors that simply cannot be programmed into your brain at birth- like the fundamentals of cellular biology that help you understand the need to clean and disinfect a cut. Or how to grow crops and feed yourself and your family. Or how to use tools that build homes and provide shelter. Our blank baby brains are part of what makes us arguably the most successful species on earth. They give us the capacity to learn.

How amazing it is that all of these factors are tied together- the evolution of bipedal locomotion, to brain size, to pelvis width, to the capacity for in utero brain development, to the evolution of higher intelligence and consequently the ability to feel emotion. Perhaps the most beautiful factor in the evolutionary story of our brains is the ability to appreciate warm fuzzy things that seemingly have no bearing on us or our chances for survival. Like cozy, happy feelings you get from Christmas time, or a bouquet of flowers from someone who loves you, or even warm fuzzies from Facebook photos of friends holding their perfectly ripened, 40-week old bundles of helplessness.

Here’s a link to the original article, written by Stephanie Pappas of Livescience.com. She writes good stuff- read more of it.

http://www.livescience.com/22715-pregnancy-length-baby-size.html