Five studies released this week focus on different aspects of reproduction, from advances in in vitro fertilization, to understanding how genes are regulated during early development (and how this could affect cancer later) and why human eggs are prone to mistakes that cause birth defects, especially in older women. Another study even discovered a way that stress during pregnancy can influence whether daughters become obese later in life.
But the one I want to really talk about is a study that tells us how evolution has changed the timing of birth in humans.
Half of my recent travel was to visit my sister and her new baby daughter. Her daughter was born healthy and happy, but my sister would have been happier if the birth date had come a little bit sooner. Despite signs in her pregnancy early on indicating she might give birth too soon, my little niece changed her mind in the last month and decided to hang out in the womb longer than anyone expected. As you can imagine, this led to some discomfort (and impatience!) on my sister’s part.
A study released last week indicates that my sister was luckier than she thinks: humans have evolved to have shorter pregnancies. Can you imagine being pregnant for a year, or maybe longer?
The article, published online April 14 in PLoS Genetics found some forms of a gene needed for ovulation were connected with a higher risk of delivering a baby prematurely. Approximately half a million births in the U.S. each year are premature (that’s 1 in 8 babies who are premies). And as most people know, premies have a higher risk of infant death as well as other health issues later in life. Also, as the scientists point out, it is still a mystery why some people have longer pregnancies than others.
In the study, the researchers first hypothesized that humans may have evolved to have shorter pregnancies because we have very large heads compared to our body size, but also have narrow pelvises that help us to walk upright. This means that we would want to deliver a baby as early as possible to reduce the risk of dying in childbirth. But it can’t be too early, or baby won’t survive.
A comparison to other primates’ body and head sizes at birth suggested that human pregnancy lengths are much shorter than would be predicted from those of our closest relatives. So, something must have happened during evolution, at the level of our DNA, to give us shorter pregnancy lengths.
Through DNA comparison between humans and other primates, 150 genes were found to be likely candidates for why we evolved to give birth sooner. These genes all showed signatures of having evolved particularly fast in the human branch of the tree of life, but not in other branches.
One of the more tantalizing genes was Follicle Stimulating Hormone Receptor (FSHR). By comparing the DNA sequence of this gene among a cohort of Finnish mothers, the researchers found that certain variants of this gene were more common in women who had previously had a premature delivery. Follow-up studies indicate that these same variants may be associated with premature delivery in other populations as well.
The interesting thing about this result is that it shows that evolution doesn’t always stop just when we want it to. Changes in the FSHR gene (and probably many others as well) might have lead to an advantage for us (ie: not dying during childbirth). So, we really don’t want any more changes in that gene, thank you very much. Unfortunately, natural selection is not so much an intelligent process as it is a process of trial and error, and it never stops.
Fortunately, humans are always “evolving” better strategies for identifying bad mutations and developing better treatments to overcome them. The results from this study may lead to development of a genetic test that could predict whether a woman is likely to have a premature baby, which could help to prevent this from happening.