Women having an increasing number of babies in an unconventional manner—via caesarean section, or C-section—may be decreasing infant mortality rates but it is also creating some evolutionary changes in humans, a small research team has concluded.
As reported by MedicalXpress.com, the scientists—from the United States and Austria—have discovered statistical evidence that a boost in the number of mothers having C-sections over the past several decades has led to a change in evolution, with babies’ heads getting bigger even as mothers’ birth canals remain relatively unchanged.
The team, which published its findings recently in the journal Proceedings of the National Academy of Sciences, note that scientists have known for quite a while that humans have more difficulty giving birth to their young than most other animals, and that is largely due to the passage of proportionally larger babies’ heads through a birth canal that is relatively narrow.
When a baby’s head is too big to pass—a condition called fetopelvic disproportion—a surgeon must manually remove the infant via an incision in the mother’s lower abdominal area, a procedure that is called a Caesarean (named after Julius Caesar, who was believed to have been cut from his dying mother’s womb).
That said, what exactly is the evolutionary impact of performing an increasing number of C-sections when a baby cannot easily fit through its mother’s birth canal? Scientists in the U.S.-Austria team believe it has caused an evolutionary effect of babies being born with increasingly larger heads.
Evolutionary changes—or just changes in modern lifestyles?
Using logic and mathematics to reach their conclusion, the team said logic suggests if babies with overly large craniums are allowed to survive into adulthood, instead of dying at birth—which is what occurred throughout most of human history—then they would carry genes for larger skulls and more babies would then be born in the future with bigger heads than in the past. And in fact, larger babies at the time of birth have been shown to be healthier in general than smaller babies, which then increases the odds of reproducing.
In order to lend more weight to their argument, the research team crunched and then analyzed figures data numbers from the past 50 years; they discovered that the world rate of fetopelvic disproportion grew from about 3 percent of births in the 1960s to 3.3 percent of births today, which is an overall increase of between 10 and 20 percent, depending upon which births are added and which are not.
The team further suggested that the higher rate of births of babies with larger heads could also be due to evolutionary change brought about by increased C-section cases that allow such births to proceed rather than be terminated. The team did say, however, they did not discover proof or a direct link to their conclusions. They said there is the possibility that the overall increase in the size of newborns’ heads generally could be due to the modern lifestyle, which is much more sedentary and rich in calories than that of past generations.