Evolution

It seems counterintuitive for naturally occurring multiple births in humans to have been evolutionarily conserved. Carrying twins is much more energetically taxing and there is higher risk of complication for the mother. The babies typically have lower birth weight and are at an increased risk for various health problems than single births. Why has this persisted in our species?

The benefit does

not fall upon the twins, but the siblings of the twins. Single births that occur after a twin birth are, on average, 226.41 g heavier than a typical baby without twin siblings. Also, single births that occur before the birth of twins are 134.07 g more than a typical baby. In historical context, even if the twins themselves were not viable, the mother’s other children would be more likely to reach adulthood.

Though the exact mechanism is not clear, researchers speculate this difference is due to variation in the mother’s ability to regulate fetal growth with insulin-like growth factor (IGF) system. This phenomenon is only found in women with the genetic predisposition for dizygotic (fraternal) twins. There is no genetic variation that increases the odds of having monozygotic (identical) twins. Also, women who had twins (or more) with the aid of fertility treatments will not have this benefit with any natural single births.

For more information: http://rsbl.royalsocietypublishing.org/content/8/1/67.full.pdf+html

Photo credit: twinpossible.com