At first glance, there's no good reason why humans would evolve the ability to give birth to twins. Indeed, human reproductive systems make having even one child difficult. But twins are doing a big favor for their siblings' survival chances.
Giving birth to twins can be very difficult in regions without adequate healthcare - the babies themselves tend to be born underweight and somewhat less fit than single births, and mothers of twins generally experience more complications than they would otherwise. As such, it seems as though twinning is at best something of an evolutionary accident, and to some degree it's surprising that it has persisted in our species.
But University of Sheffield evolutionary biology Ian Richard thinks he's figured out why twins have stuck around in humans. Since the 1950s, the UK Medical Research Council has provided medical care in rural Gambia, recording key data on 1,889 infants over 30 years. It provides a unique data set for populations without good access to medical care, and Richard says it reveals the solution to the twin mystery.
According to the data, single babies born after their mother gave birth to twins were eight ounces heavier than average. In these circumstances, a heavier baby is pretty much always going to have a survival advantage and be more likely to survive infancy. It makes a decent amount of sense for this to occur - the stress of carrying twins to term is believed to increase blood flow in the uterus and generally make it more ready for carrying future children.
Here's where it gets really interesting - single babies born before their mothers gave birth to twins were still five ounces heavier than average. That indicates mothers with a predisposition for twins may have a general predisposition for heavier, and thus healthier babies. Richard traces this back to IGF-1, a protein that circulates in the blood that both can cause the ovaries to release additional eggs and helps the fetus grow during its development.