Becoming a Novelty

Today, twins account for about three percent of live births in the United States, with modern fertility treatments contributing to their rise.

While such births are still a novelty, tens of millions of years ago they were the norm for our primate ancestors, according to a new study.

Scientists from Western Washington University and Yale University found that some 60 million years ago prehistoric primates would regularly give birth to twins, a finding that challenges the long-held belief that singleton births were the standard among primates.

The research team studied litter size across nearly 1,000 mammal species, as well as used mathematical models and data from public databases to get a better picture of the evolutionary history of primate reproduction, Interesting Engineering noted.

Their findings showed that primates initially favored twin births but shifted to singletons around 50 million years ago, coinciding with a critical period in our evolutionary journey: Encephalization, or the expansion of brain size.

Researchers explained that this life history trait is “significantly correlated with gestation length.” They added that multifetal pregnancies – namely, twins or even triplets – were associated with “smaller brains and body sizes, short gestation, and rapid growth.”

“We propose that the switch from twins to singletons was critical for the evolution of large human babies with large brains that were capable of complex learning as infants and young children,” they wrote in the Conversation.

However, this evolutionary shift wasn’t a one-time event.

The team found that the transition from twins to singletons occurred multiple times across primate lineages, underscoring the adaptive advantage of birthing one larger, more developed baby over multiple smaller ones.

“Multifetal gestation requires more energy from the mother, and because the babies are born smaller, and often earlier, early primate ancestors who gave birth to just one large offspring may have been at a survival advantage,” the authors added.