Twins were the norm for our ancient primate ancestors, but giving birth to one baby at a time had evolutionary advantages.
Twins are rare in human history, and for this reason they may seem special. Many cultures associate twins with health and vitality, while others see them as a philosophical reminder of the duality of life and death, good and evil.
A recent one suggests that twins were actually the norm in primate evolution rather than an unusual occurrence. Although almost all living primates, including people, give birth to only one baby, our most recent common ancestor, which roamed North America about 60 million years ago, probably gave birth to twins as a norm.
In recent years, scientists have been investigating the evolution of primate litter sizes — the number of babies that grow during each pregnancy. To study mammalian evolution and reproductive life history, we use collections of skeletons, both fossil and more recent.
Past litter size reconstruction
The best way to reconstruct the history of litter size is to map the known litter sizes of as many species as possible on the mammalian family tree and then use mathematical algorithms to look for patterns.
But aside from the rare cases in which entire families of animals are fossilized together, it is extraordinarily difficult to assess the litter sizes of extinct species from the skeleton alone. So instead, data was collected on as many living mammals as possible.
The number of offspring a species has in a litter is phylogenetically conserved, meaning it is most similar across the most closely related species. Deer tend to have one or two offspring, while canids and felids tend to have many more babies in each litter.
Almost all primate species give birth to only one baby, although there are exceptions. Several wet-nosed primates—including lemurs, lorises, and galagos—and nearly all marmosets and tamarins in South America give birth to twins.
Before the most recent investigation, researchers thought that these primates with distinct twins must be what evolutionary biologists call derivativesor different, from the most common ancestral characteristic. But this investigation flipped that narrative: in fact, it is the single-baby primates that are derived and distinct. Further back in evolution, two babies at once was the norm. Our ancient primate ancestors gave birth to twins.
So when did this evolutionary change in primate litter sizes occur?
The move to only children
The overwhelming majority of modern humans give birth to just one child — a very large one, with an even larger head. The size of the human brain and body is certainly linked to our ability to create and improve technologies. Paleoanthropologists have long investigated what they call encephalization: an increase in brain size relative to body size over evolutionary time.
For primates, and especially for humans, learning in childhood is crucial. We propose that the shift from twins to singletwins was fundamental to the evolution of large, large-brained human babies capable of complex learning as infants and toddlers.
Based on mathematical models, the change to twins occurred very early, at least 50 million years ago. Since then, many primate lineages, including ours, have evolved to have increasingly larger bodies and brains.
The new research also shows that the change from the birth of twins to the birth of single children occurred several times in the primate lineage—a telltale sign that it was advantageous for primates to develop only one fetus per pregnancy.
Because multifetal gestation requires more energy from the mother, and because babies are born smaller, and often earlier, early primate ancestors who gave birth to only one large offspring may have had a survival advantage.
However, the findings do not mean, the authors explain, that having twins today is a disadvantage.
Twins nowadays
As Twin rates nearly doubled in the US over the past 50 years, in part due to advances in reproductive assistive technologies. Currently, about 3% of live births are twins, although recent trends suggest a decrease in rates. The fact that women are increasingly having children in their 30s further exacerbates this situation, as women in the most advanced stages of fertility — that is, anyone over 35 — have more likely to have twins.
But having twins can be dangerous for both the mother and the babies: More than half of all twins in the U.S. are born prematurely, and many of them spend time in the intensive care unit.
Despite these risks, research shows that twins are a essential part of our genetic history.
