Throughout human history, twins have been rare, which often makes them seem extraordinary. Across various cultures, twins are symbols of strength and health, while in others, they represent opposites like life and death or good and evil.
Recent research has suggested that twins were actually quite common back during primate evolution. Almost all primates today, including people, give birth to usually just one baby.
But our most recent common ancestor, which occupied North America about 60 million years ago, likely saw twins as the norm.
A team of researchers analyzed skeletal collections to study mammal evolution and the history of reproductive life.
They mapped out the known litter sizes of species across the mammalian family tree and then used mathematical algorithms to identify patterns in order to reconstruct the evolutionary history of primate litter size.
However, it is difficult to determine litter size for extinct species from the fossils alone. So, the researchers also gathered data for nearly a thousand mammal species that are alive today. Afterward, they performed a series of statistical tests.
The number of offspring a species produces per litter is typically consistent in more closely related species, a pattern known as phylogenetic conservation.
For example, deer tend to have one or two young, while canids and felids often give birth to much larger litters.
Nearly all primate species give birth to one baby at a time, although there are some exceptions. Tarsiers, including lemurs, lorises, and galagos, as well as marmosets and tamarins from South America, give birth to twins.
Sign up for Chip Chick’s newsletter and get stories like this delivered to your inbox.
Previously, scientists believed that primates giving birth to twins were derived from the more common ancestral norm of birthing one baby. But the new findings challenge that idea: it’s actually the primates that have single births that are the deviations.
Looking further back in evolutionary history, having two babies at once was the norm. Our ancient primate ancestors regularly gave birth to two babies at a time. So, when did the change in primate litter size occur?
Based on mathematical models, the change happened at least 50 million years ago. Since then, many primate lineages, including humans, evolved to have larger brains and bodies. It was beneficial for primates to develop only one fetus during pregnancy.
Twins require more energy from the mother, and the offspring are often born smaller and earlier. So, giving birth to one large offspring may have served as a survival advantage. That doesn’t mean having twins today is a disadvantage.
In the United States, the rates of twins have nearly doubled over the past 50 years, partially due to technological advances in reproductive care. About three percent of live births are twins, although recent trends have seen a drop in twin birth rates.
Having twins can also be dangerous for both the mother and the babies. In the U.S., more than half of twins are born prematurely. Despite these risks, research shows that twins are an important part of our evolutionary background.
