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The Conversation
Essay / In Flux

Were Twins the Norm in Our Primate Past?

New research uncovers how the last common primate ancestors typically birthed twins until evolutionary pressures began to favor singletons—likely driven by the advantages of birthing larger, brainier offspring.
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This article was originally published at The Conversation and has been republished under Creative Commons.

Twins have been rare in human history and for that reason can seem special. Many cultures associate twins with health and vitality, while others see them as a philosophical reminder of the dualities of life and death, good and evil. Some famous twins are credited with the birth of nations; others are described as deities.

Our recent research suggests that twins were actually the norm rather than an unusual occurrence worthy of note much further back in primate evolution. Despite the fact that almost all primates today, including people, usually give birth to just one baby, our most recent common ancestor, which roamed North America about 60 million years ago, likely gave birth to twins as the standard.

We have been researching the evolution of primate litter size—how many babies grow during each pregnancy—for the past several years. To study mammal evolution and reproductive life history, we use skeletal collections, both fossil and recently living.

In addition to being an anthropologist, one of us (Tesla) is the mother of twin girls. That has led to a personal and not just scientific interest in this topic: When did twin pregnancies become uncommon?

RECONSTRUCTING LITTER SIZE IN THE PAST

The best way we have to reconstruct the history of litter size is to map the known litter size of as many species as possible across the mammalian family tree and then use mathematical algorithms to look for patterns. But outside of rare events where entire animal families are fossilized together, it is extraordinarily difficult to assess litter size for extinct species from the skeleton alone. So we instead collect data on as many living mammals as possible.

We searched a wide variety of public databases, including AnAge: The Animal Ageing and Longevity Database, for information about how many offspring are commonly born to each species of mammal. We also noted additional data, including what the species’ average body size is at birth and at adulthood, as well as pregnancy duration.

A graphical representation of the evolutionary history between select mammal species features light orange, green, light and dark blue, and yellow branched lines. The tree highlights a predicted mean litter size of 1.7 for the last common ancestor of primates, which is marked with an asterisk. Black, filled-in shapes of animals are on the right.

A graph shows a family tree of mammals surveyed for the study, also known as a phylogeny. The branches of the tree are labeled with colors that correspond to litter sizes. Darker colors indicate larger litter sizes, while lighter colors (oranges) indicate smaller litter sizes. The animal outlines are, from top to bottom, rodents, rabbits, primates, cetartiodactyls (whales and most hooved animals), carnivores, bats, and shrews.

After gathering all these data points for almost 1,000 mammal species, we performed a series of statistical tests to quantify relationships between different traits. Our goal was to estimate the likely litter size of different mammalian ancestors: What were the odds of a singleton birth for each species at any given point in time?

The number of offspring a species has in a litter is phylogenetically conserved, meaning more similar in more 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 just one baby, although there are exceptions. Several of the wet-nosed primates—including lemurs, lorises, and galagos—and almost all of the marmosets and tamarins from South America give birth to twins.

Three small animals with tufts of white fur around their ears and gray, tan, and black fur on their bodies huddle together looking toward the camera.

Marmosets are primates that typically give birth to twins.

Tambako the Jaguar/CC BY-ND/Flickr

Prior to our work, researchers thought these distinctive twin-bearing primates must be what evolutionary biologists call derived, or different, from the more common, ancestral trait. But our research flips that narrative on its head: It’s actually the singleton-bearing primates who are derived and distinctive. Further back in evolution, two babies at once were the norm. Our ancient primate ancestors gave birth to twins.

So, when did this evolutionary change in primate litter size occur?

THE SWITCH TO SINGLETONS

Modern humans overwhelmingly birth just a single child—a rather large child with an even larger head. Human brain and body size is certainly connected to our ability to create and refine technologies. Paleoanthropologists have long been investigating what they call encephalization: an increase in brain size relative to body size over evolutionary time.

For primates, and especially humans, childhood learning is crucial. We propose that the switch from twins to singletons was critical for the evolution of large human babies with large brains who were capable of complex learning as infants and young children.

Based on mathematical modeling, the switch to singletons occurred early on, at least 50 million years ago. From there, many primate lineages, including ours, evolved to have increasingly larger bodies and brains.

Newborn twins lie side by side in a neonatal incubator, a temperature-controlled small hospital bed with a hard, clear plastic cover. They are monitored by two medical staff wearing surgical caps, masks, and blue-green scrubs.

The occurrence of human twins has increased recently, which comes with risks for parent and babies.

Sviatlana Lazarenka/iStock/Getty Images Plus

Our new research also shows that the switch from birthing twins to birthing singletons happened multiple times in the primate lineage—a telltale signal that it was advantageous for primates to develop only one fetus per pregnancy. Because 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.

Our findings don’t mean that having twins today is a disadvantage—although, as a mother of multiples, I (co-author Tesla) can certainly say it’s not easy. But having twins today is quite a different experience from our tiny primate ancestors birthing in the trees 60 million years ago.

TWINNING TODAY

Rates of twins have almost doubled in the U.S. over the past 50 years due in part to advances in assistive reproductive technologies. Today about 3 percent of live births are twins, although recent trends suggest a downturn in rates. The fact that women in the U.S. are routinely having kids in their 30s compounds this even further, since women in the later stages of fertility—that is, anyone over the age of 35—are more likely to have twins.

But having twins can be dangerous for both the mother and babies. More than half of all twins in the U.S. are born prematurely. Many of them spend time in neonatal intensive care units.

Despite these risks, our research shows that twins are a critical part of our genetic history.

 

Editors’ Note: This article has been updated to correct the list of current mammals that normally birth twins.

From the shoulders up, a photograph features a smiling person with long straight, brown hair wearing a purple collared shirt and thin beaded necklace in front of a blurred background of a lab setting.

Tesla Monson is an assistant professor of anthropology at Western Washington University in Bellingham, where she runs the Primate Evolution Lab. Her lab researches primate evolution, life history, reproductive ecology, and the growth and development of the skeletal system. In 2020, she received a Leakey Foundation research grant for her project “Evolution of the Primate Cranium: Craniofacial Modularity in Extant Colobines.”

Jack McBride is an anthropology graduate student at Yale University. He is interested in primate morphological evolution and using morphology to understand what separated the earliest primates from other lineages. In addition, he is interested in the evolutionary history of twinning in primates and the evolution of other life history characteristics, such as increased longevity relative to body size, seen in primates and other mammal groups such as Chiroptera (bats).

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