The Evolutionary Shift in Primate Reproduction
From Twins to Singleton Births: The Story of Ancient Primates
Twins are often celebrated as rare marvels, a quirk of biology that has fascinated humanity for millennia. Many cultures weave twins into myths—twin gods, twin founders of nations, twin symbols of duality in life. But what if twins were not an anomaly, but the evolutionary default? New research is reshaping our understanding of primate reproduction, revealing that ancient primates, including the earliest ancestors of humans, likely gave birth to twins as a rule rather than an exception.
A study1 of mammalian reproduction, led by evolutionary biologists using skeletal data and mathematical models, indicates that the earliest primates routinely had two offspring per pregnancy. Over tens of millions of years, that reproductive strategy shifted, paving the way for single births to become the dominant norm in modern primates—including humans. This change, scientists argue, may have been crucial to the evolution of big-brained offspring, fostering the extended childhoods needed to develop complex cognitive abilities.
A Look Back: Twins in Our Primate Past
Today, most primates—humans included—have singleton births. But a few exceptions remain: lemurs, lorises, tarsiers, and South American marmosets and tamarins still regularly produce twins. For decades, scientists assumed these primates were outliers, a curious deviation from the more “natural” singleton births seen in species like gorillas, chimpanzees, and humans. However, the new research flips that idea on its head.
“The singleton births seen in most primates today are actually a more recent evolutionary adaptation,” explains the study's lead researchers. “Our modeling suggests that the ancestral state for early primates was twin births.”
The team compiled data on the litter size of nearly 1,000 mammal species, mapping the findings onto the evolutionary tree of life. By statistically modeling relationships across lineages, they reconstructed the reproductive patterns of extinct ancestors. The results pointed to twin births as the ancestral norm for primates—dating back to a time when the primate lineage emerged roughly 60 million years ago.
Why Singleton Births Took Over
The evolutionary shift from twins to singletons was not random. It coincided with significant changes in primate biology, behavior, and ecology. The most obvious driver? Brain size.
In humans, childbirth is already a biological challenge. A baby’s large brain and skull must pass through the mother’s pelvis, requiring a careful balance of evolutionary trade-offs—large enough brains for cognitive development, but not so large as to risk the mother’s survival during birth. For primates, the evolution of larger brains meant bigger, energy-demanding offspring that required extended parental care.
“Raising twins would have been costly for early primates living in energy-limited environments,” the researchers explain. “Producing one larger, well-developed offspring per pregnancy offered a survival advantage—both for the mother and for the baby.”
The transition began as early as 50 million years ago, the models show, as primates developed longer gestation periods, larger neonates, and extended childhoods. By investing more energy into a single offspring, primates could produce young that were more likely to survive and thrive.
Evidence of Change in Modern Primates
While single births dominate among modern primates, the “twin norm” still lingers in certain species. Marmosets and tamarins, for example, typically birth twins—a trait researchers believe reflects an ancient reproductive strategy. Unlike other primates, these small New World monkeys rely on cooperative parenting, with group members sharing the burden of raising two offspring.
“Twinning in marmosets is a fascinating evolutionary holdover,” notes the team. “It highlights how reproductive strategies can persist under specific ecological and social conditions.”
Meanwhile, humans have taken a unique evolutionary path. Singleton births became firmly entrenched as brain size increased over millions of years, and parental care became more intensive. Nevertheless, twinning has never completely disappeared. Today, about 3% of births in the United States are twins, a rate that has nearly doubled over the past 50 years due to advancements in fertility treatments and changes in maternal age.
The Legacy of Twins in Human Evolution
The evolutionary transition from twins to singletons was a turning point for primates, especially humans. By enabling the birth of larger-brained babies, this shift laid the foundation for complex learning, social cooperation, and cultural development.
“Our findings reveal how changes in reproductive strategies shaped the trajectory of primate evolution,” the researchers explain. “The transition to single births was likely critical for the evolution of traits that define modern humans, including our extended childhood and reliance on learning.”
Yet twins remain a part of humanity’s biological story—a relic of an ancient reproductive strategy that served our primate ancestors well. As rates of twinning continue to rise in the modern world, they offer a glimpse into a distant past, when two offspring at once was the rule rather than the exception.
Conclusion: Twins as an Evolutionary Echo
Understanding why singleton births replaced twins provides insight into the evolutionary pressures that shaped primate biology. For early primates, single offspring became a strategy for survival in a changing world—a strategy that ultimately allowed human ancestors to develop larger brains and longer childhoods.
Today, twins are a rare but powerful reminder of this ancient past. As science continues to uncover the story of our evolutionary history, even small details like litter size reveal the profound forces that have shaped life on Earth.
McBride, J. H., & Monson, T. A. (2024). The evolution of primate litter size. Humans, 4(3), 223–238. https://doi.org/10.3390/humans4030014