Twins Were Common Among Our Primate Ancestors. What Led to the Change?
Throughout human history, twins have been rare and often considered extraordinary. Many cultures associate them with vitality or duality, symbolizing life and death or good and evil. Some mythologies even credit twins with founding nations or being deities.
However, new research reveals that twins were once the evolutionary norm for our primate ancestors, challenging the perception of their rarity as unique.
Twin Births
Although most modern primates, including humans, typically have one baby at a time, our most recent common primate ancestor, which lived about 60 million years ago in North America, likely birthed twins regularly. This suggests that singleton births are an evolutionary shift, not the original condition.
By studying skeletal remains, fossils, and data from living mammals, researchers have mapped patterns of litter size throughout mammalian evolution. They used public databases, such as AnAge, to compile information on nearly 1,000 species, analyzing traits like litter size, body dimensions, and pregnancy duration.
Their findings challenge the assumption that modern twin-bearing primates, like marmosets and tamarins, represent an evolutionary outlier. Instead, it’s the singleton-bearing primates, including humans, that diverged from the ancestral pattern.
Why the Shift to Singletons?
The transition to singleton births likely began over 50 million years ago and provided significant evolutionary advantages. Human infants, for example, are born larger, with proportionally larger brains. This shift supported increased encephalization—greater brain size relative to body size—facilitating advanced learning and complex behaviors in early childhood.
Single pregnancies allowed mothers to invest more energy into one offspring, resulting in larger, healthier babies. In contrast, twin pregnancies often require more energy and lead to smaller, sometimes premature, babies. These evolutionary pressures likely drove the preference for singleton births across multiple primate lineages.
Twinning in Modern Times
Today, twins are more common than in the past due to advancements in reproductive technologies and an increase in maternal age. Women over 35 are more likely to have twins, and twin birth rates in the U.S. have nearly doubled over the last 50 years. Currently, about 3% of live births are twins.
While modern medical care can mitigate risks, twin pregnancies are still associated with complications like premature births and extended NICU stays. Despite these challenges, twins remain a vital part of our evolutionary history, offering insights into our species’ development and adaptability.
Even though twinning is now rare, it continues to connect us to a remarkable chapter in our evolutionary story—one where multiple births were once the rule, not the exception.
Read Original Article: Science Alert
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