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 duality 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 much further back in primate evolution, rather than an unusual occurrence worthy of note. 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’s 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.
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 size. 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.
Image modified from McBride and Monson, 2024
After gathering all these data points for almost a thousand 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….
