Our ancient primate ancestors primarily gave birth to twins, while humans typically do not. What could be the reason behind this difference?

Twins are a rare phenomenon for our species. For comparison, in the United States, they account for only about 3% of births. In a new study, researchers also found that this was not always the case: for around 60 million years, the birth of twins was actually the norm for primates, according to Live Science.

Throughout human history, twins have always been uncommon, which makes them seem special. In many cultures, twins are associated with health and vitality, while others view them as a philosophical reminder of the duality of life and death, good and evil. Furthermore, some well-known twins are also attributed with a significant role in history — sometimes even described as deities.

Recent studies indicate that in the past, twins were indeed the norm in primate evolution. Today, however, almost all primates, including humans, typically give birth to only one offspring, whereas our common ancestor, which roamed North America about 60 million years ago, likely gave birth to twins as a standard.

In the last few years, scientists have focused on studying the litter size of primates: how many offspring develop during a single pregnancy. For their research, the team utilized collections of skeletons, both fossilized and recently existing. As a result, researchers became interested in the question: when did twin pregnancies become rare?

During the study, scientists mapped the known litter sizes of as many species as possible based on the mammalian phylogenetic tree and then employed mathematical algorithms to identify patterns. This enabled researchers to reconstruct the history of litter size.

Unfortunately, estimating the litter size of extinct species solely from skeletons is extremely challenging, especially when entire families of animals do not fossilize together. As a result, scientists turned to extant mammal species. The team searched for information in a variety of publicly available databases, focusing on data such as: average body size at birth and in adulthood; pregnancy duration.

The number of offspring in a litter is phylogenetically conservative, meaning it is more similar among closely related species. For example, deer typically give birth to one or two offspring, while canids have significantly larger litters. At the same time, almost all primate species usually give birth to only one offspring, although there are exceptions. For instance, several wet-nosed primates, including lemurs, lorises, and galagos, as well as nearly all marmosets and tamarins from South America, give birth to twins.

Previously, scientists thought that these distinctive twin-bearing primates were different from a more common ancestral trait. However, in the new study, researchers proved that this is not the case: in fact, the species of primates that give birth to only one offspring are derived and distinctive. In the depths of evolution, the birth of twin offspring was the norm. But when did the evolutionary change in primate litter size occur?

It is no secret that modern humans overwhelmingly give birth to only one child — relatively large with an even larger head. The size of the human brain and body is undoubtedly linked to our ability to create and refine technologies.

For primates, and especially for humans, childhood learning is crucial. The authors of the study believe that the transition from twins to single births was actually critical for the evolution of large human infants with big brains, capable of complex learning in infancy and early childhood.

Now, based on mathematical modeling, scientists estimate that the transition to single offspring occurred at an early stage, at least 50 million years ago. Since then, many lines of primates, including our own, have evolved to have increasingly larger bodies and brains. The new study also indicates that the transition from giving birth to twins to single offspring occurred multiple times in primate lineages.