Twins are a rare phenomenon for our species. For comparison, in the US alone they account for only about 3% of births. In a new study, scientists also found that this was not always the case: for about 60 million years, twin births were actually the norm for primates, Live Science writes..

In the history of mankind, twins have always been rare, and therefore seem to be something 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.. Moreover, some famous twins are also credited with a prominent role in history - the birth of nations, and are sometimes even described as deities.

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Recent research shows that twins were actually the norm in primate evolution in the past.. Today, however, almost all primates, including humans, typically give birth to just one baby, but our common ancestor, who roamed North America about 60 million years ago, probably gave birth to twins as a standard..

Scientists have spent the last few years studying primate litter size: how many babies grow in one pregnancy. For their research, the team used collections of skeletons, both fossil and recent.. As a result, scientists became interested in the question: when did twin pregnancies become rare

In the study, scientists mapped the known litter sizes of as many species as possible across the mammalian family tree, and then used mathematical algorithms to find patterns. All this allowed scientists to reconstruct the history of litter size.

Unfortunately, estimating the litter size of extinct species from skeletons alone is extremely difficult, especially when entire families of animals do not fossilize together.. As a result, scientists turned to mammal species that still live today.. The team searched a variety of publicly available databases for information such as the species' average body size at birth and adulthood.; duration of pregnancy.

The number of offspring a species has in a litter is phylogenetically conservative, that is, more similar in more closely related species. For example, deer give birth to one or two young, while felids give birth to many more young per litter. At the same time, almost all species of primates give birth to only one baby, although there are exceptions. For example, several wet-nosed primates, including lemurs, lorises and galagos, and almost all marmosets and tamarins from South America give birth to twins.

Previously, scientists believed that these distinctive twin-bearing primates were distinct from a more common ancestral trait. But in a new study, scientists have proven that this is not true: in fact, these species of primates that bear only one young are derived and distinctive. Even in the depths of evolution, the birth of twin babies was the norm. But when did the evolutionary change in primate litter size occur

It is no secret that the vast majority of modern people give birth to only one child - quite large with an even larger head. The size of the human brain and body is certainly related to our ability to create and improve technology.

For primates, and especially humans, childhood learning is critical. The study authors suggest that the transition from twins to singletons was actually crucial for the evolution of large, large-brained human babies who were capable of complex learning in infancy and early childhood.

Now, based on mathematical modeling, scientists believe that the transition to single children occurred early, at least 50 million years ago.. Since then, many primate lineages, including ours, have evolved to have increasingly larger bodies and brains.. The new study also shows that the transition from twin births to single births occurred multiple times in the primate lineage.

Focus previously wrote that scientists studied twins who grew up separately in different countries.