To imagine a world where the norm for humans would be to wear six babies both like cats is terrifying, isn’t it? Six times more diapers to change and bottles to prepare, not to mention childbirth … but unlike many mammals, almost all primates, including humans, generally give birth to a single child. The announcement of the arrival of twins can be a real upheaval for parents alone, and, in fact, the twin pregnancy remains a rather unusual phenomenon, with almost 1 baby out of 40.
However, according to a study published in July 2024, about 60 million years ago, much earlier in the evolution of primates, our most recent common ancestor gave birth to twins. Far from being a rare event worthy of a particular interest, it was then the norm. But why, nowadays, humans generally give birth to a single child? When has twin pregnancy become a rare phenomenon? The Conversation tells us more about this intriguing discovery, which suggests that the size of our ancestors has evolved for very specific reasons.
The number of descendants of a species in a range is phylogenetically preserved, that is to say that it is more similar in closely related species. For example, deer tends to have one or two offspring, while canides and felids tend to have many more babies per litter. It is the same thing among primates, except for tarsiers, including lemurs, lois and galagos, who give birth to twins.
However, if for years the researchers thought that these primates carrying twins were “derivatives” of a more common ancestral line, the study completely overthrew this idea. It is in fact the primates which carry “singleton” (a child or an animal which is the only one to be born during a birth) which are distinctive. But when did this evolve change in the size of the scope occur?
Brain size has a key role
Researchers have been studying encephalization for a long time, that is to say the increase in the size of the brain compared to that of the body during evolution. In primates, and especially in humans, having a large brain is an advantage: this allows complex learning capacities from childhood. The study suggests that, to allow human babies to have such a large and developed brain, it was necessary to go from a strategy where primates had twins to a strategy where they had only one baby at a time.
According to them, this change occurred about 50 million years ago. From that moment, certain primates lines, including the ancestors of humans, have evolved towards increasingly large bodies and brains. This made humans an exception in the reign of mammals, because they have managed to challenge the evolutionary rules which generally apply to other species.
Research also shows that the transition from the birth of twins to the single birth has occurred several times in the line of primates, which indicates that it was advantageous for primates to develop only one fetus per pregnancy. Raising twins requires more energy from the mother, and babies are born smaller and often earlier. Thus, the first primate ancestors who gave birth only to a single large child may have benefited from an advantage in terms of survival.
Although rarer today, twins are an essential element in our genetic history. And, despite the linked risks, “Having twins today remains a very different experience from that of our tiny primate ancestors who were giving birth in the trees 60 million years ago”concludes Jack McBride, anthropologist at the University of Yale (United States) and the co-author of the study.
