Twins were the norm in our primate ancestors – one baby at a time had evolutionary benefits

Twins have been rare in human history and might due to this fact be special. Many cultures associate twins with health and vitality, while others see it as a philosophical reminder of the duality of life and death, good and evil. Some famous twins are is credited with the birth of countriesothers are described as deities.

This is what our latest research suggests Twins were actually the norm much further back in primate evolution and is just not a notable unusual event. This is despite the proven fact that today just about all primates, including humans, generally only give birth to a single baby, our most up-to-date common ancestor, which roamed North America 60 million years agoprobably gave birth to twins by default.

We have researched the evolution of primate litter size – what number of babies grow during each pregnancy – lately. To study the evolution and reproductive history of mammals, we use skeletal collections, each fossil and up to date living.

In addition to being an anthropologistone among us (Tesla) is the mother of dual girls. This gave rise to a private and not only scientific interest on this topic: When did twin pregnancies turn into rare?

Reconstruction of litter size up to now

The best approach to reconstruct the history of litter size is to map the known litter sizes of as many species as possible within the mammalian family tree after which use mathematical algorithms Look for patterns. Except for the rare cases wherein entire families of animals are fossilized together, it is amazingly difficult to find out the litter size of extinct species based on the skeleton alone. Instead, we collect data from as many living mammals as possible.

We searched quite a lot of public databases including AnAge: The Animal Aging and Longevity Databaseto acquire details about what number of offspring are normally born to every mammal species. We also noted additional data, including the species' average body size at birth and maturity, in addition to gestation length.

After collecting all of those data points for nearly a thousand mammal species, we conducted a series of statistical tests to quantify the relationships between different traits. Our goal was to estimate the likely litter size of various mammalian ancestors: What was the probability of a single birth occurring in each species at a given time?

The variety of offspring of a species in a litter is phylogenetically conserved, meaning that they’re more similar amongst more closely related species. Deer are likely to produce one or two offspring Canids And felids are likely to have many more babies in each litter.

Almost all primate species give birth to just one young, although there are exceptions. Several of the wet-nosed primates – including lemurs, lorises and galagos – and just about all marmosets and tamarins from South America give birth to twins.

Before our work, researchers assumed that these distinctive twin-bearing primates have to be what evolutionary biologists call “derived” or “distinct” from the more common ancestral trait. But our research turns this narrative on its head: it is definitely the singleton-bearing primates which can be derived and distinctive. Further back in evolution, two babies directly was the norm. Our primate ancestors gave birth to twins.

So when did this evolutionary change in primate litter size occur?

The move to singletons

Most modern humans give birth to only one child – a pretty big child with an excellent larger head. Human brain and body size is actually related to our ability to develop and refine technologies. Paleoanthropologists have long been studying what they call Encephalization: a rise in brain size relative to body size over evolutionary time.

For primates and particularly humans Learning in childhood is crucial. We suspect that the transition from twins to singletons was critical to the event of huge, large-brained human babies able to complex learning as infants and toddlers.

Based on mathematical models, the switch to singletons occurred early, no less than 50 million years ago. From then on, many primate lineages, including ours, evolved to have ever larger bodies and brains.

Our latest research also shows that the switch from giving birth to twins to giving birth to singletons occurred multiple times within the primate lineage – a telltale sign that it was useful for primates to develop just one fetus per pregnancy. Due to multiple pregnancy requires more energy from the motherand since the babies are smaller and infrequently born earlierEarly primate ancestors that gave birth to just one large offspring could have had a survival advantage.

Our results don't mean that having twins today is a drawback – although as a mother of multiples, Tesla can actually say it's challenging. But having twins today is a really different experience than the birth of our little primate ancestors within the trees 60 million years ago.

Town twinning today

The variety of twins within the United States has almost doubled over the past 50 yearspartly as a consequence of advances in assistive reproductive technologies. Today, approx. 3% of live births are twins, although recent trends suggest rates are declining. The proven fact that women within the United States routinely have children of their 30s further reinforces this, as that is the case for ladies within the later stages of fertility – anyone over the age of 35 more more likely to have twins.

But having twins could be dangerous for each the mother and the newborn. More than half of all twins within the USA are born prematurely. Many of them spend time within the Neonatal intensive care unit.

Despite these risks, our research shows that twins are an important a part of our genetic history.