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. 
| Photo Credit: Tesla Monson/The Conversation

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.

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. Deer tend to have one or two offspring, while canids and felids tend to have many more babies in each litter.

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

Prior to our work, researchers thought these distinctive twin-bearing primates must be what evolutionary biologists call derived, or different, from the more common, ancestral trait. But our research flips that narrative on its head: It’s actually the singleton-bearing primates that are derived and distinctive. Further back in evolution, two babies at once was the norm. Our ancient primate ancestors gave birth to twins.

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

The switch to singletons

Modern humans overwhelmingly birth just a single child – a rather large child with an even larger head. Human brain and body size is certainly connected to our ability to create and refine technologies. Paleoanthropologists have long been investigating what they call encephalization: an increase in brain size relative to body size over evolutionary time.

For primates, and especially humans, childhood learning is crucial. We propose that the switch from twins to singletons was critical for the evolution of large human babies with large brains that were capable of complex learning as infants and young children.

Based on mathematical modeling, the switch to singletons occurred early on, at least 50 million years ago. From there, many primate lineages, including ours, evolved to have increasingly larger bodies and brains.

Our new research also shows that the switch from birthing twins to birthing singletons happened multiple times in the primate lineage – a telltale signal that it was advantageous for primates to develop only one fetus per pregnancy. Because multifetal gestation requires more energy from the mother, and because the babies are born smaller, and often earlier, early primate ancestors who gave birth to just one large offspring may have been at a survival advantage.

Our findings don’t mean that having twins today is a disadvantage – although, as a mother of multiples, Tesla can certainly say it’s not easy. But having twins today is quite a different experience from our tiny primate ancestors birthing in the trees 60 million years ago.

Twinning today

Rates of twins have almost doubled in the U.S. over the past 50 years, due in part to advances in assistive reproductive technologies. Today, about 3% of live births are twins, although recent trends suggest a downturn in rates. The fact that women in the U.S. are routinely having kids in their 30s compounds this even further, since women in the later stages of fertility – that’s anyone over the age of 35 – are more likely to have twins.

But having twins can be dangerous for both the mother and babies. More than half of all twins in the U.S. are born prematurely. Many of them spend time in the neonatal intensive care unit.

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

This article is republished from The Conversation under a Creative Commons license. Read the original article here.

Igbo-Ora:

On a normal day, a visitor might pass Igbo-Ora with little more than a double take, wondering why so many pairs of residents wear matching clothes.

But this weekend left nobody doubting what makes the town in southwest Nigeria special.

With fanfare, pageantry, talent shows and even a royal visit, hundreds of people gathered in the self-proclaimed “twins capital of the world” to celebrate its unusually high rate of multiple births.

“There’s hardly a family here in Igbo-Ora that doesn’t have a twin,” said visiting Yoruba king Oba Kehinde Gbadewole Olugbenle, himself a twin.

Yoruba culture reveres twins and their first names are traditionally fixed — Taiwo meaning ‘one that tastes the world’ for the eldest child, and Kehinde meaning ‘one that came after’ for the second-born.

The town stands out even in the wider Yorubaland region, which boasts an above-average incidence of non-identical twins, according to population experts.

The global average birth rate for twins is around 12 per 1,000 births — but in Igbo-Ora, it is thought to be closer to 50 per 1,000, according to scientific studies and hospital records.

Explanations for the abundance differ.

Many residents put it down to diet, especially okra leaf or ilasa soup with yam and amala (cassava flour).

Fertility experts — and several residents — are sceptical, saying there is no proven link between diet and the high twin rate.

Scientists are looking into genetic factors, and how the twins’ special cultural status might make them more likely to find partners and have children.

‘Gift from God’

Whatever the reason, everyone in the town agrees the abundance of twins is a blessing — even more so this year as Nigeria grapples with its worst economic crisis in a generation.

Suliat Mobolaji gave birth to twins eight months ago and said the family had been showered with gifts ever since.

“It’s changed my life,” the 30-year-old said, clutching a son in each arm.

“You can’t give birth to twins and remain down on your luck,” she beamed. “It’s a gift from God.”

Taiwo Ojewale, a research assistant specialising in Yoruba culture at the University of Ibadan, said celebrating twins was “rooted in traditional religious belief.”

Twins are seen as a gift from the supreme god Olodumare, he explained, and Igbo-Ora’s oral history describes them as a reward following a series of disasters that befell the community.

The town erupted with joy as the event began on Saturday. At the festival ground, staff rolled out a red carpet for scores of twins both young and old.

They paraded in immaculate matching outfits — from glamorous sunglasses and patterned adire fabric to a pair of toddlers sporting purple dresses and identical handbags.

Dozens more twins watched from the stands or milled side by side around town.

The festival’s organisers — themselves twins — say they eventually want to attempt a world record for the largest gathering of twins on the planet.

Taiwo and Kehinde Oguntoye, 39, also said they hope to stage a mass wedding of twin couples next year.

“Twins bring favours, fame and wealth,” said Taiwo. “That’s why we Yoruba celebrate their birth, and maybe that is the reason why God blessed us in Yorubaland with the highest number of twins in the world.”

“It’s a blessing,” the Oguntoye twins nodded, speaking in unison.

(This story has not been edited by NDTV staff and is auto-generated from a syndicated feed.)