Why humans evolved single-child pregnancies

Triplet birth numbers are dropping—and that may be a good thing

Pregnancies involving three kids or more have dropped 62% from 1998 to 2023, a trend that the Centers for Disease Control and Prevention links to fewer embryo transfers during assisted reproduction procedures—a transition prompted by more refined techniques and technical advances. Though multiple births can occur naturally, often pregnancies involving multiple births have been the result of efforts to ensure pregnancy success by transferring multiple embryos.

Why do humans have such long childhoods?

Humans have over twice as long childhoods than other animals with similar lifespans like whales and elephants. A human child can grow for nearly 20 years before they become a reproducing adult. Researchers believe there's a connection between long childhoods and human's unusually large brain size, though the science is not totally clear. Dive into what scientists know here.

Why human brains were bigger 3,000 years ago

Our modern civilisation may be the most advanced to ever exist on Earth, but around 100 generations ago, our ancestors had brains that were larger than our own.

How humans evolved from single-cell organisms

Humans have had a long history. 3.8 billion years in the making, to be precise. From the primordial puddle to the modern day, here's how humans have evolved from the first life.

Three examples of how humans are still evolving

For much of nature, natural selection and "survival of the fittest" still play a dominant role; only the strongest can survive in the wild. As little as a few hundred years ago, the same was true for humans, but what about now?

Why humans survived and smarter species did not

This interview with Joseph Henrich, a renowned professor of human evolutionary biology, unpacks human history. Henrich shares interesting information about why humans succeeded as a species (the answer isn't IQ), how cultural evolution works, why the human brain is shrinking, and how the Catholic Church played a role in the Industrial Revolution. Listen and watch for a deep dive into human evolution.

The evolution of the human brain

The human brain, in all its staggering complexity, is the product of millions of years of evolution. Read on to find out about brain size and intelligence, language and brain development, and more.

Human origins 101

Learn about the over 20 early human species that belong in our family tree and how the natural selection of certain physical and behavioral traits defined what it means to be human.

The story of the most prolific mother ever

An 18th-century Russian woman gave birth to 69 children through 27 pregnancies, with an unbelievable 16 pairs of twins, seven triplets, and four quadruplets.

Why genetic similarities between humans suggest a past near-extinction event

Researchers have found that low genetic diversity among humans—even those on opposite sides of the planet—may result from a breeding population of as few as 1,280 adults, estimated to have existed 930,000 years ago. This genetic bottleneck may have resulted from a near-extinction or from a small population of human ancestors, from which we all descend, that broke off from a larger population.

Across the animal kingdom, parenting can include adults carrying their young, providing their bodies for sustenance, searching for food and shelter for their children, and separating siblings who may eat one another.

Only about 0.0001% of sperm, on average, successfully reach the uterine cavity

We've evolved to make pregnancy a complex and difficult process. From the initial development of the blastocyst, to the energy and nutrients needed to grow a new human, our bodies are both fighting and supporting fetal development.

Couple welcomes rare quadruplet daughters who are actually a set of twins

The four healthy girls were delivered by cesarean section and joined a larger family that includes their parents and a 2-year-old older brother. Without the use of fertility drugs, quadruplet births are rare, but the chances are even slimmer that they'll be sets of identical twins. Medical professionals say the odds are around one in a million.

Understand the complex, biological journey of pregnancy with this science explainer.

Eight British children have been born via mitochondrial replacement therapy

In July 2025, a decade after British lawmakers signed off on allowing three-parent IVF, scientists announced that eight healthy babies had already been born using this approach. The news was included in a pair of research articles published in a peer-reviewed journal that noted the oldest kids were already more than two years old, and that they were hitting developmental milestones as expected. (Some users may encounter a paywall.)

Humans won't develop more than 5 fingers because it would impact too many body parts

All modern animals have five or fewer digits on each of their limbs. Roughly 348 million years ago, at least two species had eight: the Ichthyostega and the Acanthostega. According to Dollo's law, complex evolved features are very unlikely to recur in evolution. Researchers believe humans won't develop more than five digits because it would impact other body parts too much.

3-parent baby technique fuels worries of future 'designer babies'

Some critics of this approach worry that any genetic manipulation will both culturally and technologically normalize the idea of "designer babies"—embryos that are manipulated or selected based on physiological characteristics including height, eye color, athletic ability, and intelligence.

Were humans the only hominin to walk upright?

When you imagine the line human evolution has taken, you might picture the March of Progress, an image of several chimp-like ancestors marching toward the implied final form: the upright bipedal Homo sapiens. Turns out this image, popularized in the...

Extremely rarely, there can be twins with different dads—but fewer than 20 such cases are documented worldwide

The non-identical twins, who would also be half-siblings, result from a rare biological phenomenon called heteropaternal superfecundation. Britain's first known twins with different dads, featured in this article, learned of each other's existence in recent years, when they were both 45. They discovered each other after taking DNA tests from Ancestry, a genealogy company.

The role of fire in the evolution of humanity's big brains

Roughly 1.5 million years ago, the brain size of our hominin ancestors exploded. To understand why, Richard Wrangham posited the cooking hypothesis, arguing the adoption of fire as a means to cook food had a lasting impact on our ability to digest...

At one point in the 20th century, more than a million Americans had pet monkeys

This choice to domesticate monkeys proved to be trickier than expected, and thus isn't a very appealing option for humans. Monkeys breed more slowly than other animals, while also being susceptible to diseases and challenging to contain.

Where are humans on the evolutionary tree?

This interactive tree-shaped map explores the evolutionary connections between more than two million species. According to this map, humans are on the same branch as great apes, tarsiers, and treeshrews. That branch then splits into even smaller branches, with humans appearing on the same twig-like branch as chimpanzees and bonobos.

Even in recent history, adaptations to the environment drove human evolution

Through genetics, environmental pressures, and cultural influences, humans have evolved to tolerate lactose, develop varying skin pigmentation, and acquire genetic resistance to diseases like bubonic plague. The metabolic and digestive systems of Indigenous peoples have adapted to their diet, which is influenced by the availability of food and water.

Is human evolution speeding up or slowing down?

In the past 3,000 years, many populations have evolved genetic adaptations to their local environments. People in Siberia and the high arctic are uniquely adapted to survive extreme cold. The Bajau people can dive 70 meters and stay underwater for almost 15 minutes. So what are other recent changes? And will our technological innovations impact our evolution?

The science and ethics of artificial wombs

Artificial wombs may be able to help preterm babies survive their early entry into the world and possibly curb the long-term adverse health effects associated with such premature birth. This article dives into the current ethics and science behind artificial wombs and ectogenesis, which is when a fetus develops in artificial conditions.

3-parent baby approach may help women with infertility

Though the topline promise of this approach is avoiding passing on mitochondrial diseases, some researchers and companies have been eyeing its potential for helping with female infertility, since faculty mitochondria may contribute to the condition. Recent research suggests this approach could be promising.

Scientists first observed a sperm and an egg uniting in 1876

Throughout human history, scholars and researchers have speculated on the origin of human reproduction. It wasn't until the late-19th century that the physical process of egg fertilization via sperm cells was observed (in sea urchins), forming the basis for modern fertility research.

Scientists have not yet answered what the evolutionary benefit may be of menopause

Few animals are known to experience menopause, raising questions about why it would be evolutionarily advantageous to have women survive past their reproductive years. One leading hypothesis is the grandmother hypothesis—older women and other species known to experience menopause, like whales, can aid families and pass down knowledge to future generations.

Humans may have evolved to need less sleep.

See how to make a baby with 3 parents, a process called mitochondrial replacement therapy

This primer video dives into the science of mitochondrial disease risks, how this controversial mitochondrial replacement technique can work, and why proponents say this reproductive advance—already legalized in the UK and Australia—is needed in the United States.

The history of the gene: an interactive timeline

Humans didn’t need modern technology to figure out that kids tend to look like their parents. But what did humans believe about genetics before we knew about DNA or cells in general? Explore this interactive timeline to see how philosophers and scientists throughout the ages tried to explain how traits are passed from generation to generation.

The Paranthropus genus, our mysterious extinct cousins

In the last century, skull and jaw fragments have been discovered in East Africa suggesting an ancient hominin species lived, evolved, and died alongside humanity's ancestors. With large, broad skulls and strong jaws, the Paranthropus genus—which includes several related species—is an absorbing case of the effects of niche partitioning, where species coexist by adapting differently to the environment. Paranthropus died out and human ancestors endured. Find out why here.

Why our ancestors didn't have crooked teeth

The ancient human fossil record is filled with specimens with pretty nice teeth. So why do modern humans have dental problems? Scientists can't be certain, but one hypothesis suggests the evolutionary shrinking of the human jaw crowded our mouths,...

Researches have found that various monkeys have pet-like relationships with other species.

Why are there as many males as females?

Pretty much every species breaks down with an equal sex-ratio, with roughly the same number of males as females. But on the surface, it appears that most species wouldn't need many males to keep the population growing, with their energy-efficient, abundant sex cell production. So why is it equal? Scientists argue an equal ratio is the most evolutionarily stable. Learn about this quirk of biology here.

Artificial wombs and preterm births: an overview

Scientists are working to create artificial wombs that would help preterm babies survive their early entry into the world. But how much help could this technology provide, and how much of a problem are preterm births? This article details some of...

Without dinosaurs, plants could produce fleshy fruits with bigger seeds

Previously disturbed by massive herbivores, denser and darker forests shifted plant evolution, reaching the greatest diversity of seed sizes, fruit sizes, and fruit types about 50 million years ago.

Nature versus nurture: Are you the result of your genes or your environment?

Dr. Nancy Segal breaks down how research on twins has revealed just how much our genes influence our behavior and traits—but insists they aren't everything. Catch up on the latest in the nature versus nurture debate.

Ancient humans and other early hominins were closer than you might think

Evidence that human ancestors and other early hominins coexisted continues to accumulate, raising the question of just how close these ancient people truly were. This article details how evidence is changing the theory that early hominins like...

Humans are more genetically similar to cats than to dogs

While humans are not closely related to either cats or dogs from an evolutionary standpoint, scientists find that human and cat chromosomes are organized more similarly to each other than human and dog chromosomes, making cats a better model for studying human diseases.

Why South Korean women are having fewer babies

For decades, Asian countries have been combating dwindling birth rates by basically bribing families into having more children: offering government-subsidized housing, free IVF treatments, and even straight-up cash handouts. It hasn’t worked. In this BBC video, a TV producer, a cartoonist, and a private-school teacher explain their views on having children in 21st-century Korea.

The first successful IVF pregnancy was in the United Kingdom in 1978

Gynecologist Patrick Steptoe and his collaborator Robert Edwards, a biologist, attempted to fertilize a human egg cell in a petri dish 101 times before finally succeeding. This resulted in the birth of Louise Brown—at the time referred to as a "test tube" baby—in 1978, who was born into the spotlight.

Men and women didn't evolve to perform segregated hunter-gatherer duties

The often-cited theory that men evolved to hunt and women evolved to gather is wrong. This article explores the origins of the "Man the Hunter" theory and the mounting evidence that undermines it. Exercise science indicates that those assigned...

Uncovering the evolution of history's largest dinosaurs

This article details how sauropods—the dinosaurs that Little Foot would call longnecks—repeatedly evolved to attain such colossal sizes, towering over other land critters, including tyrannosaurs and mammoths. Sauropods were giant, long-necked...

In the US, 3.6 million babies were born in 2023

That year, about 40% of the babies were born to unmarried parents, and about 32% of the births were born via cesarean. On average, mothers were 27.5 years old when they had their first child.

Human anatomy provides evidence for evolution

Throughout history, scientists have used anatomy to support the Theory of Evolution in three main ways. Homologous structures, or similar physical features in organisms that serve different functions, can help scientists understand common ancestors....

Indefinite cloning of some endangered species may not sustain their longevity because cloning success rates decrease as genetic mutations accumulate.

An anthropologist analyzes the Japanese movement against having children

Broadly known as anti-natalism, the philosophical stance against having children stems from a variety of factors. In Japan, it arises amid low fertility rates, economic pressures, and ethical concerns. This essay explores the reasons some Japanese embrace the philosophy.

Understanding the global crash in fertility rates

This article breaks down the global crash in fertility rates. According to researchers, a global crash in children being born will have a “jaw-dropping” impact on societies. Women are choosing to have fewer children as a result of more women in...

Child mortality rates since ancient Rome

One of the great achievements of modern humanity has been the sea change in child mortality rates—the chance that a newborn eventually lives past puberty. For thousands of years, children had a 50% chance of not entering adulthood. About 300 years...