Homo sapiens have been around for about 300,000 years. Often, our big brains and intelligence are credited with making us the most successful species to have ever walked on Earth, but that isn’t entirely true. There was another species of human that survived on this very same planet for nearly 2 million years, which was the grandparent of so many other human species, including us. They explored new continents. They mastered tools. They may have controlled fire. And they did it all with a brain barely half the size of ours. Enter Homo erectus, a species that may force us to confront an uncomfortable possibility
 that intelligence alone isn’t what makes a species successful.

Read more: How Homo naledi is changing what we know about death

Childbirth can be extremely challenging for humans – but some other primates may have it even worse. A comprehensive analysis of primate anatomy concludes that many species must squeeze large-headed infants through too-narrow pelvises. The problem may have begun with the very first primates, which lived more than 50 million years ago.

It has been assumed for decades that evolution has left humans with unique childbirth difficulties. The conventional view is that the trouble began when our ancestors first walked on two legs, which required the pelvis to be narrow. A few million years later, hominin brains evolved to be larger and infant heads became bigger – but the pelvis was unable to expand to allow for their easy delivery.

Other primates were thought to have things easier, largely because that was the conclusion of an published by anthropologist Adolph Schultz in the 1940s. Schultz looked at a range of primate species and concluded that in the vast majority, the infant head could fit comfortably through the female pelvis.

But his analysis was flawed, says at University College London. “One of the main problems was that it applied measurements that were originally developed for the human pelvis to all primates,” she says.

Schultz identified landmark points on the human pelvis that define the maximum width and depth of a horizontal plane at the top of the birth canal. He then assumed those same landmarks would define the maximum width and depth of any primate birth canal. They don’t. The human pelvis has a very unusual shape, and when Schultz’s landmarks are mapped onto other primate pelvises, they typically define an inclined plane that sits slightly above the birth canal. This plane overestimates the size of the birth canal, because it is effectively an oblique, oval-shaped slice through a cylinder representing the birth canal.

Torres-Tamayo and her colleagues reassessed birth canal shape in 29 primate species, while also looking at data on newborn-skull size and shape in each species. They concluded that several primates have a pelvis that seems too narrow to give birth. Small primates including bush babies and tamarins have the most severe conflict. In these primates, the newborn’s head is almost twice the size of the birth canal.

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“I was not expecting to have a mismatch in quite such a large number of primates,” says research team member , also at University College London.

Birth difficulties may even be the ancestral condition in primates, says Betti, particularly considering that early primates were small.

“It’s super cool to have such a big sample,” says at the University of Zurich, Switzerland. “These species are doing very different things, living in different niches and they do tend to be quite anatomically diverse.”

Different primates have also found their own solutions to the problem. For instance, the bush babies and tamarins dislocate the bones of the pelvis, temporarily doubling the size of the birth canal. Humans can’t do this, says Betti: it would make walking unbearably painful for a large, bipedal species.

Torres-Tamayo and Betti and their colleagues also found that birth difficulties are much less likely to arise in the great apes, maybe because they are so much larger than the tiny tree-dwelling primates. In this sense, humans are still unique in having birth difficulties, because we are the only large ape with the problem, says Betti.

But Webb isn’t so sure about this point; in a study she and her colleagues published in 2024, they concluded that between the size of the birth canal and the infant’s head. “That discrepancy is strange. It’s probably a reflection of the methods used,” says Webb. “This new paper is providing a really nice incentive for us to revisit our own hypothesis.”

Physically, Homo sapiens isn’t that special in the animal world. But our species has discovered ways of beating the odds of survival in every habitat, from jungle to Arctic wasteland. In our latest interview with biological anthropologist and broadcaster Alice Roberts, we discuss the wonderful benefits bestowed on us by animals from our evolutionary past. The biochemistry in our cells goes back to the earliest single-celled creatures living in the ancient oceans, and our arms and legs date back to when the first amphibians crawled onto land around 360 million years ago.

Read more: These are the extinct humans that live on in your DNA

Homo sapiens aren’t as unique as we once thought. In fact, only about 1.5 to 7 per cent of our DNA originated in our species alone. Everything else we share with our ancestors and those human species we once coexisted with, including Neanderthals, Denisovans and even some yet-to-be-identified “ghost populations”. This rewrites the story we have told ourselves about our species. We just aren’t that unique.

When our ancestors met these other human species, they didn’t just compete with them: they mated with them. And those encounters left a permanent record in our DNA. In other words, those ancient humans we thought had vanished didn’t necessarily disappear… Some of them are still here inside you.

Read more: How Homo naledi is changing what we know about death

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