Giant eagles once ruled the skies. They snatched human children, and shaped our evolution - BBC

A child of our ancient ancestors, just three years old and still teething, wandered through the swaying grass of the South African savannah 2.8 million years ago. The young Australopithecus africanus was not aware it was being watched from above.

Without warning, an enormous raptor dived. It grabbed the infant and killed it.

Warning: Some readers may find details in this story upsetting.

It's not known for sure what kind of bird ate the early human infant, who has become known as the "Taung child". But puncture marks below his or her eye sockets are consistent with those of eagle talons which hunt their prey in this way, often storing some of their kill in a tree and taking the rest back to its nest.

The Stone Age was a perilous time to be alive. Megafauna came to rule the world. Early humans lived alongside mammoths, mastodons, giant sloths, sabre-toothed cats – and huge child-eating birds.

For tens of millions of years, between the fall of the non-avian dinosaurs and the Great Ice Age, the skies were ruled by these giants. Their reign may have shaped the course of human evolution in profound and overlooked ways. Some are thought to have left their mark in legends of child-snatching birds that have lived on in cultures around the world, long after the birds themselves died out.

Where the monstrous birds came from

When the non-avian dinosaurs were wiped off the planet, the sky was blanketed in dust and most plants died out. For the animals, there was little to eat but seeds – but this suited birds just fine.

In the absence of predators, some small surviving ground-dwelling beaked birds were able to evolve into giants.

Some were plant-eaters, such as Australia's Dromornithidae, known as the "demon ducks of doom" – much like modern-day ducks but 100 times bigger and heavier than cows. Some took to the skies to become huge ocean-going predators, such as the largest known flying bird ever to live, the South Carolina Pelagornis sandersi, with its massive 7m (24ft) wingspan.

Others, though, stepped into the roles left vacant by the extinction of top land-based predators like the Tyrannosaurs. Where there's big prey available, even bigger predators evolve.

These were the monstrous birds our ancestors had to contend with.

Modern and ancient clues

"As long as primates have been around, there have been raptors," says Scott McGraw, a primatologist at Ohio State University, US. "And raptors have always been preying on primates – including us and our relatives."

Echoes of the ancient conflict between our ancestors and giant birds can be seen in the wild today. When McGraw examined the remains of more than 600 modern-day primates killed by birds of prey, including African crowned eagles, in Ivory Coast's Taï Forest, he discovered a remarkable similarity with the talon marks left on the Taung child's remains.

In a number of skulls, the monkeys had damage to the orbital floor – a bony surface inside the eye socket – "where the eagle had either pecked out the eyes with its beak, or torn them out with a talon", says McGraw.

"One of our reviewers went down to the fossil vault, pulled out the Taung child, and found that the damage was identical."

McGraw believes raptors would have routinely eaten our human ancestors – and that predation by birds may have been one of the most important selective forces in the evolution of primates, the order to which humans belong.

"If we go back now and examine more closely the [human] fossil assemblages that have been collected over the years," he says, "there is a good chance – now that we know what we're looking for – we will find additional raptor damage. If what we're seeing in Taï is any indication, then this is normal."

How predatory birds shaped our evolution

There's little doubt in the paleoanthropological record that early humans were prey, says James Cole, archaeologist and dean of applied sciences at Brighton University in the UK. "Particularly the earlier hominins, like the baby Australopithecus africanus that was eaten by an eagle."

Cole says our early ancestors were dinner for many predatory species. "We were predated upon for sure."

The need to evade predators – from birds to leopards, hyenas and giant crocodiles – would have been an important evolutionary pressure on our ancestors. Being dinner for giant birds is thought to have shaped both our bodies and minds.