Is this humankind's earliest ancestor? Scientists argue a 7 million-year-old ape-like animal was the first to walk on two legs

It’s considered to be one of the most decisive steps in human evolution.

Now, scientists believe they have pinpointed when our ancestors made the transition from walking on all fours to standing on two legs.

An ape-like animal that lived in Africa seven million years ago is the best contender for humankind’s earliest ancestor, they say, as fresh analysis has revealed its bones were adapted to walking upright.

The fossilised remains of the species, called Sahelanthropus tchadensis, were first unearthed in the desert region of Chad, in north-central Africa, more than two decades ago.

The anatomy of the skull suggested that it likely sat directly on top of the spine – the first indication it may have walked upright.

But new analysis of the limbs confirms the species could move around on two legs, as the bones contain a feature only found in bipedal groups.

Scott Williams, an associate professor in New York University’s Department of Anthropology, led the study.

‘Our analysis of these fossils offers direct evidence that Sahelanthropus could walk on two legs, demonstrating that bipedalism evolved early in our lineage and from an ancestor that looked most similar to today’s chimpanzees and bonobos,’ he said.

For their new study the researchers identified the presence of the femoral tubercle, a part of the body that is vital for walking upright, in the fossilised remains.

This is the point of attachment for the largest and most powerful ligament in the human body – the iliofemoral ligament – which connects the pelvis to the femur and prevents the body from bending backwards too much when standing up and walking.

Previous studies have also unearthed a ‘natural twist’ in the fossilised femur – the thigh bone – which helps legs to point forward.

Meanwhile 3D analysis has indicated gluteal – buttock – muscles similar to those in our early ancestors that keep hips stable and aid in standing, walking and running.

The team argue their new discovery, along with previous findings, mean the ancient species had the ability to walk upright.

‘Sahelanthropus was essentially a bipedal ape that possessed a chimpanzee-sized brain and likely spent a significant portion of its time in trees, foraging and seeking safety,’ Dr Williams said.

‘Despite its superficial appearance, Sahelanthropus was adapted to using bipedal posture and movement on the ground.’

This discovery makes the species the oldest known member of the human lineage since we split evolutionarily from chimpanzees.

Humans and monkeys only diverged around eight to 19 million years ago, so the findings suggest that early humans became bipedal very soon after this split.

As part of the study the team compared the remains to those of other early human ancestors as well as living apes.

They found that Sahelanthropus had a relatively long thigh bone relative to a bone found in the forearm – further evidence that it walked on two legs.

They said that apes have long arms and short legs, while humans and our ancestors have relatively long legs.

Writing in the journal Science Advances they explained that bipedalism is a ‘key adaptation’ that differentiates hominins – humans and our extinct relatives – from living and fossil apes.

‘Sahelanthropus is interpreted here as an African ape-like early hominin that demonstrates some of the earliest adaptations to bipedalism,’ they wrote.

They said they believe the evolution of walking upright was a ‘process rather than an event’, in which bipedal behaviour gradually increased over time.

This means the species may have had the ability to walk on two legs on land, but also to swing through the trees like a monkey.

Other experts have previously cast doubt on the idea that Sahelanthropus is a human forebearer.

When the species was first discovered in 2001 Milford Wolpoff, a professor of anthropology at the University of Michigan, called them into question.

In a letter to the journal Nature, Professor Wolpoff argued that Sahelanthropus is ‘not on the line directly leading to humans’.

He pointed to scars on the skull left by neck muscles, claiming they showed the species walked on all fours with its head horizontal to its spine.