Neanderthals are often depicted as barrel-chested, hunched-over cavemen, but a close inspection of their fossils tells a different story.

For the first time, an international team of researchers has reconstructed the ribcage and upper spine of a Neanderthal man who died roughly 60,000 years ago.

The findings suggest that contrary to popular images, these ancient early humans may have once had straight spines and a similar-sized chest to modern humans.

"Neanderthals are closely related to us with complex cultural adaptations much like those of modern humans, but their physical form is different from us in important ways," says co-author Patricia Kramer, an anthropologist with expertise on locomotor energetics at the University of Washington.

"Understanding their adaptations allows us to understand our own evolutionary path better."

The team focused their research on the thorax, an area of the body that includes the rib cage, spine and cavity for the heart and lungs. This region is of particular importance because it can inform us about breathing and balance.

Ever since the very first Neanderthal remains were found nearly 150 years ago, scientists have been debating the size and shape of the Neanderthal thorax.

But as luck would have it, there is one Neanderthal specimen whose spine and rib cage have been remarkably preserved. 

It is one of the most complete Neanderthal skeletons unearthed to date and is known as Kebara 2, or "Moshe" for short. 

This Neanderthal skeleton, found in Northern Israel's Carmel mountain range in 1983, was what the researchers set out to model.

"This was meticulous work," says Alon Barash, a lecturer at Bar Ilan University in Israel.

"We had to CT scan each vertebra and all of the ribs fragments individually and then reassemble them in 3D."

It's commonly assumed that Neanderthals are stockier than humans, with bigger chests and lungs. But the new research reveals that the Neanderthal thorax is about the same size as the human thorax, just wider at the bottom.

This suggests that while Neanderthals may have had a similar-sized chest, they actually had a larger diaphragm. The researchers think this would have allowed Neanderthals to breathe in more air than modern humans.

"The wide lower thorax of Neanderthals and the horizontal orientation of the ribs suggest that Neanderthals relied more on their diaphragm for breathing," explains senior author Ella Been of Ono Academic College.

"Modern humans, on the other hand, rely both on the diaphragm and on the expansion of the rib cage for breathing."

While there are still many, many questions left unanswered - like how did Neanderthals breathe and why did they need powerful lungs? - the authors said the findings were full of "big surprises."

The work adds to a previous study, done by the same team, that reconstructed Moshe's spine. At the time, this model not only reaffirmed the likelihood of an upright posture, it also found a straighter spine than that of modern humans.

Together, these two models are important steps when it comes to updating theories of Neanderthal physiology.

"Thinking through all the permutations of the different fragments, it was like a jigsaw without all the pieces," says Kramer.

"People have told you it should be a certain way, but you want to make sure you're not over-reconstructing, or reconstructing it the way you think it should be. You're trying to maintain a neutral approach."

The study has been published in Nature Communications.