If you want to understand how humans evolved, there's a few species you obviously want to look at.

Chimpanzees are a good bet. After all, they're our closest relatives, so they offer clues about our ape-like ancestors. Monkeys are also worth a look, for similar reasons.

You might also want to look at unrelated animals that are noted for their intelligence and big brains. Crows or dolphins would be good.

You surely wouldn't bother studying a panda. Sure, they are cute, but they have nothing to do with us. They aren't a close relative, and they aren't packing a lot of brainpower.

That's all true, but according to a new analysis, pandas might be extremely relevant to our evolution. They could help explain one of the most peculiar things about the human body: our upright posture.

Humans and pandas are both mammals, and the majority of mammals spend most of their time on four legs. From dogs and rats to bears and elephants, that's the norm.

Walking around on four legs means that your back is horizontal, parallel to the ground. That in turn determines how the bones in your back are arranged, and how they join to your leg and arm bones.

Pandas spend a lot of time sitting on their bottoms

Humans are different. We hold our backs and spines vertically, at right-angles to the ground. Apes often do the same thing.

The question is, when and why did this upright posture evolve? To find out, Gabrielle Russo of Stony Brook University in New York and Scott Williams of New York University decided to compare humans with another species that holds its back upright.

They wanted something that wasn't an ape or a monkey, because those species have experienced a lot of the same evolutionary pressures as us. That makes it hard to disentangle which factors caused which evolutionary shift.

That's where giant pandas come in.

In common with other bears, they spend a lot of time sitting on their bottoms, back upright.

Russo and Williams wanted to find out if pandas' spines had also changed shape in a similar way to ours.

Pandas had fewer vertebrae in their lower backs, and the vertebrae were a different shape

If they had, it would suggest that pandas and humans evolved their upright postures for similar reasons. On the other hand, if pandas' spines were not like ours, that would suggest that they evolved their upright posture for different reasons.

They compared the shapes of individual backbones – vertebrae – from pandas and closely-related bears.

Compared to their closest relatives, pandas had fewer vertebrae in their lower backs, and the vertebrae were a different shape. The same change happened when our monkey-like ancestors evolved into apes.

The results are published in the Journal of Human Evolution.

If pandas and apes evolved their upright posture for the same reason, what could that reason be?

Gorillas spend a significant amount of time sitting in upright posture and eating bamboo

Traditionally, apes' spinal shapes have been explained by their habit of swinging below tree branches using their arms, or climbing vertical tree trunks. But pandas don't do either of those things, so Russo and Williams say they are unlikely to be the explanation.

Instead it might all come down to something very simple: sitting upright on one's bottom.

"Gorillas spend a significant amount of time sitting in upright posture and eating bamboo and other foliage," say Russo and Williams. Sitting upright leaves their hands free to pick and trim leaves.

The same might apply to giant pandas, which famously have to spend most of their time sitting around eating bamboo.

Russo and Williams think that apes probably began evolving upright postures 15-20 million years ago.

"There are fossil apes like Pierolapithecus that show adaptations in their lower backs to upright posture," they say.

The line that would lead to humans then split from the rest of the apes sometime between 13 and 7 million years ago.