Rock climbing is a relatively rare ability. Sure, billy goats can scamper their way up some pretty steep terrain, but humans are the only mammal to have figured out how to negotiate over-hanging cliffs with something close to ease.

I have often wondered about this.

Considering all the danger that lurks on the ground, cliff climbing seems like the kind of ability that would come in handy out there in the wilds. And lately, scientists have started to shed some light on this subject and that light has come from a strange source: the study of upright walking.

The tale of bipedalism is a tale of mice and men and, well, marsupials. The mice in question are actually kangaroo mice, the men are self explanatory and the marsupials are all members of the family Macropodidae, which includes critters like kangaroos, wallabies and pademelons. Out of the whole of the mammaliam kingdom, these are the only three categories of critter to have ever learned to walk upright.

And bipedalism has seen something of a revolution as of late. The old theory is that humans became bipedal around the same time they descended from the trees. In 1974, when the nearly complete skeleton of Lucy, a 3.2 million-year-old Australopithecus afarensis was unearthed, her arms were clearly adapted to climbing, but her leg bones showed that she clearly walked upright. And for years, Lucy was used as a barometer of sorts, leading scientists to believe that bipedalism emerged about 4 million years ago.

Then in 1994, a 4.4 million year old Ardipethicus ramidus was discovered in Ethiopia. Skull fragments seemed to indicate the skull sat cleanly atop the spine, a dead giveaway for upright walker. Then, in 2000, it was the 6 million year old Orronin tungenesis in Kenya and a femur analysis that also suggested bipedal locomotion.

Now six million years ago is about when the human line split apart form the chimpanzee line, a discovery that turned our ideas about when we learned to walk on their head.

Even more controversial was the 2002 Chad find: Sahelanthropus tchadensis, the fossils dating to between 6 and 7 million years old, and again suggested that we walked upright far earlier than anyone suspected.

So what’s going on here? Well, new work by a number of different primatologists, including Susannah Thorpe and Robin Crompton, both from the University of Liverpool, have thrown some controversial light on the subject. Crompton and Thorpe study orangutans, the primate with the most human-like posture.

What they have recently noticed is that orangutans do something known as tree walking: specifically, the use their hands to hold the branch above them while keeping their feet on another limb below them. With this four hold system, they basically walk along tree branches on two feet.

This is a style of locomotion that allows them to move across thin, flexible branches with far less energy than hanging from four limbs and scooting along like a sloth. Thus orangutans—or so the story now goes—learned to hang-and-walk because it gave them an easier way of moving from tree to tree.

These finding have led to something of a spat about the origins of bipedalism, but the general gist is that we learned to walk upright before we learned to come down from the trees and this lesson also helps explain our rock climbing abilities.

To climb a cliff face requires a very delicate negotiation. Often times, the things you’re holding onto are the size of quarters. Learning to balance on quarters is not the kind of thing that seems easy to learn—but that’s what’s so unusual here: it’s really not that hard.

In fact, it’s easy enough that novice climbers usually find they can figure it out within a few weeks of work. The ease with which we can deduce such a thing has always suggested prior knowledge, but where did that knowledge come from has been the question.

And while tree-walking may not be the answer, it’s the closest thing I’ve seen to a best guess scenario. That said, I’m open to any other ideas, so if you have them feel free to send them along.