Origins of Us - Bones
Our bones and muscle have been transformed by this new way of getting around, upright, on two legs. I'm a human anatomist - I've studied the structure of the human body and I've mainly done that through dissection. And in fact, that's exactly what anatomy means. It means to take apart. But today I'm trying out something a bit different. I'm putting the human body, or at least the skeleton, back together again.
This skeleton is, as you might expect, white, but in fact that's because these are dead bones. Living bones are pink in colour because they're full of blood. Living bone, in our bodies is constantly changing. In response to the stresses and starins we place it under. So, over a period of years, all of the bone in your skeleton is taken away and replace with new bone.
Dr Roberts with spine
But, standing up on two legs is dependent on a central yet vulnerable part of our anatomy. Right in the centre of the skeleton is this wonderful structure, the spine, built up of a series of repeating vertebrae, and it forms this beautiful double-S shape. But, all of this anatomical beauty comes at a cost.
With this isolated spine you can see the curves really beautifully but, you can also see something else and that's the increase in size of the individual vertebrae as we go down to get here, the lumbar spine, where the vertebrae are absolutely massive. That's because they're bearing the weight of everything above them. So, its not surprising that this is where we tend to get problems with our spines, and, in fact, its the most common reason for visits to GPs.
As we get older, the intervertebral discs start to dry out, and the inside of them cab pop out and press on the spinal nerves, and that can be painful. And also the weight that is borne by the spine moves backwardfs and now is loaded onto these joints at the back, so they can be painful too. So, if standing upright causes us so many problems, why do we do it?
The answer is locked away in the dark recesses of time. Six million years ago, the world's climate was becoming colder and drier, and the forests of Africa were thinning out. And where dense jungles gave way to woodlands, the apes who lived in them started to change. In an increasingly patchy woodland, being able to stand to reach fruit on the thinning branches must have been a great advantage for our ancestors.
The latest discoveries show that Toumai was the first of many bipedal apes. Over the next two million years fossils like Orrorin tugenensis and Ardipidthecus ramidus show that other apes were also adapting to their changing environment by standing upright. They were still essentially climbers, but as the dorests thinned its thought these apes were spending more time on the ground,
Its hard to know exactly when our ancestors gave up life in the trees for living on the ground
But, there is a clue hidden away in our bones. I've been watching the chimpanzees as they've been climbing and the way their ankles work. So, I want to compare that with my ankle . Most of the time I'm walking around on the ground and my foot is set at 90 degrees to my leg. But I can move the ankle like this, that's called dorsiflexion, to about 20 degrees
Now, compare that with chimpanzees. To climb efficiently on somthing vertical, you need to be able to bend your foot up much more than we can. When climbing chimps can dosiflex their ankles up to 45 degrees. The differences in ankle movement between us and them could provide vital evidence in working out exactly when out ancestors gave up climbing for walking.