Great question again Leah.
So, there are a few theories, like everything in science!
One is that we evolved the ability to stand upright as primitive apes so we could move much longer distances to find food. Also, this would help with escaping predators. This is probably because our ancestors came down out of the trees for some reason (where standing upright is pretty useless!) to moving around flatter fields called savannah in Africa. In order to find food here it’s more efficient to walk and the skill of climbing isn’t much use.
The other theory (and one which I prefer because it sounds really cool!) is that it evolved in apes who needed to wade through water to get food. It’s easier to walk through water, because you’re lighter. Then gradually their bodies changed over many generations to allow us to only walk upright. If you look at a skeleton of humans and compare it to a chimpanzee (our closest relative), it’s still very different so this change would have taken a very long time. One of the key changes was in the hip bones, or the “pelvis”. This had to tilt downwards so we could stand up straighter.
So, because we needed to find food in a habitat that was changing around us, either to less trees of more water, some of our ancestors began to walk on two feet. And because they survived and others did not, they passed these changes on to their children and so made this skill even better.
Hi Leah, great question & answer from Shane. This is an interesting article from National Geographic about many physical problems humans now have, which can be traced back to the evolutionary decision to stand upright!
For example, animals are able to deliver their little newborns with a lot more ease than female humans – and it actually connects back to a change in our bodies to allow us to walk upright!
Leah, you re asking some great questions! and great answers guys! I m only going to add a little bit to your answer. Walking upright is what makes humans unique among primates. It freed our hands up for using tools, which allowed us to evolve as humans. But unfortunately, the changes made in our pelvis for moving on two legs (in combination with babies having big brains!) makes having babies much more dangerous for humans than childbirth for other animals. The curved vertebra in the lower back, which helps us maintain our balance as we stand and walk upright, is actually the cause of many lower back pains and strains. Yikes! Great question!