It all began with one word: necessity. It may seem like walking bipedally was the first step, but the first step was a smaller jaw bone. After that the extra space was filled in with brains. When we got bigger brains, it was as if evolution was put into fast forward. From the first hominid that was bipedal, the australopithecus afarensis, to where we, homo sapiens, are, it only took around 4 millions years. I know that still seems like a very long time but if you look at it in terms of billions of years, 4 million looks like a spit in the bucket. But I guess I have not answered why we began to walk bipedally.
As I stated before, the first evidence of our lineage bipedalism was dated to about 4 million years ago. If you would look at the photo below this is a photo of the Laetoli footprints. You can see how they dug into the soil with their heels and pushed off with the toes like we do. And the evolutionary ancestor was the australopithecus afarensis. What is more is you can see that they were walking with another bipedal hominid, most likely a child australopithecus afarensis.
This evolutionary step was fortunate for us because it helped us survive. Even though we did not have the feet or hands to climb trees as fast as our brachial (tree climbing) ancestors, we had the advantage of being able to see over tall grass. Not only that but we were able to carry tools and weapons, able to hunt and gather. It also helped with our migrating. If were were still quadrupedal our backs would have gotten most of the heat from the sun and we would have had to rest more. With bipedalism we were able to cool off more quickly with the sun would only have hit our heads, that is why we have hair to protect us. Also with bipedalism we are able to do long-distance walking.
Since we have seen the humans around us walking as humans walk we do not see it as anything special. But just think, next time you go out for a walk or even get up to get a drink of water, this thing we take advantage of is one of the major reasons our species is still alive. Without this evolutionary breakthrough we would have ended 4 million years ago.