Behold the Homind

This is a great question for Intelligent Design proponents, since it's a big contradiction in their theory. However, it's not a problem for Evolution.Homo Erectus spreads around the world. In Europe / Asia it adapts to the colder climate and eventual Ice ages and becomes Neandertals. In Africa, a different group of Homo Erectus adapts and becomes Cro-Mag. Both groups exist in their seperate areas for a while, eventually Cro-Mag ventures out and spreads into Neander territory. For a while both groups exist across that range.

Humanity is not the only species to have developed bipedalism.Bipedalism has some advantages. The ability to carry children, food or tools while travelling is a big one. Reduction of exposure to sunlight is another one. Increased line of sight over tall grasses for example. Ability to reach things in branches.So, why haven't lots of creatures developed it? Well, there are disadvantages as well. Soft underbelly is exposed to danger. Much slower on two legs than on four. Young take much longer to develop the skill to travel on their own.But more importantly, evolution is random. Even if bipedalism was massively advantageous for all creatures, a species would have to randomly develop it. It's not a selected process. First thing is this - How do we know it hasn't?
Whales could be twice as smart as we are, but without thumbs they aren't about to be making any tools. Without a need for shelter, they aren't going to be building any cities.The Octopus is incedibly intelligent as a species, and if any of them lived long enough to do anything about it, they might just take over the world. Unfortunately (fortunately) they live only a few years.Second thing - There are huge advantages to having a big brain. But there are big disadvantages, too. It takes a lot of oxygen to feed the brain. So, unless the lungs increase substantially, that oxygen has to be taken from someplace else. In our case, we took it from the muscles. We are significantly weaker than chimps, for example. But they can't do crossword puzzles.

Say you're a leaf eating bug. You live on the branches of a particular type of plant. You and all your kin are green. Some of you are bigger than others, some are wider, some are taller.Your primary predator is a certain type of bird. It hunts by spotting the insects amoung the leaves and picking them out. You can't fight back. You basically just eat leaves and hope you don't get picked off.All your wide cousins get spotted very quickly, while you and your tall cousins fair slightly better.Generations pass and this pressure leads to an increase in the average height of the bugs and a decrease in width.But, all this time, the birds are getting better two. They have to eat, after all.So, now it's not enough to be just tall. Tall and jagged edged blended in better than just tall. Birds weed out the others.And on it goes. Until eventually you and I can barely distinguish the bug from the leaves among which it hides.