According to the current Savannah theory of human evolution, man became upright at the time of taking to the open plains to hunt. But isn't there a paradox in that to be protected from predators man would have needed the spear, but to develop the spear, man would have already been walking upright?
As someone else pointed out, many species of animals have evolved or invented ways of protecting themselves. Moving in packs or herds is a good start. American farmers have begun adding llamas to their flocks because half a dozen of them can stomp a wolf to death and they'll hold off a bear, a boar or a cougar long enough for the killer dogs to arrive and finish the job.
All of the existing species of apes, with the single exception of the orangutan, are pack-social; we've been pack-social since we split off from the chimpanzee-bonobo clade.
i don't know whether chimps are bipedals or not though, they're kind of half way.
"Bipedal" means to operate at highest efficiency and effectiveness on two legs, and chimps don't satisfy that definition.
Homo sapiens has many unique modifications to the lower body that both make bipedalism possible and make it impossible for us to function on all fours.
The most recognizable is the gluteus maximus, the enormous muscle that forms the familiar hemisphere on either side of our butt. It's been rerouted from the way it attaches in other apes. It provides the strength to stand with our knees locked, and also provides the power to run bipedally--and rather quickly for quite long distances.
But possibly the most interesting adaptation serves two purposes. Our enormous brains give us disproportionally large heads. Even though humans are born at a less advanced state of mental development than most mammals in order to let our brains and heads continue growing after birth, nonetheless a newborn's head is so big that our females' birth canal has to be extremely wide. This means that our hips are very wide. This might not be a big problem for a quadruped, but it is for an animal that only walks on two legs and therefore is going to be balanced on one much of the time--and then must transfer that weight to the other one with incredible grace, so inertia does not topple it over sideways.
The muscles in our thighs and hips are absolute marvels of engineering and there's nothing like them anywhere else in the animal kingdom. Humans can kick much harder than any other animal of similar mass, and this was probably a handy defense strategy for our ancestors before they invented tools.
Hmm, I should probably make an exception for the ratites. Ostriches can kick like an elephant, so I imagine a smaller ratite closer to our mass (are emus about our size?) can probably match us for leg strength. But that just reinforces my point since all birds are bipedal. The reason ratites are so strong compared to other birds is that their bodies are much denser so they have more muscle mass. Other birds have hollow bones and similar adaptations to permit flight.
Probably... I guess it looks like the spear came first. No one is more surprised than me.
You don't have to be fully bipedal to use tools. Parrots are sort of tripedal with their prehensile beaks to help climb, and their zygodactyl feet provide a good approximation of an opposable thumb, so they're quite dextrous at figuring out how to unscrew nuts and bolts and things like that.
Non-psittacine birds like crows have figured out how to use primitive tools in the wild.