Over time, our ancestors began walking on two legs, and eventually their bodies changed so that they walked this way all the time. This adaptation allowed early humans to use their hands for other things like using tools, picking and holding plants, and carrying babies. Having their heads up higher also let humans see their surroundings better. Other animals, such as some apes and birds, also use two feet at times, but humans are the only mammal that uses bipedalism as their main way of getting around.