These environmental pressures caused selection to favor bipedalism - walking on hind legs. This gave the Homininae's eyes greater elevation, the ability to see approaching danger further off, and a more efficient means of locomotion.[citation needed] It also freed the arms from the task of walking and made the hands available for tasks such as gathering food. At some point the bipedal primates developed handedness, giving them the ability to pick up sticks, bones and stones and use them as weapons, or as tools for tasks such as killing smaller animals, cracking nuts, or cutting up carcasses. In other words, these primates developed the use of primitive technology. Bipedal tool-using primates from the subtribe Hominina date back to as far as about 5 to 7 million years ago, such as one of the earliest species, Sahelanthropus tchadensis.

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