Final answer:
The straight legs of the hominins known as "Ardis" enabled them to walk upright for extended periods in the savanna, an adaptation that offered many evolutionary benefits, including freeing hands for tool use and a shift towards a more varied and digestible diet.
Step-by-step explanation:
The straight legs of the hominins called "Ardis" (4.5 - 4.3 million years ago) enabled them to walk upright in the savanna for extended periods. Species like Ardipithecus adapted bipedal locomotion, which provided several evolutionary advantages. Bipedalism was especially beneficial in the savanna, allowing for greater visibility over tall grasses, energy-efficient locomotion across open landscapes, and freeing up the hands for tasks such as carrying food and using tools. This pivotal transition to walking on two legs marked a significant evolutionary shift from earlier, more arboreal hominins.
By about 2.8 million years ago, further evolution in bipedal hominins such as Homo erectus resulted in a physique nearly as efficient at bipedal locomotion as modern humans. With a broader pelvis, longer legs, and arched feet, early Homo species were not only adept at walking but also capable of running. The changes in body structure due to bipedalism also necessitated dietary shifts from tough plants to more digestible foods such as nuts, fruits, and meat. The hands-free benefit of bipedalism led to the development of more sophisticated tools and improved dexterity in hands over time.