Final answer:
The fact that hominins are completely terrestrial can be attributed to the evolution of bipedalism and the benefits it provided, such as freeing hands for tool use and better thermoregulation. Species like Homo erectus and Australopithecus afarensis show evidence of these terrestrial adaptations. Anatomical changes suggest strong selective pressures favoring a terrestrial lifestyle.
Step-by-step explanation:
Terrestrial Adaptation in Hominins
The fact that hominins are completely terrestrial can be explained through various hypotheses surrounding the evolution of bipedalism. The transition to a terrestrial lifestyle provided numerous benefits, such as free hands for carrying tools and offspring, improved energy efficiency, and better thermoregulation by exposing more of the body's surface to the environment. The anatomical reorganization required for bipedalism suggests strong selective advantages for hominins to adapt to life on the ground.
Homo erectus, which lived about two million years ago, was one of the earliest species to exhibit complete terrestrial behavior and upright walking. This shift to a ground-dwelling lifestyle came with the necessity for dietary changes and adaptations, such as the development of dextrous hands for tool use, as well as new ways to cope with predators.
Australopithecus afarensis, known from the 3.5-million-year-old Laetoli footprints discovered by Mary Leakey, is one of the earliest known bipedal hominins. These footprints already indicated a modern striding gait. Even with different anatomical structures, like those found in H. floresiensis, bipedalism was a key characteristic that distinguished early hominins from their ancestral quadrupedal primates.