Final answer:
The key difference in locomotion between humans and reptiles involves the upright, bipedal stance of humans compared to the sprawling, low-to-ground movement of reptiles. Humans have an efficient striding gait, while reptiles exhibit less energy-efficient movements with limbs splayed out to the sides. Reptiles' scaly skin is also adaptive for their terrestrial lifestyle.
Step-by-step explanation:
A key difference in locomotion between humans and reptiles is reflected in their limb positions and body postures. Humans, being bipedal, walk on two legs with an upright stance, supporting their body weight above the ground and moving in a striding manner. This is achieved by alternately using one leg as a stance leg to support the body while the other, the step leg, moves forward. In contrast, reptiles typically have a sprawling limb posture that keeps their bodies low to the ground. This includes reptilian species like lizards with four legs used for running, climbing, or swimming, and snakes which are limbless and move by contracting their muscles in a sinuous, side-to-side motion.
Moreover, the evolutionary adaptations such as the rotation of the femur and humerus in mammals allow their limbs to swing forward in a pendulum-like motion aiding in efficient overland movement, unlike reptiles whose sprawling posture results in a gait more similar to 'walking while doing push-ups', which is less energy-efficient. Additionally, reptiles possess scaly skin containing keratin that helps prevent water loss, a crucial adaptation for terrestrial life.