Quick Answer. The function of motor neurons is to release the neurotransmitter acetylcholine, which are released at a junction known as the neuromuscular junction, that bind to postsynaptic receptors, causing the muscle fiber to respond and result into a muscle movement.
The frontal lobe is what really sets us apart from animals, which controls one important aspect of humans: Self awareness. The thinking in terms of I am.. Animals don't do that. (although they DO have sex for pleasure.
Humans have been somewhat more efficient at this mode of locomotion. Like australopithecines , early humans were light in frame and relatively short. They were only about 3 ft. 4 in. to 4 ft. 5 in. tall (100-235 cm) and weighed around 70 pounds (32 kg) The evolution of larger bodies occurred later in human evolution. The differences between australopithecines and early humans are most noticeable in the head.