Flexor Pollicis Longus Muscle
The flexor pollicis longus (FPL, Latin flexor, bender; pollicis, thumb; longus, long) is a muscle in the forearm and hand that flexes the thumb. It lies in the same plane as the flexor digitorum profundus.
This muscle is unique to humans, being "either rudimentary or absent" in other primates.
Read more about Flexor Pollicis Longus Muscle: Evolutionary Variation, Additional Images