[A] When a person is standing in the medical anatomical position (where the palm is facing to the front), the thumb is the outermost digit.

[2] Linguistically, it appears that the original sense was the first of these two: *penkwe-ros (also rendered as *penqrós) was, in the inferred Proto-Indo-European language, a suffixed form of *penkwe (or *penqe), which has given rise to many Indo-European-family words (tens of them defined in English dictionaries) that involve, or stem from, concepts of fiveness.

Moving a limb back to its neutral position is called reposition and a rotary movement is referred to as circumduction.

The spider monkey compensates for being virtually thumbless by using the hairless part of its long, prehensile tail for grabbing objects.

[9] Darwinius masillae, an Eocene primate transitional fossil between prosimian and simian, had hands and feet with highly flexible digits featuring opposable thumbs and halluces.

[citation needed] In addition to these, some other dinosaurs may have had partially or completely opposed toes in order to manipulate food and/or grasp prey.

[26] A ventral forearm muscle, the flexor pollicis longus (FPL) originates on the anterior side of the radius distal to the radial tuberosity and from the interosseous membrane.

It passes through the carpal tunnel in a separate tendon sheath, after which it lies between the heads of the flexor pollicis brevis.

It is innervated by the anterior interosseus branch of the median nerve (C7-C8)[27] It is a persistence of one of the former contrahentes muscles that pulled the fingers or toes together.

It uses the dorsal tubercle on the lower extremity of the radius as a fulcrum to extend the thumb and also dorsiflexes and abducts the hand at the wrist.

Passing through the first tendon compartment together with the abductor pollicis longus, it is attached to the base of the proximal phalanx of the thumb.

There are three thenar muscles: The abductor pollicis brevis (APB) originates on the scaphoid tubercle and the flexor retinaculum.

[29] The first dorsal interosseous, one of the central muscles of the hand, extends from the base of the thumb metacarpal to the radial side of the proximal phalanx of the index finger.

[31] It has been suggested that the variation is an autosomal recessive trait, called a hitchhiker's thumb, with homozygous carriers having an angle close to 90°.

[32] However this theory has been disputed, since the variation in thumb angle is known to fall on a continuum and shows little evidence of the bi-modality seen in other recessive genetic traits.

[9] In humans, the distal pads are wider than in other primates because the soft tissues of the finger tip are attached to a horseshoe-shaped edge on the underlying bone, and, in the grasping hand, the distal pads can therefore conform to uneven surfaces while pressure is distributed more evenly in the finger tips.

[citation needed] The climbing and suspensory behaviour in orthograde apes, such as chimpanzees, has resulted in elongated hands while the thumb has remained short.

However, in pronograde monkeys such as baboons, an adaptation to a terrestrial lifestyle has led to reduced finger length and thus hand proportions similar to those of humans.

It can thus be difficult to identify hand adaptations to manipulation-related tasks based solely on thumb proportions.

[42] This, however, is the suggested result of evolution from Homo erectus (around 1 mya) via a series of intermediate anthropoid stages, and is therefore a much more complicated link.