Table 2.

Muscular characters that were likely present in the hand of the Pan-Homo LCA in comparison with the derived condition present in modern humans

Muscle or myological character	Inferred condition of the Pan-Homo LCA	Derived condition in modern humans	What can be said from fossil evidence
Abductor pollicis longus	Inserts strongly into the base of the trapezium, trapezoid, and scaphoid and variably into the base of the first metacarpal. Its largest potential torque is probably in flexion (see Discussion).	Insertion is almost exclusively into the base of the pollical metacarpal and its largest potential torque is in extension (see Discussion).	Little can be inferred, but it may be possible to identify carpal insertion markings. Changes in the mechanics of the muscle mechanics may be identifiable based on carpal geometry.
Adductor pollicis (oblique and transverse heads)	Probably a relatively small PCSA* for the oblique head, but the dimensions of the muscle have not been measured in other apes. Both the transverse and oblique heads probably act as extensors and adductors of the trapeziometacarpal joint (see Discussion).	Relatively enlarged PCSA* for the oblique head. Both heads primarily act as flexors of the trapeziometacarpal joint (see Discussion).	The derived mechanics of this muscle may be identifiable based on carpal geometry, but more research is required (see Discussion).
Flexor pollicis brevis	Single superficial head that originates from the trapezium.	Expansion of superficial head origin to the flexor retinaculum; distinct deep head originates from the trapezoid, capitate and palmar ligaments.	Little can be inferred. It may be possible to identify carpal insertion markings for the derived deep head, but more likely such markings will be difficult to distinguish.
Flexor pollicis longus	Absent, degenerate or ligamentlike tendon slip with no separation of belly from the flexor digitorum profundus.	Distinct muscle belly and strong tendinous insertion into distal pollical phalanx.	Derived condition is probably difficult to determine from distal pollical phalanges (see Discussion).
Opponens pollicis	Probably a relatively small PCSA*, but the muscle's dimensions have not been measured in other apes.	Relatively enlarged (PCSA* approximately twice that of Pan).	Little can be inferred. The relationship between PCSA* and the muscle marking for the opponens pollicis on the lateral aspect of the pollical metacarpal shaft should be explored.
Forearm flexor mass relative to forearm extensor mass	Flexors > Extensors	Flexors = Extensors	Little can be inferred. With future work it may be possible to predict forearm muscle masses from bony markings. See Discussion regarding skeletal correlates of the flexor digitorum superficialis.
Dorsal interossei	First dorsal interosseus first metacarpal origin mostly restricted to proximal portion of pollical metacarpal shaft.	First dorsal interosseus has expanded distal origin along the pollical metacarpal shaft and covers a large area.	The derived condition can be identified in the fossil record (see Discussion).
Palmar interossei	First palmar interosseus absent	First palmar interosseus present	Little can be inferred. It may be difficult to distinguish palmar interosseus markings from those of the dorsal interosseus.

^*PCSA, physiological cross-sectional area. This is an estimate of the number of muscle sarcomeres acting in parallel and is therefore directly proportional to the tension-production capability of a muscle (see Marzke et al. 1999).