The superficial part is inserted with one or more tendons into the radial side of the base of the first metacarpal bone, and the deep part is variably inserted into the trapezium, the joint capsule and its ligaments, and into the belly of abductor pollicis brevis (APB) or opponens pollicis.

[6] The APL insertion on the trapezium and the APB origin on the same bone is the only connection between the thumb's intrinsic and extrinsic muscles.

)[10] The only primates to have an APL completely separated from the extensor pollicis brevis are modern humans and gibbons.

In all these primates, the muscle is inserted onto the base of the first metacarpal and sometimes onto the trapezium (siamangs and bonobos) and thumb sesamoids (crab-eating monkeys).

It is possible that these differences are due to the supinated position of the trapezium in humans which, in its turn, is a result of the expansion of the trapezoid on the side of the palm.

[13] A small, lens-shaped radial sesamoid embedded into the APL tendon is a primitive state found in all known Carnivora genera except in the red and giant pandas and the extinct Simocyon where it is hypertrophied (enlarged) into a sixth digit or a so-called "false thumb", a derived trait that first appeared in ursids.