Separate extension of fingers in the hand of primates is performed by 3 muscles: m. extensor pollicis longus, m. extensor digiti secundi, and m. extensor digitorum lateralis. Here it is proposed to consider them as parts of the extensor digitorum profundus muscular complex. The diversity in structure of these muscles in primates is examined based both on original anatomical study of New World monkeys and analysis of extensive published data on primates from different taxonomic groups. It is shown that in these muscles there are 2 main types of structure variations -the division of the muscle belly into several heads which give rise to separate tendons, and the split of the single terminal tendon into several branches. The first type of modification ensures the possibility of a separate management of the fingers, and the second, on the contrary, ensures the coupled control of extension of fingers. A scheme of evolutionary transformations of muscles belonging to the complex of the deep extensors of fingers is proposed.

During dissection of a juvenile specimen of howler monkey (Alouatta seniculus), we discovered a supernumerary muscle. This muscle originates from the lateral epicondyle of the humerus, distal to the m. brahioradialis and beneath the m. extensor carpi radialis longus, and runs deeply to insert on the proximal part of the radius adjacent to the m. supinator.To determine homology of the unusual muscle, we compared it with the known abnormal extra muscles in the group of preaxial forearm extensors in humans and other primates, as well as other mammals. The only similar muscle is the so-called m. brachioradialis accessorius, which is very rarely found in humans medial to the n. radialis r. superficialis. Both in howlers and humans, its unique topological interrelations with the n. radialis suggest that this muscle is fundamentally different from all surrounding forearm extensors including the proper m. brachioradialis. At the same time, its innervation by the n. radialis confirms that it is a true extensor, contrary to the reptilian m. tractor radii.The general problem of identifying homology of anomalies and novelties is considered. As the enigmatic muscle departs from rules of myological architecture of the tetrapod forelimb, we failed to establish its general homology and, instead, suggest naming it as m. contrahens cubiti. This means that the muscle acts as the elbow flexor although it intimately belongs to extensors.

Manipulatory abilities and activities of primate hands are crucially important for humanity's understanding of our own species, and they have been very well studied in diverse aspects. However, there remains a surprising ignorance as to the variety of flexion creases on primate palms and their functional and evolutionary significance. Here we present the first extensive attempt to fill this gap. Major attention is paid to New World monkeys known for the diversity of their grasping adaptations, which exceeds that of prosimians, Old World monkeys and apes together. The presented analysis is based on the assumption that the palmar flexion creases border groups of fingers which are used togetherwhich we suggest naming "finger linkage groups". We show that this assumption is in agreement with what is known about hand usage in locomotion and manipulation by different primates. Here we show that the Life Line bordering the thumb is present in all primates manifesting the basic ability of thumb opposition, which can be enhanced (like in humans or capuchins) or reduced (like in marmosets or spider monkeys). Longitudinal creases (Fate Line) seem to have appeared early in evolution, as they are present in lemurs, marmosets, and some Old World monkeys; typically, two longitudinal creases are present (humans show one as a variation). The longitudinal creases were replaced in simian evolution by transversal ones, of which the most common is the Simian Line bordering fingers II-V and facilitating a power grip by these four fingers against the palm. This four-finger crease is the only type of transversal creases known in Old World monkeys and apes (the last have two such creases). In marmosets the transversal crease borders all five fingers, while in schizodactylous New World Monkeys it borders only the three last fingers. Apart from them, such a three-finger crease, known as the Heart Line, is found in humans only. However, its origin is different. In human evolution, it was most likely produced by a breakage of the ancestral Simian Line into two, the second half being represented by the unique human Head Line. This breakage dislinked the index finger from the power grip, allowing for pointing gestures and individual usage of this finger.