Palmar flexion creases and finger linkage groups in New World monkeys — functional and evolutionary palmistry

Abstract

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 together — which 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.