Hands are some of the most enduring images at Upper Palaeolithic rock art sites across the world. They have been recorded across the Americas, Africa, Arabia, Australia, East and South Asia, and Europe.
     Ethnographic accounts suggest that hand images were produced by hunter-gatherers for a variety of reasons, and by men, women, and children - to commemorate a visit, claim territory, mark the number and direction of travellers, as the signature of an artist, a message to spirit ancestors, or as part of initiation and healing ceremonies.
     Some are pecked into rock, but most use paints. Prints are positive images made when a hand coated with pigment is pressed against a surface. Stencils are negative images made by placing a hand against a surface and applying pigment around it. Prints and stencils preserve a record of the size and shape of the hand used to make them, and the correct identification of the sex and age of hand images is a necessary first step to understanding this most common of Palaeolithic image forms.
     Recent studies based on hand size and finger length have produced conflicting results, but researchers in England and South Africa believe a new strategy can sex hand stencils with 90 percent accuracy.
     Patrick Randolph-Quinney, a forensic anthropologist at the universities of Central Lancashire and Witwatersrand, says: "The problem with focusing on hand size and finger length is that two different shaped hands can have identical linear dimensions and ratios. To capture shape, we applied geometric morphometrics, a technique used in forensic studies that had never been tested on hand stencils before."
     Researchers translate 2D stencils into 3D hands using computer models based on data from known-sex hand stencils and the hands that created them, identifying a series of 2D landmarks linked with hand shape and form and highlighting the correlations most predictive of sex. The method has the advantage of being able to detect differences in both complete and partial hand stencils.
     Randolph-Quinney explains: "This geometric approach is very powerful as it allows us to look at the palm and fingers independently. It revealed that the shape of the palm is actually most indicative of the sex of the individual, rather than the finger size or length."
     The earliest hand stencils are in Indonesia, and are at least 40,000 years old, while the earliest found in the caves of Europe are thought to be 37,000 years old.
     Scientists want their technique to be tested on a variety of ethnicities and populations to confirm its validity, and have not yet used it to analyse prehistoric hand stencils.
     Anthony Sinclair, an archaeologist at the University of Liverpool, says: "We would encourage other researchers to apply this method to different human populations so we can build a more global understanding of hand variation."

Edited from ScienceDirect, UPI (13 December 2016)