Pete Oxford / Head-hunted human skulls, collected by the Konyak Naga tribe, North East India, October 2014.

Pete Oxford / Konyak Naga tattooed Ang or Chief. Only someone who has killed and taken a human head can be tattooed in this manner. Mon district. Nagaland, North East India, October 2014.

Paul D Stewart / Chromolithograph illustration of Cromagnon 'Skull of an old man' Plate 1, Section C. Edouard Lartet and Henry Christy. 'Reliquiae Aquitanicae' 1865-1875, Williams and Norgate, London 1875..

Juergen Freund / Very old human skulls on a ledge in the karst limestone walls of a Raja Ampat island. Raja Ampat, West Papua, Indonesia, February 2010

Doc White / Skull of Japanese Sailor. Truk Lagoon, Chuuk islands, Federated States of Micronesia.

Ashley Cooper / Pile of bones, the remains of hunted Beluga Whales (Delphinapterus leucas) at Bourbonhamna Svalbard, Norway, July 2013

Ashley Cooper / Pile of bones, the remains of hunted Beluga Whales (Delphinapterus leucas) at Bourbonhamna Svalbard, Norway, July 2013

Ashley Cooper / Pile of bones, the remains of hunted Beluga Whales (Delphinapterus leucas) at Bourbonhamna Svalbard, Norway, July 2013

David Pattyn / Human skulls and bones in wooden troughs, traditional burial site from an ancient culture within cave in Tsingy rocks, Anjajavy Private Reserve, north west Madagascar, August 2016.

David Pattyn / Human skulls and bones in wooden troughs, traditional burial site from an ancient culture within cave in Tsingy rocks, Anjajavy Private Reserve, north west Madagascar, August 2016.

Steve O. Taylor (GHF) / Traditional medicine trader holding Western lowland gorilla (Gorilla gorilla gorilla) and Chimpanzee (Pan troglodytes) skulls and talking about what they are used for, Libreville, Gabon, 2008.

Enrique Lopez-Tapia / Human and orangutan skulls at the Camp Leaky museum, Tanjung Puting National Park, Borneo, Central Kalimantan, Indonesia.