A demonstrator shows her arm with the name “Marielle” written on it, as she performs during a protest against the murder of councilwoman Marielle Franco in Rio de Janeiro, Brazil, Tuesday, March 20, 2018. Franco's murder came just a month after the government put the military in charge of security in Rio, which is experiencing a sharp spike in violence less than two years after hosting the 2016 Summer Olympics. (Photo by Leo Correa/AP Photo)

An Afghan boy jumps off the turret of a Soviet tank on a hilltop on the the outskirts of Kabul, Sunday, March, 4, 2018. (Photo by Rahmat Gul/AP Photo)

An OV-10 Bronco aircraft, decorated with World War One commemoration motifs, flies over Flanders international airport, ahead of the world's first Short Take Off & Landing competition on sand, in Wevelgem, Belgium May 8, 2018. (Photo by Francois Lenoir/Reuters)

Emma Sees, Valor Christian, #6, accidentally grabs Windsor's Alexa Kopren's hair while both were chasing after the ball in the second half of the Colorado State girls 4A soccer championship at Dick's Sporting Goods Park May 23, 2018. (Photo by Andy Cross/The Denver Post)

Prince Philip, Duke of Edinburgh practicing his bicycle polo technique, Windsor Great Park, Berkshire, 1964. (Photo by Norman Potter/Daily Express/Hulton Archive/Getty Images)

Teens got their glad rags on as they hit the town in Newcastle city centre, United Kingdom on August 16, 2018, as they celebrate their exam results. (Photo by North News and Pictures/NNP)

A member of the Pakistani Anti-Narcotic Force (ANF) takes a selfie beside a pile of burning drugs and liquor during a ceremony at the Kacha Ghari army firing range, on the outskirts of Peshawar on October 16, 2018, after the contraband was seized in raids across the province. (Photo by Abdul Majeed/AFP Photo)

The secretive indri (Indri indri) of Madagascar, the largest living lemur. It is also critically endangered and highly evolutionarily distinct with no close relatives, which makes its branch one of most precarious on the mammal evolutionary tree. In the likely event that the indri goes extinct, we will lose 19m years of unique evolutionary history from the mammal tree of life. (Photo by Pierre-Yves Babelon/Aarhus University)