The tusk of a woolly mammoth that lived some 18,000 years ago, from which researchers sequenced the extinct mammal's entire genome, is seen in northeastern Siberia in this photo from 2015. (Photo by Love Dalen/Handout via Reuters)

A man rows a boat on the Dal Lake as a woman holds an umbrella during snowfall in Srinagar February 19, 2015. (Photo by Danish Ismail/Reuters)

Young gymnasts practice pull-up on a bar at a gymnasium of a sports school in Jiaxing, Zhejiang province April 11, 2015. (Photo by William Hong/Reuters)

A clothing shop displays its merchandise in the rebel-controlled city of Idlib, Syria November 7, 2015. (Photo by Ammar Abdullah/Reuters)

A helicopter drops water over a wildfire as a firefighter works to contain it in Benahavis, southern Spain, May 19, 2015. (Photo by Jon Nazca/Reuters)

A participant smiles at the annual Christopher Street Day parade on Kurfuerstendamm in Berlin, Germany, June 27, 2015. (Photo by Fabrizio Bensch/Reuters)

Four-month-old fox cubs squaring up to each other in Walthamstow, England on August 7, 2015. (Photo by Greg Morgan/Barcroft Media)

Attendees at Wizard World Comic Con walk through hallways in Chicago, Illinois, United States, August 22, 2015. (Photo by Jim Young/Reuters)