Chen Zhe, a researcher with the Nanjing Institute of Geology and Paleontology at the Chinese Academy of Sciences, shows the earliest known footprints left by an animal on earth, which date back at least 541 million years, in Nanjing, Jiangsu Province, June 7, 2018. Researchers from the institute and Virginia Tech University in the United States found the tracks in the Three Gorges area and published their report in the US journal Science Advances. (Photo: China News Service/Yang Bo)

Chen Zhe, a researcher with the Nanjing Institute of Geology and Paleontology at the Chinese Academy of Sciences, shows the earliest known footprints left by an animal on earth, which date back at least 541 million years, in Nanjing, Jiangsu Province, June 7, 2018. Researchers from the institute and Virginia Tech University in the United States found the tracks in the Three Gorges area and published their report in the US journal Science Advances. (Photo: China News Service/Yang Bo)

Chen Zhe, a researcher with the Nanjing Institute of Geology and Paleontology at the Chinese Academy of Sciences, shows the earliest known footprints left by an animal on earth, which date back at least 541 million years, in Nanjing, Jiangsu Province, June 7, 2018. Researchers from the institute and Virginia Tech University in the United States found the tracks in the Three Gorges area and published their report in the US journal Science Advances. (Photo: China News Service/Yang Bo)

Chen Zhe, a researcher with the Nanjing Institute of Geology and Paleontology at the Chinese Academy of Sciences, shows the earliest known footprints left by an animal on earth, which date back at least 541 million years, in Nanjing, Jiangsu Province, June 7, 2018. Researchers from the institute and Virginia Tech University in the United States found the tracks in the Three Gorges area and published their report in the US journal Science Advances. (Photo: China News Service/Yang Bo)