As we honor the 70th anniversary of Pearl Harbor Day, I thought I’d share a story that highlights how World War II affected the study of human evolution. It’s one of the great mysteries in the history of paleonanthropology: how boxes full of hominid fossils disappeared during the war.

The story begins a couple of decades earlier. While working in an area about 30 miles southwest of Beijing in the 1920s, paleontologists discovered the teeth and bones of primitive humans in the caves of Zhoukoudian, or Dragon Bone Hill. At the time, the known human family tree was sparse. Scientists had discovered only three extinct species of hominids: Neanderthals in Europe, Pithecanthropus erectus (now called Homo erectus) in Indonesia and the disputed Australopithecus africanus in South Africa. Canadian anthropologist Davidson Black believed the fossils from China represented a new hominid species that should be added to the list: Sinanthropus pekinensis, more commonly known as Peking Man.

Black and other researchers continued to unearth new Peking Man fossils through the 1930s. Black died in 1934, and German anthropologist Franz Weidenreich, who had been working in the United States, took over his research at Zhoukoudian. After Japan invaded China, work at the caves ceased, and local authorities worried about the safety of China’s most important fossil collection. To protect the Peking Man fossils, the Chinese asked the United States to take the nearly 200 fossils out of China.

This is where the mystery begins. The bones were last seen in December 1941, when they were packed into boxes that were supposed to be handed over to U.S. Marines stationed in China at the onset of the war. No one knows what happened to the boxes. But author Paul M. Edwards outlines several possibilities in his 2010 book Between the Lines of World War II. Here are just a few:

The bones are in Japan: Japanese soldiers might have intercepted the boxes and brought them home.

The bones are buried at sea: The Japanese might have packed the stolen cargo aboard a ship that sank during the war.

The bones are in the United States: The Marines might have successfully carried out their mission, but now, for some inexplicable reason, the United States won’t admit it has them.

The bones are buried in China: Someone who didn’t recognize the bones’ value may have tossed the boxes away at some point during the fossils’ journey to where the Marines were stationed.

Fortunately, all was not lost 70 years ago. Before the Peking Man bones went missing, Weidenreich made casts of the fossils. And after the war, in 1949, excavations at Zhoukoudian resumed. Since then, scientists have uncovered numerous additional fossils and stone tools, dating Peking Man’s inhabitance at the site to 780,000 to 400,000 years ago. Researchers have also recognized that Peking Man, like Pithecanthropus erectus, was really a member of the species Homo erectus.

But the original fossils, collected between 1927 and 1937, have not been forgotten. In 2006, the local government near the fossil site established a search committee to track down the missing bones. Despite their efforts, the mystery of the missing hominid fossils remains a cold case.