It's a whodunit that dates back 430,000 years.
Deep in a cave in northern Spain, archaeologists have uncovered fragments of a skull with protruding eyebrow ridges and a smaller brain area than you would find in a modern-day human. Researchers say it belongs to an early human relative that predates even the Neanderthals.
It took scientists years to reconstruct the ancient cranium from 52 bits of bone, each about the size of a quarter. But when they finally did, they noticed something strange: The skull had two holes just left of the center of the forehead.
The pattern of fractures revealed that the bone broke while still enmeshed in living tissue. It was also clear the victim, probably a male in his early 20s, could not have survived long after the blows were inflicted.
The two wounds were almost identical, and could not have been caused by a fall, an attack by a predator or an accidental collision with a tree or rock, the archaeologists said. And so they came to a dark conclusion: This ancient skull is evidence of the earliest known murder among our ancestors.
The findings, published this week in the journal PLOS One, suggest that interpersonal violence may be baked into the human experience.
"One implication of the study is that murder is a very ancient human behavior," said Rolf Quam, a paleoanthropologist at Binghamton University in New York who worked on the study.
The pieces of bone that make up what the team calls "Cranium 17" were unearthed over the course of several years at an underground cave known as Sima de los Huesos, or the "Pit of Bones."
Using forensic techniques similar to those on "CSI," they determined that the victim was alive when the attack occurred because the fractures on the forehead were characterized by oblique angles and smooth surfaces. Fractures that occur after death have right angles and jagged surfaces because the bone has started to dry out.
Based on the impact trajectories, the researchers believe the victim was facing his assailant when the violence occurred.
"We don't know whether it is face-to-face combat or an ambush," said Quam. "What we can definitely say is it is intentional."
The authors are not sure what type of weapon was used, but a wooden spear or stone hand-ax are likely candidates, the researchers said. Metal tools had not yet been invented.
"We are pretty sure that these two fractures are the result of two repeated blows with the same implement," Quam said. "And that implies a clear intent to kill."
There is evidence of cannibalism among early humans dating back 900,000 years, but evidence of murder has been harder to find.
Researchers have previously found a 25,000-year-old Homo sapien fossil from the Russian plains with a puncture wound on one of the vertebrae. That wound surely would have been fatal, but it might have been the result of a hunting accident rather than willful violence.
There's also a Neanderthal that lived 75,000 to 100,000 years ago in present-day Iraq. This individual had a life-threatening puncture wound to one of the ribs, but it looks like it had started to heal. That suggests the Neanderthal did not die immediately from this injury.
Cranium 17 was discovered at the bottom of a vertical shaft that lies about 0.3 of a mile from the entrance to the underground cave system. The only way to enter the cavern was by falling 45 feet through a hole in the floor.
The archaeologists say it is unlikely that any of the 28 early humans who wound up at the bottom of the shaft got there by accident. And since the victim could not have sustained the lethal injuries in the cavern, he must have been thrown into the pit after death — perhaps as an early form of burial.
Clues about how our earliest ancestors lived are rare, which is why experts say this discovery is so provocative.
"Anthropologists are always asking what makes us human, and are humans inherently violent?" said Danielle Kurin, a forensic anthropologist at UC Santa Barbara who was not involved in the research. "This study contributes to that debate by suggesting intentional assault between two people has deep roots in our hominid human history."
Lead author Nohemi Sala of the Complutense University of Madrid is still looking for evidence that some of the other 27 hominids in the pit may have been murdered as well. She has not turned up anything yet.