A near-perfectly preserved ancient human fossil of the largest known skull is the skull of a newly discovered human species named Homo longi, or "Dragon Man", say scientists.
Their findings appear in The Innovation and suggest that the Homo longi lineage may be our closest relatives, which could reshape our understanding of human evolution.
"The Harbin fossil is one of the most complete human cranial fossils in the world," says author Qiang Ji, a professor of palaeontology of Hebei GEO University.
"This fossil preserved many morphological details that are critical for understanding the evolution of the Homo genus and the origin of Homo sapiens."
The massive skull was reportedly discovered in the 1930s in Harbin City, China.
It could hold a brain comparable in size to modern humans but had larger, almost square eye sockets, thick brow ridges, a wide mouth and oversized teeth.
"While it shows typical archaic human features, the Harbin cranium presents a mosaic combination of primitive and derived characters setting itself apart from all the other previously named Homo species," says Prof Ji, leading to its new species designation of Homo longi.
Scientists believe the cranium came from a male, about 50 years old, living in a forested, floodplain environment as part of a small community.
"Like Homo sapiens, they hunted mammals and birds, and gathered fruits and vegetables, and perhaps even caught fish," says author Xijun Ni, a professor of primatology and paleoanthropology at the Chinese Academy of Sciences and Hebei Geo University.
Given that the Harbin man was probably very large in size, as well as the location where the skull was found, researchers suggest Homo longi may have been adapted for harsh environments, allowing them to disperse throughout Asia.
Using geochemical analyses, Prof Ji, Prof Ni and their team dated the Harbin fossil to at least 146,000 years, placing it in the Middle Pleistocene, a dynamic era of human migration.
They hypothesise that H. longi and H. sapiens could have encountered each other during this era.
"We see multiple evolutionary lineages of Homo species and populations co-existing in Asia, Africa and Europe during that time," says author Chris Stringer, a palaeoanthropologist at the Natural History Museum in London.
"So, if Homo sapiens indeed got to East Asia that early, they could have a chance to interact with Homo longi, and since we don't know when the Harbin group disappeared, there could have been later encounters as well."
Looking farther back in time, the researchers also found that Homo longi is one of our closest hominid relatives, even more closely related to us than Neanderthals.
"It is widely believed that the Neanderthal belongs to an extinct lineage that is the closest relative of our own species," Prof Ni says.
"However, our discovery suggests that the new lineage we identified that includes Homo longi is the actual sister group of Homo sapiens."
Their reconstruction of the human tree of life also suggests that the common ancestor we share with Neanderthals existed even further back in time.
"The divergence time between Homo sapiens and the Neanderthals may be even deeper in evolutionary history than generally believed, over one million years," Prof Ni says.
If true, we likely diverged from Neanderthals about 400,000 years earlier than scientists had thought.
The researchers say that findings gathered from the Harbin cranium could rewrite major elements of human evolution.
Their analysis into the life history of Homo longi suggest they were strong, robust humans whose possible interactions with Homo sapiens may have shaped our history in turn.
"Altogether, the Harbin cranium provides more evidence for us to understand Homo diversity and evolutionary relationships among these diverse Homo species and populations," says Mr Ni.
"We found our long-lost sister lineage."