A million-year-old human skull found in China has opened paths for new discoveries about human evolution.
The new study suggests that Homo sapiens began to emerge at least half a million years earlier than what was initially believed.
It also showed that we coexisted with other sister species, including Neanderthals, an extinct group of archaic humans, for much longer than we have come to believe.
The scientists claim their analysis "totally changes" our understanding of human evolution, and, if proven right, it would rewrite a key early chapter in our history.
Published in the leading scientific journal Science, the discovery shocked the research team, which included scientists from a university in China and the UK's Natural History Museum.
When scientists found the skull, named Yunxian 2, they assumed it belonged to an earlier ancestor of ours, Homo erectus, the first large-brained humans.
That is because it dated back about a million years, long before more advanced humans were thought to have emerged.
Homo erectus eventually evolved and began to diverge 600,000 years ago into Neanderthals and our species, Homo sapiens.
However, the new analysis of the skull, which has been reviewed by experts independent of the research team, suggests that it is not Homo erectus.
It is now thought to be an early version of Homo longi, a sister species at similar levels of development to Neanderthals and Homo sapiens.
Genetic evidence suggests it existed alongside them, so if Yunxian 2 walked the Earth a million years ago, say the scientists, early versions of Neanderthals and our own species probably did too.
This startling analysis has dramatically shifted the timeline of the evolution of large-brained humans back by at least half a million years, according to Prof Chris Stringer of the Natural History Museum, a co-lead on the research.
He said there are likely to be million-year-old fossils of Homo sapiens somewhere on our planet that we just have not unearthed yet.
There are two ways to pinpoint the species of an early human and work out when it walked the Earth: analysing the shape of the skull and its genetic data. In the case of Yunxian 2, both methods were used, and each came to the same conclusion.
The earliest known evidence for early Homo sapiens in Africa is 300,000 years ago, so it is tempting to conclude that our species might have evolved first in Asia.
The earlier timeline means that the three species of humans coexisted on the planet for around 800,000 years, much longer than previously thought, perhaps interacting and interbreeding in that time.
