A fossilised, battle-scarred skull belonging to a previously unknown species of primate has been unearthed that sheds light on the evolutionary origins of apes, including humans.
The creature was no bigger than a baboon and lived in the canopy of a warm, damp forest near what is now the Red Sea in western Saudi Arabia, between 28m and 29m years ago.
The remains, which include a partial skull and teeth, were recovered from ironstone sediment during an expedition to the site in February 2009, but only now has a detailed description of the fossil been published.
The discovery, reported in Nature, is being hailed as one of the most significant finds in decades because it illuminates a critical moment in evolution when ancient primates split into two separate lineages.
One group of primates gave rise to the Old World monkeys, which include baboons and macaques, while the other led to the great apes and, ultimately, Homo sapiens.
The skull and teeth show that the animal, named Saadanius hijazensis, had similar teeth to Old World monkeys. Unlike apes, it lacked a frontal sinus (which is responsible for the feeling of "brain freeze" sometimes caused by eating ice-cream).
By comparing the remains of Saadanius with other ancient primates, the researchers put the date of the evolutionary split at between 29m and 27m years ago.
"The roots of apes, humans and monkeys go back a long way. We were interested to know when these ancient primates diverged because, in a way, that's when we got our start," said William Sanders, an author on the paper at the University of Michigan's Museum of Palaeontology.
"Knowing the date is important because you can then look at what the conditions were like at that time and place and get some idea of what was driving their evolution," he added.
The Red Sea had yet to form when Saadanius was alive and the landscape was changing with the arrival of animals and plants from Eurasia.
Iyad Zalmout, lead author of the study, spotted the damaged skull of Saadanius lying upside down in the sediment with its teeth glinting in the sun.
Serious wounds on the front of the skull suggest the creature met a violent end. "He got in the way of a big carnivore and died in a horrible way," Zalmout said. "The puncture marks in the skull suggest he was seized by the head, got chewed around a bit, and was then thrown away."
Brenda Benefit, professor of biological anthropology at New Mexico State University, said: "For me this discovery is one of the most significant in my lifetime. Until now we have not had a very perfect fossil ancestor for the Old World monkeys and apes."
"Some palaeontologists, inlcuding myself, thought that this is exactly what the common ancestor to Old World monkeys and apes would look like, based on resemblances between Miocene fossil Old World monkeys and apes, whereas others thought they would be shorter snouted and more round-headed like modern gibbons.
"Saadanius resolves this debate and demonstrates the importance of the fossil record for knowing what our ancestors looked like."
