It is the oldest fossil belonging to homo sapien found outside of Africa and the Levant (a historical region encompassing what is now Syria), and the first ancient human fossil discovered in Saudi Arabia.
Scientists believe early people left Africa a few times after evolving there at least 300,000 years ago.
The discovery is "a dream come true" for Professor Michael Petraglia, an archaeologist at the Max Planck Institute for the Science of Human History, who led the project that found the bone.
Numerous fossils were discovered at the site of Al Wusta, an ancient fresh-water lake located in what is now the hyper-arid Nefud Desert.
The team recognised the bone as human on sight, and later confirmed this by comparing it to finger bones of other humans, extinct hominins like Neanderthals, and other primates such as gorillas.
Uranium-Thorium and paramagnetic resonance analysis revealed that the finger bone found in the Arabian Desert is between 86,000 and 95,000 years old.
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"All of these studies agreed that the fossil belonged to Homo sapiens", Groucutt said at the news conference.
Although some say it's hard to identify our species, Homo sapiens, by a single bone, the findings appear unimpeachable, says John Shea, an anthropologist at the State University of NY in Stony Brook who studies human origins, but wasn't involved in the study.
The bone, 3.2 centimetres long, is thought to be the middle bone of a middle finger, and is likely to have belonged to an adult. The team also dated a hippopotamus tooth, stone tools and sediments, which provided similar date ranges of about 85,000 to 90,000 years.
But why were these African animals in Arabia at this time? This place lies in one of Saudi Arabia's desert regions.
"And, of course, hunters and gatherers would have been following those animals", Petraglia said. We also CT-scanned the Al Wusta fossil to produce a 3D computer model. Because the country has historically closed itself off from foreign researchers, its role in modern humans' early migration story has been largely under-represented.
This 85,000-year-old fragment, however, has just helped to revolutionise our understanding of early human history because it should not have been in Arabia for another 25,000 years.
However, even though the finger bone is much younger than the jawbone, it's still a momentous find, Groucutt said. We don't yet know if people survived long term in the Levant, which is a very small area. While the Levant was then a wooded area with winter rainfall, Al Wusta, about 400 miles (650 kilometers) away, was a grassland that received summer rain.
How did these ancient humans respond to the dramatic environmental change which dried out lakes such as that at Al Wusta? The earliest human fossils in the Levant date to 177,000 years ago from a fossil in Misliya Cave in Israel.