Today, the Al Wusta archaeological dig site is in the barren Arabian desert, but 90,000 years ago the landscape was much different: lakes and rivers weaved through a lush grassland. A newly unearthed ancient fossilized human finger bone suggests the area was also home to some of the earliest humans to have left Africa, and it could hold the key to understanding how our species began to encompass the entire world.

Scientists say the 3.2-centimeter finger bone likely belongs to the middle digit of an adult. It is the oldest “directly dated” Homo sapiens fossil to be found outside of Africa and the Levant – a region containing Israel, Syria, Lebanon, and Jordan.

Researchers created 3D images of the bone using CT scans to compare its shape, dimensions, and other proportions to the finger bones of Homo sapiens, extinct hominins, and other species of non-human primates. The results, which are published in Ecology & Evolution, indicate the finger’s long, slender shape likely belonged to H. sapiens rather than the more “flared” shape of Neanderthals. Uranium series dating, which uses a laser to measure the ratio between tiny traces of radioactive elements in the bone, revealed the fossil was around 88,000 years old.

DNA testing is probably unlikely – the bone is completely mineralized – but the scan revealed a bony lump on the finger called an enthesophyte, which forms after repeated physical stress where the ligaments and tendons attach to the bones. Scientists say it suggests a lot of hard work, possibly from making tools.

The Arabian Peninsula sits at the crossroads of Africa and Eurasia. Geological and sediment records indicate the area shifted from an arid climate to a more humid one around the same time that evidence of early humans began showing up. Along with the human finger, archaeologists uncovered Middle Palaeolithic stone tools that likely came from Africa with their owners. Other animal fossils, including a hippopotamus and tiny freshwater snails, were also found.

The movement of H. sapiens from Africa used to be portrayed as a single migration, but a wave of new discoveries has challenged that idea, suggesting the exodus was much more complicated, filled with stops, delays, multiple routes, and a variety of groups. The human finger adds to a growing body of work that suggests our migration out of Africa was much more expansive and convoluted. 

"This discovery for the first time conclusively shows that early members of our species colonized an expansive region of southwest Asia and were not just restricted to the Levant," said lead author Dr Huw Groucutt in a statement. "The ability of these early people to widely colonize this region casts doubt on long-held views that early dispersals out of Africa were localized and unsuccessful."