Fabrice Demeter. A skull (left) and jaw from the same cave in Laos have forced a rethink of human evolution in southeast Asia.

The oldest modern human fossils known from Southeast Asia have been reinterpreted to indicate that our ancestors were much more diverse than previously recognized. An adult jawbone that combines features associated with humans and Neanderthals is even smaller than one of Indonesia's “Hobbits.”

When a skull was found in the Annamite Mountains of Laos in 2010, it rewrote our understanding of human migration in the region, demonstrating that we had been living in the area for thousands of years longer than we thought. The skull was dated back to 46,000-63,000 years ago, while the oldest confirmed finds of modern humans elsewhere in east Asia are around 36,00-40,000 years old.

A year later, a jawbone was found in the same cave. The skull and jaw date back to roughly the same time, but size and other features rule them out from coming from the same individual. The skull has features consistently associated with modern humans, but the jaw is harder to place than the skull.

Credit: Demeter et al. The 46,000-63,000 year old human jaw and teeth from several angles.

"In addition to being incredibly small in overall size, this jaw has a mixture of traits that combine typical modern human anatomy, such as the presence of a protruding chin, with traits that are more common of our archaic ancestors like Neandertals—for example, very thick bone to hold the molars in place," says Dr. Laura Shackelford of the University of Illinois.

Such combinations of features associated with different species can trouble anthropologists. "Some researchers have used these features as evidence that modern humans migrating into new regions must have interbred with the archaic populations already present in those regions." Shackelford says. However, she sees things differently, saying, "But a more productive way to look at this variation is to see it as we see people today—showing many traits along a continuum.”

In PLOS ONE, Shackelford and an international team explore this theory. They note that much older fossils have been found in the region with features suggesting they may be from early members of our own species, or from a predecessor. Meanwhile, other specimens demonstrate that other species from the genus Homo were in the region as far back as 190,000 years ago.

The authors conclude that the jaw came from an adult of unknown sex. Nevertheless, it is smaller than “all modern and archaic samples.” “The only other Homo fossils that are similarly small in bigonial breadth and dental arcade breath” are the “hobbits” found on Flores.

“Genetic studies of Late Pleistocene hominins indicate increasingly complex population dynamics throughout the Old World, but particularly in Asia,” the paper notes. “The mixture of archaic and derived morphology in the earliest modern humans has implications for population dynamics in the region.” The authors believe that because samples from this era are so scarce, we are failing to see the continuity, instead wondering if differing features suggest crossbreeding with related, now extinct, species.

While numerous animal fossils have been found in the cave, so far no more human remains have helped fill in the picture.