The last days of Homo erectus

Skulls from central Java may come from the last surviving population of Homo erectus, suggests a new study dating the fossil bed and the surrounding landscape. The population’s death roughly coincides with dramatic changes in the environment, which may have caused the species’ extinction long before the first Homo sapiens reached Southeast Asia.

The “last stand” of Homo erectus?

University of Iowa anthropologist Russell Ciochon and his colleagues dated fossils and sediment layers from a site called Ngandong in a naturally terraced valley carved out of the surrounding hills by the Solo River. In the 1930s, archaeologists unearthed the tops of a dozen Homo erectus skulls, along with two tibiae (shin bones). These fossils seem to be different from older Homo erectus fossils in some important ways, like much larger cranial capacity (which suggests bigger brains) and higher foreheads.

“Ngandong Homo erectus has the largest brain size and highest foreheads of any known Homo erectus,” Macquarie University geochronologist Kira Westaway, a co-author of the study, told Ars. “This indicated an important evolutionary change. The timing of this change is crucial to our interpretation and understanding of our distant cousins.”

In a bid to make sense of it all, Ciochon and his colleagues used uranium-series dating on some newly excavated mammal fossils from the same layer as the Homo erectus skulls. To piece together the whole area’s geological history and see how it might relate to the Homo erectus fossils, they also used other dating methods on sediment and rocks from Ngandong and other sites in the river valley. The results suggest that the bone bed (and therefore the collection of Homo erectus fossils) is between 117,000 and 108,000 years old.

That makes Ngandong the last-known trace of Homo erectus in the world.

“There is always a possibility that someone will find new Homo erectus evidence that is younger and therefore that becomes the last appearance—but this is science! At present, we make an interpretation based on the evidence that we have, and this is that Ngandong represents the last appearance of Homo erectus,” Westaway told Ars.

Of course, that doesn’t mean these were definitely the last of their kind in the world. The fossil record is patchy and imperfect, and we haven’t actually found all of it yet. “Our work provides the age of the last-known appearance of Homo erectus, but this does not mean that it is the age of extinction,” Ciochon told Ars. “Small groups of Homo erectus may have lived longer without leaving fossil evidence. We know that there are no living Homo erectus, but it is difficult to prove when the extinction event happened.”

Close encounters of the hominin kind

The Ngandong dates also strongly suggest that Homo erectus may have gone extinct, at least in Indonesia, long before our species made it that far. The most adventurous Homo sapiens explorers were probably somewhere around the Levant or the Arabian Peninsula at the time and didn’t make it to the islands of Southeast Asia until around 73,000 years ago.

On the other hand, Ciochon and his colleagues’ timing leaves plenty of room for Homo erectus to have encountered Denisovans. That would help explain why the Denisovan genome contains a tiny fraction of genetic material from a much older species (just like many modern people’s genomes contain fragments of Neanderthal and Denisovan DNA).

“This older species is likely Homo erectus,” Ciochon told Ars. “There is considerable speculation about where and when the Denisovans meet Homo erectus and what the results of those interactions were. Our dates support the genetic evidence that Homo erectus could have interbred with the Denisovans.”

But it’s too soon to say for sure. “This is yet to be proven, but establishing a solid chronology for Homo erectus is the first step in this investigation,” Westaway told Ars. “The possibility of intermixing with Denisovans is an exciting prospect well worth exploring.”

Listing image by Rizal et al. 2019

https://arstechnica.com/?p=1634575