Using modern dating techniques, scientists have dramatically narrowed the age of Homo erectus fossils found in Java during the early 1930s. Confirmed as the youngest known Homo erectus fossils, they’re providing critically important insights into the final chapter of these archaic humans.
By the time our species emerged in Africa some 300,000 years ago, a separate human species had been wandering this good Earth for well over a million years. Such is the incredible story of Homo erectus, one of the most successful hominins to have ever lived. These archaic humans boasted the largest geographical spread of any hominin until we came along, a range that included Africa, China, India, Europe, and Java.
They were also an exceptionally long-lived species, though the exact length of their tenure has been a matter of considerable debate. We know they emerged around 2 million years ago, but their expiry date has remained unclear.
New research published today in Nature is providing some of the most accurate dates yet for the youngest H. erectus fossils in the archaeological record, showing that these hominins were still around between 117,000 and 108,000 years ago and that they likely made their last stand in the Solo River region of central Java in Indonesia.
Our story begins at the Ngandong site in Java during the early 1930s, when scientists found 14 fossils belonging to H. erectus, a major score that included 12 crania (skull caps without the mandible) and two lower leg bones. It remains the largest assemblage of H. erectus fossils found at a single location, but the complex geology of the site, plus various mistakes and omissions committed by the original researchers (such as failing to document where they located the bones), made it difficult to properly date the bone bed in which the fossils were found.
This resulted in a huge range of possible dates, with estimates stretching from 550,000 to 27,000 years ago. This unacceptably large window has been a major headache for scientists, as Kira Westaway, a co-author of the new study and a geochronologist at Macquarie University in Australia, explained to Gizmodo.
“Knowing when a species was alive and when they eventually died out is important for understanding where they sit in the evolutionary tree, who they interacted with, and why they became extinct,” she said in an email. “If the evidence was young, then Homo erectus could have interacted with modern humans and could have been wiped out by competition, but if it was older, then it’s more likely they could have interacted with another human species—the Denisovans—and the changing environmental conditions could have caused their extinction… Timing really is everything in this human story.”
The legwork for this new study, led by Russell Ciochon from the University of Iowa, began in 2008 when an expedition revisited the Ngandong site. Equipped with a 1934 map used by the original researchers and intent on making better sense of these enormously important fossils, Ciochon and his colleagues relocated the original bone bed layer in a river terrace extending 20 metres above the adjacent Solo River and two distinct spots from which the bones were likely pulled.
In addition to extracting numerous samples from the site and the surrounding region, the researchers also studied the geology and topography of the area, which allowed for a fresh view of the site and an improved understanding of its geological context.
The next step was to date the sediments in which the fossils were buried, along with other stratigraphic layers and clues found at the site. In total, the researchers used five different dating techniques.
In case you’re wondering, the Homo erectus bones could not be directly dated, as radiocarbon dating only works for organic materials younger than 60,000 years. That’s why archaeologists have to date the materials in which these older fossils are found, but different dating methods require different types of datable material.
“Our study directly dated mammal fossils from the bone bed and sediments from the river terraces,” said Ciochon. “The different dating methods also have different limitations, such as some produce maximum ages or minimum ages, while others produce age ranges.”
Two of the five dating techniques involved luminescence dating, in which dates are determined for light-sensitive buried materials, such as quartz. Westaway, who is one of the world’s leading experts on luminescence dating, said she and her colleagues were driven to get it right.
“There have been so many inconclusive and unsatisfying ages present for Ngandong that we were determined to finally nail the chronology,” Westaway told Gizmodo.
The results from the various dating methods were consistent, according to Ciochon, which allowed the team to combine them with a mathematical technique known as Bayesian modelling, resulting in the date range between 117,000 and 108,000 years old.
“This is a very solid piece of science,” said John Kappelman, an anthropologist at the University of Texas at Austin, in an email to Gizmodo. “Dating fossils is often tough work, and so the authors came at the question of the age of the site from several different directions.”
Kappelman, who wasn’t involved with the new research, was most impressed by how the authors focused on dating the regional landscape and the geology associated with the Solo River and its terraces.
“This study is an excellent example of what collaboration is all about,” he said. “No one person could have individually completed this study… the whole of the science is greater than the sum of the individual pieces.”
Westaway was excited by the new timeline for the Ngandong site, but she said the “age of the site was not surprising, as we all suspected it would be in this ball park due to the age of another site in Java called Punung.”
Indeed, the timing of the Punung site—a former rainforest—suggests it was climate change and the associated environmental shifts that contributed to the eventual extinction of H. erectus. These archaic humans had been living and thriving in an open woodland environment, but as the region transitioned to a hot and humid rainforest, the population of H. erectus suffered and could not adapt quickly enough. As Westaway pointed out, the new dates show a distinct overlap between the Punung and Ngandong sites.
“We suspected that the Ngandong population would have been wiped out… at the start of this environmental change, but the new timeline suggests that the Solo River valley containing the site of Ngandong probably persevered for longer than expected,” explained Westaway, who said these final stragglers found refuge in a slightly drier area at a high, inland location. Eventually however, even this “relic” population of H. erectus died out, putting an end to a reign that lasted roughly 2 million years.
As a striking demonstration of how climate change was affecting this population, the large assemblage of Ngandong fossils appear to have been the product of a mass death.
“A large flood caused the remains of Homo erectus and the other mammals found at Ngandong to be swept into the river,” Ciochon told Gizmodo. “The evidence indicates that [the Homo erectus individuals] died shortly before the flood, and that very little time passed between [their deaths] and the flood that transported the remains to Ngandong.”
Importantly, and as Ciochon pointed out, the new study provides the age of the last known appearance of H. erectus, which doesn’t necessarily speak to the timing of their extinction. Small groups may have lived longer without leaving fossil evidence, he said.
Kappelman said this presumed last stand of H. erectus can be “tested by dating other fossil sites, but Ngandong has been the best candidate for the most anatomically advanced form of this species for nearly 90 years.” By “most anatomically advanced,” he is referring to distinctive features of late H. erectus, such as their large skulls and long foreheads. “Time will tell as to whether or not other isolated Homo erectus populations hung on even longer. There are still lots of surprises out there, and if it was me, I’d focus on regions that were geographically isolated. Islands are a good place to start.”
As Westaway hinted earlier, the new dates also overlap with something else: the presence of other humans on Earth, namely Homo sapiens, Neanderthals, and Denisovans. Of these groups, the Denisovans were most likely to interact with H. erectus, as our species was still confined to Africa and Neanderthals did not venture south of Asia.
To be clear, no evidence exists to suggest H. erectus intermingled with Denisovans or any other hominin for that matter, but it now remains a tantalising—if difficult to prove—possibility.