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."
The end of an Ice Age
Now that paleoanthropologists have a more reliable date for something that looks like the "last stand" of Homo erectus, it's easier for them to start working on hypotheses about why the species went extinct. And Ciochon and his colleagues already have a culprit in mind: climate change.
"The new timeline for the Homo erectus extinction occurred at a pivotal period of environmental and climatic change during the last interglacial, when the climate became warmer and wetter in between ice ages," Westaway told Ars. Since their arrival on Java around 1.5 million years ago, Homo erectus had lived in a landscape of open woodland—the perfect sort of place for hunter-gatherers to make a living. But by the time the unlucky Ngandong individuals died and were washed downstream in a flood, the world around them was changing. A long ice age, which had kept much of the world relatively cool and dry, was coming to an end.
As conditions got warmer and wetter, corridors of rainforest stretched across Southeast Asia, displacing the plants and animals Homo erectus had lived alongside (and eaten) for millennia. In fact, Ngandong is about the same age as layers from another site on Java, called Punung, where excavations have found the bones of animals like orangutans. This suggests that parts of the island were already covered in rainforest—a much harder place for hunter-gatherers to thrive.
Westaway told Ars that, thanks to its higher altitude and inland location, the Solo River Valley probably acted as a land refuge for Homo erectus and the open woodland ecosystem they called home. But by 117,000 to 108,000 years ago, change had overwhelmed the valley and its inhabitants, too. "I would be very surprised to discover that Homo erectus managed to survive throughout the interglacial phase on Java—especially after the evidence we found at Ngandong and the scale on which the Ngandong population were wiped out," Westaway told Ars.
Paleoanthropologists in several regions of the world have suggested that Homo erectus may have overlapped in space and time with our species. Westaway, on the other hand, says that if a later set of fossils turns up, they probably won't be on Java.
What happened here?
Excavations at nearby sites have found stone tools old enough to have belonged to Homo erectus, which shows that hominins lived in the area. Ciochon and his colleagues suggest that, not very long after these Homo erectus died, a flood swept down the Solo River valley, washing bones and other debris downstream. When the river narrowed and rounded a bend at what is now Ngandong, it slowed enough to drop the bones, and more flooding quickly buried the remains in a layer of mud. Today, the bone bed is buried in a layer of sediment about 20 meters (65.6 feet) above the modern river.
Ciochon told Ars that a flood would explain the types of sediment at Ngandong, along with the decades-old mystery of why the site contained a bunch of skull caps and not many other body parts. Floodwater would tend to sort bones by weight and buoyancy as it carried them along.
Since the fossils were unearthed in the 1930s, paleoanthropologists had debated their age. In the late 1990s, a group of researchers dated fossilized mammal bones from the same bone bed to between 53,000 and 27,000 years old. The researchers used a method called uranium-series dating, which measures the ratio of uranium to thorium in a bone or sediment sample. Another study dated the Homo erectus fossils themselves to around 40,000 years old. Those dates were recent enough that Homo erectus on Java could have lived alongside members of our species. But studies of sediment from the site, along with other sites downstream, suggested much older dates, ranging from 60,000 years to 546,000 years old.
Ciochon and his colleagues' results are quite a bit older than the youngest suggested dates from earlier studies, and it totally rules out the older dates. They knew that the Homo erectus fossils couldn't be older than 316,000 years old—before then, the terraces of the Solo River valley (which reflect the level of past floodplains) hadn't started forming, according to radiometric dating of the sediment layers. And the Ngandong terrace formed between 140,000 and 92,000 years ago, according to the same radiometric dating, so the fossils probably weren't any later than that.
Uranium
And when Ciochon and his colleagues uranium-series dated bones and teeth from ancient cattle, unearthed from the same sediment layer as the Homo erectus bones, the remains dated to between 134,000 and 106,000 years old. So those dates line up. Statistical modeling narrowed the age of the bone bed down further, to between 117,000 and 108,000 years old.
"We believe that our bone-bed chronology is much closer to the actual age of Ngandong Homo erectus," wrote Ciochon and his colleagues. And that's science. These new findings may open up more debate about the fossils' age, but eventually, the evidence will tell the tale. And ultimately, understanding how the Homo erectus branch of the family tree evolved could help us better understand our own origins, too.
Nature, 2019. DOI: 10.1038/s41586-019-1863-2 (About DOIs).
