Fossils From Some of the Last Homo Erectus Hint at the End of the Long-Lived Species

Homo erectus was a very successful early human, spreading across the ancient world and surviving Earth’s changing environments for nearly two million years—at least five times longer than our own species has been around.

Now scientists may have pinpointed where and when Homo erectus made a final stand. The youngest known fossils of the long-lived species were identified on the Indonesian island of Java, where a dozen skulls found before World War II have finally been definitively dated to between 108,000 and 117,000 years ago.

Those dates mark the end of a long run. Homo erectus was the first known human species to evolve modern body proportions—including shorter arms and longer legs that indicate an upright walking lifestyle that permanently traded the trees for the ground. The close relative to Homo sapiens was also the first hominin known to leave Africa, and Homo erectus spread more widely than any other human species except our own. The fossils of H. erectus have been found in Western Asia (Georgia), Eastern Asia (China), and, thanks to a land bridge during a glaciated era of low sea levels, the islands of Indonesia, where the species persisted longest.

The new dates from Ngandong, Java, place the species’ end days in context. “When Homo erectus was living at Ngandong, Homo sapiens had already evolved in Africa, Neanderthals were evolving in Europe, and Homo heidelbergensis was evolving in Africa,” said co-author Russell Ciochon, a paleoanthropologist at the University of Iowa. “Basically, Homo erectus sits back there as the ancestor of all these later hominins.”

In a new study in Nature, Ciochon and colleagues have written what, at least for now, appears to be the hominin’s final chapter. “Of course it would be presumptuous for us to say we’ve dated the very last Homo erectus,” he says. “We’ve dated the very last evidence that we have, the last appearance of Homo erectus. We don’t know if on some neighboring island Homo erectus existed for a little longer after our date.”

The fossils in question have their own long and complex history. They were unearthed near the muddy banks of the Solo River in the early 1930s by a Dutch team that spotted an ancient rhino skull sticking out of the eroding sediments of a riverside terrace.

The bones puzzled scientists over the succeeding years. Along with thousands of animal remains, a dozen human skull caps were found, but just two lower bones, which made experts wonder how the skulls came to be isolated without their attending skeletons.

Because the bones were excavated nearly a century ago, it has been difficult to date them. The team tackled the problem by dating the wider geological context of the river system and the bone bed where the skulls were found, which sits some 20 meters above the current river thanks to thousands of years of erosion.

Ciochon and colleagues began excavations in 2008, launching the comprehensive study more than a decade in the making. “We’ve dated everything that was there, the river terraces, the fossils themselves, the bone bed, and the stalagmites that formed in the karst caves,” he says.

The geological work suggests that the dozen Homo erectus individuals died upriver and were washed downstream by monsoon flooding, then were caught in debris jams where the ancient river narrowed at Ngandong. At that spot, they were further buried by channels of flowing mud.

At least their skulls were. The research team also offers an explanation for why the rest of the Homo erectus skeletons went missing

“Where burials were in terraced deposits, once water eroded them out the skulls seemed to separate from the limb bones,” Ciochon says. “Limb bones are heavy and they dropped to the bottom of whatever water was moving them, but the skulls float. That may be why the skulls at Ngandong ended up separated from all but two of the long bones.”

Although most of the ancient skeletons were lost to the river, the skulls’ strange journey and fortunate discovery provided plenty of evidence for the team to examine.

“They’ve done some extensive excavations and geological studies, and they’ve done a tremendous job integrating a variety of dating techniques to show very tight age constraints for that fossil bed and by inference the last appearance of Homo erectus,” says Rick Potts, a paleoanthropologist and head of the Smithsonian’s Human Origins Program. “We have evidence for terrace formation, we have evidence for these flood deposits and rapid deposition, all the fauna is coming from that bed, and so it’s most likely that Homo erectus did, too.”

Homo erectus survived so long in present-day Indonesia that the species ended up sharing the planet with new groups of humans. Our own species is among these, but the new dates suggest that that we never lived side by side. Homo sapiens lived in Africa 117,000 years ago, but there’s no evidence they reached Java before about 73,000 years ago—at least 35,000 years after the last known Homo erectus died out. (African H. erectus are thought to have vanished some 500,000 years ago.)

What finally finished Homo erectus off after nearly two million years of survival? Ciochon and colleagues theorize that climate change played a role. The bone bed at Ngandong was also filled with animal remains, especially deer and the large bovid ancestors of water buffalo and Java’s banteng wild cattle. These large mammals thrived in open woodland ecosystems like the African homeland of Homo erectus.

“Ngandong was an open country habitat, with a little woodland, somewhat like the savannas of East Africa,” Ciochon said. “Then around 120,000 or 130,000 years ago, we know that there was a change in the climate, and this rainforest flora spread across Java. Homo erectus was not able to adapt. Other than Homo sapiens, no other early human was adapted to living in a rainforest.”

Though Homo erectus did finally fade away, it will always retain a prominent place on the family tree of human ancestors.

“Homo erectus is one of the iconic species in human evolutionary history,” Potts says. “It’s perhaps the most important species that indicates how branchy the human family tree is, because Homo erectus persisted through all of those other species, including eventually Homo sapiens, coming into being from earlier populations of Homo erectus.”

Though this branch of our ancestral tree survives only in the distant past, the dates of Homo erectus’ “last stand” show the species enjoyed a longevity that only we might match—if we can survive another 1.5 million years.

Brian Handwerk | READ MORE

Brian Handwerk is a science correspondent based in Amherst, New Hampshire.