Researchers have discovered the youngest fossils of Homo erectus in Central Java, Indonesia, an ancient human species that went extinct before modern humans evolved. The researchers say that their findings confirm when the species went extinct.
The study published Wednesday in the journal Nature.
The fossils were found at the Ngandong site, where climate change likely signaled the end for Homo erectus. The Ngandong site was first excavated in the 1930s by a Dutch team, recovering more than 25,000 fossils in a bone bed -- 14 of which belonged to Homo erectus, including 12 skull caps and two lower leg bones. The majority of fossils belonged to animals.
Previous studies from the site produced different timeline results, leading to confusion over when the species met its end. But new and improved dating techniques by the researchers helped them determine dating for the bone bed itself, as well as the animal fossils contained in the site.
"This confirms that Ngandong is the youngest Homo erectus site found anywhere in the world," said Russell Ciochon, study author and paleoanthropologist at the University of Iowa. "We have ended a long controversy over the age of this important site in human evolution."
The research project to understand when Homo erectus died out began in 2006. Ciochon's research team combined forces with geochronologist and quaternary scientist Kira Westaway's team also working at the site.
Long history
Indonesia has a long history with Homo erectus fossils. Homo erectus was able to reach Java because it was connected to mainland Asia by a land bridge when sea levels were low during glacial periods, the researchers said. The first fossil was found at another site named Trinil in 1891. Around 200 have been found across Java since then. The oldest Homo erectus fossils on Java date to 1.7 million years ago. Determining the age of the youngest fossils shows how long the species endured before dying out.
"Homo erectus was an incredibly long-lived species with a massive geographic distribution which makes it one of the most successful hominins that ever lived," Ciochon said.
They were the oldest early humans to have body proportions similar to modern humans, including an expanded brain case. But the Ngandong fossils take it a step further.
"The Ngandong Homo erectus fossils have the largest cranial capacity of any Homo erectus fossils," Ciochon said. "But without additional evidence for behavior, we are unable to say that they were smarter than other Homo erectus groups. Due to the large brain size, Ngandong Homo erectus is referred to as the most derived, advanced, Homo erectus."
But the site tells another story of how even the most successful species can end.
"Our research indicates that Homo erectus likely went extinct due to climate change," Ciochon said. "Homo erectus was found with a collection of animal fossils that lived in an open woodland environment similar to the environment in Africa where it evolved. The environment at Ngandong changed, and the open woodland was replaced by a rainforest. No Homo erectus fossils are found after the environment changed, so Homo erectus likely was unable to adapt to this new rainforest environment."
Mass death
Another intriguing factor to the site is the fact that the fossils discovered came from a mass death event that occurred upstream. A flood washed the remains to the site, where they were found. Unfortunately, the animal fossils recovered during the Dutch excavation were lost, so the true diversity of animals from the site is unknown.
The researchers don't know what caused the mass death, but given that the site is associated with climate change, theories abound including a mudslide triggered by a volcano, Ciochon said.
During the glacial and interglacial periods that allowed the land bridge to be exposed or covered, a rainforest crept in, replacing the natural habitat of both Homo erectus and the animals in the area.
Modern humans are the only hominins that have been found in a rainforest environment, the researchers said, and that's likely due to the fact that we can adapt to live there.
"They might not have been able to find food sources they normally ate, or they might have been more vulnerable to the predators in the rainforest," Ciochon said.
This finding changes our perspective of both Homo erectus and human evolution, the researchers said.
"Now that we have a convincing timeline for the last known appearance of Homo erectus, we can start to understand where they sit on the evolutionary tree, who they interacted with, and start to explore the potential cause of extinction," Westaway said.
New possibilities
The new timeline helps establish that Homo erectus couldn't overlap with modern humans because they didn't arrive until 35,000 years ago on Java. But it opens up new possibilities for interactions with other species: the Denisovans, a mysterious ancient human ancestor known only from a few bones.
"This is a human species more known for its genetic make up rather than actual fossils, but we suspect that this species roamed as far as Southeast Asia and may have interacted with the Ngandong Homo erectus," Westaway said. "This is yet to be proven but the possibility of intermixing with the Denisovans is an exciting prospect well worth exploring."
The authors cautioned that exact dates for extinction are hard to prove.
"Our work provides the age of the last know appearance of Homo erectus, but this does not mean that it is the age of extinction," Ciochon said. "Small groups of Homo erectus may have lived longer without leaving fossil evidence."
But having an idea of when Homo erectus went extinct in a specific area is vital for understanding how they died out -- a challenge Westaway and her team want to solve.
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2019-12-19 07:18:00Z
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