They discover remains of those who were the last of the first humans to walk upright

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(CNN) - About 108,000 to 117,000 years ago, the first humans to walk upright stood up for the last time.

Researchers have discovered the youngest fossils of Homo erectus in Central Java, Indonesia. It is an ancient human species that became extinct before modern humans evolved. The researchers say their findings confirm when the species became extinct.

The study was published Wednesday in the journal Nature.

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The fossils were found at the Ngandong site, where climate change probably marked the end of 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 upper parts of skulls and two lower bones of the legs. Most fossils belonged to animals.

Previous studies of the site produced different results in the timeline, which generated confusion about when the species came to an end. But the researchers' new and improved dating techniques helped them determine the dating of the bone bed, as well as the fossils of animals contained in the site.

"This confirms that Ngandong is the youngest site of Homo erectus found anywhere in the world," said Russell Ciochon, author of the study 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 was extinguished began in 2006. The Ciochon research team combined forces with the team of geo-scientist and quaternary scientist Kira Westaway who also worked on the site.

A long story

Indonesia has a long history with fossils of Homo erectus. The 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 called Trinil in 1891. Since then about 200 have been found in Java. The oldest fossils of Homo erectus in Java date back 1.7 million years ago. Determining the age of the youngest fossils shows how long the species endured before disappearing.

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"Homo erectus was an incredibly long-lived species with a massive geographic distribution that makes it one of the most successful hominids ever," said Ciochon.

They were the first oldest humans to have body proportions similar to those of modern humans, including an expanded cavity for the brain. But Ngandong fossils go one step further.

"Ngandong Homo erectus fossils have the highest cranial capacity of any Homo erectus fossil," said Ciochon.

“But without additional evidence of behavior, we cannot say that they were more intelligent than other groups of Homo erectus. Because of the large size of the brain, Homo erectus from Ngandong is known as the most derived and advanced Homo erectus. ”

But the site tells another story of how even the most successful species can end.

"Our research indicates that Homo erectus probably became extinct due to climate change," said Ciochon. “Homo erectus was found next to a collection of fossils of animals that lived in an open forest environment similar to the environment in Africa where it evolved. The environment in Ngandong changed, and the open forest was replaced by a rainforest. No Homo erectus fossils were found after the environment changed, so Homo erectus probably could not adapt to this new rainforest environment. ”

Mass death

Another intriguing factor of the site is the fact that the fossils discovered come from a massive death event that occurred upstream. A flood dragged the remains to the site, where they were found. Unfortunately, the fossils of animals recovered during the Dutch excavation were lost, so the true diversity of animals on the site is unknown.

Researchers do not know what caused the mass death, but since the site is associated with climate change, theories abound that include a landslide caused by a volcano, Ciochon said.

During the glacial and interglacial periods that allowed the land bridge to be exposed or covered, a tropical rainforest dominated, replacing the natural habitat of both Homo erectus and animals in the area.

Modern humans are the only hominids that have been found in a rainforest environment, the researchers said, and it is probably due to the fact that we can adapt to live there.

"They may not have been able to find the food sources they normally ate, or they could have been more vulnerable to predators in the jungle," Ciochon said.

This finding changes our perspective of both Homo erectus and human evolution, the researchers said.

View of Lawu volcano from the Solo river valley.

"Now that we have a compelling timeline for the last known appearance of Homo erectus, we can begin to understand where they sit in the evolutionary tree, with whom they interacted and begin to explore the possible cause of extinction," said Westaway.

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New possibilities

The new timeline helps establish that Homo erectus could not overlap with modern humans because they did not reach Java until 35,000 years ago. But it opens up new possibilities for interactions with other species: the Denisovans, a mysterious ancient human ancestor known only to a few bones.

"This is a human species best known for its genetic composition rather than real fossils, but we suspect that this species roamed as far as Southeast Asia and may have interacted with the Ngandong Homo erectus," said Westaway. "This has not yet been proven, but the possibility of mixing with the Denisovans is an exciting perspective worth exploring."

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The authors warned that the exact dates of extinction are difficult to prove

"Our work provides the age of the last appearance of Homo erectus, but this does not mean that it is the era of extinction," said Ciochon. "Small groups of Homo erectus may have lived longer without leaving fossil evidence."

But having an idea of ​​when Homo erectus became extinct in a specific area is vital to understand how they became extinct, a challenge that Westaway and his team want to solve.

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