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The banks of the Solo River, on the Indonesian island of Java, kept for years the fossils of the last known Homo erectus , the most recent and evolved of the first hominid species to walk fully upright. But, after the discovery of these important remains, which occurred between 1931 and 1933, it has yet to wait another almost 90 years to determine what time they lived.
The latest technologies, together with an interesting change of method, have finally allowed us to solve the puzzle: Homo erectus survived until 108,000 to 117,000 years ago in the Ngandong region (Central Java). The bad conditions of both the terrain and the bones had so far prevented reliable dating, which has been achieved after digging again to access the original site and thoroughly study the environment.
The population of Homo erectus to which the remains belong - 12 skulls and two pimples - survived thanks to finding refuge in the environment of the Solo River. The area remained relatively dry in the midst of climate change, which transformed the most open landscapes in which this species lived in humid and warm forests, which arrived on the island of Java 1.7 million years ago .
The Homo erectus of Ngandong are important not only because they are the last known specimens of their species, but also because they are the most anatomically advanced: their brains were larger and their fronts were wider, all of which indicates that, at some point, there was an important evolutionary change.
"The big question is: did this evolutionary change happen in isolation or was it the direct result of a mixture with another human species?" Asks Kira Westaway, one of the main authors of the study, published this week by the magazine. Nature The exact dating of the fossils, explains this researcher, will help answer this question, since it places the latest and most modern Homo erectus in a specific time frame, in which they might be able to mix with some species, but not with others.
"At this time, there is no evidence of Homo erectus and modern humans agreed, but there were modern humans in Africa at the same time that Homo erectus from Ngandong lived in Java," said Westaway, a researcher at the Macquarie University of Sydney. To the Indonesian island, however, modern humans did not arrive until 36,000 years ago, which "eliminates any possibility that (current) humans are direct descendants of Homo erectus .
"Really, timing is everything in this human story," Westaway summarizes. The last Homo erectus , in fact, lived at about the same time as the Homo floresiensis - known as Hobbits - of Indonesia and the newly discovered Homo luzonensis of the Philippines, and with both shared some anatomical features.
The three species, says Russell Ciochon, a researcher at the University of Iowa and another of the study's authors, "represent three evolutionary trajectories other than Homo on the islands of Southeast Asia, and all of them ended up becoming extinct ."
Previous studies had yielded extreme and contradictory dates about the time in which the Homo erectus of Ngandong lived: some dated too recently - 53,000 to 27,000 years ago - while others threw a much farther period - 147,000 to 500,000 years ago - . "After many unconvincing efforts to date the site, we knew we needed to try a different approach," Westaway explains.
The key, scrutinize the landscape
"The key to this approach has been to be able to date buried sediments. Instead of focusing only on fossils, in themselves, we have come to their place in the landscape." That is, scientists have considered bones as part of a larger puzzle , to see how they fit into a changing environment whose vestiges have been analyzed at different levels of sediment.
To do this, they had to date different levels of sediment deposits and limit, above and below, the dates to which the fossils should belong. "By doing this, we could see that an earlier or earlier date was not possible for Ngandong," adds Westaway, who also indicates that they had the latest infrared stimulated luminescence (IRSL) dating technologies, which was not yet available. when the investigation began, in the year 2008.
Finally, the scientists excavated the original site again, which gave them the definitive confirmation: "Finding the original bone bed was crucial for this study. That way, there could be no confusion or doubt about whether the material we dated was directly associated with the original discovery of Homo erectus, "concludes Yan Rizal of the Bandung Institute of Technology.
The hominid fossils show that they must have died at the same time as the fauna that surrounded them, indicating that a great catastrophic event that "swept this stressed and declining population" must occur, says Westaway. "At the moment, we don't know what this event consisted of."
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