(Inside Science) -- The earliest undisputed ancestor of modern humans, Homo erectus, likely survived up to at least 117,000 years ago, before going extinct when the environment in its last refuge changed from woodland to rainforest, a new study suggests.
H. erectus appeared in Africa about 2 million years ago and was the first known human species to leave the continent, reaching the Indonesian island of Java more than 1.5 million years ago. In contrast, the oldest known specimens of our species, H. sapiens, date back only about 300,000 years, and the earliest signs of modern humans leaving Africa date back roughly 194,000 years.
Scientists have long disputed how long H. erectus lasted. Previous work suggested the latest known fossils of the species were a dozen skull caps and two shinbones found in the 1930s near the village of Ngandong in central Java. The age of the Ngandong fossils had proved difficult to determine -- with estimates ranging from between 27,000 to 550,000 years old -- because multiple groups have conducted excavations at the site over the past 90 years. This sowed confusion as to whether scientists were dating samples from the fossil bed or material from elsewhere in the area that had contaminated the site.
The oldest evidence of modern humans in this region date to between 63,000 and 73,000 years ago from the neighboring island of Sumatra -- if both species overlapped in time, that would raise the possibility that our species might have driven our ancient cousins extinct.
In the new study, the researchers carefully pinpointed the ages of multiple layers throughout the entire Ngandong site using modern dating techniques. They found the fossils dated between 108,000 and 117,000 years old. "H. erectus did not survive late enough to interact with modern humans on Java," said study co-author Russell Ciochon, a paleoanthropologist the University of Iowa in Iowa City.
"Our research indicates that H. erectus likely went extinct due to climate change," Ciochon added. "H. 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 H. erectus fossils are found after the environment changed, so H. erectus likely was unable to adapt to this new rainforest environment."
However, these findings suggest H. erectus may have overlapped with a different human lineage -- the mysterious Denisovans, an extinct group currently known only from fossils unearthed in Siberia and the Tibetan plateau but whose genetic influence has been seen as far away as New Guinea. The possibility that Denisovans interbred with H. erectus, potentially explaining some archaic DNA seen in their genes, "is an exciting prospect well worth exploring," said study co-author Kira Westaway, a geochronologist at Macquarie University in Sydney, Australia.
The scientists detailed their findings in the Dec. 19 issue of the journal Nature.