Between 117,000 and 108,000 years the end was nigh for the hominins we call Homo erectus . They were the first of our ancestors to stand tall and walk upright, but even this ancient human species could not keep going. New research shows the last known members of their group met their fate in Central Java, Indonesia. So many years later, we are finally discovering more about the end of their kind and it is all thanks to a bone bed found by a river.

A Homo erectus Bone Bed

The Pleistocene epoch is the time Homo erectus called earth home. They evolved around two million years ago to change hominin history, but there has been some confusion about when they went extinct. Things began to change when a bone bed containing 12 Homo erectus skull caps and two lower leg bones was identified in the 1930s at a site above the Solo River in Ngandong, Central Java. A new study published in the journal Nature by Dr. Russell Ciochon of the University of Iowa, USA and Dr. Kira Westaway of Macquarie University, Sydney, Australia, says this is where the species known as Homo erectus took its last stand.

Palaeoanthropologist Russell L. Ciochon of the University of Iowa in Iowa City with a collection of Homo erectus fossil replicas from Ngandong. (Tim Schoon/University of Iowa)

But researchers had a problem. While the bones were identified as Homo erectus , the dates seemed to be in as much of a jumble as the fossils. Leading experts have been baffled and their dates for the bones range from 550,000 to 27,000 years ago. According to the Nature study, the confusion is due to the complex stratigraphy of the site. As Dr. Kira Westaway explained to Ancient Origins:

“As with most human evolution sites the biggest issue at Ngandong is timing. Without a convincing timeline this site has been shrouded in uncertainty and doubt. The interpretation of the fossils has always been clear as a younger Homo erectus - but the timing has huge implications. The age of the site tells us who they could have interacted with but more importantly the potential causes of extinction. If it was young then Homo erectus could have interacted with modern humans and could have been wiped out by competition, but if it was older than its more likely they could have interacted with Denisovans and the changing environmental conditions could have caused their extinction. Timing really is everything in this human story.”

A new approach was needed and Dr. Russell Ciochon, Dr. Westaway, and their colleagues created it. Dr. Westaway told Ancient Origins about the methods used in the study

“We were able to succeed where others did not because we tried a different approach. The key to this approach was being able to date the burial sediments. Instead of just focusing on the fossils themselves we used their place in the landscape system to constrain them on multiple levels - as a part of river deposits - in a former floodplain surface (terrace) - in a series of terraces as an uplifted section of the Kendeng Hills and within a newly created landscape by the much earlier diversion of the Solo River. Each stage constrains the one above and below by doing this we could see that a younger or older age for Ngandong is not possible.”

Solo River by Ngandong including exposed river terraces on far bank. (Copyright Kira Westaway)

Their reanalysis of the site and its surroundings show that the bone bed dates to between 117,000 and 108,000 years ago. These dates would mean that the Homo erectus population at Ngandong were the last of their kind and helps clarify which roles the ancient hominin species played in human evolution in that region.

Dr. Ciochon gave Ancient Origins some insight on the excavation site, stating:

“Our excavations at Ngandong were conducted in 2008 and 2010. Our most exciting discovery was finding the complete bovid scapula. The scapula is a fragile bone, and it is unusual to find one complete. Further excavations at Ngandong are not practical, because the only unexcavated area at this site is under a local road. There have been several other excavations in the region that have produced stone tools such as the Sembungan excavation that was presented in our paper. These excavations provide more evidence about Homo erectus on Java and demonstrate the need for further excavations of terrace deposits in the area.”

Excavations underway at Ngandong in 2010. (Copyright Russell L. Ciochon)

Bones Washed in After a Mysterious Mass Death Event

The researchers explain in their journal article that the Homo erectus fossils washed upriver of Ngandong as the environment changed from woodlands to a rainforest. They state that “the fossils are part of a mass death event,” and Dr. Ciochon named the possible cause of the extinction, as he told Ancient Origins:

“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. Our research indicates that Homo erectus likely went extinct due to climate change . 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.”

Study co-author Dr. Westaway concurred on the impact the environment may have had on the extinction of the Homo erectus population. She also provided Ancient Origins with information on what’s next in the team’s research:

“Now that we finally have a convincing timeline for the Ngandong site and for the last appearance of Homo erectus we can start to explore the potential cause/s of extinction. The new timeline for the Homo erectus last known appearance certainly occurs at a pivotal period of environmental and climatic change during the last interglacial (a warm phase ~120,000 years ago) - when rainforest corridors enticing rainforest fauna extended down through a connected Southeast Asia (due to the lowered sea level) at this time. The encroaching warmer and more humid environment would have caused widespread vegetational changes and caused problems for the essentially open woodland fauna associated with Ngandong. We suspected that the Ngandong population would have been wiped out at the cusp of the last interglacial at the start of this environmental change, but the new timeline suggests that the drier conditions in Solo River valley containing the site of Ngandong probably persevered for longer than expected, this supported a relic Ngandong population as a refugia in a slightly drier and more open environment afforded by its higher and in-land location. But by 117-108,ooo years ago this refugia could no longer be supported by the prevailing climatic and environmental changes and the Ngandong erectus and its associated fauna disappeared from the fossil record. As much of the fauna died within a very close timeframe it’s likely that the already stressed population was tipped over the edge by some large event - at present we have no proof of what that event was - this is a next area of research.”

Exposed bone bed in Facies A and C in Pit G09 from 2010 excavations at Ngandong. (Copyright Russell L. Ciochon)

Westaway expressed her excitement to face that challenge, telling Ancient Origins, “The cause of extinction is the next big step - this may be an even greater challenge than the timing!”

Homo erectus’ Possible Neighbors

The researchers’ Nature article mentions that Homo floresiensis and Homo luzonensis populations were living on outlying islands, close to the time Homo erectus was alive. Since the relationship between these three ancient hominin species is often questioned, Ancient Origins asked the co-authors of the current study to weigh-in on the debate.

Dr. Ciochon provides the following insight, “Based on the current evidence, there is enough morphological variation for H. luzonensis and H. floresiensis to be classified as separate species from H. erectus, but they are likely insular dwarfs of H. erectus. More fossil evidence and research is need to help clarify the relationships between H. luzonensis, H. floresiensis, and H. erectus.”

For her part, Dr. Westaway suggests:

“If Homo erectus was the ancestor to Homo floresiensis and Homo luzonensis then they would have diverged from the Homo erectus branch much earlier than 1 million years ago. Once separated on the evolutionary tree their fates played out on different islands with Homo floresiensis surviving the interglacial changes and out surviving Homo erectus by at least 50,000 years. This means that they both evolved in isolation for at least 1 million years. This type of endemic evolution creates a new species that is sufficiently different from the ancestor to remain as a species in its own right.”

This means that the researchers still have some difficult work ahead of them, but the hard work may pay off with even more exciting insight into a critical timeframe in human evolution.

Top Image: Representation of Homo Erectus. Source: crimson

By Alicia McDermott