140,000-year-old bones of our ancient ancestors found on sea floor, revealing secrets of extinct human species

Bones from an extinct human ancestor have been recovered from the seafloor, revealing a previously unknown Homo erectus population in Southeast Asia that may have interacted with more modern humans, new studies find.

The H. erectus bones were among a cache of more than 6,000 animal fossils hoovered up as part of a construction project off the island of Java in Indonesia. This is the first time scientists have seen fossils from the submerged parts of the Indonesian archipelago, which connected islands like Java to the Asian mainland during the last ice age, when sea levels were lower.

These lost lands, called drowned Sundaland, were once vast open plains interspersed with rivers around 140,000 years ago. The newly discovered fossils revealed the rivers were teeming with fish, turtles, river sharks, hippos and other marine life, while terrestrial giants such as elephants, the elephant-like Stegodon and water buffalo populated the plains, according to the studies.

H. erectus' presence on this landscape confirms that our ancient ancestor was taking advantage of drowned Sundaland's fertile hunting grounds, at least between Java and another, smaller island called Madura. This region, once a valley, is now submerged in a body of seawater called the Madura Strait.

The researchers found cut marks on some of the fossils that confirmed the Madura Strait hominins (humans and our close relatives) were hunting turtles — the earliest evidence of this in Southeast Asia — and large game. The remains also suggested that these hominins were selectively targeting cow-like bovids in their prime, which Indonesian H. erectus isn't known for. This hunting strategy is associated with more modern humans on the Asian mainland, raising the possibility that the newly discovered H. erectus population copied the strategy from other human relatives.

"The Madura Strait hominins may have developed this hunting strategy independently," study lead author Harold Berghuis, a researcher studying H. erectus at Leiden University in the Netherlands, told Live Science in an email. "But the other possibility is that we are looking at a kind of cultural exchange."

The researchers shared their findings in four separate studies published last week in the journal Quaternary Environments and Human.

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H. erectus is an important part of our evolutionary history. Emerging at least 2 million years ago, it was the first species to develop human-like body proportions and the first human species to migrate out of Africa, eventually finding its way to Southeast Asia. Other ancient humans followed in H. erectus' footsteps, but the extent to which these different species interacted in Southeast Asia is unknown, and the genetics of these different hominins is uncertain.

The human family tree is complicated, particularly in Southeast Asia. Berghuis noted that by around 350,000 years ago, H. erectus was being replaced on what is now the Asian mainland by a more modern human population, which included the mysterious Denisovans and the Neanderthals.

Fossil evidence suggests that H. erectus continued to survive on the island of Java until around 117,000 to 108,000 years ago, when the species eventually went extinct. Our species, H. sapiens, arrived in Southeast Asia around 77,000 years ago.

Fossil island dream

The latest fossil discovery was uncovered because of a large construction project in the Madura Strait. From 2014 to 2015, contractors pulled around 177 million cubic feet (5 million cubic meters) of sand and sandstones from the seabed near the port city of Surabaya to create an artificial island, according to one of the new studies published May 15.

To remove the sediment, the contractors used a trailing suction hopper dredger, which is a ship that drags a steel structure along the seabed, breaking up the sediment and mixing it with water. A suction pipe then hoovered up the sediment. This process, called dredging, can negatively impact marine life, but the amount of damage varies depending on the location and the species involved — invertebrates, eggs and larvae are most vulnerable to the practice. Berghuis noted that under Indonesian legislation, the dredging was subject to environmental impact assessments and supervision.

The hoovered-up sediment was discharged at a land reclamation site to create a 250-acre (100 hectare) sandy island. Berghuis had access to the site as a geotechnical consultant for the port of Surabaya, and spent many weeks searching on hands and knees for fossils. He told Live Science that he "dreamed" of finding a hominin fossil, but it wasn't until his very last day of collecting that he finally spotted one.

"It was already getting dark and I sat down to enjoy [the] sunset," Berghuis said. "And then, right beside me, lay this fossil that reminded me so much of the only Dutch Neanderthal. This is a well-known fossil in my country, dredged from the North Sea."

Berghuis took the fossil, a skull fragment, back to his hotel room and compared it with images of the famous Dutch Neanderthal. Its pronounced brow ridge was similar to Neanderthals and other ancient humans. Berghuis and his colleagues later determined it belonged to an adult or adolescent H. erectus.

The team also identified another H. erectus skull fragment in the fossils Berghuis recovered. Based on the thickness of the second fragment, the team determined this individual had not reached adulthood, according to one of the studies. The researchers couldn't determine how the individuals died.

Related: When did modern humans reach each of the 7 continents?

Land of dragons

The H. erectus discovery was just the tip of a mountain of findings documented in the new studies. Researchers identified 36 different species in a total of 6,372 recovered fossils. These included fossils of Komodo dragons (Varanus komodoensis).

Komodo dragons are giant lizards capable of (slowly) killing large animals, including water buffalo, with bacteria and venom-laced bites. Today, they are an endangered species restricted to a few Indonesian islands, but the new studies suggest they could have ruled the Sundaland plains.

"Komodo dragons may have been the most important predators," Berghuis said.