The first archeologists to find strange stone artifacts on Naxos were French researchers working on the Greek island in 1981.

Naxos, the largest in a cluster known as the Cyclades that dot the Aegean Sea, is rich in the type of archeology many would recognize from classical exhibits in museums: 5,000-year-old, beautifully proportioned white marble figurines; 3,000-year-old, strikingly patterned pottery vessels.

These scrappy pieces of rock looked much, much older.

“The stone tools they were finding on the site looked nothing like the stone tools that had ever been found before on prehistoric sites in the Cycladic Islands,” said Tristan Carter, an archeologist at McMaster University in Hamilton.

Archeologists have long believed that the first people to colonize the region were early farmers who arrived by boat approximately 9,000 years ago. Only humans who had made the leap from a hunter-gatherer subsistence to organized agriculture — a major revolution in the history of our species, one that saw a lurch forward in technological and social complexity — could have accomplished the sea crossing.

But the stone tools on Naxos appeared to be hewn by Paleolithic people — much more ancient humans, perhaps not members of our species at all.

Since 2013, Carter has co-directed a new round of investigations on Naxos. He and a handful of others working in the region have begun to furnish evidence that humans reached the islands of the Aegean Sea 250,000 years ago and maybe earlier. If those dates are confirmed, it means the first people there were Neanderthals, their probable ancestors, Homo heidelbergensis or maybe even Homo erectus.

Could these archaic hominins have travelled by boat?

Sea voyaging is supposed to be an indicator of “behavioural modernity,” the suite of capabilities that distinguishes us from our primitive, now-extinct human cousins and from primates. A crossing of any major distance requires the tool-making and co-operation necessary to build a craft, the navigational savvy to pilot it and, perhaps most significantly, the imagination and daring to conceive of the journey in the first place.

Other researchers insist that much better evidence needs to be discovered to attribute such complex behaviours to Neanderthals and other hominins: a rewriting of a significant chapter in the evolution of our genus.

“If things like erectus were deliberately zooming around bodies of water 1.5 million years ago, that would be an absolutely massive deal for overturning our received understanding for how these creatures think and behave,” said Tom Leppard, Renfrew fellow in archeology at the University of Cambridge, and a cautious voice in the debate.

In recent years, many of these behavioural firewalls have crumbled. Archeologists have found indications, some still disputed, that Neanderthals carved cave symbols, painted their bodies with pigment, created musical instruments and jewelry, and intentionally buried their dead — all practices thought to be exclusive to us. Most stunningly, scientists have shown that living people with European or Asian ancestry carry a small amount of Neanderthal DNA, evidence of prehistoric interbreeding.

“Research over the last couple decades has just perpetually shown us that all of these characters are more capable, more complex that we thought,” Carter said. “It’s all about the archeology of ego in us.”

Still, he is the first to say that with the stakes so high, his team must be incredibly careful in how they work and what they claim: “There’s going to be a very high bar for us.”

The French team on Naxos was not the first working on Aegean and Mediterranean islands to report artifacts older than the Neolithic, as the era that began with the adoption of agriculture is known. But the older discoveries were piecemeal and never stood up to scrutiny.

Then, in 1988, archeologists began excavating a collapsed rock shelter on the southern shore of Cyprus. They found about 1,000 bladelets and small tools typically associated with pre-Neolithic people.

“There was a lot of skepticism at first,” said Alan Simmons, an anthropologist at the University of Nevada Las Vegas who was involved in the work. “But once we had all the radiocarbon dates, it came to be accepted.”

The site pushed the peopling of Cyprus back to 12,000 years ago — only a few millennia, but enough to break the Neolithic barrier and establish the presence of hunter-gatherers. Today, the distance to mainland Turkey is about 75 kilometres. Sea levels have fluctuated and the crossing was once shorter, but Cyprus has always been an island.

The discoveries on Cyprus overturned the idea that hunter-gatherers were incapable or unwilling to travel by sea. But the debate was still confined to the activities of our species, Homo sapiens.

In 2008, a Greek-American team of archeologists began searching on the southwest coast of Crete for pre-Neolithic artifacts. They found many from roughly the same era as those on Cyprus. But they also found rough quartz hand axes and cleavers that appeared to be much more ancient.

The team discovered artifacts eroding out of a layer of soil that dated to at least 130,000 years ago, and the tools themselves looked like those archeologists associate with archaic hominin sites on the mainland — ones that are at least 250,000 years old.

“That put the cat amongst the pigeons,” Carter said. Like Cyprus, Crete has always been an island.

Archaeology magazine declared the finds one of its top 10 discoveries for 2010.

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The discoveries on Crete are still roiling the field, with critics arguing the researchers’ conclusions have leapt well ahead of what they actually found. Because most of the tools were discovered on the surface, rather than buried in layers of sediment, the artifacts have been difficult to date definitively.

“It’s not enough. They have to find better pieces, more pieces,” said Albert Ammerman, an archeologist who worked on Cyprus.

Better and more pieces is exactly what Tristan Carter’s team is after. Even for those who can envision seafaring hominins, it takes more than one site to overturn a long-standing paradigm.

Two years ago, Carter and project co-director Demetris Athanassoulis obtained permission to excavate the formerly French site on Naxos, known as Stelida.

“I had always had a sneaking feeling, having visited Stelida a few times during my PhD many moons ago, that maybe this site fit into these new discoveries.”

The site is challenging. The researchers believe it was a quarry, where people came to retrieve the material to make stone tools for hundreds of thousands of years.

“It’s like going to a factory and only seeing the stuff they threw out,” Carter said.

After years of work, Carter’s team has found loads of artifacts, including distinctive tool types that on mainland Greece have been exclusively associated with Neanderthals and Homo heidelbergensis and could be more than 250,000 years old.

But that won’t be enough to convince the skeptics: they need artifacts buried in clearly defined layers of soil that can be accurately dated.

“A lot of people are going, “Aha! You have proven you have Neanderthal seafarers,’” Carter said. “We’ve always said, first of all, we’ve got to wait until we’ve got really, really good scientific dates.”

Carter is relying on a method known as optically stimulated luminescence. Unlike radiocarbon dating, optically stimulated luminescence works on extremely old soil deposits, but takes months to process. Early lab results have dated some of the Stelida artifacts to at least 50,000 years ago. But the team is still waiting for results from the lower layers of the site, where the pre-Neanderthal-looking tools were found.

Solid dates are important to estimate sea levels. Unlike Crete and Cyprus, Naxos is not a “true” island: when sea levels fell, it was connected to its neighbours to form one mega-Cycladic island. The region’s active tectonics don’t help: modellers disagree on whether that Cycladic land mass was connected to the mainland at times, allowing hominins to reach what is now Naxos by foot.

Discoveries half a world away from the Aegean complicate the picture. In 2003, archeologists on the Indonesian island of Flores found bones belonging to a hobbit-like species of human that stood only four feet tall and survived until 18,000 years ago, called Homo floresiensis. Stone tools later discovered on the island have been dated to as far back as 1.1 million years ago.

To some researchers, this is clear evidence that the origins of seafaring began well before Homo sapiens. But others argue that Flores may have been a short hop from the mainland. A few haphazard jaunts in Indonesia and the Aegean is not evidence of deliberate, established sea voyaging. That would explain why Paleolithic artifacts in the Aegean are so scant: millennia of human habitation should have left a more indelible mark.

But others believe that archeologists just haven’t been looking until now.

Because most working in the region are specialists in the cultures that produced those exquisite ivory figures and pottery, “a lot of the archeologists really weren’t trained to look for scatters of broken rocks, basically, because it wasn’t very exciting,” Alan Simmons said.

The artifacts may not look very exciting, but the implications are. If archaic hominins could reach the Aegean islands by boat, where else might we find them?

“We’ve always looked at the sea as a barrier,” Simmons said. “But what some people are arguing is maybe it wasn’t a barrier — maybe it was a highway.”