Inner Workings: Fishing for artifacts beneath the waves

Fisherman Mike Anderson has accidentally hauled up all manner of oddities in his fishing gear—bits of pottery, lumps of coal, even a silver spoon. But he’d never seen anything like the heavy, jagged block pulled up by his scallop dredge as he cruised near the New England coast in the spring of 2013.

The research on Anderson's find is among the latest efforts to uncover prehistoric homelands drowned by rising seas. Some archaeologists are mapping submerged terrain, trying to resurrect the rivers and estuaries that would have attracted humans eons ago. Others are sifting seafloor sediments for DNA, pollen, and similar clues to the ecology of swamped landscapes. For all, the ultimate prize would be an actual archaeological site, perhaps a Neolithic village or an early trash heap.
Proponents of underwater archaeology note that humans have always favored seaside real estate, with its abundant food sources and temperate climate. Ancient humans, after all, both migrated and settled along coasts. Looking beneath the waves may be essential to grasping the early history of Homo sapiens. "If we want to understand how America was colonized, if we want to understand how northwest Europe worked, how rice agriculture originated . . . you've got to understand what's happening on these coastal shelves," says landscape archaeologist Vincent Gaffney of the University of Bradford in Britain. Even so, the endeavor is costly, and archaeological sites are elusive. But if the risks are great, so too are the potential rewards.

Into the Deep
By peering into the watery expanse off New England, researchers may have revealed more than a mammoth's missing molar. The Paleoindians who lived in the northeastern United States at the time are thought to have hunted caribou, not primitive elephants. But the new finds highlight the possibility that these early Americans were more eclectic in their diet, according to the study of Anderson's haul. "A mammoth or mastodon could feed a whole village," says coauthor Stefan Claesson, a maritime archaeologist at the Institute of Maritime History based in Tall Timbers, MD. The study doesn't show whether Pleistocene humans hunted mammoth, but "there is a landscape preserved [underwater] that might be able to tell us." Claesson would like to conduct a targeted search of the sea floor in an area where ancient stream and river banks seem to have survived intact, though so far no funding has come through.
On the other side of the continent, researchers are engaged in an ambitious effort to chart lands engulfed by the Pacific Ocean since the depths of the last Ice Age. Many archaeologists think the first humans to populate the Americas took a coastal route, moving down the edge of the continent via boat. Evidence to support that theory has been scant, perhaps because of rising seas. "The sites of the earliest migration may be underwater," says archaeologist Todd Braje of San Diego State University.
Thanks to more than $1.2 million in funding from the US Bureau of Ocean Energy Management and the National Oceanic and Atmospheric Administration, Braje and his colleagues are deploying sonar to reveal ancient landscapes buried by sediment and inundated by saltwater. Their efforts have focused on a patch near the Channel Islands, west of Los Angeles, and a separate sector off the central Oregon coast.
Sonar surveys in 2017 have already picked out features that might be buried river channels and floodplains, says archaeologist Loren Davis of Oregon State University, who heads the Oregon survey effort. The next step is to extract plugs of sediment, known as cores, from some of those features to double-check the sonar results. The California team has also seen several shadowy features that could be piles of mollusk shells, discarded at a prehistoric waterway.
The search for direct evidence of ancient peoples on the sea floor is like trying to find "a needle in a few haystacks," Davis concedes. But he is undaunted. "Once we know what those landscapes look like, it's only a matter of time before we find those sites."

Controversy at Sea
If those sites are found, they can yield stunning results. But such findings often spur skepticism, as might be expected for an approach still getting its sea legs.
Just off Britain's southern coast, sediment cores recently served up revelations from a drowned countryside at a prehistoric site called Bouldnor Cliff. Today it lies 16 meters underwater, but 8,000 years ago it was an encampment set amid an oak forest. When researchers analyzed a sediment core from Bouldnor, they found DNA from a completely unexpected source: wheat, a result published in 2015 (3).
Most researchers had thought farming-and therefore wheat-didn't appear in Britain until several thousand years later. A lack of wheat pollen in the sediment bolstered the idea that the grain was imported rather than grown locally. Perhaps, the authors hypothesize, the Bouldnor wheat was imported by boat from France or Spain.
But shortly after the study's publication, Science published a commentary calling the date of the wheat into question (4). In response, the Bouldnor team examined a twig from the same sediments as the wheat DNA and found that it dated to roughly 8,000 years ago, confirming the team's original results (5).
Then, a few months after the commentary, an article in eLife questioned the authenticity of the Bouldnor DNA (6). The wheat DNA does not bear the classic ravages of old age, suggesting it was a modern contaminant, the article said. That set off a new round of research by the Bouldnor team.
Additional experiments have since shown that the marine environment's stable temperature and salt actually preserve DNA, says evolutionary geneticist Robin Allaby of the University of Warwick in Britain, one of the authors of the original Science article. As a result, ancient DNA from undersea sediments is more pristine than ancient DNA extracted from other environments, notes Allaby, who hopes to publish his results in the coming months. A separate analysis of how environmental conditions affect ancient DNA also shows the Bouldnor DNA shouldn't suffer as much degradation as critics claim, Allaby says (7).

Beating the Odds
The challenge of assessing submarine evidence is matched by the challenge of getting it in the first place. Researchers who want to map underwater landscapes ideally have access to two important assets: a large, sophisticated ship and a tolerance for large, complicated snafus. Storms can halt research, and expensive equipment can end up on the seafloor. Earlier this year, Davis and his team had collected data for less than an hour when their ship's steering broke, forcing them to return to port. After a repair, the researchers returned to sea and got the data they sought. Garnering details about the ocean bottom requires complex technology, and "every single thing has a chance to go wrong," Davis says. "When they don't, it's amazing." Even assuming perfect weather, ample funding, and flawless equipment, finding an actual archaeological site on the sea floor will always be a long shot. Many prehistoric human populations were small, their footprint on the land light.
Even so, underwater archaeology has yielded sites important for understanding humanity's march across the globe, says archaeologist John Ives of the University of Alberta. He favors an approach that focuses on landscape features of the appropriate age. But it's still not clear "how effective that can be as a strategy," Ives adds. Finding evidence of ancient peoples "is hard enough on dry land, and it's incredibly difficult to do underwater," says Jonathan Lothrop, curator of archaeology at the New York State Museum.
The odds have not deterred those convinced that scouring the sea floor is essential for understanding human history. Gaffney, Allaby, and their colleagues, for instance, are embarking on extensive analysis of more than 80 sediment samples off Britain's east coast. Now covered by the North Sea, the region was once an unbroken plain stretching all the way to continental Europe. Perhaps hunter-gatherers here were dabbling in the crops and domesticated animals used by neighboring farmers. The researchers will sift the cores for DNA, pollen, beetles, and more, hoping for a prize such as the DNA of sheep or goats. These domesticated animals are not native to Britain, so their presence would point to the arrival of agriculture.
Meanwhile, since 2015, archaeologist Jon Adams of the University of Southampton in Britain and his colleagues have been crisscrossing the Black Sea off the Bulgarian coast, using sonar to detect ancient shorelines and dunes. Having already uncovered the apparent remnants of an ancient shoreline 110 meters deep, they hope to use dating material from sediment cores to determine whether the sea rose gradually or, as in the so-called deluge model, in a rush.
Finding evidence at the bottom of the sea is "so difficult . . . that people have found it convenient not to bother with it," Gaffney says. But "somewhere out there is the transition between farming and huntergathering," he adds. "It has to be out there."