The Coastal Crossing Theory proposes that the earliest human populations entered the Americas by following the Pacific coastline of Beringia and North America. This hypothesis suggests migrants used watercraft to navigate the coastal margin, often called the “kelp highway” due to its ecological productivity, rather than waiting for an inland passage to open. Evidence, such as the early date of the Monte Verde site in Chile, suggests humans were south of the ice sheets before the interior corridor was viable. However, direct proof of the coastal migration route remains elusive. The scientific challenges in verifying this theory are immense, stemming primarily from the geological and taphonomic consequences of the last Ice Age.
Submergence of the Pleistocene Shoreline
The most significant barrier to proving the Coastal Crossing Theory is that the ancient landscape of the migration route is now deep underwater. During the Last Glacial Maximum (LGM), vast amounts of water were locked in continental ice sheets, causing a global sea level drop of approximately 100 to 120 meters. This exposed a massive stretch of coastal plain and continental shelf, which served as the habitation and travel corridor for early migrants.
As the glaciers melted at the end of the Pleistocene epoch, the sea level rose rapidly, inundating this newly exposed land. This process was not always gradual; for instance, Meltwater Pulse 1A caused the sea level to rise by 16 to 25 meters in just a few centuries around 14,600 years ago. This swift marine transgression submerged the ancient coastal settlements and campsites that hold the direct evidence for the theory. The former shoreline, once a flat, accessible plain, is now covered by an immense volume of ocean water.
Archaeological sites are not only beneath a hundred meters of water but are also typically buried under thick layers of marine and terrigenous sediment. Over millennia, river runoff and ocean currents have deposited deep layers of silt, sand, and mud over the former land surface. This overburden effectively seals off potential sites, making them inaccessible to visual survey. It is also difficult to detect these sites even with advanced remote sensing technology. Consequently, the Pacific continental shelf is now one of the most inaccessible archaeological environments on the planet.
Poor Preservation of Coastal Archaeological Materials
Even if a submerged site is successfully located, the destructive nature of the marine environment severely limits the preservation of archaeological materials. The high-energy zone of the former coast, with its fluctuating water levels and strong wave action, promotes mechanical destruction. This constant movement of water and sediment causes “battering and rounding” on stone tools and bones, leading to the disintegration and dispersion of artifacts.
Furthermore, the chemical environment is often hostile to key organic and inorganic remains. Bone and shell, composed primarily of calcium carbonate, are highly susceptible to dissolution in acidic conditions. Although deep ocean water is generally alkaline, localized areas with high organic decay can become acidic enough to dissolve these materials. Submerged bone often develops a chalky surface texture and can be stained by iron oxides or sulfides, indicating significant chemical alteration.
Crucial evidence, such as watercraft, wooden tools, basketry, and clothing, are organic materials that decay rapidly. These items would definitively prove maritime adaptation. The high-energy, fluctuating water conditions of a former coastal zone are poor for long-term organic preservation, unlike the stable, anoxic conditions sometimes found in deep-sea environments. The destruction of these perishable items removes the most direct proof of the sophisticated maritime technology required for the coastal journey.
Difficulty in Identifying and Dating Diagnostic Evidence
The specific types of evidence needed to prove a coastal migration are inherently rare and difficult to verify once found. Diagnostic evidence would include specialized maritime toolkits, such as bone bipoints or gorges used for fishing, or evidence of processing marine resources. However, the earliest stone tools are often crude and generalized. This makes them difficult to distinguish from naturally fractured rock or “geofacts,” especially after being tumbled by wave action.
When organic materials are present, accurately determining their age is complicated by the Marine Reservoir Effect (MRE). Marine organisms incorporate carbon from the ocean that is significantly older than atmospheric carbon, due to the slow mixing of deep, carbon-14 depleted water with surface layers. This effect causes radiocarbon dates from marine samples to appear hundreds of years older than their true age, typically by about 400 years. In Arctic regions, this discrepancy can potentially exceed a thousand years.
Correcting for the MRE requires a precise, regional correction value, which is extremely challenging to establish for a submerged, ancient coastline where the local oceanographic conditions are unknown. Additionally, organic material can be contaminated by modern carbon from seawater or older carbon from geological sediments, further skewing the results. This combination of ambiguous artifacts and unreliable dating makes it nearly impossible to conclusively verify a site’s age and cultural origin.
Logistical and Financial Constraints of Submerged Research
The practical hurdles of conducting research over the vast, submerged continental shelf severely limit the frequency and scope of archaeological investigation. Locating potential sites first requires extensive, expensive remote sensing surveys using specialized equipment. This technology includes multibeam sonar to map seafloor topography and sub-bottom profilers to penetrate sediment layers. All these surveys require dedicated, large research vessels.
Once a potential target is identified, Remotely Operated Vehicles (ROVs) or Autonomous Underwater Vehicles (AUVs) are deployed for close-up visual inspection and documentation. The advanced, deep-rated models needed for this research often exceed $50,000, with some professional systems costing over $200,000. Furthermore, the operational cost of deploying a large research vessel with a full crew and specialized equipment can run into tens of thousands of dollars per day.
The sheer scale of the area that needs to be surveyed is another prohibitive factor, as thousands of miles of former coastline must be systematically searched. This expense and logistical complexity limit deep-sea archaeological expeditions to infrequent, short-duration projects. Consequently, the rate at which data can be collected to prove or disprove the Coastal Crossing Theory is extremely slow. This ensures that the critical evidence remains largely hidden beneath the waves.