In 2019, a survey crew mapping a dried lake bed in New Mexico noticed faint depressions in the cracked mud that resembled human footprints. The team initially assumed the prints were recent, left by hikers days earlier. Radiocarbon and pollen dating later placed them at roughly 23,000 years old, thousands of years before humans were believed to have been present in that region.
The site is White Sands, New Mexico, located on the bed of Lake Otero, which dried up thousands of years ago. Because tracks in this environment typically fade within two or three years of being exposed, researchers used ground-penetrating radar and magnetometers to map buried features before excavation began.
For roughly two decades before this discovery, the leading explanation for how humans reached the Americas was known as the coastal migration route, sometimes called the kelp highway. This theory proposed that people moved from Siberia along the Pacific coast, traveling by boat and on foot, and arrived south of the ice sheets around 16,000 years ago. Marine archaeologist John Erlandson coined the term in 2007, and by the 2020s it had become the standard explanation in textbooks and museums.
Starting in 2019, researchers excavated hundreds of footprints along the old lake shore, forming a single trackway nearly a mile long. Just over 400 prints were recorded, with roughly a third fully excavated and mapped. The tracks show a woman or adolescent carrying a toddler, occasionally setting the child down. Other tracks show human footprints stepping directly into the fresh prints of a giant ground sloth, as well as a mammoth calf walking beside its mother. Local Acoma Pueblo cultural monitors worked alongside the scientists, with sisters Bonnie Lino and Kim Charlie uncovering several of the clearest sloth and human prints in 2021.
The dating process presented challenges. The team first dated seeds from an aquatic plant found in layers above and below the prints, returning an age of 21,000 to 23,000 years. This timeframe falls within the last glacial maximum, the coldest period of the ice age, when both proposed migration routes into the Americas are considered to have been impassable. The paper was published in Science in 2021 and generated significant debate.
To understand the significance of this finding, it helps to consider the theory it challenges. For much of the 20th century, archaeologists largely accepted the Clovis first model, which held that hunters crossed the Bering Land Bridge around 13,000 years ago and moved south through an ice-free corridor. This theory was treated as established fact for decades. Any finding suggesting older human presence was typically met with skepticism about the dating methods rather than reconsideration of the theory.
That changed with Monte Verde, a site in southern Chile. Archaeologist Tom Dillehay spent years excavating wooden hut foundations, knotted tent stakes, a child’s footprint, and seaweed carried from the coast. Radiocarbon dating put the site at roughly 14,500 years old, older than the Clovis model allowed. After a skeptical review panel validated the site in person in 1997, Clovis first was abandoned. The coastal migration theory then emerged to explain how people could have reached South America that quickly.
One reason direct evidence for the coastal route has been difficult to find is that sea levels during the ice age were substantially lower than today. The coastline those travelers would have used is now underwater, buried beneath sediment. Marine archaeologists have only begun surveying small portions of this submerged shelf. Much of the supporting evidence for the coastal theory comes from sites located far inland, such as Cooper’s Ferry in Idaho and Paisley Caves in Oregon, rather than from the coast itself.
The White Sands footprints raised a difficult question for the coastal theory. If people were walking in the interior of North America roughly 23,000 years ago, they may have arrived before the coastal route was believed to have opened. This leaves two possibilities: either humans arrived earlier than most models allow, or the ice age coastal route opened sooner than currently thought, with its evidence now sitting under hundreds of feet of ocean.
The initial dating of the White Sands footprints faced criticism. Archaeologists David Madsen, Loren Davis, and Charles Oviatt pointed out that the aquatic plant seeds used for the first dates can absorb old carbon dissolved in lake water, a phenomenon called the hard water effect. Another archaeologist, Vance Haynes, suggested the seeds might have been moved from older sediments by water.
The original team, including USGS geologists Jeff Pigati and Kathleen Springer, responded by dating materials less susceptible to these problems. Conifer pollen from land plants, which absorb carbon from the air, produced the same age of about 23,000 years. Optically stimulated luminescence on quartz sand, which measures when buried grains were last exposed to sunlight, returned a minimum age of 21,500 years. A third team led by geologist Vance Holliday dated the lake sediment itself and placed the same layer at roughly 22,400 years old. These three independent methods, run by separate teams, all produced results within the same narrow timeframe.
Some critics remain unconvinced. Davis and colleagues published a 2024 paper titled “Unresolved,” arguing that the newer samples came from slightly different layers, a claim the original team disputes. The debate continues, but the convergence of multiple dating methods has strengthened the case for the footprints’ age.
The coastal migration theory has also faced complications from oceanographic research. In 2023, oceanographer Summer Praetorius and her USGS team modeled ice conditions and currents along the ice age Pacific coast. They found that during the coldest periods, currents pushed northward, working against anyone traveling south by water. Only two brief windows appeared to offer viable travel conditions: one from 24,500 to 22,000 years ago, and another from 16,400 to 14,800 years ago. A separate 2024 study by researcher Martina Steffen found gaps in the fossil record of large mammals along that coastline during the coldest periods, suggesting limited resources were available.
The first window identified by Praetorius overlaps closely with the White Sands dates, suggesting the possibility of an earlier migration window rather than a continuous open route.
The other pillar of the coastal theory has also faced challenges. In 2026, archaeologist Todd Surovell published a paper in Science arguing that an 11,000-year-old volcanic ash layer beneath the occupation deposits at Monte Verde meant older material had washed downward, inflating the site’s apparent age. His revised estimate placed the site between 4,200 and 8,200 years old. Dillehay and more than 20 colleagues disputed the finding, questioning the ash layer’s existence and the analysis. Geneticist Jennifer Raff noted that the broader migration debate does not rest on Monte Verde alone. The dispute remains unresolved.
Several considerations affect how this debate is understood. The actual ice age coastline is inaccessible, buried underwater and under sediment. The only known site where the coastline remained available for direct study is Triquet Island off British Columbia, where archaeologist Duncan McLaren excavated a hearth with charcoal dated to roughly 14,000 years old. The Heiltsuk Nation, whose territory includes the island, had oral histories describing the area as an ice age refuge for their ancestors before the archaeological work began.
The question of who controls the narrative around ancient remains also remains relevant. In 1996, a skeleton found along the Columbia River in Washington, later known as the Kennewick Man or the Ancient One, became the subject of a legal dispute. Local tribes requested the remains for reburial, while a group of scientists sued to study them. In 2015, genetic analysis showed the remains were most closely related to modern Native Americans, specifically tribes of the Columbia Plateau. The remains were returned and reburied in 2017 after 20 years in storage. Current federal law requires consultation with potentially affiliated tribes before extensive study of newly discovered remains.
The coastal migration theory has not been abandoned. A major 2025 genetics study of over 1,500 whole genomes traced how early populations split apart as they spread south roughly 13,900 years ago, providing evidence for a fast expansion. However, the study does not specify which route those populations took.
As it stands, the theory has been complicated by findings from two directions: the White Sands footprints suggesting a much earlier presence in the interior, and the disputed dating at Monte Verde. The direct evidence that could settle the question remains underwater. The footprints at White Sands, which first drew attention in 2019, have likely faded by now, as most tracks there erode within a few years of exposure. The data collected from those prints, however, remains part of an ongoing scientific debate about when and how humans first reached the Americas.
