Recent studies, bolstered by innovative dating methods, indicate that the Americas saw human habitation about 23,000 years ago, revising prior estimates of a 14,000-year history. The timeline of human migration into the Americas has always been a topic of much debate.
Throughout the 20th century, it was widely accepted by archaeologists that the Americas weren’t inhabited until roughly 14,000 years ago. However, the recent findings paint a different picture, pointing towards human presence in America around 23,000 years back.
Previously, it was believed that human arrival coincided with an ice-free passage created between two large ice masses covering modern-day Canada and the northern US.
This passage, formed by ice melt towards the end of the last Ice Age, was theorized to facilitate human movement from Alaska deep into North America. This prevailing view began to shift over time.
Over the past few decades, evidence started pointing to human presence as far back as 16,000 years ago, aligning with the last Ice Age’s conclusion. In September 2021, our team unveiled a study in Science, revealing fossil footprints in New Mexico, dated at approximately 23,000 years ago, during the peak of the last Ice Age.
These imprints, found near present-day White Sands by a group journeying past an ancient lake, extended the known human history on the continent by 7,000 years, reshaping our understanding of American prehistory.
This suggests that if humans were present during the Ice Age’s climax, the icy terrains were not as prohibitive, or humans had been on the continent for a significantly longer period, possibly from a prior melt phase.
These findings faced scrutiny, but subsequent evidence reinforced these early-date claims. To many, the term “pollen” evokes seasonal allergies and discomfort.
However, ancient pollen can offer invaluable scientific insights. In our 2021 research, radiocarbon dating was applied to common ditch grass seeds, found in sediment strata above and below the discovered footprints.
Radiocarbon dating evaluates the radioactive decay of carbon-14 in recently deceased organisms, within the past 50,000 years. Some questioned the accuracy of these radiocarbon dates, attributing potential distortions to the “hard water” effect, where carbon-rich groundwater can skew dating results by having its carbon-14 partially decayed.
They argued that the aquatic ditch grass seeds might have absorbed this old water, thus altering the dating outcomes. Such skepticism is an essential component of scientific discourse.
Radiocarbon dating remains a reliable method, applicable to diverse organic substances, given a sufficient quantity. Hence, team members Kathleen Springer and Jeff Pigati from the United States Geological Survey embarked on dating pollen grains.
Given the minuscule size of pollen grains, usually around 0.005 millimeters, substantial quantities are required. To achieve this, we employed flow cytometry, a method typically used to count and segregate individual human cells, to sort and concentrate fossil pollen for radiocarbon dating.
Flow cytometry harnesses cell fluorescence, instigated by a laser, to segregate and accumulate individual cells within a fluid stream. With ample pollen grains present in the sediment layers surrounding the White Sands footprints, dating became feasible.
The abundant pollen also enabled us to isolate unaffected species, like pine trees, from the old water influence. Intense and costly laboratory efforts over a year resulted in dates based on pine pollen, corroborating the footprint’s original timeline, demonstrating no old water effects at the location.
Additionally, the pollen provided insights into the vegetation present when the footprints were formed, revealing expected Ice Age flora in New Mexico.
For verification, we utilized another dating method, optically stimulated luminescence (OSL), which determines the age of buried quartz grains based on stored energy from omnipresent background radiation. The detected energy levels indicate the last time these grains were exposed to light.
To gather the quartz, protective metal tubes are used to extract samples without light exposure. Laboratory exposure to light reveals the quartz grain ages.
The data from OSL aligned with results from other methods. Ultimately, ancient pollen, combined with advanced medical techniques, validated the footprint dating and the timeline of human arrival in the Americas.