Big feet. Little feet. A heel here. A toe there.
Stamped across the shoreline of Calvert Island, British Columbia, are 13,000-year-old human footprints that archaeologists believe to be the earliest found so far in North America.
The finding, which was published Wednesday in the journal PLOS One, adds support to the idea that some ancient humans from Asia ventured into North America by hugging the Pacific coastline, rather than by traveling through the interior.
"This provides evidence that people were inhabiting the region at the end of the last ice age," said Duncan McLaren, an anthropologist at the Hakai Institute and University of Victoria in British Columbia and lead author of the study. "It is possible that the coast was one of the means by which people entered the Americas at that time."
Dr. McLaren and his colleagues stumbled upon the footprints while digging for sediments beneath Calvert Island's beach sands. Today, the area is covered with thick bogs and dense forests that the team, which included representatives from the Heiltsuk First Nation and Wuikinuxv First Nation, could only access by boat.
At the close of the last ice age, from 11,000 to 14,000 years ago, the sea level was six to ten feet lower. The footprints were most likely left in an area that was just above the high tide line.
"As this island would only have been accessible by watercraft 13,000 years ago," Dr. McLaren said, "it implies that the people who left the footprints were seafarers who used boats to get around, gather and hunt for food and live and explore the islands."
They found their first footprint in 2014. While digging about two feet beneath the surface in a 20-square-inch hole, they saw an impression of something foot-shaped in the light brown clay.
In 2015 and 2016, they returned and expanded the muddy pit. They discovered several more steps preserved in the sediment. The prints were of different sizes and pointed in different directions. Most were right feet. When the team was finished they had counted 29 in total, possibly belonging to two adults and a child. Each was barefoot.
The researchers think that after the people left their footprints on the clay, their impressions were filled in by sand, thick gravel and then another layer of clay, which may have preserved them.
That would make them the oldest preserved human footprints in North America.
"It's not only the footprints themselves that are spectacular and so rare in archaeological context, but also the age of the site," said Michael Petraglia, an archaeologist from the Max Planck Institute for the Science of Human History in Germany who edited the paper for PLOS One but was not involved in the work. "It suggests an early entrance into the Americas."
Dr. Petraglia said the footprints also provided strong evidence for the coastal movement hypothesis and he added that they may have rode the so-called "Kelp Highway," a hypothesis that underwater kelp forests supported ecosystems down the North Pacific coast that helped ancient seafaring people hunt, develop and migrate.
"The work is important because it shows the 'real' people, not just artifacts or skeletal remains," said Steve Webb, a biological archaeologist at Bond University in Australia. "However, the footprints are limited in number and don't shed light on activities or movement that tell us very much."
He added that future hunts for footprints should keep in mind that not everyone from this time period walked around barefoot. If anthropologists are too busy searching for soles, toes and arches, they might miss clues from those who wore animal skin shoes.