Two years ago, I wrote about human footprints discovered in New Mexico’s White Sands National Park that appeared to be over 20,000 years old. That was a big deal for archaeologists, because, as I wrote at the time:
current scientific thinking holds that people first arrived in the Americas sometime around 16,000 years ago. Back then, Russia and Alaska were connected by land, but ice sheets blocked the way south through Canada, so most archaeologists who study this stuff think the earliest people to arrive south of the glaciers came along the coast by boat. The new findings—if they hold up to the inevitable scrutiny—suggest that people had made it well into what’s now the lower 48 before the ice sheets formed. For archaeologists, that opens many new possibilities about timing, routes, migration patterns, ancient climates, how people interacted with extinct species like mammoths and for how long, and on and on and on. This is an exciting, and kind of scary, place to be as a scientist!
The peopling of the Americas, as this area of study is known, is perhaps the most contentious, meticulous, and skeptical subfield in all of archaeology. So it was no surprise that one paper from one site suggesting an older date didn’t immediately change everyone’s minds about the story. This week, the team of White Sands fired its second shot at the current paradigm: Different dating techniques performed in independent labs on new kinds of samples agreed the footprints were between 21,000 and 23,000 years old. From my story:
So the White Sands team returned to the site to collect new kinds of samples. They extracted pollen from the same layers of sediment targeted in the first paper, which lie underneath, between, and on top of various sets of footprints. They also collected quartz grains from a layer of clay just above the oldest tracks. In different labs than those used in the first paper, researchers isolated and radiocarbon dated pollen from pine trees—terrestrial plants with no danger of absorbing old carbon. They also performed optically stimulated luminescence (OSL) dating on the quartz grains, a method that measures when the grain was last exposed to sunlight. The pollen returned dates between 23,000 and 21,000 years old, and OSL showed the quartz grains were buried between 21,400 and 18,000 years ago, the team reports today in Science. Ages from both methods “are statistically indistinguishable from our original seed ages,” says co-author Jeffrey Pigati, a geologist at USGS.
And now we enter a place all archaeologists and archaeology reporters know well—the phase of intense and possibly years-long debates about stratigraphy:
But Davis still has concerns. The sediment covering the footprints must have been carried in by a stream or blown in by the wind, he says, and this burying layer could have eroded out of much older rocks. He also notes the clay that yielded the OSL samples is likely the remnant of the ancient lake floor. As the lake dried up, the older clay could have eroded and been redeposited on top of the younger lakeshore mud that captured the footprints, leading to out-of-order layers. The co-authors insist there is no evidence of such erosion. But Davis would like to see more OSL dates from samples taken directly from the sediments that contain the footprints.
There’s another thing that could push the debate forward, though: Finding more sites! One of the most fascinating pieces of this story, to me, is that archaeologists in North America haven’t really been looking for signs of human activity before around 16,000 years ago. Why would they, if it was impossible to get to the continent? Academics look where they think they’re most likely to find interesting things, and salvage archaeologists who come in before construction projects aren’t required to examine sediments that old. I predict a lot more people are going to be looking now. Whatever they find, I look forward to writing about it.
Can’t decide if I’m more amazed by the date or by how OSL works 🤓. Also - those toes! 🥰