The “ghost prints” of White Sands National Park are among the century's most evocative and newsworthy discoveries. Thousands of fossilized footprints capture the movements, and encounters, of Ice Age animals – mammoths, ground sloths, camels, dire wolves – and Ice Age people. But two years ago, an international scientific team made a stunning announcement: these prints dated to between 21,000 and 23,000 years ago. That's some 8,000 years earlier than most archeologists believed humans occupied this continent.
Paradigm shifts are never simple, and while many hailed the findings, there were also skeptics. Now, the team has published new findings. They strengthen the case that these Chihuahuan Desert footprints are the oldest known evidence of people in the Americas.
U.S. Geological Survey scientists Jeff Pigati and Kathleen Springer are spearheading the dating work.
“The funny thing was,” Pigati said, “that almost every one of the comments critical of our work ended the same way: 'Man, this site is really cool. It could be really old. But what we really need you to do is radiocarbon dating of pollen and luminescence dating to see if those seed ages are accurate.' And we were like, 'Great idea!' Because we were literally doing that the entire time.”
Desert erosion exposes the prints at the surface – and then erases them. Researchers are rushing to document the prints, and the stories they tell – of sloth hunts, of family interactions – before they're lost.
But for dating, the team used ground-penetrating radar to locate prints underground. Excavating, they found the seeds of an aquatic plant – Ruppia cirrhosa, or spiral ditchgrass – embedded in human prints. Those seeds provided the first dates.
But aquatic plants can take up carbon from surrounding rock, rather than the air, and that can skew dating. For confirmation, the team pursued other approaches.
From sediment in the prints, scientists at Indiana University and the Lawrence Livermore National Laboratory used cutting-edge techniques to isolate tiny pollen grains. Most of the pollen was of pine, which in itself reveals a vanished world, Springer said.
“That is where the pine forest would be back then,” Springer said, “as well as a sagebrush steppe on the piedmont surrounding the actual basin, which held a lake, and a lake edge, where the people and the animals were hanging out.”
The team also pursued the “luminescence dating” of quartz crystals. In their chemistry, these crystals reveal when they were buried.
Both techniques are painstaking, Pigati said.
“It's a really long, really involved process,” he said, “and it was excruciating to wait those last few weeks till we got the results – but, thank goodness, everything came out all right.”
The dates converged. And they reinforced the initial finding – that the prints record the movement of people and animals over a 2,000-year period, between 21,000 and 23,000 years ago.
At that time, glaciers covered all of what's now Canada and Alaska. Archeologists have long assumed the first Americans migrated from Asia via an “ice-free corridor” some 15,000 years ago.
“Immediately the question everyone asks is, 'How did they get there?” Springer said. “'There were 3,000-foot ice sheets in the way.' Yep. So there's people that are studying that very question.”
Dating work will continue at White Sands. But researchers might also explore ancient lake beds found elsewhere in the West, including West Texas.
“Footprints on a surface are unequivocal evidence of the presence of humans,” Springer said, “and I think people are going to start looking more and more at these lake basins all over the West. Because I guarantee you they're there.”
Pigati agreed.
“It's a treasure trove just waiting to be discovered,” he said.
