James Adovasio stood on a platform on an overcast day last week, about forty feet up the side of a steep-sloped, wooded valley outside the tiny town of Avella, Pennsylvania, about thirty miles southwest of Pittsburgh. Adovasio, a sixty-nine-year-old archaeology professor, watched as a handful of archaeologists, mostly young ones, gently probed and poked at slabs of exposed rock. Torrential rains in mid-July had partially flooded the site, known as the Meadowcroft Rockshelter, and the group was here to assess the damage.

Adovasio began excavating this site back in 1973. A young archaeologist at the University of Pittsburgh, he intended to use Meadowcroft to train students. But what he found here helped demolish his colleagues’ long-held ideas about the timing of humans’ first steps in the New World. Since the nineteen-thirties, the conventional wisdom had held that humans crossed over into North America from Siberia around thirteen thousand years ago, then spread over the next five hundred years through North and South America—wiping out mammoths, mastodons, and other large mammals as they went. This hypothesis became known as the “blitzkrieg model” of species extinction.

But Adovasio, now a professor at Mercyhurst University, in Erie, Pennsylvania, discovered evidence that humans had camped at Meadowcroft, under a protective rock overhang, sixteen thousand years ago—a few thousand years before the Siberian crossing. “Nobody believed it,” said the scientist, who is trim and keeps a gray beard. “They said our dating had to be wrong.” A couple of years later, however, archaeologists working at a site known as Monte Verde, in Chile, found evidence of a human presence at least fourteen thousand eight hundred years old. Later discoveries at Cactus Hill, in Virginia, the Paisley Caves, in Oregon, and several other sites have indicated older dates of migration as well.

Nevertheless, it took decades for the archaeological establishment to began to accept these findings, in part because several converging lines of evidence supported the previous model. For one thing, the spear tips that, for decades, were the oldest known human artifacts in North America—called Clovis points, because they were first discovered near Clovis, New Mexico—dated from about thirteen thousand years ago. For another, New World mastodons and other so-called megafauna went extinct around that time, too. And, although glaciers still covered parts of North America at the time, receding ice sheets opened up a corridor along the eastern Rockies that would have allowed Siberians to travel south from Alaska with relative ease.

The key problem with this whole scenario, Adovasio said, is “that five-hundred-year sprint down the hemisphere. People don’t just go running over the landscape. It takes a while to figure out what to eat, when to eat it, which plants will kill you, and so forth.” In part, he attributes the longstanding acceptance of this implausible story to the fact that, until relatively recently, most archaeologists were men. “I mean, who but a male would think that the ancestors of modern Native Americans sprinted to South America and killed everything in their path?”

If the first immigrants did arrive much earlier, the ice-free corridor through the glaciers wouldn’t have been available. But there’s an alternate route: they could have travelled down the coast in boats. “The colonization of Australia occurred even earlier,” Adovasio said. “It’s, in my opinion, simple racism that we never recognized before that the earliest populations in the Americas were capable of building boats.”

Despite that logic, and the discovery of additional pre-Clovis sites, it has taken nearly a quarter of a century for the conventional wisdom to begin to change. In 1997, a delegation of senior archaeologists travelled to Chile to examine the Monte Verde site—two decades after it was first identified. They pronounced the site authentic—and, by extension, laid the blessing of credibility on the pre-Clovis model established by Meadowcroft and other pre-Clovis sites.

The dating of the Meadowcroft site itself remains controversial, however. The Associated Press recently noted that, in a poll of archaeologists, thirty-eight per cent accepted Adovasio’s dating, while another thirty-eight per cent remained unsure and twenty per cent disagreed. “You can find people,” Adovasio said, “who would sit here with you today and say, ‘None of these pre-Clovis sites are real. They’re all nonsense. There’s something wrong with every one of them,’ although that number keeps shrinking.”

Exactly when the first Americans did arrive is something Adovasio doesn’t claim to know, but focussing on a particular date may be misleading. “It has become clear,” he said, “that there have been multiple incursions from Siberia by ethnically, linguistically, genetically, and technologically different populations. Some of these pulses may have begun before the last glacial maximum, before twenty-two thousand years ago. Not all of them would have succeeded.”

These days, Meadowcroft, a field school for young archaeologists, is mostly fulfilling the role Adovasio first envisioned for it. It’s unlikely, after forty years, that it will yield any startling new insights. If not for the federal budged sequester, in fact, Adovasio wouldn’t even be here. “Normally,” he said, “this is the time I’d be in the Gulf of Mexico with the underwater folk looking at the continental shelf.”

At the end of the last ice age, so much water was frozen in glaciers that sea level was three hundred feet lower than it is today. “Forty per cent of ice-age Florida is now underwater,” he said, and if humans were there at the time, they might well have settled at river mouths that are now deeply submerged. “Now we’re going to those landscapes that we know date back to those times, and looking specifically there for early material. And we’re finding it.”

Photograph: Universal History Archive/Getty.