I've always had an interest in when humans first stepped on the North American continent. The needle was stuck at 10-12,000 years before present (YBP) for decades. The early Clovis people had come across the Bering land bridge and, in an amazingly short time, managed to populate both continents (and, most theories have it, wipe out the megafauna they found in abundance.)
There have always been arguments for sites like Meadowcroft Rock Shelter in Pennsylvania ( where a sizable minority of archaeologists accepts 16-19,000 YBP) and Monte Verde in Chile (argued to be from 14,800 to 33,000 YBP), Other sites have generally pushed back the 12,000 YBP consensus to around 14,000, but vigorous debate continues on older sites. A new paper published in 2017 is sure to spark more debate: it puts remains found near Old Crow, Alaska, back as far as 24,000 years.
None is as contentious as the Cerutti Mastodon site near San Diego, where a mastodon shows strong signs of being butchered and the long bones broken apart for marrow. Hammerstones and other human stone implements surround it. How old is it? According to a letter published in the top journal Nature, 131,000 YBP. Or to quote the letter, "Th/U [thorium-uranium) radiometric analysis of multiple bone specimens using
diffusion–adsorption–decay dating models indicates a burial date of
130.7 ± 9.4 thousand years ago." That's a startling claim indeed.
How would humans have come so far south so early? While shorelines on Western North America have receded since the Ice Ages, submerging many known and possible sites, we don't have hard evidence that any pre-Clovis people (or, in this case, pre-pre-pre-Clovis people) used boats or rafts to come down the coast, and there's not a chain of known settlements close enough to each other (or dated closely enough) to indicate a land migration. Yet here we are.
If they were really there 130,000 years ago, who were they? Early modern humans (EMH, a term replacing the too-limited "Cro-Magnon" with many scientists) existed only in Africa. It wasn't until 100,000 YBP they'd even pushed out as far as Israel. What are we left with? As far as we know, the Neanderthals never reached eastern Asia. A surprisingly advanced batch of Homo erectus? The still-mysterious Denisovians, who lived in Siberia and Southeast Asia? With just stones and mastodon bones, we don't have any human DNA to settle this with.
Critics think the discoverers got the dating right but suggest the human artifacts - if that is what they are - were left in the same area much later. That raises a host of questions that can only be answered by similar finds - or, to a degree, by a long search yielding no new finds. Some of the area archaeologists would like to inspect, though, is buried under suburban homes and parking lots.