Ancient humans settled in North America around 130,000 years ago, suggests a controversial study — pushing the date back more than 100,000 years earlier than most scientists accept. The jaw-dropping claim, made in Nature, is based on broken rocks and mastodon bones found in California that a team of researchers say, point to human activity.
Their contention, if correct, would force a dramatic rethink of when and how the Americas were first settled — and by who. Most scientists subscribe to the view that Homo sapiens arrived in North America less than 20,000 years ago. The latest study raises the possibility that another hominin species, such as Neanderthals or a group known as Denisovans, somehow made it from Asia to North America before that and flourished.
“It’s such an amazing find and — if it’s genuine — it’s a game-changer. It really does shift the ground completely,” says John McNabb, a Palaeolithic archaeologist at the University of Southampton, UK. “I suspect there will be a lot of reaction to the paper, and most of it is not going to be acceptance.”
The study focuses on ancient animal-bone fragments found in 1992 during road repairs in suburban San Diego. The find halted construction, and palaeontologist Tom Deméré of the San Diego Natural History Museum led a five-month excavation.
His crew uncovered teeth, tusks and bones of an extinct relative of elephants called a mastodon (Mammut americanum), alongside large broken and worn rocks. The material was buried in fine silt left by flowing water, but Deméré felt the rocks were too large to have been carried by the stream.
We thought of some possible explanations for this pattern, and the process we kept coming back to was that humans might be involved,” he says. Attempts in the 1990s to date suggested that the ivory was some 300,000 years old, but Deméré was sceptical: the method his colleagues used was problematic, and the age seemed so improbable for humans to be living in California.
Over the past decade, archaeological research and studies of modern and ancient DNA have reached the conclusion that humans from Asia crossed the Bring land bridge into Alaska some 20,000 years ago and reached the southern tip of South America around 14,000–15,000 years ago.
Some archaeologists, however, maintain that humans arrived earlier. They point to sites containing rocks that resemble stone tools as well as large animal bones that have damage apparently inflicted by humans.
Deméré’s co-authors Kathleen Holen and her husband Steven Holen, archaeologists at the Center for American Paleolithic Research in Hot Springs, South Dakota, have put forward several sites in the US Midwest as evidence for a human presence in the Americas up to 40,000 years ago. But many scientists have viewed these claims with scepticism.
After hearing about the San Diego mastodon, the Holens visited Deméré in 2008 to see the boxed-up remains. “We were looking at something very, very old, but it had the same fracture patterns that we had seen before,” says Kathleen Holen.
The bones looked as though they had been set on a large ‘anvil’ stone and struck with a ‘hammer’ rock. The team contends that the rocks recovered from the site were used either to extract the mastodon’s bone marrow or for making more-delicate bone tools. There are no obvious cut marks on the mastodon bone, suggesting that the animal wasn’t killed or butchered for its meat.
Using refined dating methods, the researchers tried again to determine the age of the site. They couldn’t use radiocarbon dating on the mastodon remains because the bones lacked carbon-containing collagen protein. A second method was too imprecise.
A third technique, which measures relative levels of radioactive uranium and thorium in bone, suggested that the remains are 130,000 years old. “I’m sure that many of our colleagues are going to be quite sceptical.
I would expect that. This is far, far older than most archaeologists expect hominins to be in North America,” says Steven Holen. “I say that even for myself.”
- Nature