It had been thought that humans first colonized "The New World" — the Americas — around 16,000 years ago. But now, in two papers published in the journal Nature, that date has been pushed back, way back.
Stone tools excavated from a cave in central Mexico date to 33,000 years ago. In different levels of the cave, over 1,900 artifacts have been found, and scientists think the cave was occupied for a staggering 20,000 years.
Chiquihuite Cave is located in the Astillero Mountains in the Mexican state of Zacatecas. The cave sits at 1.7 miles (2,740 m) above sea level, and about 0.62 miles (1,000 m) above the valley floor.
According to the lead author of one of the Nature studies, Ciprian Ardelean, who is an archeologist at the Universidad Autonoma de Zacatecas, radiocarbon dating of the oldest artifacts found in the cave date to between 33,000 and 31,000 years ago. They also differ from artifacts found anywhere else in the Americas.
Scientists also discovered tiny pieces of charcoal in various layers of the Chiquihuite Cave's sediment, dating from 12,000 years ago all the way back to 32,000 years ago. Another unusual find located in a sediment layer dating to 28,000 years ago was a Douglas fir tree.
That tree is no longer native to Mexico, but as Ardelean explained in a National Geographic article, the Mexican climate back then was much cooler and wetter than it is now, more like the climate of British Columbia.
Clovis Culture was not the first
Prior to the find at Chiquihuite Cave, the most widely accepted theory was that humans first arrived in the Americas around 16,000 years ago, traveling along a land bridge that connected what is today Alaska with Russia. Known as Beringia, the land bridge extended from Russia's Lena River on the west, to Canada's Mackenzie River on the east.
The people who crossed that bridge are known as the Clovis Culture, and it is thought that they dispersed across North America, with one branch going east into southern Ontario, Canada, and another branch heading south through Central America and into South America.
A companion study that appeared in the same issue of Nature found that artifacts collected from 42 sites across North America showed the presence of humans dating from the end of the Last Glacial Maximum (LGM). This was the period when vast ice sheets covered much of North America, Northern Europe, and Asia.
If not across Beringia, then how?
Between 33,000 and 16,000 years ago, the ice sheet across North America made land travel across Beringia impossible. Therefore, the people who occupied Chiquihuite Cave either had to have used the land bridge before then, or else they had to have arrived by sea.
If they traveled from Asia by boat, they would have created settlements along the Pacific coast, however, no settlements have been found. A possible answer to this problem in U.S. history lies in the rise of sea level that occurred following the end of the last ice age. Sea levels rose by as much as 400 feet (120 m), and this would have put any settlements under water.
No matter how they arrived, the presence of homo sapiens in North America can be inferred by the extinction of many native animals that took place around 15,000 years ago. These include mammoths and ancient horses and camels.
Where does Native American DNA come from?
In optimal conditions, such as very cold ones, ancient DNA can survive for up to 1 million years. The oldest complete genome ever sequenced is that of a horse that was unearthed in Yukon, Canada that dates to between 560,000 to 780,000-years-old.
The oldest DNA ever sequenced from Homo sapiens was extracted in 2016 by researchers excavating the Sima de los Huesos cave in Spain. They were able to sequence around 50,000 base pairs which showed that the hominins living in the cave 430,000 years ago were Neanderthals.
In 2015, scientists were able to isolate enough DNA from the jawbone of a modern human who lived in Romania 40,000 years ago to determine that the man had a Neanderthal ancestor only four to six generations earlier.
The latest archaeology discoveries have revealed that Native American ancestors split off from people living in Siberia and appeared in the Pacific Northwest between 17,000 and 14,000 years ago.
A recent study by He Yu and her colleagues at the Max Planck Institute for the Science of Human History in Germany described how they extracted DNA from the fossilized tooth of a man who lived near Lake Baikal in southern Siberia about 14,000 years ago. The ancient man's DNA was closely related to that of Native American people.
In 2014, the DNA was extracted from the skeleton of a 12,500-year-old infant, commonly referred to as Anzick-1, that was found in Montana. The infant had been buried with several Clovis artifacts, and his DNA showed strong similarities with that taken from Siberian sites. The baby's DNA was also the same as DNA taken from other ancient sites within the Americas, indicating that Native Americans are descended from people who lived in or near Siberia.
It was only about 500 years ago that the DNA of Native Americans began showing the contribution of Europeans, and while Eskimos also derive their DNA from Siberia, it reflects that of a later migration.
In 2015, a study showed that Native Americans living in the Amazon shared their ancestry with Australasians, who are natives of Australia, Melanesia, and the Andaman Islands. A 2005 study showed that the Americas were colonized by a surprisingly small number of individuals, numbering only about 80.
The findings from Chiquihuite Cave are already proving controversial, but the chance to open a window on how and when the first people arrived in the Americas is an important one.