How was western North America formed? Geologists have long held that as North America broke away from Pangea and headed west, it ran into the Farallon plate. Subducted under the North American plate, the The Farallon acted almost like a conveyer belt, creating the extensive area of elevated topography that dominates the West—the coastal ranges, the Rocky Mountains and the high plateaus in between. This mountainous area consists of dozens of crustal blocks of varying age and origin, welded onto the American continent over the past 200 million years.

But something was missing in this explanation, says Karin Sigloch, of Ludwig Maximilian University in Munich. “How these blocks arrived in North America has long been a puzzle.”

So with colleague Mitchell Mihalynuk of the British Columbia Geological Survey, Sigloch went to work on the puzzle. The scientists used a technique called seismic tomography. Seismic tomography makes it possible to probe the geophysical structure of Earth’s interior down to the depth of the lower mantle by analyzing the propagation velocities of seismic waves. The method can image the remnants of ancient tectonic plates at great depths, revealing ocean floor that subducted a long time ago, disappearing from the surface and sinking back into the mantle.

The new data from their study suggest that the Farallon Plate was far smaller than had been assumed, and underwent subduction well to the west of what was then the continental margin of North America. The researchers also determined that there was likely another, previously unrecognized oceanic plate involved in the formation of the West.

As the North American plate moved westward, the initially met and consumed the previously unknown oceanic plate, now detected seismologically beneath east coast of modern North America. Only then did the continent begin to encounter the Farallon plate. On its westward journey, the scientists conclude that North America overrode one intervening island arc after another—annexing ever more of them for the construction of its wild, wide mountains of the West.

The study is published in this week’s Nature.

Image: Karin Sigloch

Thirty years ago, a farmer discovered an adult male mastodon skeleton in Washington state. At the time, archeologist Carl Gustafson excavated the fossil and found some peculiar things about it. Ed Yong explains in the Guardian:

[He] noticed a pointed object embedded in its rib. Gustafson took a fuzzy x-ray and interpreted the object as a projectile point made of bone or antler… By dating organic matter around the fossil, he estimated that it was about 14,000 years old.

Other researchers challenged the man-made instrument and date—one reason was that it pre-dated the Clovis culture by about 1,000 years. Clovis is the name given to the distinctive tools made by people starting around 13,000 years ago. Gustafson’s finding would have rewritten the history of people on our continent.

Technology has changed quite a bit since the discovery of the mastodon bone thirty years ago. Enter Michael Waters of Texas A&M. As we described in an article about his research last spring, he’s not afraid to rewrite history. Waters contacted Gustafson about performing new tests on the rib with the bone point.

Waters modern tests confirmed Gustafson’s suspicions. New radiocarbon dates confirmed that the site was 13,800 years old. High resolution CT scanning and three-dimensional modeling confirmed that the embedded bone was a spear point, and DNA and bone protein analysis showed that the bone point was made of—get this—mastodon bone.

“The evidence from the site shows that people were hunting mastodons with bone weapons before the Clovis stone spear point,” says Waters.

In addition, the new evidence supports extinction theories of large mammals at the end of the last Ice Age. During the last cold period, herds of mammoth, mastodon, camels, horses and other animals roamed Texas and North America. At the end of the Ice Age, these animals became extinct.

“While these animals were stressed by the changing climate and vegetation patterns at the end of the Ice Age, it is now clear from sites like [this one] that humans were also hunting these animals and may have been a factor in their demise,” Waters adds.

Timing is indeed everything. The current research was published last week in the journal Science.

Image courtesy of Center for the Study of the First Americans, Texas A&M University

During the last century, the story about the peopling of the Americas seemed relatively straightforward. But over the past few decades, new evidence has challenged all we thought we knew about the route, timing, and origins of these first Americans. Now an archaeological discovery in central Texas has turned everything on its head once again.

Nearly 80 years ago, North American archaeologists identified what they believed to be the earliest stone tools in the New World. Characteristic of the Clovis culture, stone tools with “fluted” spear points first appeared about 13,000 years ago and were found throughout North America.  However, early versions of these stone tools have never been found outside North America, in places like northeast Asia or Alaska, where the first Americans started their journey into the New World. This lack of continuity of Clovis culture tools outside North America has led some to question whether this culture really did accompany the First Americans.

In this week’s issue of Science, a team of North American archaeologists report the first concrete evidence debunking the so-called “Clovis First” archaeological model of the peopling of the Americas.

The team of archaeologists, led by Michael Waters of Texas A&M University’s Center for the Study of the First Americans, has been excavating at the Debra L. Friedkin archaeological site northwest of Austin for five years. This site contains stone tools and other artifacts that span American prehistory, including thousands of Clovis points.

But whereas the appearance of Clovis points in most archeological dig sites marks the earliest evidence of human occupation, artifacts discovered at this site predated the Clovis points.

As Waters recalls, “The kicker was the discovery of nearly 16,000 artifacts below the Clovis horizon that dated to 15,500 years ago.” That’s 2,500 years older than any Clovis points unearthed in the Americas.

Among the 16,000 artifacts excavated are various types of small blades, choppers, and scrapers made of chert. Their small size has led some to theorize that these tools represent a ‘mobile toolkit’ that could easily be packed up and moved. Now identified as the Buttermilk Creek Complex, this assemblage represents the oldest archaeological site in North America.

The early age of this site coincides with recent genetic and paleoenvironmental evidence that people first crossed into the Americas much earlier, perhaps as early as 30,000 years ago, and spread into North America by about 17,000 years ago. But the archaeological record in support of an early arrival has, until now, been limited.

Many experts believed more archaeological evidence was needed to support the model of an early arrival. The Buttermilk Creek Complex provides that evidence. The discovery of a huge assemblage of stone tools that predate Clovis culture also puts to rest the conventional wisdom that the first Americans brought the Clovis culture with them from Asia.

As Waters explains, “It is now time to abandon once and for all the ‘Clovis First’ model and develop a new model for the peopling of the Americas.”

Anne Holden, a docent at the California Academy of Sciences, is a PhD trained genetic anthropologist and science writer living in San Francisco.

Image courtesy of Michael R. Waters