The Evidence by the Freeway
If I leave my house and head towards the Pacific Ocean, within a few minutes I’m driving west on San Diego’s 54 freeway. There’s a certain spot I pass not far from my home that always gives me pause. Could this be the earliest known site of human habitation in the Americas?
When the freeway was widened back in 1992 and 1993, earth-moving machinery began to uncover numerous ancient animal bones. That’s not unusual, in a region where fossil remains are frequently discovered during construction work. And as required by law, paleontologists — in this case, from the San Diego Natural History Museum — were on hand to monitor the work and recover unearthed remains.
The discovery of mastodon bones and teeth was almost routine. Yet something seemed odd to these experts. The remains, evidently from a single creature, did not to them seem randomly placed. One tusk was in a vertical position. Five large stones mixed in with the bones puzzled them too; three had clusters of bone flakes around them. To these paleontologists, this didn’t look like a disorganized jumble. Instead, they believed the site showed signs of being arranged — as it would if humans had been breaking open bones to make them into tools, or to extract nutritious marrow. The rocks also showed signs of wear similar to those known to have been used as hammers and anvils.
Bones can be dated with accuracy by measuring their level of radioactive decay. And this is where things got crazy.
The dates came back as 130,000 years old, plus or minus 9,400 years.
That’s over a hundred thousand years earlier than humans, or any other hominin group, were believed to be in the area.
Rocks can, of course, be worn down by natural processes, such as erosion by wind or moving water. But the fine silt surrounding these rocks seemed to have settled slowly on a swampy bank of a river. The placement of the rocks also suggested to the investigating team that they were purposely moved to the site, rather than being carried there in a fast-moving current.
Bone can also be broken by nature’s forces and by scavenging animals. But here, only the larger bones appear to have been broken, and telltale spiral fractures show they were apparently broken very soon after the animal died. The paleontologists saw signs that suggested to them they had been deliberately smashed by a hard object with strategically concentrated force. No scavenger could break bones in this way. And the bones looked like they were broken right where they were found. Smaller bone pieces were no longer there — as if, once broken, they were taken elsewhere to be worked into tools, as ancient humans were known to do.
It wouldn’t be the first time that opinions have shifted as to the date of human activity in the Americas. As late as the 1970s, the general consensus in the archaeological community was that humans did not reach the continent earlier than 13,000 years ago. It took decades before overwhelming newer evidence changed general opinion. Recently, another site in the Americas is showing evidence that could push the date back to 24,000 years.
Natural processes, however, cannot be entirely ruled out at the San Diego site. Although some believe it would take a large series of coincidences all the features could, possibly, have occurred without humans. In the absence of any definitely-human-crafted objects, the evidence is maddeningly inconclusive to many, and hotly debated by experts. It would not be the first time that a promising site has proven to be a dead end.
I used to work with famed archaeologist Dee Simpson at the San Bernardino County Museum. She had begun studying artifacts seemingly made by early humans at a site in the Calico Mountains of California in the 1940s, looking at rocks that appeared to have been shaped into tools up to 200,000 years ago. This attracted the attention of legendary paleoanthropologist Louis Leakey, whose discoveries in Tanzania had greatly expanded knowledge of human origin and evolution. He and Simpson began excavation work at Calico in 1963: they continued researching the site until each of them passed away. Other experts, however, are certain that the thousands of samples Leakey and Simpson cataloged as human-made stone tools are, in fact, rocks broken in floods in an ancient alluvial fan. The focus on the Calico site damaged Louis Leakey’s reputation forever, and today his conclusions about the site are discounted by the academic field.
The experts I talked to who discount the Calico site are much more intrigued by the San Diego site. In fact, a number of them (although not all) find it hard to imagine how the site could have been made without hominins. When visiting the San Diego Natural History Museum, the exhibition about the discovery is pretty persuasive for the general visitor. The bones, rocks, and other evidence are not locked away in some archive. They are on open display, with signage, video, and hi-res 3D digital models telling the story. Close-up photos show microscopic chips and scratches that seem to support the hypothesis of human involvement. Detailed mapping of the objects’ relation to each other when found helped me make sense of the discovery. Seeing the two tough, ball-like heads of the mastodon’s femur bones, it’s hard to imagine how they could have been separated from the rest of the bone without seriously concentrated force. I could see how they were found side by side, inches from the rock believed to have been used as an anvil. Much like detailed evidence collected and presented in court from a crime scene, the paleontological team make their case. It’s fascinating to see the video of the site as it was being excavated, and hear the expert explanations. When wandering the rest of the museum and seeing more complete mastodon skeletons, what I had just learned allowed me to look at the strong leg bones and greater understand their reasoning.
So as I drive on the freeway by the San Diego site, about five miles from the ocean, I look at the side of the road, and the wide embankment between the traffic and the nearest houses. Some paleontologists think there might be more evidence in there. I might be driving past the key to the earliest history of human habitation of this continent. Or I might be driving past a pile of dirt that will reveal nothing.
Either way, isn’t it fascinating to think about? I’m relatively new to the neighborhood. What if this is the oldest known neighborhood on the continent?
Francis French is an internationally-recognized, bestselling science author whose work can be found at www.francisfrench.com
