This is a review of an article that just ran in the journal Nature, titled, “A 130,000-year-old archaeological site in southern California, USA.”
The core of the claim is that mastodon bones and a few stone cobbles are evidence that a species of Homo were once residents of an area that would become San Diego in 130,000 years. The bones were solidly dated to about this time using uranium-thorium dating and the stone cobbles lying near by had damage the authors are stipulating is the result of breaking the large mastodon bones open to get at the marrow. The bones themselves show signs of “green-bone” breakage, such as spiraling that occurs shortly after death and defleshing.
Up front, I’d like to point out three things: 1) I’m not a paleontologist or a physical anthropologist regardless of the extent of my knowledge and understanding of bones and butchery/kill sites; 2) I’m not against the idea that North America was visited by any species of Homo before the 15,000-20,000 years or so ago that most people recognize today; and 3) I’m skeptical of the claim by Holen and his colleagues in Nature all the same.
And here’s why.
I see a few problems with the claim, which more or less amount to “extraordinary claims require extraordinary evidence.” At best, we’re given some very mediocre evidence.
1) Modern humans probably did not expand out of Africa until 50-80 kya
“All non-Africans descend from a single migration of early humans from Africa. The estimates from the studies point to an exodus somewhere between 80,000 and 50,000 years.”
Non-Africans in the world appear descended from a single migration of early humans from Africa somewhere between 80-50 kya. In a single volume of Nature in 2016, there were several articles based on studies that all arrived at roughly this conclusion. 
Therefore, if Holen et al are correct, then the species of Homo that they’re referring to isn’t that of Anatomically Modern Humans. And, if they weren’t human, then they would have to be Neanderthals or another extinct species. This, I think, places a bit of stress on their conclusions to suggest that species like Neanderthal or Denisovans migrated to California. I definitely wouldn’t say it’s impossible–just not the most parsimonious idea.
2) Speaking of parsimony, what about simpler explanations for damage to the bones?
The bones and stones were found in a 20-30 cm thick stratum of silt within a 12 meter thick sequence of Pleistocene sediment according to the article. Plus however many meters of overlying Holocene deposition.
The authors note that the Pleistocene sediments were fluvial and upwardly fining, which essentially means that this was the probable result of a meandering stream or river in the Pleistocene. Over time, deposition of sediment, piling sand and silt up to create the site, burying with it the bones and stones. The authors note the green-bone breakage but fail to explore other possible explanations.
While it’s true that the upward fining stratigraphy is suggestive of a low-energy fluvial deposition (meandering stream or river), most people who live near these types of streams and rivers will tell you that they aren’t always “low-energy.” There are times when they’re raging rapids, only to return to their normal lazy, meandering natures for years until the next odd weather event.
Also, I noticed in the paper that the dire wolf, horse, and camel bones found nearby didn’t exhibit the same fracturing as the mastodon. This I found to be curious. wouldn’t these be easier bones to crack open? It leaves me to wonder also what the order of discovery was. Were the mastodon bones discovered first, putting a halt to the excavation activity. Then further, more careful excavation revealing the bones of the smaller animals?
If so, this suggests that the green-bone fracturing might not have been so green. Certain soil conditions can allow for preservation of bone that, when fractured several millennia later, present as green-bone fractures. The several tons of heavy excavation equipment in the overlying sediment could very well have provided the force needed to crack and fracture these bones in a manner consistent with green-bone fractures. 
3) Where are the butchery marks?
If these alleged people are cracking the bones open for marrow, why no marks where they butchered the bones for meat which exposed the bone? It’s likewise curious that there are no gnaw marks from rodents or carnivores. Could this be because the mastodon, the dire wolf, the horse, and the camel found themselves literally stuck in the mud only to be drowned?
4) Alleged stone tools aren’t convincing.
There just isn’t anything shown that can’t be imagined to be natural much less diagnostic. The few nicks and scratches in the cobbles might be created by the same force that brought the rocks there. While it is possible that force was a species of Homo, it’s also possible that it was a force of nature.
The generally upwardly fining sands of the excavated site is no guarantee that 30 or 100 meters away in a lateral direction there isn’t (or wasn’t) evidence for a more energetic type of deposition for which the mastodon site is on the statistical periphery. Or that the meandering stream or river wasn’t punctuated by one or more short-term, high-energy events that were capable of depositing a few cobbles.
I’m certainly not saying it isn’t possible that an early species of Homo didn’t migrate to California from Asia. In fact, I’m hoping this is true. Maybe we’ve just never found this type of evidence because we, as archaeologists, don’t look at pre-Holocene strata. But I took special offense to Holen et al when they wrote that they had “unquestionable artefacts […] found in primary context.”
I question the hell out of these artifacts.
References and notes:
- Mallick, Swapan; et al (2016). The Simons Genome Diversity Project: 300 genomes from 142 diverse populations. Nature, 538(7624), 201-206.
Pagani, Luca; et al (2016). Genomic analyses inform on migration events during the peopling of Eurasia. Nature, 538(7624), 238-242.
Malaspinas, Anna-Sapfo; et al (2016). A genomic history of Aboriginal Australia. Nature, 538(7624), 207-214.
- Haynes, Gary (2015). Bone Breakage and Other Disturbances at the Inglewood Mammoth Site. Technical Report found online at www.academia.edu.
Haynes, Gary (2016). Taphonomy of the Inglewood mammoth (Mammuthus columbi) (Maryland, USA): Green-bone fracturing of fossil bones. Quaternary International, 30, 1-13.