Humans didn’t get to California 130,000 years ago. But if they did…
A team led by Steven R. Holen of the Center for American Paleolithic Research just published in Nature the find of an 130,000 year old kill site in California. You can read the original research here (paywall, sorry), and an accessible summary here.
The first thing to know is that these findings are almost certainly wrong. First things first, I would not hang any hats on uranium-thorium dating as the final word, as bones are phenomenal sponges for weird elements. When I dug dinosaurs in high school, we found an allosaur skull with a geiger counter because it had absorbed so much uranium from groundwater. The idea behind the dating technique is that once the bone has absorbed all the uranium it can, the decay into thorium (which bone does not absorb). The ratio of thorium to uranium can be, in theory, a great dating method for those bones. The problem is that the bone can keep absorbing uranium for a long period of time. And it isn’t an easy thing to model, because the process can go on for thousands of years. And dating aside, I’m not convinced the break patterns have to be from stone tools, or that the stone tools are themselves stone tools.
But, let’s put aside the criticism of the article and consider only the implications — what is a plausible model for these findings?
Let’s first look at how phenomenal it would be. Right now the oldest widespread evidence we have for humans in the Americas is the Clovis culture. They are widespread and show a remarkable similarities in their giant Clovis points that spread from coast to coast in the US and extend south into South America. But note the qualifier — the oldest widespread evidence. There are flashes in the pan of an earlier coastal culture, in Monte Verde in Chile and Page-Ladsen in Florida at around 15,000 years ago. Finding the earliest humans in the Americas is tough, primarily because the most hospitable places for humans would have been the coasts, and the coasts America had 15,000 years ago are about 300 feet underwater from our present shorelines. Studying those sites is about as easy as it’s going to be to vacation in Palm Beach 200 years from now.
There has been more reports of even early human occupations in places like Meadowcroft, PA and an older habitation in Monte Verde at around 20,000–30,000 years ago, but the evidence is not compelling and most archaeologists don’t buy it. Everything we have suggests that humans came across the Pacific via a coastal route sometime around 15,000 years ago, and didn’t have large population numbers until 13,000 years ago, when we start to see signs of the Clovis culture just about everywhere. That’s the problem with archaeology/paleontology — you don’t find the first of anything, you only find out when it was common. Future archaeologists wouldn’t find out about the Beatles from their demo recordings — they’d find about about them from Sgt. Peppers or the White Album. Generally, for something to have a chance for surviving thousands of years, it has to be common and widespread to even qualify. Hipster archaeology just doesn’t work.
But the claims by Holen et al leapfrog this debate about a 20,000–30,000 years in a massive way — THEY ARE ARGUING FOR HUMANS AT 130,000 YEARS AGO. That is nuts. It is crazy. But, it is not 100% impossible. It just wouldn’t have been a population we’re prepared to study.
Let’s rewind the clock to 130,000 BC. The first thing you need to know is that what we think of as humans today — Homo sapiens sapiens — doesn’t exist yet. Homo sapiens does exist — kinda — in small isolated populations in Africa and possibly as far north as present-day Israel/Palestine. In fact, they were probably a constantly endangered species. They also wouldn’t have been the only human-ish species around. In Africa, there may have still been multiple species of humans — the best candidate to co-exist with Homo sapiens would be isolated populations of H. ergaster (erectus if you’re in Asia) and possibly even populations of H. naledi, the recent paleoanthroplogical wünderkid from South Africa.
In Europe, H. neanderthalensis — Neanderthals — would have been in their prime. In Siberia, the recently discovered H. sp. altai — Denisovans — would have likely been around. We don’t know a lot about this group — the only discovered remains so far are a pinky finger and a tooth. But, scientists have been able to recover quite a bit of DNA from Denisovans, so while we have a poor understanding of what they looked like, we know a lot about their genetics. In fact, it is highly probably we do have more remains of Denisovans, but they are probably classified as H. erectus fossils, and we’d be unable to prove the association without more genetics. And finally, in present-day Indonesia, we would have had H. floresienses — Hobbits — in scattered islands and possibly a bit on the Southeast mainland.
So the hominids in California — which I can’t stress enough, probably weren’t there — were almost certainly not modern H. sapiens. If I had to guess, it would be either Denisovans or (as a distant second possibility) Hobbits. This also means they may not have left traces we’d recognize archeologically — we don’t have a lot of pre-H. sapiens archaeological sites that record what people were doing. In fact, we don’t have a lot of ancient H. sapiens sites that fit the bill — there are only about 17 megafauna kill sites in the Americas when we’ve got a comparatively abundant record for Clovis and Folsom people. Generally, we only have their bones, with only a few very rare exceptions. The findings of Holen et al. would be hard to evaluate for that reason. This would have been a rare find in Europe/Siberia/Africa since it is a site which records behavior, not just bones. This isn’t to say there isn’t any analogues — there is some evidence for stone tools used in Olduvai Gorge in Tanzania, and some butchery at Laetoli in Tanzania and Koobi Fora in Kenya. We do have evidence for butchery among Neanderthals in Europe, but those are typically in cave environments where they lived, not the place where the kill and processing took place. But in most of these contexts, hominid fossils are also found. I can’t stress this enough, we don’t have a great idea for what pre-H. sapiens archaeological sites should look like.
To me, what is more striking is the time — 130,000 years ago — because if I had to make a random guess, that’s exactly when I’d guess humans could have made it over. In fact, it is one of the only plausible windows for any animal to migrate to the Americas. This is because of the ice age cycle. For the entirety of human evolution, we have been going from long, cold glacial periods to short, warm interglacial periods. We are in the middle of one now — and with CO2 emissions, we are inadvertently doing our darn hardest to break out of that glacial cycle. The point is — you can’t get from Siberia to America except for a brief window when:
a) sea levels are low (land bridge exists)
b) giant glaciers are out of the way, and
c) you can live on either side of the land bridge for generations
Modern humans came over 15,000 years ago not just because they were technologically well prepared to do so, but it was the only window. Humans had been in Siberia for 30,000–40,000 years before, but for the time before there were continent-wide ice sheets that would have prevented any meaningful attempt to expand. It is difficult to move enough people from one continent to another as it is — but then you have to have a large enough population move over so that the species is viable. That means thousands and thousands of individuals who are able to reproduce. If you don’t have enough, you will end up with high inbreeding and thus the abundance of recessive, deleterious genes increasing in a small population.
But, 130,000 years ago, this would have been possible, because that would have been during the Eemian interglacial — which was actually warmer than the Holocene period we currently live in. It began 130,000 years ago, and ended 115,000 years ago. That’s the window for the species to make it.
We also know, from a combination of paleontology and genetics, that some animals crossed between Siberia and America at that time. The cheetah (Acinonyx jubatus) evolved in the Americas, and migrated to Asia and eventually Africa during this time.
This, incidentally, is why American pronghorn (Antilocapra americana) are one of the fastest land animals on Earth — they were the first to have to deal with cheetahs, and have had a mostly smooth ride since their extinction around the time Clovis people were roaming around the continent.
The initial migration of American Bison (Bison bison) also may have occurred in this window.
All this is to say, it is plausible that either Denisovans or Hobbits wandered to North America from Siberia at the same time other animals like bison and cheetah were. Though like the cheetahs, they would have faced genetic bottlenecks in the process. But this one site in California is insufficient to justify rewriting the history books just yet. We’ll need more than one site — and bones — to do it. My alarm bells are still up given the profound weirdness of finding a pre-H. sapiens archaeological site. But the picture of human evolution has been getting more complicated every year, and it may be that hominids were much more widespread than we ever could have realized.