Scientists like to be really sure. That is pretty much what the scientific method is all about – systematically controlling for all possibilities, all confounding factors, all variables and all alternative interpretations. We feel more confident when multiple lines of evidence converge on one explanation, and when rigorous attempts to disprove that explanation fail.

I like seeing that process in action over specific claims, especially when the claim itself is interesting.

One such interesting question is, what was the earliest human presence in the Americas? Any question about first or earliest is always tentative in paleontology. It simply refers to our current evidence, but it is statistically unlikely that we will have found the literal earliest example of a species or an occupation. So conclusions about “earliest” are always changing, moving back as more evidence is found.

There is solid archaeological evidence for human occupation near the  Bering Strait 14,000 years ago. There is strong but not universally accepted evidence for this occupation going back 24,000 years. So that is the current range of possible dates for the earliest presence of humans in North America.

Solid evidence of human occupation is human bones with reliable dating. Obviously if there are human remains in North America, then humans were in North America. The only question then is about dating. Human bones are considered direct unequivocal evidence.

But there are lots of types of indirect evidence. The next step down the evidence ladder in terms of reliability are unequivocal human artifacts, such as stone tools. Tools that have been extensively shaped in such a way that only human action could have done so is also great evidence that humans were present. The tools themselves cannot be dated, but the substrate in which they are found sometimes can be (depending on local conditions).

We can also find evidence of human occupation. For example, the Monte Verde site in Chile has evidence of wooden structures, hearths, and stone tools. The site dates to 12,500 years ago, within the accepted time frame of human occupation.

As we go further down the evidence pyramid we get to equivocal and disputed indirect evidence. This is evidence that suggests human occupation, but archaeologists are generally unwilling to accept this evidence as definitive, especially if it moves back the date of first occupation. The two main types of such equivocal evidence are possible artifacts and possible evidence of activity, such as processing bones.

The evidence for human occupation in Alaska at 24,000 years ago is based entirely on marks on bones that are consistent with stone tool marks. This came from an exhaustive survey of thousands of bones in the Bluefish caves, with 14 showing evidence of stone tool marks. This evidence is interesting, but not universally accepted.

There is also the Topper site, which some archaeologists (most notably Goodyear) claim contains human artifacts from 50,000 years ago. The “human artifacts”, however, are possible “rudimentary” stone tools – in other words, they are rocks.  Such evidence is generally considered unconvincing.

New Evidence for 130,000 Years

Researchers are now claiming evidence for human occupation in North America dating to 130,000 years ago. The evidence is not human remains, and not unequivocal artifacts. It is possible stone artifacts and possible evidence of working bone – the weakest form of evidence for occupation. As the BBC reports:

Thomas Deméré, Steven Holen and colleagues examined material from the Cerutti Mastodon site near San Diego. The site was originally uncovered in 1992, during highway construction work. Possible stone tools were discovered alongside the smashed up remains of a mastodon (Mammut americanum) – an extinct relative of mammoths and living elephants.

“Possible stone tools” means rocks. They are unworked – they haven’t been reshaped for use. The evidence that they are tools comes from wear marks on the rocks.

The evidence that the  Mastodon bones were broken deliberately by smashing them with rocks is due to the pattern of breaks. The spiral pattern is consistent with being broken up while fresh. The researchers also reproduced breaking up elephant bones with large rocks and the resulting pattern of breaks was similar to the fossil finds.

The BBC did a good job of reporting this item. They did what science reporters are supposed to do – they spoke with experts in the field who are not the researchers making the claims. They found general skepticism among those experts. The consensus is that this evidence is “provocative” but extremely thin and unconvincing.

Skepticism is higher because of the nature of the claims. Moving back first occupation to 130,000 years is a huge change. That could mean they were not even modern humans, but Neanderthals. There is nothing impossible about this, but that is a massive claim based on a thin reed of evidence.

I think it is reasonable for paleoarchaeologists to be conservative in this regard. Human remains and unequivocal human artifacts are solid evidence of human occupation. The lesser forms of indirect evidence are suspect. As some of the scientists pointed out, there are lots of ways for stones to be worn and bones to be broken, and many of those could look like human activity.

This type of evidence relies heavily on an argument from ignorance – not knowing another way that the bones could have been marked or broken the way that they were. That is a far cry, however, from proving that there was no other possible way for the observed breaks to have occurred.

This type of reasoning is common in science, which is why we prefer direct or corroborating evidence. Inference to the best conclusion is fine, but always a weak and tentative conclusion without direct evidence to back it up. In this case I think that the skepticism of the scientific community is fully warranted.

As a side note – I hope this analogy is not lost of regular readers who have been following the discussion about the historicity of Jesus. (And sorry to beat this horse if it was obvious.) We have a similar situation, in which the conclusion is based upon indirect evidence and reasoning based heavily on an argument from ignorance. Nature has many ways to break bones, and there are many ways for stories to emerge and spread. Not knowing exactly how a story emerged does not mean it was based on a real historical figure. We need corroborating direct evidence, which is lacking.