Why hominin fossils matter

Yesterday Dienekes had a post up, Homo erectus soloensis fades into the past…. In it he states:

Every year or so there seems to be a redating of a key fossil in human evolution. It’s nice to see scientific self-correction in action, and soon after Neandertals got a little older, casting doubt on their supposedly long co-existence with modern humans, we now have a redating of Homo erectus soloensis from Java to about 150-550 thousand years ago, but certainly long before there were any anatomically modern humans in the area.

I think Dienekes is jumping the gun a bit in terms of the solidity of any given finding in knocking down prior consensus. That being said, the very young ages for Southeast Asian H. erectus, on the order of ~30-50,000 years B.P., always seemed strange to me. The paper Dienekes is referring to, The Age of the 20 Meter Solo River Terrace, Java, Indonesia and the Survival of Homo erectus in Asia, is rather technical in the earth science, as it involves dating and interpreting confounds in the stratigraphy. But this section of the discussion gets to the gist of the matter if you can’t follow the details of fossil dating:

If the middle Pleistocene 40Ar/39Ar ages better reflect the age of the Solo River 20 meter terrace deposits and hominins, the site of Ngandong remains a relatively late source of H. erectus; however, these H. erectus would not be the contemporaries of Neandertals and modern humans, and their chronology would widen the gap between the last surviving H. erectus and the population from Flores – whose source population has been argued to be Indonesian H. erectus…although this point is contested…Instead, the Ngandong hominins would be contemporaries of the H. heidelbergensis from Atapuerca, Spain and elsewhere in Europe, and, possibly the archaic H. sapiensspecimen from Bodo (Ethiopia), which might favor arguments that they are more closely affiliated with these taxa and differ from H. erectus…Such ages for Ngandong would suggest that a series of geographically relatively isolated lineages of hominins lived during the middle Pleistocene.

About three years ago I went to a talk given by Tim White, famed paleoanthropologist who was the maestro behind Ardi. He began by observing that fossils are real, concrete, and they matter. My own suspicion that is that White was taking a mild swipe at the prestige accorded to genetic inference in reconstructing the human past. In some ways White’s assertion would seem to be be validated. Svante Pääbo subsequently gave a talk at the conference where he very confidently ruled out Neandertal admixture into the modern human lineage. Pääbo famously did an about face on this issue later, because new data compelled him to do so. But, observe that the inferences generated are sensitive to the inputs you have available at any given moment.

Fossils are concrete and can be absorbed in a Gestalt fashion. You can visually see how much more robust earlier hominin lineages were when you look at their skulls! But the reality is that this sort of innate understanding of the general nature of a fossil has its limitations. Recall the conflicts of interpretation between paleoanthropologists which raged across the decades. Part of the issue is the matter of sample size. So much hinges on just a few specimens. And very few people alive have a command of the details of all these specimens to make an intuitive assessment of the issues at hand.

And just because the fossil is concrete, it doesn’t mean that all the contextual issues around the fossil are so cut and dried. Just like geneticists make inferences of facts unseen from patterns of variation they do see, so paleontologists need to make inferences as to the geology of a locale to accurately assess dates and fossil succession. Because of modern radiometric dating the errors are far less than they were in the past, but when you’re talking human evolutionary history the margin is far less than in other situations. A few tens of thousands of years here, and a few tens of thousands of years there, and pretty soon you’re talking a very long span indeed.

With the confused state of the understanding of modern human origins today fossils are even more important in pegging some of the parameters to fixed values in our models. Consider the possibility of admixture across human lineages. Genetics can tell us that two different lineages which were isolated from each other eventually co-mingled at some point in the future, but it’s a much sketchier thing to pinpoint where that event occurred. To do so is a matter of elimination of the possibilities. If the fossils tell us that other hominin lineages were thick on the ground in eastern Eurasia then the possibilities for admixture naturally present themselves. On the other hand, if it seems that hominins were very rare or extinct before modern humans, then they do not.

There are two extreme scenarios for the exotic ancestral elements in modern humans, the ~2.5% Neandertal in all humans outside of Africa, as well as the ~5% Denisovan in Melanesian populations. In one model the admixture event occurred outside of Africa, after modern humans left that continent. The distinctiveness between the two genetic lineages is a simple matter of geographic distance. Another model, which I believe Dienekes’ favors, is that the admixture is simply a reflection of the structure within the continent at the time of the “Out of Africa” event. In other words, ancient Africans were themselves compounds of different hominin lineages.

Obviously the nature of the fossil record matters a lot in weighing the probability of the disparate scenarios. Right now much of the world is a large blank spot. West and Central Africa, large swaths of East Asia after the earliest erectus finds, for example. So it is critical that the fossils that we do have are well dated. And because of the paucity of the fossil record much can ride on a solid date for a few specimens here and there. As a civilian it can get very frustrating, because it’s not really possible to adduce which side hast he right of a matter if there’s a conflict. But often that’s just how science is.

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