Three papers that caught my attention this week in the world of human evolution:
It is now well-established the early “modern” humans (that term will remain in scare quotes until I resolve myself on something better) admixed with Neandertal populations, in addition to several other archaic human lineages, on multiple occasions. The establishment of such admixture between these diverse lineages has resolved a long-standing argument in paleoanthropology. Not yet resolved is what happened after such admixture events. In particular, a fascinating set of conflicting results has popped up as to whether and how Neandertal alleles that entered into the gene pool of expanding “modern” human populations were subsequently selected against via natural selection. At issue is whether or not Neandertal variants were largely selected against very quickly (on the order of ~1000 years) or whether there was a slow, gradual decline in the amount of Neandertal ancestry carried by “modern” humans (Fu, et al., 2016; Harris & Nielsen, 2016).
The basic idea is that Neandertals had an excess of deleterious alleles relative to “modern” humans, at least in part, because Neandertal (effective) population sizes were so small as to limit the effectiveness of selection to act on these alleles. Once they found themselves on the background of a less demographically-constrained genetic background (provided by “modern” humans), they were suddenly more available for the action of selection. Rather than go into the technical details of the paper, I’ll refer you to a great explainer thread on twitter from one of the paper’s authors, Benjamin Vernot:
Our new paper is out, with @fleventy5 @jfkelso and Svante! We looked at the role of selection on introgressed Neandertal DNA. There might be a TL;DR at the end, but first I want to talk not about the final result, but the process of making the sausage. https://t.co/gObigkQpoC 1/n
— Benjamin Vernot (@melanoidin) January 15, 2019
This matters because it provides additional framing for how we think about the demographic and evolutionary context of admixture between paleo-lineages of humans. It is potentially important in the increasingly nuanced argument about whether or not Neandertals (and other archaic lineages) represent different species of humans, or simply divergent (and largely extinct) lineages of Homo sapiens.
This paper is a really wonderful overview/review of how we can address questions of evolutionary change and evolutionary process through study of morphology. This review builds off a lot of exciting work within the field over the past decade, including some of von Cramon-Taubadel’s own work (e.g. von Cramon-Taubadel, 2014). I thought the paper did a great job of presenting how we ask questions of morphology alongside the limitations on those questions. I took out a couple of quotes from it on twitter. For example:
“…It is important, therefore, to constantly bear in mind that just because we measure a bone a certain way (and have been doing so for decades) does not imbue that measurement with any inherent evolutionary information…”
This question of how evolutionary information is distributed within, among, and between fossils and fossil samples has been on my mind a lot of late, and is often an issue left unaddressed in work. von Cramon-Taubadel also goes on to make a very clear distinction between the various ways in which “functionality” gets interpreted in morphology, and how those varying definitions play into arguments about the action of natural selection versus genetic drift:
“So, the overall phenotype is, of course, “functional,” but the question is whether variation within the phenotype can be thought of as “adaptive,” in the sense that natural selection is required to explain the diversification of skeletal form across lineages…Rather than pitching stochastic and selective explanations as alternative and competing hypotheses, we need to recognize neutral models as helpful null hypotheses of explanation on which we can build more elaborate adaptive scenarios should they be needed.”
That paper is definitely going onto my syllabus.
* This last one is cheating, because it is not a paper, but instead a special issue featuring a whole batch of papers. The online journal, Paleoanthropology, released a special 2018 volume with a series of papers on Australopithecus sediba, the set of fossils recovered from the site of Malapa, South Africa, a decade ago.
The volume includes eight research papers on different aspects of Au. sediba‘s anatomy, in addition to an introduction by Scott Williams, Jerry DeSilva, and Darryl De Ruiter (2018). With all the excitement about the Rising Star hominin fossils over the past several years, it is easy to lose track of the other amazing batch of fossils discovered over the past decade, not far down the road.
I love Paleoanthropology as a journal because it is entirely open-access and its online-only format means that there are no space constraints on publications. The journal actively encourages you to think of ways of publishing raw data! So you can go to this paper on the cranial remains by De Ruiter, et al. (2018), and immediately find yourself immersed in 17 pages of comparative data tables. I love it!
Works cited:
– De Ruiter, et al. “The Skull of Australopithecus sediba.” Paleoanthropology 2018: 10.4207/PA.2018.ART112
-Fu, Qiaomei, et al. “The genetic history of ice age Europe.” Nature 534.7606 (2016): 200.
-Harris, Kelley, and Rasmus Nielsen. “The genetic cost of Neanderthal introgression.” Genetics (2016): genetics-116.
-Petr, Martin, et al. “Limits of long-term selection against Neandertal introgression.” Proceedings of the National Academy of Sciences (2019): 201814338.
– von Cramon-Taubadel, Noreen. “Evolutionary insights into global patterns of human cranial diversity: population history, climatic and dietary effects.” J. Anthropol. Sci 92.4 (2014).
– von Cramon-Taubadel, Noreen. “Multivariate morphometrics, quantitative genetics, and neutral theory: Developing a “modern synthesis” for primate evolutionary morphology” Evolutionary Anthropology (2019). https://doi.org/10.1002/evan.21761
– Williams, DeSilva and De Ruiter. “Malapa at 10: Introduction to the Special Issue on Australopithecus sediba” Paleoanthropology 2018: 10.4207/PA.2018.ART111
