Palaeolithic is the new Neolithic
In the last six years we have seen an explosion of human ancient DNA studies most of them focused in a controversial and long standing question, the Neolithization of Europe (for example here). This process was ‘culminated’ by the publication last year of 230 low coverage ancient genomes across Europe raging from the Mesolithic to the Iron Age. The majority of them, however, came from Neolithic sites. The Eurasian Bronze Age also got in the spotlight during 2015 with over hundred low coverage ancient genomes, and just a few weeks ago two interesting papers (this one and this one) revealed details on the relationships between ancient Britons from the Iron Age, Anglo-Saxon and Roman periods, and modern Europeans. So far, older times, such as the Upper Palaeolithic have deserved less attention, and hence the dynamics and between relationships the first modern humans inhabiting Europe remain largely unknown.
There are at least two reasons for this scarcity of human ancient DNA research before the Neolithic. First, we tend to think that whatever makes us European, both biologically or as society, must have happened when farming entered Europe. This in part makes sense as so many changes happened in the Neolithic transition. No just the way we obtained and consumed food, but also the turn to a more sedentary life style, they way we organized our societies and interacted between groups, the upheaval in culture, etc. It is very easy to be interested on what happened with human populations in this period.
The second reason is technical. Population density during the Palaeolithic was much lower than any other time after. On top of that, it is more difficult to find remains of the peoples that inhabited Europe 40,000 years ago than 4,000 simply because they are older and the remains have less chances to survive that period. But even if we are lucky enough to find remains, it is generally more complicated to sequence Palaeolithic than Neolithic DNA. The older the remains the more degraded the genetic material on them will be, and hence the lower are the chances to obtain usable sequences. New technology is developing lightning fast, but sequencing ancient genomes older than 10,000 years is still a huge challenge.
Whatever the reason, the lack of ancient human genomes from Palaeolithic times began to change a couple of years ago. Nowadays we have a bunch of Mesolithic ancient genomes from Europe (see here), but still not that many from before 10,000 years, apart from the two Upper Palaeolithic genomes (Satsurblia from the Caucasus and Bichon from Central Europe) very recently introduced by Jones et al. 2015.
The figure at the head of this article belongs to an important paper on human Palaeolithic populations by Posth et al. that was published last week. In this paper we don’t have information from nuclear genomes, only mitochondrial DNA, but the main result suggest something very interesting: a strong bottleneck during the last glacial maximum (LGM) followed by a massive genetic turnover (extinction of several mitochondrial lineages and recolonization by others) at the Late Glacial is the demographic model that fits best the data (see figure on the left). You’ll find good discussions about this elsewhere on the web (here, here or here for example). What I want to remark about it is that, in my opinion, Posth et al. results highlight a very important question that remains widely unanswered: what shaped the structure and genetic diversity of the European human populations before the Neolithic.
Ancient genomes from the Mesolithic suggest that there was lot more genetic diversity among settlers before farming arrived to Europe. For example, if we look a the PCA from Haak et al. 2015 (figure above), we can see that the Mesolithic hunter-gatherer genomes from Europe (EHG, SHG and WHG) are lot more sparse than the ones from the Neolithic (EN and MN), at least before the steppe influx on the late Neolithic. Even though this effect is probably also driven by the fact that the PCA space is dominated by modern samples and the ancient samples are just projected onto it, it still is a relevant clue suggesting the presence of high levels of population diversity and structure in the pre-farming Europe.
But what happened before? What elements forged that diversity? What ecological or climatic, cultural or behavioural traits drove population structure in these early European human populations? Posch et al. 2016 is important because, although using only the sex-biased information contained in the mitochondrial DNA, it has already started addressing these matters. With the major questions related to the Neolithic already outlined, I am sure that in the following years we will see a lot more Palaeolithic ancient genomes and a shift of focus of ancient DNA research from Neolithic to the pre and post-LGM. Palaeolithic is the new Neolithic.
