Pearl Millet genomics and domestication centre

Modern genomes have the potential to reveal alot about evolutionary history and past geography of a crop. Of course there are some limitation to the degree that it hard to account for extirpated past populations, such as the wild rice that used to grow in central China or the wild pearl millet that used to grow what is now the Sahara. A new study of pearl millet (Pennisetum glaucum) by Bugarella et al (2018) provides a convincing set of deductions from genetic history that infer an origin of the crop in the western Sahel/Sahara around what is today northern Mali/northeast Mauretania (map at left), followed by early differentiation between those of the far west (Mauretania/Senegal), and the eastern Sahel (e.g. Sudan) from those in the core zones of western Africa. Based on spatial simulations that take into account a few archaeological data points they also estimated the onset of the expansion of pearl millet as crop out of its centre of origin a starting ca. 4800-4900 years ago. This fits nicely with current archaeobotany. The earliest, already domesticated, pearl millet is from northeastern Mali in the lower Tilemsi valley between 2500 and 2000 BC. Perhaps a parallel trajectory of dispersal is represented by the Tichitt Tradition of Mauretania (from ca. 1700 BC). What remains an open question is whether these two area represent distinct domestication trajectories (a point suggested by MacDonald et al 2009; Manning and Fuller 2014), much as we see the West Asian Fertile Crescent as a mosaic of domesticators across the region as more or less the same time (e.g.here or there ). The zone delimited by the genetic study could well represent a sort of "West African Fertile Crescent" in which more than cultural groups were in the process of cultivating and domesticating pearl millet during the middle Holocene. The differentiation of a far western genetic groups would then represent dispersal first through the Tichitt-Oualata traditions of Mauretania and onwards to the Sengal valley- which fits with Brunken, De Wet and Harlan's old taxonomic differentiation of a western race leonis (Economic Botany 1977). A rapid and early spread easterns to the eastern Sahel, which was followed by local introgression with local wild populations, is also implied in this genetic analysis, and fits with the albeit limited archaeobotanical evidence for pearl millet (both morphologically domesticated and wild) alongside cultivated sorghum in the Kasala region of eastern Sudan around 1850 BC, just recently published by Beldados et al. (2018). 

Pearl millet demographic modelling: 3rd millennium BC and importance of flowering time

A new article in Molecular Biology and Evolution by Clotault et al "Evolutionary History of Pearl Millet (Pennisetum glaucum [L.] R. Br.) and Selection on Flowering Genes since Its Domestication" has some exciting and intriguing results. They have done demographic modelling, using a number of different scenarios of gene flow, based on 20 random genes, and they have also looked for selective sweeps, finding evidence for strong selection on flowering-time related genes. Although they have only modelled single domestication scenarios (which is by no means a done deal in Pearl millet), they have nevertheless taken into explicit account the notion of protracted domestication process (sensu Allaby et al 2008 or Allaby 2010), with gradual fixation of domestication traits, with exponential rather than instantaneous population growth after the bottleneck. 

They find a bottleneck strength and reduction of genetic diversity that fall amongst those estimated for other crops. Their estimated time for domestication returned an intriguingly plausible 4800 years ago, just a few centuries before the earliest archaeological evidence for domestication pearl millet in the Tilemsi valley (see Manning et al, blogged previously). The evidence they have found for selection on flowering related genes makes a whole lot of sense, because the dispersal of pearl millet from a Sahelian zone southwards crossed many different ecological zones, for which adjustments in seasonality would indeed have been important.

Debating early African bananas

Neumann et al. in a new Quaternary International article "First farmers in the Central African rainforest: A view from southern Cameroon", report a combination of archaeobotanical, apynological and historical linguistic evidence for the nature of early Bantu economies on the northwestern rainforest along margins of central Africa in the First Millennium BC. This includes updated and important discussions of pearl millet, tree nut use (like Canarium and oil palm). These societies brought savanna millet agriculture with them and took advantage of drier conditions to cultivate millet in marginal forest environments, while utilizing (and managing?) forest tree resources as well. Of relevance to those who have argued that bananas were fundamental to early Bantu economies in the rainforest zone (e.g. Blench 2009), however, is the lack of evidence for bananas in these newer excavations. The article includes a short paragraph on bananas, with some quite  critical comments on the issue of early African banans ("act two" in the history summarized below on this blog). They note that study in their samples "several thousand phytoliths already counted, no evidence for Musa could be detected. This sheds further doubt on the banana phytoliths from the contemporary third millennium BP site Nkang" and also they argue that, "There are also ecological arguments against cultivation of banana during this period. As is shown in the following, the climate was much more seasonal in the second half of the third millennium BP and thus unfavourable for plantains which require a humid climate without any major oscillations"

So the debate is out in the open. I don't think there is question of whether the reported phytoliths of Nkang are from Musa, but the worry is surely whether these phytoliths are actually of Iron Age date. They are not directly dated, and the possibility of intrusive or contaminating material from later, when bananas are such a prominent part of the present landscape, contamination is what we need to worry about. On the other hand Nkang is not in exactly the same area as the site studied by Neumann et al, so supporters of the early banana hypothesis might point to diverse and varied economies in the Iron Age. All the more reason to chase more archaeobotanical sampling in the region: we are still reliant on a just a few sites.

African Archaeobotany 2011

I am still trying to see through my Holiday period intention of flagging some of the archaeobotanical highlights of 2011. Africa, as a continent, remains one of the archaeobotanically least known and so it worth noting a number of contributions over the past year.

One of the best itegrated studies (from anywhere, not just Africa) of wood charcoal alongside seeds, pollen and other lines of evidence for the study of changing cultivation practices, including shifting cultivation in Burkino Faso by Hohn and Neumann, which is in press but on-line.

A important book released in 2011 was Marijke van der Veen's monograph in the Journal of African Archaeology series on Consumption, Trade and Innovation, which reports the archaeobotanical evidence from the Red Sea trade port of Quesir, an site with both Roman and Islamic era evidence as a port site [table of contents PDF]. It has excellent illustrations and straight forward quantitative analyses that highlight the different food traditions of the Roman and Islamic periods, highlights trade in foods (fruits, nuts, spices), and the much more 'globalized' of pan-Indian Ocean in character, with lemons, eggplants and watermelons consumed for their seeds. I am particularly taken by the evidence for two-way flow. This is not just about importing spices and bananas from the East, but I am struck by how many European imports, like hazelnuts, were also consumed in the Egyptian desert. The evidence for imported spices in Roman and Medieval western Europe, reviewed by Livarda 2011, is remarkably Asian-centric (black pepper, cardamon, a single medieval nutmeg) but some African Melgueta pepper has been reported from European Medieval contexts.

Some small contributions from the lab here in London came out, too:

Manning et al. reported the earliest archaeological Pearl Millet, with direct dates between 2500 and 2000 BC from the Tilemsi valley in NE Mali, and evidence from chaff impressions for non-shatteing domestication traits. The discussion (and supplement) include a database summarizing the whole archaeological record of pearl millet.

Giblin and Fuller reported the first archaeobotanical results from the first flotation in Rwanda, dating from ca. 400 AD to 1200 AD, with sorghum, pearl millet and cowpea from the earliest samples. Of note is the recurrence of finger millet, and this article includes a discussion and supplementary database of the whole archaeological record of finger millet.

Nixon, Murray and Fuller published the archaeobotany from Tadmekka/ Essouk in NE mali the trans-Sahaan trade route. It was Islamic era trade city, with excavated material between 700 and 1400 AD. Amongst the staples were pearl millet and wild grains (Echnichloa, Brachiaria), but there is also evidence for wheat (imported or locally irrigated) and cotton processing (imported or locally irrigated), and several fruits. It includes an effort to tabulate the sum of archaeobotanical evidence for medieval west Africa.

Ruas and Tengberg (from the Paris archaeobotany lab) published an archaeobotanical study from Igiliz in southern Morocco, the first ever from the region, which included a detailed consideration of Argan oil production (blogged previously).

Although not strictly Africa, another study from the Paris lab, Bouchard, Tengberg and Pra report evidence from Mada'in Salih in Saudi Arabia, which expands our archaeological evidence for cotton cultivation, and is discussed alongside that from Bahrain, in relation to the role of arid Arabia in producing cotton as part of date palm oasis cultivation systems in Antiquity.

The last article taken with Sarah Walshaw's article on Swahili era Pemba from the previous year (World Archaeology 2010: "converting to rice"), frames the northern and southern limits of early Old World cotton production.

One looks forward to the new contributions at this years African Archaeobotany Workshop. For my part, we are making some progress on flots from SE Kenya, SW Ethiopia, and some from Mali from Kevin MacDonald's Sorotomo project.