1 million years ago, a migratory wave of humans (homo erectus) and animals (large mammals) from the east swept across the northern Mediterranean in a bid to conquer new territories. At the time, the ecosystems they were targeting to ensure their survival were wetlands, veritable oases of life and potential food in a generally arid Mediterranean environment. The tufa of Marseille (Bouches-du-Rhône, France), with its ecological diversity, its edible plants including proto-cereals, fruits and herbaceous plants, and its water resources, was a favourable site for this migratory dynamic.

A scientific article published in Geosciences by Valérie Andrieu, professor at the University of Aix-Marseille and member of CEREGE, and her colleagues from the Universities of Aix-Marseille, Montpellier and Malta, proposes a reconstruction of the palaeoenvironment of Marseille at the beginning of the Pleistocene, 1 Ma ago, thanks to a multidisciplinary study of the fluvial tuffs in this basin.

Palaeomagnetic measurements have identified the Jaramillo magnetic inversion and dated the Marseille tuff to between 1.06 and 0.8 Ma. The sedimentological data show the existence of a varied depositional environment including natural dams formed by accumulations of plants stabilised by carbonate precipitation, favouring the development of upstream bodies of water within marshes.

Carbon isotope ratios indicate that the Marseille tuffs are not true travertines but tuffs deposited by cold water springs.

Climatic reconstructions based on pollen data indicate a climate that is slightly cooler (especially in winter) and wetter than the current one.

Analyses of fossil pollen indicate a semi-arboreal, diverse, mosaic plant landscape, dominated by a Mediterranean forest of pine and oak with beech, fir and spruce, species that are now rare or no longer grow at low altitude in Provence, mainly due to human occupation. The presence of the chestnut tree is unexpected in a limestone environment, but this tree was able to grow on the decarbonate clays of the Oligocene, which outcropped everywhere in the Marseille basin.

Along the rivers, the riparian forest was diversified and included walnut and plane trees, as is the case today in the eastern Mediterranean, and trees such as alder, willow, hazel and ash. The potential diet of the first hominins, which we have reconstructed from pollen and plant macroremains, was varied and included the fruits of chestnut, hazelnut and walnut trees, as well as arborescent Rosaceae such as various species of plum and apple trees. Remains of vines have also been found, showing that grapes were already part of the diet of frugivores, including hominins.

Among the many edible herbs identified are the Compositae, which include many salads, nettles and mallow, a plant particularly popular in North Africa. Populations of hominins could potentially feed on resources from the sea, which were diversified at the time, and on land resources, including large herbivores.

The most surprising discovery was the presence of cereal pollen (proto-cereals because of their age), including rye, which was identified. These proto-cereals, which grew as part of the steppe herbaceous procession, were able to substantially enrich the diet of mammals (including hominins) that frequented the Marseille basin a million years ago with carbohydrates.

The Marseilles basin is the third site after those of Acigol and Kocabas (Andrieu-Ponel et al., 2021), in south-western Anatolia, to show the presence of proto-cereal pollen well before the start of the Neolithic period 12,000 years ago. The identification of spores of coprophilous fungi shows the presence in situ of herds of large herbivores.

It is possible that, as in Anatolia, the disturbance of ecosystems by large herbivores was responsible for the genetic mutation of Poaceae and the appearance of cereals. These sites show that the appearance of cereals was not caused by human populations, but rather by a natural process linked to biotic interactions between populations of large herbivores and steppe ecosystems.

In the Neolithic period, mankind, having become farmers out of necessity due to the reduction in mammalian fauna, would have cultivated edible plants that pre-existed in herbaceous ecosystems. This new discovery of proto-cereals requires a new vision of the history of human nutrition, as we suggested earlier (Andrieu-Ponel et al., 2021).

With the aim of gaining a better understanding of our future, an international collaboration involving researchers from the Centre de Recherche et d'Enseignement en Géosciences de l'Environnement (AMU/CNRS/IRD/INRAE/Collège de France) is exploring ancient warm periods in the Earth's history. In their study, published on 29 January in the journal Nature Geoscience, they show that an increase in greenhouse gases in the atmosphere 56 million years ago led to a sudden change in rainfall and vegetation in Central Asia.

Today, vast treeless steppes characterise the landscape of Central Asia. The rare rainfall that reaches the interior of Asia is mainly due to the monsoon in summer or to mid-latitude westerlies in winter and spring. These steppes are particularly sensitive to changes in temperature and precipitation, with all the consequences that this implies for their inhabitants, such as the Saiga antelope or Przewalski's horse, an endangered species”, explains Dr Niels Meijer, before adding: “One of the major uncertainties of global climate change is how the Asian monsoon and the Central Asian regions that depend on it will react to future climate change.”

As part of an international research team that has been working in the region for more than twenty years, Niels Meijer recently focused on the warm period of the early Palaeogene, where he identified a hyperthermal event of extreme warming precisely dated to 56 million years ago.

In order to reconstruct the precipitation patterns of this period, the researchers combined their expertise to develop an innovative multi-proxy approach in which they combined fossil pollen and spores with geochemical data from fossil soils. “During the hyperthermal event we studied, precipitation temporarily doubled due to higher temperatures and the regional steppe was replaced by a forested landscape,” explains Meijer, describing the results and adding, “But most importantly, we were able to use geochemical data to show that the soils dried out in winter, meaning that, contrary to expectations, most of the precipitation fell during the summer period - which is comparable to the modern monsoon.”

Scientists associate this hyperthermal event with the “Paleocene/Eocene temperature maximum”, a phase of global warming associated with a considerable increase in greenhouse gases in the Earth's atmosphere and oceans. During this period, the global temperature rose by an average of six degrees Celsius in the space of a few thousand years. Their research shows that in Asia this event was accompanied by exceptionally wet conditions and an inland expansion of precipitation, which they have called a proto-monsoon.

“The abrupt greening of the Central Asian steppe desert as a result of the monsoon climate probably also allowed the spread of new mammal species and may also have played a role in feedbacks in the global carbon cycle”, Meijer sums up before concluding: “Our work shows an abrupt and non-linear response of the Asian monsoons to extreme greenhouse conditions. Although the seas and mountains of Asia 56 million years ago were very different from those of today, these data highlight the possibility of abrupt changes in precipitation and ecosystems in Central Asia under future global warming. Current warming is imposing extreme temperatures and increased drought on the Central Asian steppe and its fragile flora and fauna, which are already under threat from anthropogenic land use. Even more dramatic abrupt changes are expected if temperatures continue to rise”.

29.01.2024