• For the first time we demonstrate that C4 grasses were the dominant herbaceous element of the Pikermi Formation. Our habitat reconstruction suggests fire-prone woody grasslands and woodlands within a savannah biome for Pikermi and Pyrgos and, thus, provides unambiguous evidence for the early environmental conjectures of Gaudry. Given the potential hominin nature ofGraecopithecus freybergi, our habitat reconstruction for the Pikermi Formation further supports the “Savannah Hypothesis” put forward to explain earliest hominin emergence. Analysis of both potential hominin sites indicates that Graecopithecus inhabited different habitats, be it open braided-river landscapes in Azmaka, or the wooded grassland of Pyrgos.

• The Tortonian-Messinian transition in the Mediterranean appears to represent a period of significant environmental and climatic changes. During the latest Tortonian (~7.4–7.25 Ma) C4 grass ecosystems progressively penetrate the Balkan Peninsula and constitute the environment of the mammal fauna of Pikermi, which contradicts earlier assumptions . The classical Pikermi fauna is terminated at the beginning of the Messinian (7.25–7.10 Ma) by a significant faunal turnover (post-Pikermi turnover), accompanied by massive increase of Saharan dust and salt accumulation with profound effects on soil salinity and nutrition.

• Our results reveal formerly unrecognized Mediterranean environmental changes during the Tortonian-Messinian transition, which provide important constraints for the evolution of Graecopithecus freybergi. At the Tortonian-Messinian boundary (7.25 Ma), water-stress levels increased and wildfire frequency decreased, which can be interpreted as increasing aridification. Rather than representing a local phenomenon, aridification occurred on a larger scale. We demonstrate that aeolian dust accumulation was widespread at the northern Mediterranean coast and that large amount of salt-laden mineral dust and marine-based aerosols were blown from dried lake beds in North Africa toward Europe, where ~30-m-thick red silts were deposited in southern Greece and southern France. We relate this dust accumulation to progressive late Tortonian Mediterranean aridification and cooling, which started at around 7.4 Ma and culminated during the earliest Messinian, when Mediterranean Sea surface temperature dropped by about 7˚C to values comparable to the present-day (Fig 4). Modelling studies have shown that Middle Miocene Tethyan seaway closure and accelerated Late Miocene uplift of the Iranian plateau provided key boundary conditions for north African aridity. We hypothesize that the ~700 kyr cooling episode, combined with the long-term eccentricity minimum between 7.3 and 7.2 Ma, acted as a final trigger for substantial north African aridization, which resulted in the initial formation of a large Saharan and Arabian desert belt. Furthermore, mineral dust in Attica was rich in soluble evaporites (halite, gypsum) in the earliest Messinian and especially during two pronounced insolation seasonality minima at 7.18 and 7.157 Ma, which suggests an orbitally driven progressive Sahara desertification. We suppose that a latest Tortonian to early Messinian dust- and salt-laden atmosphere over the Mediterranean may have further accelerated cooling and aridification via absorption of incoming solar radiation and, thus, may partially explain regionally accentuated Mediterranean cooling.

• The documented environmental changes were likely to have caused a significant faunal transition. Our dating ofGraecopithecus and the taxonomy of its accompanying large mammals indicate that, during culmination of cooling at the base of the Messinian, the post-Pikermi turnover replaced part of the Pikermi fauna. Several newcomers like the elephantoid Anancus or the boselaphidTragoportax macedoniensis have Asian affinities and we hypothesize that Eastern Mediterranean aridification played an important role in the westward shift of their habitats.Graecopithecus, as part of this new post-Pikermi fauna, lived in a warm-temperate and dusty environment unlike any other known hominid (except for our own genus).

• We subdivide Upper Miocene sediments of the Athens and the Mesogea Basins (Figs 1 and 2) into the terrestrial to alluvial Pikermi Formation (new formation) and the palustrine to lacustrine Rafina Formation (new formation). The Pikermi Formation represents an up to 30-m thick sequence of predominantly reddish silts with subordinate clastic channels of conglomerates and sandstones, which contains a rich and exclusively terrestrial vertebrate fauna. The formation rests discordantly upon the ‘lower limestone unit’ (palustrine to lacustrine grey marls and coals) and is concordantly overlain by the Rafina Formation (palustrine to lacustrine clay, coal, and platy limestone). Based on transport mechanisms, sediment colour, and palaeosol development, the Pikermi Formation can be subdivided into two members: the Red Conglomeratic Member (new member) characterized by debris flows and the fluvio-alluvial Chomateri Member (new member). The lower part of the Pikermi Formation (Red Conglomeratic Member) represents an alternation of red silts with a weak pedogenic overprint and debris flow deposits (Fig 3). These debris flows contain clasts of the nearby Attica unit of Mt. Pendeli, which indicates a strong topographic gradient. Silts from the lower Red Conglomeratic Member include the classical Pikermian bone accumulations. The upper Pikermi Formation (Chomateri Member) represents an alternation of reddish to yellowish silts with fluvial channels and channel-fill trains (Fig 3) that are indicative of small migrating streams during times of increased surface run-off. Away from channels, silts can contain well developed calcareous palaeosols rich in mammalian fossils. In the Athens Basin the Pikermi Formation is best known from the Pyrgos outcrop (Fig 2). In both basins, the Pikermi Formation is concordantly overlain by palustrine clays and coals, and lacustrine marls and limestones of the Rafina Formation.