In southeastern Algeria, at more than 1,500 meters altitude, a sandstone plateau extends over 72,000 square kilometers -- an area comparable to Belgium multiplied six times. This plateau, the Tassili n'Ajjer ("plateau of rivers" in Tuareg), is today a desert of rock, sand, and columns sculpted by erosion. But its cliffs and rock shelters contain some 15,000 engravings and cave paintings dating from 10,000 BCE to the early centuries CE -- the world's largest open-air prehistoric art museum. These works are witnesses to a Green SaharaGreen SaharaA name for the Sahara during the "African Humid Period" (c. 14,500 to 5,000 years ago), when increased monsoon rainfall sustained lakes, rivers and savannas, making the region habitable before its gradual desiccation.→.1

The Green Sahara: A Climatic Reality

Between 11,000 and 5,000 BCE, the Sahara experienced a climatic episode scientists call the "African HoloceneHoloceneThe current geological epoch, begun about 11,700 years ago at the end of the last ice age; the setting of all post-glacial history.→ Climatic Optimum" or "Humid Sahara." The African monsoon, shifted northward by the Earth's orbital parameters (Milankovitch cycles), regularly watered what is today a desert. Lakes occupied the depressions of the eastern Erg; temporary rivers, oueds, ran at the foot of the cliffs. Vegetation was dense enough to feed cattle herds -- and human communities of several thousand individuals.

Rock engraving of Bubalus antiquus at Tassili n'Ajjer, Algeria, 8000 BCE
Engraving of a Bubalus antiquus (giant African buffalo) at Tassili n'Ajjer (Bubaline period, c. 10,000-7,000 BCE). The presence of large aquatic mammals proves the Sahara was then a humid wooded environment. (Photo: Hocine Zirari / CC BY-SA)

The Great Artistic Periods

Tassili art is divided into several successive styles corresponding to the major climatic and cultural phases of the Sahara:

The Large Fauna period or "Bubaline" (10,000-7,000 BCE) is the oldest. It features monumental engravings, often large-scale, depicting animals now extinct or absent from the Sahara: Bubalus antiquus (a giant buffalo with 2-meter lyre-shaped horns), hippopotamus, rhinoceros, elephant, giraffe, and crocodilian species. These animals unambiguously prove the Sahara was then a humid environment with permanent water bodies and savanna vegetation.2

The Round Head period (8,000-6,000 BCE) is the most mysterious. It features paintings of human figures with spherical heads, without distinct facial features, sometimes wearing horns or mushroom-shaped structures. Their bodies float in suspended poses, surrounded by lines and geometric forms. These figures inspired popular extraterrestrial theories (without scientific basis). They more likely represent masked beings during shamanic rituals, analogous to "sorcerer" representations in other world rock art.

The Pastoral period or "Bovidian" (7,500-3,500 BCE) corresponds to the Green Sahara's zenith and the arrival of the first cattle herders. Paintings show herds of graceful-horned cattle, enclosures, milking scenes, and daily life. These works constitute an exceptional ethnographic document on the pastoral societies that occupied the Sahara before its desiccation. They are often polychromatic and of high artistic quality.

The Horse period (2,000-700 BCE) marks the horse's entry into the Sahara, probably from the north (Carthaginian or Egyptian influence). Chariots pulling warriors appear, testifying to contacts with Mediterranean civilizations. Progressive desiccation is visible: cattle rarify, herds diminish.

The Camel period (from 100 BCE onward) reflects a desiccated Sahara. The camel (dromedary), perfectly adapted to heat and lack of water, replaces the ox as beast of burden. The more schematic engravings and paintings of this period mark the end of Tassili rock art as an expression of sedentary or semi-nomadic culture.3

A Threatened Heritage

Listed as a UNESCO World Heritage Site since 1982, Tassili n'Ajjer is today threatened by several factors. Tourism, even supervised, wears painted and engraved surfaces. Modern graffiti cover some works. Clandestine excavations have removed engraved sandstone blocks. Current climateClimateThe long-term average atmospheric conditions of a region; its variations (glaciations, aridifications) shaped migrations, agriculture and the collapse of prehistoric societies.→ change could also modify local rainfall patterns, accelerating wind and water erosion of the rock faces.

Tassili n'Ajjer nonetheless remains one of the most evocative sites on the planet: its forests of erosion-sculpted sandstone columns, deep canyons, 2,000-year-old Saharan cypresses (Cupressus dupreziana, a critically endangered endemic species), and prehistoric frescoes compose a unique landscape -- the painted memory of a world that no longer exists.