Open landscapes resembling savannas existed in Northwest Africa about 15 million years ago, well before the period generally associated with their expansion.
This ancient history is difficult to reconstruct directly on land. Soils and plant remains disappear over millions of years. Researchers nevertheless have another archive: sediments accumulated on the ocean floor offshore from the continent. They continuously receive dust and materials transported from nearby land.
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Certain molecules produced by plants are particularly resistant to this long-term preservation. By studying these traces in sediment layers of different ages, scientists can determine which broad types of vegetation once occupied the land. This method complements traditional fossils, which are very unevenly distributed across Africa.
The new analyses indicate that a marked change was already underway during the Middle Miocene. Around 15 million years ago, the landscapes of northwestern Africa contained a significant proportion of open vegetation. Savannas therefore did not necessarily wait for the much more recent transformations often associated with their major expansion across the continent.
To understand this difference, it is necessary to distinguish the emergence of a savanna from the later dominance of certain grasses. Savannas encompass environments where grasses play an important role, alongside more or less widely spaced trees. Their vegetation also changed over time. Ancient landscapes therefore did not necessarily resemble the great African savannas of today.
Previous data had notably shown a later sharp increase in so-called C4 grasses. This term refers to a particular way of capturing carbon during photosynthesis. It benefits many grasses in high temperatures and certain dry environments. Their rise does not necessarily mark the birth of the earliest open landscapes, however.
This new chronology also places Miocene animals in a different setting. Earlier studies have shown that African herbivores and thorny plants were already diversifying more than 10 million years ago. More open spaces may have altered food resources and relationships between plants and animals long before modern savannas became dominant.
Marine sediments now make it possible to track this transformation over a long period without relying solely on rare terrestrial fossil sites. Researchers can thus compare the evolution of vegetation with changes in climate and animal life. This chronology should notably help determine when different types of savanna became established in Northwest Africa.