University of Tübingen: Desertification drove mammals from Eurasia to Africa

The formation of deserts on the Arabian Peninsula has had a decisive impact on the migratory movements and evolution of large mammals and our human ancestors over the past millions of years. This is the result of an international team of researchers led by Professor Madelaine Böhme from the Senckenberg Center for Human Evolution and Palaeoenvironment at the University of Tübingen in a new study. The scientists reconstructed the climatic history of the northern Arabian Peninsula between 12.5 and 2.5 million years ago using data they obtained from rocks in Mesopotamia. This resulted in new indications of the causes of the animal migration. The results were published in the journal Nature Communication Earth & Environment released.

The evolution of today’s African savanna fauna took place in relative isolation over the past five million years. This had been known for a long time – as was the fact that the ancestors of many savannah animals such as rhinos, giraffes, hyenas and big cats came from Eurasia. So far, however, it was unclear what prompted the animals to move between the continents over a large area.

Rock stores climate data
The northern Arabian Peninsula is the gateway to Africa. Today it includes desert areas such as the Syrian Desert, the Israeli Negev Desert and the Saudi Nefud Desert as well as more humid steppes and semi-deserts in Mesopotamia, the greater part of which is in what is now Iraq. The research team examined the 2.6 kilometers thick rock layers at the foot of the Zagros Mountains in what is now Iran, on the edge of Mesopotamia, using chemical, physical and geological methods.

It found evidence of four short periods of desertification in Mesopotamia, each lasting only a few tens of thousands of years. These phases 8.75 million, 7.78 million, 7.5 million and 6.25 million years ago were each interrupted by sections with a more humid climate. “5.6 million years ago, at the same time as the temporary drying out of the Mediterranean, there was an extreme drought that lasted 2.3 million years in Mesopotamia,” says Madelaine Böhme. This exceptionally long period with a desert climate – called NADX (Neogen Arabian Desert climaX) by Boehme’s team – was only ended by global warming 3.3 million years ago.

Stages of migration and isolation
Mighty strata of rock in the northern Arabian Peninsula
The researchers examined 12 million years of climatic history of the northern Arabian Peninsula on 2.6 kilometers of rock.
“Contrary to what we expected, these desert phases on the Arabian Peninsula did not match those in the African Sahara,” reports Böhme. Desertification in the Sahara is causally linked to polar ice formations, while those on the Arabian Peninsula and Mesopotamia, according to the results, are linked to a low water level in the Caspian Sea. “The reciprocal emergence and disappearance of deserts in the Sahara in northern Africa on the one hand and on the Arabian Peninsula in western Asia on the other hand resembles a kind of swing, a desert swing,” says the researcher.

The research team suspects that these changing and initially short-term desert formations in Mesopotamia, as push factors, were the driving force behind the spread of mammals from Eurasia to Africa. In the following extremely long desert phase NADX, however, the African continent was cut off from immigration and exchange with Eurasia for 2.3 million years. “During this time, today’s African savanna fauna emerged from the Eurasian immigrants, and the Australopithecids, our human ancestors, developed,” explains Böhme. With the global warm period 3.3 million years ago, the deserts on both continents gave way and ended the isolation of Africa. A mutual exchange developed between the fauna of Africa and Eurasia. The first dogs appeared in Africa,

Explanation for two phenomena
“Our study provides climatological explanations for two central phenomena for the first time,” summarizes Böhme. On the one hand, these underpinned the ‘Out-Of-Europe’ hypothesis she formulated, according to which the ancestors of African great apes and humans developed in Europe, but migrated south six to seven million years ago, so that their further evolution in Africa played. On the other hand, it could explain why the evolution of the African savannah fauna, including human ancestors, took place in a long phase of isolation.