Land bridges in the Middle East: Red Sea, Tethys and wandering hominids

How a new young ocean, the Red Sea, opened during the final spasms of an ancient ocean, the Tethys; and what that meant for migrating species such as our distant human ancestors and hominid relatives. Nowhere but in the Middle East was geography so pivotal in shaping how we became human. A clearer picture of this history is slowly crystallizing, yet a few pieces of the puzzle are still waiting to be untombed on sun-beaten Arabian shores.

By Kathelijne Bonne.

Africa and what is now the Arabian Peninsula used to be one uninterrupted landmass, one large continent-island, populated by a multitude of primates, including our own distant ancestors. This ancient "Greater Africa" (also called Afro-Arabia) was separated from Eurasia by the Tethys Ocean. But plate tectonics caused Africa to drift northward towards Eurasia. The Tethys became narrower, and eventually both continents connected by a land bridge, a corridor for migration. But meanwhile, a new marine barrier to migration formed: As the Red Sea and Gulf of Aden opened, the Arabian Peninsula and its inhabitants got isolated from Africa.

Wild tide

The forming and vanishing of crucial land bridges and marine straits are subject to the turbulent tide of interactions between plate tectonics, climate and sea level fluctuations, which play out on disparate timescales. But the animal species including hominids that took these geographical hurdles, were, during their individual lives, blissfully oblivious to all those planetary vicissitudes. Yet their wanderings and ways of living, adapting and adjusting were completely determined by it, through the unbroken chain of thousands of generations.

It shaped us into who and how we are today. 

There are still gaps in our understanding of hominid and human evolution, as seen in through the lens of our planet's geodynamic processes. Fossil evidence is fragmentary, and we find ourselves at the interface between different disciplines - a rough tidal zone as well, a borderland, a no man's land.

I've been given the opportunity to add a few links to the study of the environments and continents through which ancient hominids roamed. Or maybe I just like to wander around in that still wild tidal region between the disciplines.

Whatever the reason, here is a brief history of the migratory corridors and straits between Africa and Eurasia to help supporting later ideas and interpretations. We'll cover the timespan from thirty to five million years ago (Oligocene-early Pliocene). 

30 Ma: Afar plume, Red Sea and Gulf of Aden

The landmass of Greater Africa started to show its first cracks 30 million years ago (30 Ma). A huge mantle plume rose from the Earth's mantle where the Afar Triangle is today. The Afar "superplume" caused the land to bulge, massive volumes of lava accumulated in thick layers. The typical basalt traps still form Ethiopia's high and varied landscapes.

From the Afar, large rift valleys developed in three directions, forming a so-called triple junction. The three branches can now be recognized as: (1) the Gulf of Aden, (2) the Red Sea + Gulf of Suez, and (3) the Ethiopian rift. All these structures are connected through Ethiopia to the East African Rift System, which stretches all the way to the Smoke-that-thunders.

The Gulf of Aden formed first, shortly after the eruption of the Afar plume. It widened and deepened until it was flooded by the ocean, becoming an embayment of the Tethys.

Around 24 Ma (Chattian, end of Oligocene), the second branch formed: the Gulf of Suez and the Red Sea, forming one large valley. It was flooded from the west by the Tethys (now Mediterranean Sea).

Bab-El-Mandeb isthmus

The waters of Red Sea and Gulf of Aden were not connected at that time. There was still land between them, the isthmus of Bab-El-Mandeb, at the place where Afar volcanism had taken place. Millions of years later, it would become a narrow marine strait. Today, the Strait of Bab-El-Mandeb – meaning gate of sorrow – is one of the world's most important and infamous marine gateways. If it were infamous for its treacherous currents only …

Greater Africa, then still quite far from Eurasia, was home to several species of apes and monkeys. The great ape taxon had split off ~25 Ma ago. Another taxon, the "new world monkeys" had migrated long before (in the Eocene) to South America – that is, over sea (!), on a natural raft, e.g. a broken off piece of mangrove or vegetation island. 

24 to 15 Ma: The Mesopotamian seaway and earliest crossings

Meanwhile, Africa closed in on Eurasia. The last stretch of the Tethys Ocean between both continents, the Mesopotamian seaway, was getting narrower. In the Early Miocene it was about 500 km wide, but narrower and shallow in places. The strait gradually silted up as it was filled by sediment from the rising Zagros mountains to the north. By 20 Ma, circulation and mixing of Indian Ocean and Mediterranean waters was already greatly reduced.

Early great apes of Africa presumedly made their way across this seaway, perhaps even before there was a fully-fledged land bridge. The oldest known great ape fossil outside Africa, a Griphopithecus, has an age of 17 or 16 Ma (Burdigalian, early Miocene, found in Germany).

Whether those early great apes crossed by natural raft or via islands or shallow tidal zones is not known. We only know that they reached Eurasia, alive. They initially stranded somewhere in southern Turkey or the Zagros.

Among these travelers were ancestors of humans.

15 Ma: New Gondwana, new tensions

At the end of the Langhian (Middle Miocene, 15 Ma), Africa collides with Eurasia in one of the mightiest phases of mountain building in recent geological history. Such an event is usually smeared out over thousands of years or more, yet there must have been one good day when the beaches on both sides of the Tethys fused to form a permanent land bridge. The Mesopotamian seaway had closed forever. The formation of the new land bridge is actually the first step towards a new supercontinent: Afreurasia. Or is New Gondwana a better name?

The continental collision had created new tectonic tensions, which determined the location of the land bridge. The large transform fault running from the Gulf of Aqaba through the Dead Sea and Lebanon into Turkey became active. This was accompanied by uplift along the fault line. Therefore it is plausible that the earliest land bridge to Eurasia was along this fault, perhaps in the area of the notorious Turkish triple junction (near Aleppo, Gaziantep), which is the the ultimate place of linkage between the East African rift and the Alpine-Himalayan orogen.

The closure of the Mesopotamian Seaway split the Tethys into two: the Mediterranean Sea became a nearly enclosed basin and the Persian Gulf became an inlet of the Indian Ocean. But the closure also had global repercussions. Ocean circulation changed dramatically, contributing to a rather sudden global climate cooling and permanent glaciation of Antarctica.

The Mesopotamian corridor and dispersal of species

Land life migrated unimpeded across the new Mesopotamian corridor since about 15 Ma; animals no longer have to float in agony on a vegetation island to hopefully bump into land, barely alive, on nearby shores.

In the sea, marine fauna on both sides of the new land bridge got separated. Their descendants would only meet millions of years later during temporary marine transgressions, and in the modern era when the Suez Canal is dug. But they will have transformed beyond recognition into new species.

Creatures that preferred living along coasts also parted ways. Possibly, the closure of the Mesopotamian seaway led to a split in great apes, which according to the Waterside Hypothesis lived near coasts. One group of great apes migrated east via Asian coasts to Southeast Asia and some of them became the pongines (including orang-utan) [1].

The hominids including human ancestors, and dryopithecids (an extinct taxon of great ape) migrated to Europe via Turkey and Mediterranean coasts [1].

14 to 5 Ma: hyper-saline seas

The Red Sea and Gulf of Suez (RZ/Suez) were still well connected to the Mediterranean until about 14-13 Ma (Middle Miocene), but the marine passage between both (near Sinai) began to silt up. A smaller land bridge emerged: the Sinai corridor. The RZ/Suez was cut off from the Mediterranean and became an enclosed basin with highly saline waters. Thick salt layers of halite and anhydrite precipitated on the seafloor. This inhospitable environment persisted for millions of years but was interrupted occasionally by inflows from the Mediterranean, e.g., during high sea levels or temporary lowering of the Sinai corridor.

At the end of the Miocene, the Mediterranean evaporated almost completely because the only connection (Gibraltar) to the world ocean had closed. This was the start of the Messinian Salinity Crisis. Hence, the adjacent Red Sea which didn't have any other oceanic connection, didn't receive any 'new' water either, and became a highly saline basin as well, prone to massive evaporation. 

5 Ma: Natural floodgates: Gibraltar, Sinai, Bab-el-Mandeb

Around 5.3 Ma the hot, dry, salty episode came to a rather sudden end. Natural floodgates opened through a breach at Gibraltar; the Zanclean mega-flood filled the Mediterranean, ending the Messinian crisis. The Med rose to global sea level. In the east its waters flooded across the Sinai corridor (second floodgate) and into the Gulf of Suez and the Red Sea, so that an open-sea environment was once more established there too.

Further east a third floodgate opened, around 5 Ma (early Pliocene). With the advancing dislodging of the Arabian and African tectonic plates, the Bab-el-Mandeb isthmus had lowered and fell below global sea level, becoming a marine strait. Waters and fauna of the Indian Ocean and Mediterranean mixed. 

Eurasia and Arabia were then, for some time, completely separated from Africa by interconnected seas. 

Homo/Pan split on Red Sea shores

Between all those shifting seas and lands, an important split had happened in the family tree of hominids, namely that between the ancestors of humans (genus Homo) and chimpanzees (genus Pan). Although this was one of many splits, it is crucial for us as humans because chimps are our closest living relatives. All later splits became extinct.

Exactly how and where the Homo/Pan split occurred has long been a mystery. But a clearer picture is starting to crystallize thanks to an interdisciplinary group of researchers including the proponents of the Waterside hypothesis.

One plausible hypothesis for the Homo/Pan split posits that a group of common ancestors on the Arabian Peninsula became isolated from their family members due to the vanishing of landbridges along the Red Sea [2]. A group had ended up on the African side, while the others in Arabia could no longer join them. There, on Arabian shores, at the dawn of the Pliocene, began the story of our human lineage, of which modern humans are the only surviving species.

The human in me yearns to know the individual, lived and felt details of this pivotal separation of species. But we can only guess how it happened. Which group stayed behind, who went out, perhaps to forage some distance away, never to see the others again?

Did they look over their shoulder one more time, uttering a final call not knowing it was the last?

In my imagination – a time-machine would be needed to either confirm or refute it – the answer to that question is: yes, they somehow did say goodbye.

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Millions of years later, humans and chimps looked each other into the eyes again. That happened in an acutely conscious way when a wild baby chimpanzee in Tanzania reached out its tiny hand to the outstretched hand of a slightly odd-looking blonde female ape called Jane. She felt then how that unbridgeable biological barrier that had been separating 'us' from 'them' over countless generations crumbled. But that is another story.

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Dive deeper into the roots of humanity through the post on the Waterside hypothesis, about how and why great apes started walking upright, got bigger brains and lost their fur. The posts on the Great Rift of East Africa and the Mediterranean Sea give an idea of the Miocene world in which the human clade arose. 'Africa, the journey of an ancient continent' is about the even more ancient history of Africa and supercontinent Gondwana. Massive magma outpourings like those in the Afar (sometimes called 'super-plumes') occur every ten of millions of years and cause the break-up of continents, see Zambezi River and the Smoke-that-thunders. But they can also affect climate, like the Siberian Trapps that led to the mass extinction known as the Great Dying. And by now we know very well that anthropogenic climate change is much faster than that caused by any superplume. What does it say about our Homo sapiens brains that we're not acting fast enough?

Kathelijne: I am intrigued by how earth, life, air, ocean and societies interact on geological and human timescales.

Why I started GondwanaTalks.

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