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Stone tools discovered in the inhospitable desert of Saudi Arabia indicate that members of our genus Homo had ventured beyond the known borders of Africa and the Levant between 300 000 and 500 000 years ago. And according to climate data captured in the bones of animals found on the site, the environment in which they settled may not have been very different from the one they left in Africa from the East. This could help anthropologists better understand the role of the environment – and the ability to adapt to new challenging landscapes – to shape human evolution and global expansion.
The things they left behind
The archaeologist Patrick Roberts of the Max Planck Institute for the Science of Human History and his colleagues recently discovered a handful of stone tools in a sandy soil layer under dry traces from a shallow Pleistocene lake to Ti & al Ghadah, in the Nefud desert in northern Saudi Arabia. The soil layer dated from 300 000 to 500 000 years ago, and it also contained fossilized remains of grazing animals, water birds and predators such as the Hyena and the jaguar. Many bones seem to carry the marks of skinning by hominines brandishing tools.
Archaeologists had found other fossils on the site with possible marks of cuts, but, without stone tools, it is difficult to determine whether a notch in a fossil vein was put there by a human hand and not by a other predator or natural process. The tools – six brown chert flakes and a scraper – make the case much clearer. Roberts and his colleagues say they are the oldest hominin artefacts radiographically dated from the Arabian Peninsula, 100,000 years older than the previous candidate.
Sequins show signs of attack from a prepared stone core, a relatively advanced technique generally attributed to modern humans or Neanderthals. But Roberts says that 300,000 to 500,000 years ago, toolmakers were more likely to be members of older hominins like Homo erectus (The oldest human fossils found in Africa date back 200,000 years). Old environmental records in the bones that littered the long-abandoned tools suggest that the Nefud was a very different place at the time.
When Arabia was green
Our species was not the first hominin to settle in Europe and Asia. When modern humans began to slowly spread around the world some 100,000 years ago, they met other members of the genre. Homo who had ventured much earlier, starting with Homo erectus about 1.9 million years ago. Some paleoanthropologists, including Roberts and his colleagues, claim that our predecessors were attached to diverse landscapes of grasslands and trees, located near lakes or rivers, while modern humans possessed a unique talent for adapting to a particular landscape. wide range of extreme environments, from deserts to tropical forests to cold forests of Siberia. But others have pointed out the widespread spread of some missing groups as proof that they were, in fact, just as adaptable as us.
To solve this debate, scientists must understand what the environment looked like hundreds of thousands of years ago during the Middle Pleistocene. The fossilized animals at Ti 'al Ghadah may have something to say about it because the proportion of certain isotopes in their tooth enamel preserves information about the plants that they eat and eat. about the climate in which they grew. Roberts and his colleagues used these chemical signatures to reconstruct an ancient environment that looked surprisingly like the humid savannah of modern East Africa.
Photosynthesis does not work in exactly the same way for all plants. Most shade-tolerant trees, grasses, shrubs and grasses store carbon using a chemical pathway called C3, while most herbs and sedges use a different route called C4. Each method gives a different carbon-13 isotope ratio to other stable isotopes of carbon in plant tissues, and these ratios are transmitted to animals that graze on plants. In Ti's al Ghadah, the tooth enamel of 21 fossilized herbivores belonging to different species contained 13 carbon levels that corresponded almost exactly to a C4 grasses diet. This suggests a wide band of open meadows around the shores of the shallow lake gone.
It is a landscape very different from today's reddish sand dunes. Oxygen-18 ratios relative to other oxygen isotopes present in tooth enamel of Ti's al Ghadah fossils suggest a much wetter climate 300,000 years ago in Nefud. The oxygen 18 is a little heavier than other oxygen isotopes. As a result, when water evaporates, more oxygen 18 tends to be left behind. Oxygen-18 ratios can reveal information on a complex set of factors, including temperature, moisture, and precipitation source. And at Ti's al Ghadah, these ratios suggest an environment very similar to a wet savannah.
Like at home
These ratios are in line with climate models that suggest a more humid and hospitable climate in Arabia, due to the change of the monsoon in Africa during periods of global warming called interglacials. They also help make sense of the collection of animals found on the site: elephants, oryx, hartebeest and others that would have flourished in a savanna. This means that, during the first migrations out of Africa, the Middle Pleistocene pioneers would not have faced the challenge of adapting to life in today's arid and hot desert.
And that, according to Roberts and his colleagues, means that Homo erectus and the other Middle Pleistocene hominins would not have needed much adaptability to make a living in the Arabian Peninsula. Instead, it seems that our parents – and perhaps the first members of our own species – have spread to an Arabian peninsula temporarily filled with welcoming grasslands. They did it with other species, which the fossil record clearly indicates to have moved to Eurasia around the time of the discoveries of Ti's al Ghadah.
Modern humans who came later did not have life so easy. "Although the data are not yet complete, the subsequent migrations of our own species into the Arabian Peninsula seem to be associated with drier conditions," Roberts told Ars Technica. Modern humans would also have benefited from wetter periods to penetrate the Arabian Peninsula. But according to Roberts and his colleagues, our species has managed to penetrate deeper into a more difficult territory, "penetrating the dunes and living in perhaps harsher conditions than their Middle Pleistocene predecessors".
More questions to answer
Historical climate data – etched in sediments at the bottom of lakes and in layers of mineral deposits in caves – suggest that the Arabian Peninsula experienced several milder and moister phases of climate over the course of two million years of hominin movements in the area. But between these phases, the region has dried up and the desert has recovered.
"Between these phases, I think it's clear that the Arabian Peninsula would have looked something like today and that the existence of the hominin would have been impossible in most of inside, "Roberts told Ars Technica. "Indeed, this is the reason why we end up with fossil assemblages like the ones we have here, presumably the product of a slowing climate and the disappearance of local populations." That is, the migration of hominins, Homo erectus at Homo sapiens, crossing the geographical crossroads of the Arabian Peninsula probably went through a series of legumes.
But to understand this process in detail will require better data on old environments. According to Roberts, one of the big questions is how much the Arabian Peninsula was "green" during its most hospitable phases. New samples of lake sediment cores in the area could help answer this and other questions, as shown by Ti's al Ghadah.
"Really, we want to emphasize that ancient fossil discoveries must be accompanied by detailed environmental information," Roberts told Ars Technica. "When we discuss migration, it's probably the most interesting part in terms of studying the challenges and capabilities of different populations."
Nature Ecology & Evolution, 2018. DOI: 10.1038 / s41559-018-0698-9 (About DOIs).
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