The stone tools unearthed in the inhospitable desert of Nefud in Saudi Arabia indicate that members of our Homo genus had ventured beyond the family boundaries of Africa and the Levant sometime between 300,000 and 500,000 years ago. And according to the climatic data captured in the bones of the animals found on the site, it is possible that the environment they moved to has not been so different from what they left in East Africa. This can help anthropologists better understand the role of the environment, and the ability to adapt to new challenging landscapes, in the configuration of human evolution and global expansion.
The things that they left behind
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 layer of earth beneath the dry footprints of a shallow Pleistocene lake in Ti Al Ghadah, in the Nefud Desert, in the north of Saudi Arabia. The soil layer dated from 300,000 to 500,000 years ago, and also contained fossilized remains of grazing animals, waterfowl, and predators such as the hyena and the jaguar. Many of the bones appear to have the killing marks of hominids with tools.
Archaeologists found other fossils at the site with possible cut marks, but, without stone tools, it is difficult to determine if a notch in a fossil rib was placed there by a human hand and not by another predator or natural process. The tools (six sharp brown chert flakes and a scraper) make the box much clearer. Roberts and his colleagues say they are the oldest hominin-dated radiometric artifacts in the Arabian Peninsula, which outperform the previous contender for 100,000 years.
The flakes show signs of having been struck from a prepared stone core, which is a fairly advanced technique that is 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 earlier hominin species. Homo erectus (The first modern human fossils found in Africa date back to 200,000 years ago). The old environmental records in the bones lying next to the tools discarded for a long time suggest that Nefud was a very different place at that time.
When Arabia was green
Our species was not the first hominin to move to Europe and Asia. When modern humans began to spread slowly around the world about 100,000 years ago, they found 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 adhered to patches of familiar landscapes of grasslands and trees, located near lakes or rivers, while modern humans have a unique ability to adapt to a wide range of extreme environments, from deserts to tropical. Forests to the cold of Siberia. But others have pointed to the widespread diffusion of certain extinct groups as evidence that, in fact, they were as adaptable as we are.
To solve this debate, scientists must understand what the environment was like hundreds of thousands of years ago, during the middle Pleistocene. Animals fossilized in Ti al Ghadah may have something to say on the subject, because the proportions of certain isotopes in their tooth enamel preserve information about the plants they ate and the climate in which they grew. Roberts and his colleagues used those chemical signatures to reconstruct an ancient environment that looked strikingly like the humid savannah of modern East Africa.
Photosynthesis does not work in exactly the same way for all plants. Most trees, herbs, shrubs and shade-tolerant grasses store carbon using a chemical route, called C3, while most grasses and sedges use a different route, called C4. Each method results in a different ratio of the carbon-13 isotope to other stable isotopes of carbon in the tissues of plants, and those relationships are transmitted to animals that graze on plants. In the Ti to the Ghadah, the tooth enamel of 21 fossilized herbivores of different species contained 13 carbon proportions that almost exactly coincided with a diet of C4 grasses. That suggests a large swath of open grassland around the shores of the shallow lake gone.
That's a very different landscape from the reddish sand dunes of today, and the proportions of oxygen-18 to other oxygen isotopes in the tooth enamel of the Ti fossils in Ghadah suggest a much more humid climate in Nefud 300,000 years ago . Oxygen-18 is a little heavier than other oxygen isotopes, so when water evaporates, more oxygen-18 tends to lag behind. Oxygen-18 ratios can reveal information about a complex set of factors including temperature, humidity, and the source of rainfall. And in You's to the Ghadah, those proportions suggest an environment very similar to that of a wet savannah.
Like at home
Those indexes align with climate models that suggest a more humid and hospitable environment in Arabia, courtesy of a change in the African monsoons during the warmer global climate periods called interglacials. They also help make sense of the collection of animals found on the site: elephants, oryx, hartebeest and others that would have thrived in a savannah. That means that, during the first impulses of migration from Africa, the Middle Pleistocene pioneers would not have faced the challenge of adapting to life in the arid and arid desert of today.
And that, according to Roberts and his colleagues, means that Homo erectus and other Middle Pleistocene hominids would not have needed much adaptability to make a living in the Arabian Peninsula. Instead, it seems that our relatives, and possibly the first members of our own species, expanded to an Arab peninsula that was temporarily filled with cozy meadows. They did so with other species, that the fossil record clearly shows the move to Eurasia at the time of Ti's findings in Ghadah.
The modern humans who came later did not have it so easy. "While the data is not yet available, subsequent migrations of our own species to the Arabian Peninsula appear to be associated with drier conditions," Roberts told Ars Technica. Modern humans would also have needed to take advantage of more humid periods to move to the Arabian Peninsula. But according to Roberts and his colleagues, our species managed to push more into more challenging territory, "to penetrate the dune fields and live in conditions that were perhaps harder than their predecessors of the Middle Pleistocene."
More questions to answer
The ancient climatic records, recorded in the sediments at the bottom of the lakes and in the layers of mineral deposits in the caves, suggest that the Arabian Peninsula enjoyed several phases of milder and more humid climate during two million years of hominin movements. in the region. But between those phases, the region dried up and the desert closed again.
"Between these phases, I think it is clear that the Arabian Peninsula would have looked something like today, and that the existence of hominids would have been impossible in most of the interior," Roberts told Ars Technica. "In fact, this is the reason why we end up with fossil assemblages like the ones we have here, presumably the product of a deceleration of the climate and the disappearance of local populations." That means that the migration of hominins, of Homo erectus to Homo sapiens, Through the geographical crossroads of the Arabian Peninsula, it probably happened in a series of pulses.
But understanding that process in detail will require better data in older environments. A big question, says Roberts, is how "green" the Arabian Peninsula really was during its most hospitable phases. The new samples of lacustrine sediment cores in the region can help answer that question and others, since Ti & # 39; s al Ghadah provides another piece of that puzzle.
"Actually, something we want to emphasize is that early fossil finds must be accompanied by detailed environmental information," Roberts told Ars Technica. "When we discuss migrations, this is possibly the most interesting part in terms of studying the challenges and capacities of different populations."
Ecology of nature and evolution, 2018. DOI: 10.1038 / s41559-018-0698-9 (About DOI).