Stone tools unearthed in Saudi Arabia’s inhospitable Nefud Desert indicate that members of our genus Homo had ventured beyond the familiar borders of Africa and the Levant sometime between 300,000 and 500,000 years ago. And according to climate data captured in the bones of animals found at the site, the environment they moved into may not have been that different from the one they left behind in East Africa. That may help anthropologists better understand the role of environment—and the ability to adapt to challenging new landscapes—in shaping human evolution and global expansion.

The things 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 soil beneath the dry traces of a shallow Pleistocene lake at Ti’s al Ghadah, in the Nefud Desert of northern Saudi Arabia. The soil layer dated to between 300,000 and 500,000 years ago, and it also contained fossilized remains of grazing animals, water birds, and predators like hyena and jaguar. Many of the bones seem to bear the marks of butchering by tool-wielding hominins.

Archaeologists had found other fossils at the site with possible cut marks, but, without stone tools, it’s difficult to determine if a notch in a fossil rib was put there by a human hand and not another predator or natural process. The tools—six sharp brown chert flakes and a scraper—make a much clearer case. Roberts and his colleagues say they’re the oldest radiometrically dated hominin artifacts in the Arabian Peninsula, edging out the previous contender by 100,000 years.

The flakes show signs of being struck from a prepared stone core, which is a fairly advanced technique usually attributed to modern humans or Neanderthals. But Roberts says that, at 300,000 to 500,000 years ago, the toolmakers were more likely members of earlier hominin species like Homo erectus (the earliest modern human fossils found in Africa date to just 200,000 years ago). Ancient environmental records in the bones that lay alongside the long-discarded tools suggest that the Nefud was a very different place at the time.

When Arabia was green

Our species wasn’t the first hominin to move into Europe and Asia. When modern humans began slowly spreading across the globe sometime before 100,000 years ago, they encountered other members of the genus Homo who had ventured forth much earlier, starting with Homo erectus around 1.9 million years ago. Some paleoanthropologists, including Roberts and his colleagues, say that our predecessors stuck to familiar patchwork landscapes of grasslands and trees, situated near lakes or rivers, while modern humans had a unique knack for adapting to a wide range of extreme environments, from deserts to tropical forests to the cold of Siberia. But others have pointed to the wide spread of certain extinct groups as evidence that they were, in fact, every bit as adaptable as we are.

To settle that debate, scientists need to understand what the environment was like hundreds of thousands of years ago, during the Middle Pleistocene. The fossilized animals at Ti’s al Ghadah may have something to say on the subject, because the ratios of certain isotopes in their tooth enamel preserve information about the plants they ate and the climate they grew in. Roberts and his colleagues used those chemical signatures to reconstruct an ancient environment that looked surprisingly like the humid savanna of modern East Africa.

Photosynthesis doesn’t work exactly the same way for all plants. Most trees, herbs, shrubs, and shade-tolerant grasses store carbon using one chemical pathway, called C3, while most grasses and sedges use a different pathway, called C4. Each method results in a different ratio of the isotope carbon-13 to other stable isotopes of carbon in the plants' tissues, and those ratios get passed along to animals that graze on the plants. At Ti’s al Ghadah, tooth enamel from 21 fossilized herbivores of different species contained carbon-13 ratios that almost exactly matched a diet of C4 grasses. That suggests a large swath of open grassland around the shores of the vanished shallow lake.

That’s a very different landscape from today’s dunes of reddish sand, and the ratios of oxygen-18 to other oxygen isotopes in the tooth enamel of the Ti’s al Ghadah fossils suggest a much wetter climate in the Nefud of 300,000 years ago. Oxygen-18 is a little heavier than other oxygen isotopes, so when water evaporates, more oxygen-18 tends to get left behind. Ratios of oxygen-18 can reveal information about a complex set of factors including temperature, humidity, and the source of rainfall. And at Ti’s al Ghadah, those ratios suggest an environment very similar to a humid savanna.

Just like home

Those ratios line up with climate models that suggest a wetter, more hospitable environment in Arabia, courtesy of a shift in Africa’s monsoons during periods of warmer global climate called interglacials. They also help make sense of the collection of animals found at the site: elephants, oryx, hartebeest, and others that would have thrived in a savanna. That means that, during the early pulses of migration out of Africa, the Middle Pleistocene pioneers wouldn’t have faced the challenge of adapting to life in today’s hot, arid desert.

And that, according to Roberts and his colleagues, means that Homo erectus and other Middle Pleistocene hominins wouldn’t have needed much adaptability in order to make a living on the Arabian Peninsula. Instead, it looks like our relatives—and possibly early members of our own species—expanded into an Arabian Peninsula that was temporarily filled with welcoming grasslands. They did so with other species, which the fossil record clearly shows moving into Eurasia at around the time of the Ti’s al Ghadah finds.

Modern humans who came later didn’t have it so easy. “While the data is not quite there yet, later migrations of our own species into 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 into the Arabian Peninsula. But according to Roberts and his colleagues, our species managed to push further into more challenging territory, “penetrating into the dune fields and living under conditions that were perhaps harsher than their Middle Pleistocene predecessors.”

More questions to answer

Ancient climate records—etched in sediments at the bottom of lakes and in layers of mineral deposits in caves—suggest that the Arabian Peninsula enjoyed several phases of milder, wetter climate during two million years of hominin movements through the region. But between those phases, the region dried up and the desert closed in again.

“In between these phases, I think it is clear that the Arabian Peninsula would have resembled something like today, and hominin existence would have been impossible throughout most of the interior,” Roberts told Ars Technica. “Indeed, this is why we end up with fossil assemblages like we have here, presumably the product of a downturn in climate and demise of local populations.” That means hominin migration, from Homo erectus to Homo sapiens, through the geographical crossroads of the Arabian Peninsula probably happened in a series of pulses.

But understanding that process in detail will require better data on ancient environments. One big question, Roberts says, is how “green” the Arabian Peninsula actually was during its more hospitable phases. New core samples of lake sediments in the region may help answer that question and others, as Ti’s al Ghadah provides another piece of that puzzle.

“Really, something we want to emphasize is that finds of early fossils should be accompanied by detailed environmental information,” Roberts told Ars Technica. “When we discuss migrations, this is arguably the most interesting part in terms of studying the challenges and capacities of different populations.”

Nature Ecology & Evolution, 2018. DOI: 10.1038/s41559-018-0698-9 (About DOIs).