As the earth shook and the climate swung between extremes, early humans in East Africa underwent a radical shift in cultural behaviour, according to researchers.
Archaeologists reconstructed the climate of the Olorgesailie Basin and its inhabitants' behaviour over hundreds of thousands of years, and found the residents became better toolmakers and trade partners when the going got tough.
Not only did the teams unearth the earliest evidence of pigment use so far, they found sophisticated tools made of materials that were sourced from outside the area.
This happens to be around the time anatomically modern Homo sapiens appear in the fossil record.
There are two theories for the dawn of culture in hominins, according to Smithsonian Institution Museum of Natural History palaeoanthropologist Rick Potts.
The first is that it gradually happened hundreds of thousands of years ago. The other states that it happened late — around 70,000 years ago — and swiftly.
Work published in a trio of papers in Science today indicated the cultural shift in this region happened quickly at least 305,000 years ago.
"You have this package of behaviours — not just new ways of making stone tools, but evidence of social networks and the potential for symbolic use of colour — that are emerging right at the cusp … of us becoming Homo sapiens," Dr Potts said.
Some scientists think environmental pressures, which, for instance, made it harder to find food, forced early humans to develop social networks to get through those tough times.
To find out, archaeologists need sites with preserved evidence in the form of fossils and artefacts like stone tools. The sites also need a way to recreate the past climate.
One such site is the Olorgesailie Basin in southern Kenya.
Lying on the East African Rift, the Olorgesailie Basin is a hotbed of ancient artefacts, with the first evidence of ancient hominins there being 1.2 million years old.
But human fossil remains there are rare, said Dr Potts, who has worked in the Olorgesailie Basin since 1985.
So to get a handle on what the basin's residents' life was like, Dr Potts and colleagues analysed sediments to piece together the region's climate history.
They found the basin was a floodplain until around 800,000 years ago, when it suddenly turned into grasslands and started fluctuating between dry and wet conditions.
Embedded in the sediments were fossilised bones of small animals — signs, they claimed, of climate instability.
They also found evidence of volcanic flows and earthquakes from around 500,000 years ago.
Ian Moffat, an archaeologist at Flinders University who wasn't involved with the work, said there's an art to this sort of climate reconstruction.
"You take some really disparate threads of evidence and you use that to construct an idea of what the climate's like at a particular time," he said.
"Looking at the sediments laid down you can say, for example, that a place was a lake at a particular time, or it was much drier, or maybe it was an active river, or maybe it was a sand dune."
Story told by artefacts
Scattered through the Olorgesailie Basin sediment layers were thousands of bones, plant matter and tools.
A team led by Alan Deino, a geochemist at the Berkeley Geochronology Centre, looked at tools to piece together the transition from bulky, teardrop-shaped "Acheulean" handaxes to more advanced weapons with finer blades or the ability to be attached to a stick.
These advanced tools marked the onset of the Middle Stone Age, where early humans expressed a little more flair when it came to toolmaking, according to University of New South Wales archaeologist Darren Curnoe, who wasn't involved with the research.
"With the Acheulean, it's this global uniformity. You generally find the same kinds of pieces [everywhere around the world] with the same template underpinning them," he said.
Using argon and uranium dating, Dr Deino and his crew found this transition occurred 320,000 to 305,000 years ago at the latest in the Olorgesailie Basin, pushing back the beginning of the Middle Stone Age at least 20,000 years.
What the tools were made of, too, changed during this time.
Where the Acheulean handaxes were made almost exclusively of rock from the immediate area, more than 40 per cent of the advanced Middle Stone Age tools were made of volcanic glass called obsidian.
The sources of obsidian, according to a third paper led by George Washington University archaeologist Alison Brooks, were some 25 to 50 kilometres away.
"But there was one piece we tested that very clearly and only matched a source that was 95 kilometres away," said Dr Potts, who was also a co-author on this study.
The team also found black and red rocks, likely a used as colour pigment.
One of the red rocks had distinct chisel marks that suggested it was a lump of ground ochre pigment.
When Dr Potts and his team found it, they thought it was a lump of limestone painted red.
"But then when we actually did the analysis, we realised that the entire rock was ochre, all the way through," he said.
Dr Curnoe said exotic pigments may have been used to "paint a wide range of materials, not just rock paintings, but also painting wooden objects, stone objects, and painting the body, particularly as part of ceremony.
Because the obsidian and pigment rocks originated tens of kilometres away, it's thought that they were traded by other populations living in the area.
Causation or coincidence?
So environmental changes may have forced humans to become better toolmakers and more sociable.
But there's a big chunk missing from the Olorgesailie Basin timeline, from around 500,000 to 320,000 years ago, where the corresponding sediment layers were eroded away completely.
"That gap was pretty frustrating," Dr Potts said.
In 2015, Dr Potts led another study that modelled the climate of that period, and found it relatively stable between 350,000 and 460,000 years ago.
The shift from Acheulean to Middle Stone Age could well have happened in that time, he said.
"It is quite possible. The relationship to climate is unclear," Dr Potts.
Dr Curnoe agreed that local environmental fluctuations and the onset of Middle Stone Age technology and pigments "could just be a coincidence".
Dr Potts hopes to fill in part of that 180,000-year gap soon. He's drilled a core of sediments from a site near these excavations to try to nail down how much the climate changed, if at all.
