A Pink, Dried-Up Lake in Kenya Shaped Human Evolution 500,000 Years Ago

There’s a great pink lake in the southwest of Kenya that’s sometimes dry and sometimes wet, depending on the season. Not that Lake Magadi was always so fickle: Long before modern humans walked the Earth, it was full of water that supported ancient hominins and the animals that lived alongside them. But about half a million years ago, report scientists in a new Proceedings of the National Academy of Sciences study, the lake suddenly started drying up, reshaping the landscape and the hominin populations that called it home.

A team of scientists led by Hong Kong Baptist University geology professor Richard Bernhart Owen, Ph.D., read evidence of this shift in sediment cores pulled from the bottom of the lake. The cores took them back in time, showing that a change in climate kicked off a long period of aridification 575,000 years ago. By comparing the timing of those great changes with the tools left behind by nearby hominins, the team found a link between the drying lake, the disappearance of the local Homo erectus population, and — just maybe — the sudden appearance of a far more sophisticated species.

“There are no fossil skeletons to give a definitive answer, but there are some interesting comparisons,” Owen tells Inverse. Ancient tools found near Lake Magadi suggest that Homo erectus living in the area started disappearing around the same time as the drying period and that another unknown species — a particularly smart one with far superior tools — just happened to show up some 180,000 years later.

By shedding light on how local hominins responded to their changing world, this study hints at whether those mystery toolmakers were our oldest ancestors.

" We didn’t know from that site exactly what was going on in terms of the environment. And that’s where our study really fills in that gap.

To figure out what happened to the ancient hominins around Lake Magadi during that drying period, we must understand the harsh world they lived in. Information about the environment can be gleaned from cores extracted from the bottom of a lake, but to make sense of how those conditions affected humans, scientists need an archaeological record too. Fortunately, Lake Magadi is just 15 miles away from Olorgesailie, an archaeological site jam-packed with hominin artifacts.

Lake Magadi’s sediment cores showed that a long-term drying trend began 575,000 years ago, intensified into an particularly intense early aridification period between 525,000 and 400,000 years ago, tapered off, and then intensified again 350,000 years ago. The archaeological record suggests the early aridification period was a bad time to be a hominin.

Olorgesailie is renowned among archaeologists for its wealth of stone age tools. Together, these tools provide a faithful record of the hominins that lived there over the millennia, but as study co-author and University of Arizona geoscience professor Andrew Cohen, Ph.D., tells Inverse, for a long time there was a curious gap in the record, right around the early aridification period, when there were no tools left behind.

“There’s an unconformity of about 200,000 years from about 500,000 to 300,000 years ago,” says Cohen, who directs the Hominin Sites and Paleolakes Drilling Project (HSPDP), which pulled up the Lake Magadi core. “We didn’t know from that site exactly what was going on in terms of the environment. And that’s where our study really fills in that gap.”

The team pieced together temporal data from the sediment core and the Olorgesailie archaeological record to show that early in the aridification period, around 500,000 years ago, local large grazing animals started to go extinct. Around the same time, the primitive, pear-shaped Acheulean hand-axes usually wielded by Homo erectus stopped appearing.

Life, it seems, took a turn for the worse, leaving a void in the archaeological record.

"Together, the evidence suggests an advance in cognitive abilities.

But fast-forward to about 320,000 years ago, and the world changes again. Lake Magadi’s intense early drying period has ended. Finer, sharper tools — the varied, delicate precursors to spears, arrows, and fish hooks — show up at Olorgesailie, as a breakthrough Science paper showed in March. The tools are a huge upgrade from the chunky hand-axes of Homo erectus, which were the only technology that hominins from Africa to Eurasia used for over a million years.

This new, so-called “Middle Stone Age toolkit” is made from a variety of rock types, suggesting that whoever made them was capable of traveling greater distances.

“Together,” says Owen, “the evidence suggests an advance in cognitive abilities.”

We can’t definitively identify the Homo species that made these new tools, but paleoanthropologists can’t help but notice that they just happened to appear around the same time that history-changing events were ongoing in present-day Morocco.

Nobody can be certain of how long Homo sapiens has been around, but the oldest known specimen was found at Jebel Irhoud in Morocco and is about 315,000 years old. By that point, other Homo species like Neanderthals and Denisovans had already settled in Eurasia, and Homo erectus had spread across Africa. It’s not clear whether our big-brained ancestors were big on travel just yet, but it’s hard not to wonder whether they were the mystery toolmakers at Olorgesailie, even though Jebel Irhoud is about a 3,700-mile walk to the northeast of Lake Magadi.

“Although very distant from Olorgesailie this does suggest they could have been present at Olorgesailie when the Middle Stone Age tools first appeared in the study region, so it is possible (though not proven) that H. erectus were replaced by H. sapiens,” says Owen.

"There’s no smoking gun that we can say definitely that modern Homo sapiens was the only one that was capable of making these things, but the timing is sure suspicious about that.

“There’s no smoking gun that we can say definitely that modern Homo sapiens was the only one that was capable of making these things,” adds Cohen, “but the timing is sure suspicious about that.”

As Middle Stone Age tools began appearing at Olorgesailie, Lake Magadi entered its second aridification phase, which continues to shape its pink-crusted surface today. Homo erectus seems to have fled the scene entirely during the early dry period, though their fate remains a mystery. In one hypothesis, the difficult conditions forced them to travel farther away and interact with other bands of hominins to trade goods, information, and technology. And though Homo erectus, like the Neanderthals and Denisovans, is considered a separate Homo species to sapiens, it remains entirely possible that they mated with our ancestors, entangling our evolutionary pasts.

“We don’t really know what happened to Homo erectus,” says Cohen. “They might have gone extinct, or they might have evolved into modern *Homo sapiens.”

To untangle our complicated evolutionary family tree, all we can do is make the most of the clues left behind beneath ancient lakes and in the sand. With every new glimpse into the past, we understand our present a little bit more. “The correspondence of the arrival of the Middle Stone Age in time with the earliest modern Homo sapiens,” Cohen says, “is interesting at the minimum.”