![]() Unlike protected caves, tufa forms in the open where sunlight, dust, and debris can contaminate the ages. This is because dating tufas is not straightforward. This method is routinely applied to cave deposits like stalagmites and flowstones but has not been used very much on tufa. ![]() By precisely measuring how much uranium and thorium is in the tufa today, we use the known decay rate to calculate when the 'clock' started. The uranium-thorium system acts like a clock that starts when the tufa is formed. When tufa forms, uranium is 'locked' into the crystal structure and begins to decay to produce thorium. Uranium is radioactive, meaning that it decays at a constant rate over time and produces 'daughter' elements thorium is one of them. To find out how old the tufas are and when these wet periods occurred, we used a method called uranium-thorium dating. Knowing when the tufas formed at Ga-Mohana tells us when it was wetter there. Similar structures are growing today at places like Sitting Bull Falls, New Mexico in the US. But the fossil tufas represent periods in the past when there was more water available. The tufa system is no longer active, apart from small drips during the rainy season. Over time, these waters precipitate calcium carbonate and form tufa.įossil tufas occur in several parts of the world. This occurs when underground aquifers are recharged with rain water and begin to overflow. The Ga-Mohana hillside is draped in deposits called tufa these form from water leaking out of cracks in the bedrock. Some kinds of rocks preserve traces of the past environment. The fact that the climate was very different then opens up possibilities about why this previously under-appreciated region must have played an important role in our species' evolutionary history.Īrchaeological and geological fieldwork allowed us to piece together this story. At some points, Ga-Mohana was much wetter than today, with pools of standing water and waterfalls tumbling down the hillside. Our work contradicts the idea of an arid and empty interior. ![]() The southern Kalahari is often considered too arid to be important for human evolution. To do so, we looked at Ga-Mohana's rocks. And so, a major part of our research centered on working out what the area's climate was like 105,000 years ago. Reconstructing past environments allows us to understand this context. Survey team from University of Cape Town and University of Queensland. What brought people there in the first place, at that time, to begin using those tools and collecting those crystals? Early humans bringing crystals into Ga-Mohana suggests innovation in how people interacted with each other and their environment.īut such ancient innovation didn't occur in a bubble: there is context to when and where innovation occurs. They may have been collected for their aesthetic properties, or included in ritual activities.Ĭrystals are collected by many people around the world to this day for ritual purposes. ![]() Why the crystals were brought there is unknown they are not modified and do not seem to have a functional purpose. Large calcite crystals from several kilometers away were found in the cave alongside stone tools. Identifying the ancient roots of symbolism is limited to what preserves over time. Humans use symbols as a shortcut to communicate important ideas. Our international team, made up of researchers from South Africa, Canada, the UK, Australia and Austria, has found evidence for complex symbolic behaviors 105,000 years ago. It's in the Northern Cape that we studied and described a new archaeological site, Ga-Mohana Hill, for research just published in Nature. The Kalahari is a huge expanse of desert in southern Africa, stretching across Botswana and into the northernmost part of South Africa's Northern Cape province. Ga-Mohana Hill in South Africa’s Northern Cape province.
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