How we calculated the age of the caves in the cradle of humanity – and why it is important

As a species, we humans have always been fascinated by their origin. Initially, it was thought that humans could not be from Africa.

This misconception began to evolve slowly from 1925, when the modern discipline of paleoanthropology – the study of our origins – was born in South Africa. It was at this point that Professor Raymond Dart of the University of the Witwatersrand, in Johannesburg, recognized the importance of a small fossil skull that had been brought to him in Taung, in the North West Province of the country.

Dart realized that the skull belonged to a creature who was neither a monkey nor a man: he represented one of our distant ancestors, whom he called Australopithecus africanus, meaning "monkey from southern Africa".

In the years that followed, paleontologist Robert Broom discovered more adult fossils from Sterkfontein and other caves in the area, now known as the Cradle of Humankind, right next to Johannesburg. . His work confirmed that Africa was the birthplace of humanity, although many European scientists took years to get there.

Since the 1960s, the fossil record of this important region has largely given way to East African discoveries. Indeed, we did not know how old the caves of the cradle of humanity were, so we could not provide conclusive dates for the many fossils they contain. The geological setting of the cradle is very different from the rift system of East Africa, where layers of volcanic ash lie between layers of fossils; the layers of ash can be dated, giving ages to fossils. The caves of South Africa do not have such volcanic layers.

But there are other types of rocks in the caves. Working with them, my colleagues and I used a method called leaded uranium dating to establish the cradle-age cradle of humanity. This means that we can reduce the entire human record from cradle to a few brief periods from one to three million years ago.

One of the most interesting things about this research is that we can – for the first time – compare South African hominins to their cousins ​​in East Africa.

Unlock flowstones

The caves are full of special rock types. There are stalagmites growing on the ground and stalactites hanging from the ceiling. As water drips into the cave flows on the ground, rocks known as flow stones form – and in doing so, the mineral uranium is locked up inside, crystal crystal. This creates a kind of "clock" that tells us how old are the flowstones.

We collected these flow stones in the various caves of the cradle, brought them back to the laboratory, and then extracted and concentrated the minute amount of uranium present in the interior, as well as an even larger amount tiny lead produced from the decay of uranium. . These isotopes of uranium and lead allow us to read the "clocks" of creep stones – technically known as the dating of uranium-lead.

This lead-uranium dating is not new. It is well established and has been used by geologists for decades. its how we know how old the earth has. Nevertheless, the process was not easy. My colleagues and I had to adapt the existing uranium-lead dating method specifically for South African flow stones. The challenge was that in the rocks of a few million years only – young according to geological standards – there was not much time to accumulate lead, the product of the girl's disintegration of 39; ;uranium.

It took us 13 years to reach this point. But it was worth the wait.

In our recent article on Nature, we have compiled the largest dataset ever listed on ages for the Cradle Caves. We have been able to analyze them thoroughly and look for patterns. Specifically, we asked if all the caves were open to the surface to allow the fossils to infiltrate and collect at the same time – or each act as their own small ecosystem.

The cradle is a relatively small place (about 10 x 15 km) and we expect the same events to be recorded in all the caves at the same time.

And that's exactly what we found. We dated 29 runoff stones, coming from eight caves, and we found that they all date from the same narrow time window. For example, 2 million years ago, all of the cradle 's significant cave sites were closed – nothing was infiltrated – with a flowstone forming inland.

We also know that flow stones can only form when it is raining more. So, by dating the flow stones, we choose these times in the past. For the first time, we know that our human ancestors have experienced great changes in the local climate. The sediments containing the fossils inside the caves are all sandwiched between flow stones. We interpret this model, flowstone-sediment-flowstone, as a sign of these climate changes, with sediments representing drier times.

This means that all cradle fossils, hominin and other animals, have accumulated during dry periods.

Dating from the indateable

The layers of Flowstone in the caves are the equivalent of the ash layers in the Rift Valley. With the leaded uranium ages of these flow stones, South African fossils can come out of the shadow of their non-dating and their impregnation.

This will allow the world to turn its attention to the country's incredibly rich fossil record, understanding better when these fossils have been formed and what it tells us about human evolution.

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