Monday , August 15 2022

How did we calculate the age of caves in the Leagan of Mankind – and why it matters


Dr Pickering in a modern cave, smiling at the beautiful drop on the cave floor. Credit: Gavin Prideaux

As a species, we humans have always been fascinated by where we came from. It was originally thought that people could not have come from Africa.

This misconception began to change slowly from 1925 when the modern discipline of paleoanthropology – the study of our origins – was born in South Africa. Professor Raymond Dart at the University of Witwatersrand in Johannesburg recognized the significance of a small fossil skull brought to him from Taung, in the northwestern province of the country.

Dart realized that the skull belonged to a creature that was neither fully monkey nor man: it was one of our distant ancestors, Australopithecus africanus, sense "the southern monkey of Africa".

In the following years, paleontologist Robert Broom has found several adult fossils from Sterkfontein and other caves in the area we now know as the Leagan of Humankind, just outside Johannesburg. His work cemented the fact that Africa was the birthplace of mankind, although many European scientists had years to come to this.

Since the 1960s, the fossil record of this important area has largely taken the back seat for discoveries in East Africa. That's because we did not know how old caves in the Leagan of Mankind were and could not provide conclusive data for the many fossils found in them. The geological setting in Cradle is very different from the Rift system in East Africa, where there are layers of volcanic ash between fossil beds; ash layers can be given, giving fossils age. Caves in South Africa do not have such volcanic layers.

But there are other types of rocks in the caves. Working with these, my colleagues and I used a given method of uranium-lead to establish the centuries of caves in the Leagan of Mankind. That means we can narrow the entire early Cradle record to several short windows between one million and three million years.

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

Unlocking flower stones

The caves are full of special types of rocks. There are stalagmites growing from the floors, and stalactites hanging from the ceilings. As the drainage water from the cave flows along the floor, there are formed rocks known as flower stones – and as they do, the mineral uranium is stuck inside them, the crystal crystal. This creates a sort of "clock" that tells us how old the flowers are.

We collected these flowers from the various places in Christmas, we took them back to the lab, then extracted and concentrated the small amount of uranium present inside, as well as the even lower amount of lead that was produced from the disintegration of uranium. These uranium and lead isotopes allow us to read the "clocks" of flowers – technically known as uranium lead dating.

This uranium-lead encounter is not new. It is well established and has been used by geologists for decades; how we know how old the earth is. However, it was not an easy process. My colleagues and I had to adapt our current uranium-lead dating method, especially for South African flowers. The challenge was that in the rocks of only a few million years – young people according to geological standards – there was not much time to accumulate lead, the product of the uranium decolorizing iron.

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

In our recent Nature, we put together the largest set of age data for Cradle Caves and we managed to look carefully at these things and look for models. Specifically, I asked if all the caves were open to the surface so the fossils could wash and collect fossils at the same time – or if everyone acted as their own little ecosystem.

Cradle is a relatively small place (approximately 10 x 15 km), and we expected the same events to be recorded in all caves at the same time.

And that's exactly what I found. I gave her 29 stones, eight caves, and found that all the stones of flowers dated to the same six narrow windows. For example, 2 million years ago, all the important places in the Cradle Cave were closed – nothing was washed in them – with a stone that formed inside them.

We also know that the layers of flowers can only form in times when it rains more. So, by meeting with the flower stones, we choose these times in the past. For the first time, we know that our early ancestors lived through great changes in the local climate. Fossil sediments inside the caves are all adorned with flowers. We interpret this model, flow-sediment-flowstone, as a signal of these climate changes, with sedimentation of dry times.

This means that all fossils from the swing, ominous and other animals accumulated during the dry season.

Dating the undatable

Cave rock layers are equivalent to ash layers in the Rift Valley. With uranium-lead ages for these flower stones, fossils in South Africa can come out of the shadow of undetectable and undetectable.

This will allow the world to turn its attention to the country's rich fossil record with a better understanding of the moment when the fossils were formed and what they tell us about human evolution.

Continue to explore:
Evolution: The hominin record in South Africa is a good weather friend

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