Fossils In 'Cradle Of Humankind' Could Be Older Than 'Lucy', Oldest Known Human Ancestor

Human Ancestor Fossils: Scientists have recently found a wealth of fossils relating to human evolution in the "Cradle of Humankind" in South Africa. The fossils are older than Dinkinesh or Lucy.

Scientists have recently found a wealth of fossils relating to human evolution in the "Cradle of Humankind" in South Africa. Researchers have studied these fossils of early human ancestors and their long-lost relatives for decades. A Purdue University geologist has developed a dating method which just increased the age of some of these fossils found at the site of Sterkfontein Caves by more than a million years. 

Since the age of the fossils has been pushed back more than a million years, it means they are older than Dinkinesh, also called Lucy, the most famous Australopithecus fossil in the world. 

An international team of scientists including researchers from the University of the Witwatersrand in Johannesburg and the University Toulouse Jean Jaurès in France have made interesting discoveries related to the "Cradle of Humankind". The study describing the findings was recently published in the journal Proceedings of the National Academy of Sciences. 

What Is The "Cradle Of Humankind"?

The "Cradle of Humankind" is a United Nations Educational, Scientific and Cultural Organization (UNESCO) World Heritage Site in South Africa that comprises various fossil-bearing cave deposits, including at Sterkfontein Caves, made famous by the discovery of the first adult Australopithecus, an ancient hominin, in 1936. 

Hominins include humans and their ancestral relatives. However, other great apes are not hominins. 

Since 1936, hundreds of Australopithecus fossils have been found at the Sterkfontein Caves, including the well-known Mrs Ples, and the nearly complete skeleton known as Little Foot. 

In order to understand human and environmental evolution over the past four million years, palaeo anthropologists and other scientists have studied Sterkfontein and other cave sites in the Cradle of Humankind for decades.

Darry Granger, one of the researchers involved in the study, devised a method for dating buried cave sediments which is now used by scientists all over the world. He had previously worked at Sterkfontein where he dated the Little Foot skeleton to about 3.7 million years old. However, scientists are still debating the age of other fossils at the Sterkfontein Caves.

All Australopithecus-Bearing Cave Sediments Date From About 3.4 To 3.7 Million Yrs Old

The scientists involved in the study have discovered that not only Little Foot, but all of the Australopithecus-bearing cave sediments date from about 3.4 million to 3.7 million years old, rather than two to 2.5 million years old as researchers had previously theorised. 

Lucy Is About 3.2 Million Years Old

According to the study, the age places these fossils toward the beginning of the Australopithecus era, rather than near the end. Dinkinesh is 3.2 million years old and hails from Ethiopia. Her species, Australopithecus africanus hails back to about 3.9 million years old.

The study said that Sterkfontein is a deep and complex cave system that preserves a long history of hominin occupation of the area. The rocks and bones tumbled to the bottom of a deep hole in the ground, because of which understanding the dates of the fossils can be tricky. Also, there are few ways to date cave sediments.

Many hominin fossils have been found in East Africa. The Great Rift Valley volcanoes in East Africa lay down layers of ash that can be dated. The age of a fossil can be estimated by studying those layers.

How Researchers Estimate Age Of Fossils In South Africa

However, scientists do not have that luxury in South Africa, especially in a cave. Researchers mostly use other animal fossils found around the bones to estimate the age of a particular fossil. They also use calcite flowstone deposited in the cave. These are typically sheetlike deposits of carbonate minerals like calcite. Flowstone is a  mineral deposit found in “solution” caves in limestone. They are composed of sheetlike deposits of calcite or other carbonate minerals.

However, bones can shift in the cave, and young flowstone can be deposited in old sediment. This makes those methods of using animal fossils around bones or calcite flowstone to determine the age of other fossils potentially incorrect.

Why The Researchers Dated The Actual Rocks In Which The Fossils Were Found

Dating the actual rocks in which the fossils are found is a more accurate method to determine the fossils' age. Granger and his colleagues analysed the concrete-like matrix that embeds the fossil, called breccia.

In a statement released by Purdue University, Granger said Sterkfontein has more Australopithecus fossils than anywhere else in the world, but it is hard to get a good date on them. He added that people have looked at the animal fossils found near them and compared the ages of cave features like flowstones and have obtained a range of different dates. The data obtained by the scientists resolved these controversies. The data shows that these fossils are old — much older than "we originally thought", Granger explained.

Scientists Measured Radioactive Nuclides Found In The Rocks

The researchers measured radioactive nuclides in the rocks using accelerator mass spectrometry, a technique for measuring long-lived radionuclides that occur naturally in our environment. The team also used techniques such as geologic mapping to measure the radioactive nuclides in the rocks. Geologic mapping is a scientific process that can produce a range of map products for many different uses including assessment of groundwater quality and contamination risks. 

They also tried to understand how cave sediments accumulate in order to determine the age of the Australopithecus-bearing sediments at Sterkfontein. 

What Cosmogenic Nuclides Revealed

Granger and his colleagues at the Purdue Rare Isotope Measurement Laboratory (PRIME Lab) study cosmogenic nuclides, which are extremely rare isotopes produced by cosmic rays. They also analyse what cosmogenic nuclides can reveal about the history of fossils, geologic features and rock.

The incoming cosmic rays have enough energy to cause nuclear reactions inside rocks at the ground surface, and can create new, radioactive isotopes within the mineral crystals. For instance, aluminium-26 is an isotope of aluminium which is missing a neutron and slowly decays to turn into magnesium over a period of millions of years. Aluminium-26 could be formed only when a rock is exposed at the surface, and not after it has been deeply buried in a cave. Therefore, PRIME Lab researchers can date cave sediments and fossils within them by measuring levels of aluminium-26 in tandem with another cosmogenic nuclide, beryllium-10.

Hence, the scientists were able to date the fossils at Sterkfontein based on cosmogenic nuclides. 

Significance Of The Study

The researchers also made careful maps of the cave deposits and showed how animal fossils of different ages would have been mixed together during excavations in the 1930s and 1940s. This could have led to decades of confusion with the previous age.

Granger said what he hopes is that this will convince people that the dating method gives reliable results. According to him, this method can be used to more accurately place ancient humans and their relatives in Africa and elsewhere in the world.

The age of fossils is important to know because it determines scientists' understanding of the living landscape of the time. Some burning questions among researchers include how and where humans evolved, how they fit into the ecosystem, and who their closest relatives are and were. According to Purdue University, putting the fossils at Sterkfontein into proper context is one step towards solving the entire puzzle.