Hominins link the great apes to modern humans on the evolutionary tree. Ancestors mark a crucial transition in the history of human evolution and have fascinated paleoanthropologists for decades.
In 1936, South African physician and paleontologist Robert Broom made a historic discovery in the Sterkfontein Caves in South Africa. Broom discovered the first adult specimen of the genus australopithecus, a group of early hominins from which our own genus, Homo, surfaced.
Since 1936, the Sterkfontein Caves have become Ground Zero for Australopithecus Research and fossil finds. The complex cave system runs 60 meters deep and has revealed hundreds of caves Australopithecus fossils in its sediment. Out of these rocks came remarkable discoveries, such as the almost complete skeletons of specimens named “Little Foot” and “Mrs. Please.”
Age is just one method
The cave consists of six areas or members: members 1 to 3 lie underground, while members 4 to 6 are exposed to the air due to erosion in the cave roof. Most Australopithecus Fossils are located in Member 4. The Sterkfontein Caves are part of a World Heritage Site with an evocative name – the cradle of mankind.
The complex cave system still holds many secrets, but the discoveries already made hold their own mysteries. One of the most debated topics is the age of the fossils found in Member 4. Researchers have estimated the age Australopithecus in the lower portion of Member 2 at 3.7 million years, which agrees with the estimated age of the fossils found higher in the cave. Researchers originally estimated the age of the fossils in Member 4 at between 2 million and 2.4 million years. The cave’s geological features challenge traditional methods of aging fossils and cast further doubt on the accuracy of these estimates.
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Purdue University’s Darryl Granger is among the researchers who questioned Member 4’s age Australopithecus. Recently, Granger and a team of scientists from France and South Africa tried to redate the famous fossils using a new method. They published their results in Proceedings of the National Academy of Sciences.
The team discovered that scientists were right to doubt the original results. That Australopithecus The fossils in the cave sediments of Member 4 date from 3.4 to 3.7 million years ago, about a million years older than originally thought. Now that the famous fossils have finally been placed in the correct timeline, scientists can better envision the lives of these early hominins, including the environmental conditions in which they lived. The findings also increase the geographic range and diversity of our earliest ancestors, reigniting debates about the history and timeline of human evolution.
Dating fossils in cave sediments
Accurate dating of fossils in caves is difficult. Unlike surface rock, cave sediments do not accumulate in neat strata. Large rocks, animal fossils, and debris from different time periods can fall into the cave, confusing paleontologists. Also, long after the cave’s formation, water can percolate downward to form younger stalactite deposits in older sediments.
Experts dated calcite boulder deposits in the cave to provide the original age estimates for Member 4 Australopithecus fossils. However, Granger and his colleagues point out that multiple lines of evidence indicate that this stalactite may be younger than the cave sediment itself – including the rocks in which it is found Australopithecus was deposited.
The team carefully analyzed the geological features in Member 4 to conclude that the sinter is much younger than the surrounding rock containing the fossils. The authors claim that the dating of stalactites using standard uranium isotope methods is correct, but that the estimated depositional dates are accurate for the stalactite, not the fossils.
The best dating methods examine the rock in which fossils are found, called breccias. To date the fossils, Granger transported the breccias back to his lab, the Purdue Rare Isotope Measurement Laboratory or PRIME Lab, where he and colleagues developed a new technique for determining fossil ages using so-called cosmogenic nuclides. These nuclides are isotopes produced by cosmic rays—energetic particles that can come from the sun, from outside the solar system, and even from other galaxies. These rays cause reactions within rocks that produce specific types of radioactive atoms with different numbers of neutrons from the element’s stable form. However, underground rocks are safe from these intergalactic particles. Researchers can therefore use the decay of aluminum-26 and beryllium-10, isotopes formed by cosmic rays, to this day when the rocks and the fossils they contained were buried in the cave.
The researchers combined their isotope results with an extensive geological survey of the area and concluded that the fossils are between 3.4 million and 3.7 million years old – at least a million years older than the original estimate of 2 million to 2, 4 million years. Member 4 makes this estimate Australopithecus the wise elders of the most famous australopithecus, “Dinkinesh” (also known as “Lucy”), dated by experts at 3.2 million years. The results are also consistent with the age of the older specimens found in the lower reaches of the cave. With the discrepancy corrected, we now know the whole thing Australopithecus The aggregation at the South African site is between 3.4 million and 3.7 million years old.
The startling results lead researchers to reconcile the ages of early hominins with our current understanding of human evolution. For example, because members of our genus, homoappeared about 2 million to 2.8 million years ago, the original South African Australopithecus Fossils were thought to be too young to be their ancestors. So, of course, the researchers assumed that homo evolved in East Africa, where we’ve found older fossils like Lucy.
These findings challenge the recognition of East Africa as the first place where humans evolved. Suddenly South Africa is a contender for the most likely area of early human evolution – and it really lives up to the Cradle of Humanity moniker.