Out of Africa: dating our earliest ancestors

Posted on December 6, 2011


La Trobe archaeologist helps estimate the age of African fossil finds –

Our earliest ancestor? Dr Herries examines a model of the skull of 1.98 million year old Australopithecus sediba. On screen is an image of what Au. sediba may have looked like. (Reconstruction by John Gurche.)

Dr Andy Herries has helped firm up a date of almost two million years ago for a number of South African fossils that made world headlines last year when hailed as some of the most complete early human fossils ever found.

He is part of a global team which includes scientists from the University of Melbourne and James Cook University, Queensland, whose work was published recently in a series of five papers and on the cover of the international journal Science.

The initial discovery of the fossil in cave sediments at Malapa in South Africa was featured last April as the cover story in Science. Suspected then to be at least 1.8 million years old, it generated wide-spread speculation as to whether it is the fossil link between more primitive ape-like human species (Australopithecus) and fossils that are more human-like and in our own Genus, Homo.

Since then, the team have been engaged in more detailed analysis of the cave site and have identified more than five specimens, including babies, juveniles and adults.

Lessons from the Earth’s magnetic field

Dr Herries is a specialist in ‘Archaeomagnetism’ which aims to understand the past by looking at the magnetic mineralogy and fossil direction of magnetisation (palaeomagnetism) preserved in archaeological and fossil bearing sediments and materials.

The Malapa Cave sediments in which the Australopithecus sediba fossils were found can be dated using archaeomagentism because they preserved the direction of the Earth’s magnetic field at the time they were deposited.

At certain times in the past, says Dr Herries, the Earth’s magnetic field has reversed itself by roughly 180o. The age of many of these reversals is known from the study of deep sea sediments and lava sequences.

Dr Herries explores a cave in South Africa. Such ‘flowstones’ can be used to date the age of fossils and preserve evidence of the climate, vegetation and geomagnetic field at the time they were formed.

Most precise dates ever achieved

‘Sometimes the field is stable in one direction for a long period; at other times it is more unstable and reverses itself a number of times, or reverses itself and then returns back to its original position over a very short period, what is known as an “excursion”.’

He says the Au. sediba fossils appear to have been deposited in the Malapa cave during one of these very short – about 6-3,000 year long – ‘excursions’ around 1.98 million years ago.

This age has been confirmed by Dr Robyn Pickering at the University of Melbourne by dating the decay of uranium to lead in ‘flowstone’ layers above and below the fossil. ‘The results of this study present arguably the most precise dates ever achieved for any early human fossils,’ says Dr Pickering.

‘Mrs Ples’ and other ancestors

Drs Herries and Pickering have provided the first dates for several of our earliest ancestors in Africa, including Au. sediba’s ancestor, Australopithecus africanus.  And Dr Herries was the first to provide an age for the famous and most complete fossil of Au. Africanus – also known as ‘Mrs Ples’ – from Sterkfontein Cave.

Like Au. sediba from Malapa, ‘Mrs Ples’ was deposited in the cave during a short magnetic ‘excursion’. She (although the fossil is probably of a he) is only slightly older than Au. Sediba, about 2.05million years, but Dr Herries has dated other fossils of this species back as far as three million years.

Recently, he began a National Geographic funded project to date the first specimen of Au. africanus ever located, a child’s skull from the site of Taung in South Africa, found by Australian anatomist Raymond Dart in 1924. This research has shown that South African caves often preserve reversals and excursions of the Earth’s magnetic field, many of which have either never been documented, or whose age is uncertain.

Dr Herries’ Archaeomagnetism laboratory at La Trobe is also beginning to try and understand, not only the age of these ‘reversals’ and ‘excursions’, but also what happens during these events, and if such changes influence climate or the behaviour, extinction and development of humans and other species.

More cosmic radiation reaches the Earth

‘We don’t known exactly what effect such reversals in the Earth’s magnetic field have on biological organisms, but during these shifts the magnetic field decreases causing more cosmic radiation to reach the Earth’s surface. 

A model of the ‘Australopithecus sediba’ skull, left. On the right is the most complete specimen of Australopithecus africanus, (also known as Mrs Ples) the species from which Au. sediba evolved, The skull, centre, is perhaps the oldest specimen of Homo ergaster, from Swartkrans Cave, dated to about 1.8million years ago. Au. sediba shares traits with both these specimens.

‘The link between such an unusually rich fossil assemblage and such a short geomagnetic field event is intriguing, but may just be pure coincidence. Only future research may answer the question over whether there is a link.’

Dr Herries says that the Earth’s magnetic field has been declining rapidly over the last 2,000 years. Given that the last major geomagnetic field reversal took place 780,000 years ago, we are overdue for one – but probably not for at least 2,000 years.

‘Caves also provide us with the potential to date and understand these short geomagnetic field events in a unique way.  Our ability to date and correctly identify these events is crucial as it will enable us to both better date fossil and archaeological sites as well as understand the physical workings of our own planet and its core.’

What does Au. sediba tell us about our evolution?

The new Au. sediba fossils comprise two partial skeletons  and a well-preserved juvenile skull. It was found by South African archaeologist Lee Berger from the University of the Witwatersrand – or to be more precise, by his then nine year old son after finding the site on Google Earth.

Dr Herries says Au. sediba has a mix of characteristics typical of older Au. africanus and the younger species Homo ergaster, which eventually led to Homo sapiens.  The skeletons are a vital clue that could illuminate the complex transition from more ape-like to more human-like bipedal primates. 

The fossils, with their Homo-like traits, are considered by his colleagues as one of the most complete candidates for such a transitional species.

‘The question is,’ says Dr Herries, ‘whether these similarities mean that Au. sediba is the direct ancestor of Homo ergaster – the first species to leave Africa, which most researchers agree  with – or whether Au. sediba is a side branch in evolution that shows evidence for the parallel evolution of certain anatomical features researchers have traditionally used to define the Genus Homo.’

Origins of the Australian seaside lifestyle

Excavations at the 164,000 year old Pinnacle Point Cave. The cave may have been a refuge for the first modern humans who learnt to harvest shellfish, a stable source of food during a harsh, cold climate. Dr Herries is abseiling in front of the cave while exploring older, higher caves in the cliff face.

Dr Herries, originally from Liverpool in the UK, has worked in South Africa for the last 15 years on important cave finds relating to human ancestry going back five million years. 

He joined La Trobe early this year where he set up the new Archaeomagnetism Laboratory, the first of its kind in Australia.

In 2005, he and Professor Curtis Marean of Arizona State University received a $2.5 million National Science Foundation (NSF) grant to excavate a number of sea caves at Pinnacle Point on the southern Cape coast of South Africa,

Abut 164,000 years ago a small group of modern humans, perhaps our direct ancestors, began collecting shellfish off the rocks at Pinnacle Point and, for the first time, used red ochre to make a pigment, suggesting the beginnings of symbolic thought.

‘Here they took their first steps to a coastal lifestyle that would eventually lead us out of African and all the way to Australia,’ says Dr Herries. This work was published in Nature in 2007.

Later the same team discovered the oldest evidence for the heat treatment of stone tools at 72,000 years ago and this work was published in Science.

Discovery of oldest rock art in China

The work at Pinnacle Point has pushed back the earliest known evidence for when humans began to think and behave like we do. Recently Dr Herries and colleagues obtained another $1 million NSF grant for this project.

Since 2007 he has also worked in Yunnan Province southern China as part of an ARC Discovery Project with colleagues from Griffith University, the University of New South Wales, ANSTO and the Yunnan Institute of Cultural Relics and Archaeology.  This work has so far discovered a number of new hominin fossils – and the oldest rock art in China.

Along with his projects in Australia, the China research aims to understand what happened to humans at the other end of their journey, after they left Africa.

‘Did they bring the skills they developed in Africa with them, such as heat treatment, and who did they meet along the way? What else can we learn from these sites about who we are and where we come from?’ (ER)

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