Teacher background information

This elaboration provides students with a context for consolidating their understanding of the structure of atoms, and how natural changes in the nuclei of atoms of some elements allow materials to be dated. Students learn how methods based on the phenomenon of radioactivity, in conjunction with other, more recently developed dating methods, have allowed archaeologists to confirm what First Peoples have long held to be true: that Aboriginal people have been present on the Australian continent for more than 60,000 years.

Elements are made up of atoms. Atoms are made up of smaller particles called protons, neutrons and electrons. The atoms of each element contain the same number of protons in their nuclei. The number of neutrons in these atoms may, however, vary. Atoms of the same element, but with different numbers of neutrons are called isotopes. The element carbon, for example, exists naturally as three different isotopes: carbon-12, carbon-13 and carbon-14, generally denoted in the form of their chemical symbols 12C, 13C and 14C. All of these atoms contain 6 protons – this is what makes them carbon atoms. The number of neutrons in each of these isotopes is 6, 7 and 8 respectively. Some isotopes, such as 14C, are unstable. In a process called radioactive decay, the composition of their nuclei changes to become more stable nuclei. The rate at which this decay occurs differs for each particular radio-isotope. In the case of 14C, the rate of decay is such that half of the 14C nuclei in any given sample will decay in 5,730 years. This duration is called the half-life of 14C. 

Carbon-14 atoms are formed in the Earth’s upper atmosphere by the interaction of cosmic ray neutrons with nitrogen-14 atoms (14N). The 14C atoms combine with oxygen to form carbon dioxide which then enters the biosphere primarily via photosynthesis. While an organism is alive the ratio of 14C : 12C isotopes in its body is similar to that in the atmosphere and remains relatively constant. When the organism dies, there is no further uptake of 14C, so the ratio of 14C : 12C changes as the 14C atoms radioactively decay to form 14N again. Therefore, by measuring the 14C : 12C ratio in biological artefacts, it is possible to calculate the amount of time that has passed since the organism died. Since the half-life of 14C is 5,730 years, this method can be used to measure dates up to about 40,000 years in the past with reasonable accuracy. 

Radiocarbon dating procedures have been used by archaeologists to study the migration patterns of early humans and the antiquity of the habitation of the Australian continent by Aboriginal Australians. While people of European descent have been living in Australia for a little over 200 years, in the first half of the 20th century it was generally accepted by the scientific community that Aboriginal peoples have been living in Australia continuously for about 20,000 years. The discovery of human remains at Lake Mungo in western New South Wales, however, lengthened this timeline. These remains, informally referred to as ‘Mungo Lady’ and ‘Mungo Man’, were unearthed in 1969 and 1974 respectively. The original radiocarbon dating of Mungo Lady’s remains revealed that they may have been as old as 26,500 years. Mungo Man’s remains were estimated, using stratigraphic comparison with Mungo Lady, to being between 28,000 and 32,000 years old. In 2003, a panel of experts using a range of more refined dating techniques agreed that the Lake Mungo remains are probably about 40,000 years old.  

Based on recent archaeological evidence, it is now clear that the Australian Aboriginal peoples have occupied the Australian continent for even longer than the Lake Mungo calculations suggested. 

Analysis of excavations at the Madjedebe or Malakunanja II rock shelter in Arnhem Land using a combination of radiocarbon dating and other methods based on thermal and optical luminescence has revealed evidence of continuous habitation of Australia by Aboriginal peoples for at least 65,000 years. 

This elaboration provides opportunities for students to investigate the advantages and disadvantages of a range of dating techniques, in particular, the complications associated with radiocarbon dating. Investigation could take into account the fact that the 14C : 12C ratio in the atmosphere has not always been constant, or the impact of contamination by ‘younger’ carbon on the age determination, and the methods scientists are using to minimise these limitations. 

Furthermore, students will have an opportunity to investigate competing viewpoints about the spread of people to Australia and gain an appreciation of the vast timescale of Aboriginal habitation on the Australian continent.