Rock drawings of an ancient bird twice the size of an emu have been identified in an Aboriginal reserve in remote northern Australia and are being considered as the oldest art in the world. Rock art experts believe the paintings depict the Genyornis, a giant bird that allegedly went extinct 40,000 years ago. Any time scientists try to date things from thousands of years ago, however, they have to depend on assumptions that may or may not be correct.
Arnhem Land in Northern Territory is known for its ancient rock art. A wide variety of paintings on the sandstone rocks have survived for thousands of years, depicting ancient animals no longer around today. The area's painted menagerie includes a variety of extinct megafauna like the giant echidna and giant kangaroo, as well as the Tasmanian tiger. Now, archeologists believe they've found the painting of a Genyornis, a huge flightless bird with a distinctively rounded beak like a parrot. Those studying the red-ochre paintings assumed they were of emus – until they got to that beak.
"If you were to draw a Genyornis this is they way it would look," said Dr Peter Murray, an anthropologist and paleontologist retired from the Museum and Art Gallery of the Northern Territory.
"The details on this painting indicate that it was done by someone who knew that animal very well," archaeologist and rock art specialist Ben Gunn told Australian national broadcaster ABC. "Either the painting is 40,000 years old, which is when science thinks Genyornis disappeared, or alternatively, the Genyornis lived a lot longer than science has been able to establish."
Until now, the oldest ancient art in the world has been believed to be the spectacular rock art in the Chauvet Cave in southern France, radiocarbon dated to about 30,000 years ago.
While these paintings are no doubt very old, other scientists are not so quick to give them a 40,000 year date. For instance, the red ochre paint has preserved quite well for so great an age without having been protected inside a cave. Convenor of the International Federation of Rock Art Organisations Robert Bednarik suggested that the painting is just 5000 years old and expressed doubt that it represented a Genyornis.
"I am not aware of any painting or even petroglyph (carving) of an animal anywhere in the world that is more than 10,000 years old located outside of caves.”
Gavin Prideaux, a megafauna expert with Flinders University in Adelaide noted that scientists still debate when exactly the Genyornis went extinct. Radiometric dating has not yet been done on the paintings, yet scientists expect it soon to give an absolute date to the rock art.
How Old Is It Really?
Archeologists and geologists use a variety of methods to determine the ages of items from rocks to bones to pottery shards. In this case, they've examined paintings of an animal that supposedly died out 40,000 years ago and therefore believe the paintings must be at least 40,000 years old.
Scientists can also turn to radiometric methods in order to get a more absolute age for an archeological site. While the Genyornis paintings have not been subjected to radiometric dating, the debate arises in the first place because the youngest fossils of Genyornis were dated to 40,000 years ago through radiometric dating methods.
Ancient items like rocks or bones or pottery shards or torch markings – or the rock they are found in – all contain certain chemical elements. By measuring the radioactive elements in an item as well as the daughter material those elements break down into, scientists believe they can measure the age of the item with a fair degree of accuracy.
The isotopes of certain elements, or nuclides, are unstable and they spontaneously break down at known decay rates. The parent material is radioactive, and its final daughter product is stable. The time it takes for half of the atoms in a nuclide to break down into its daughter material is called its “half-life.” Perhaps the most well known dating method involves carbon-14, an isotope with six protons and eight neutrons that decays into nitrogen-14 through beta decay. By measuring the amount of parent material (carbon-14) in an item and comparing that to the amount of daughter material (nitrogen-14), scientists believe they can determine the age of anything containing carbon – up to a certain point. The half-life of C-14 is 5,730 years, and is considered to be useful for dating carbonaceous materials up to about 60,000 years in age.
Many other radioactive isotopes can be used, depending on the ages scientists expect to get. With its half life of 1.25 billion years, potassium-40's decay into argon is measured to determine the age of material believed to be 100,000 years old at the least and billions of years old at the most. Uranium-lead with its half-life of 4.5 billions years is used for samples expected to be more than 1 million years old. These are the best known methods, but others include iodine-xenon (I-Xe), lanthanum-barium (La-Ba), lutetium-hafnium (Lu-Hf), rhenium-osmium (Re-Os), and dozens more.
Assumptions?
Despite the geologist's great faith in radiometric dating, these methods involve a variety of assumptions. In order for the measurements taken to give an accurate date, the following all have to be true:
(1) the radioactive element being measured must decay at a constant rate and must have decayed at the same rate since the material being analyzed came into being;
(2) the material being analyzed cannot be contaminated by additional daughter product;
(3) the material cannot have already contained some of the end product when it was formed;
(4) parent element cannot have leached out of the sample material.
One way to double-check an age-date is to use a variety of different dating methods and see whether they give the same dates. Scientists often claim they have done just this and have gotten corroborating results. Others have argued that these scientists throw out the unexpected results that do not "fit" and their conclusions are therefore faulty.
Another way to double-check the age dating methods is to do a large number of tests on samples from the same site and see whether the range of dates produced are fairly close to each other - or whether they are widely disparate. Checking samples from sites where a date is already known from history can also shed light on the accuracy of various dating methods.
[To be continued next week, when we will examine studies that test the dependability of different radiometric dating methods...]
Related Links:
• Aboriginal Rock Art May Be 40,000 Years Old - The Times