When and how did the first humans arrive and settle in Australia? These questions have vexed scientists for a long time.
For the most part, it was largely assumed that early people moved around the coasts and gradually made their way inland. But that idea has now been turned on its head by the discovery of microscopic plant fossils from a remote and little-known highland valley in Papua New Guinea.
It opens for us a window on the distant past, revealing what the environment was like thousands of years ago.
The minute traces of pollen, starch, silica skeletons and charcoal reveal that humans were sufficiently adaptable to climate extremes to survive more than 45,000 years ago across a range of harsh environments on the ancient ice age continent of Sahul, comprising New Guinea, Australia and Tasmania.
Local botanists Michael Lovave and Thomas Magun, far left and far right, with Professor Glenn Summerhayes and student Sindy Luu at Kosipe Mission site.
"We are now much closer to understanding what drove the first Australians to rapidly disperse and permanently settle the wide range of environmental zones across this continent, from the southerly, cold reaches of Tasmanian wilderness to the scorching deserts that border Australia's north-west coast," says University of New South Wales archaeologist Judith Field.
"The evidence makes us pause and reconsider the novel and largely unpredictable ways that humans respond to uncertain climates, new landscapes and new ways of using those landscapes."
Their movement from the coast to the highlands indicates that the first Australians were not completely dependent on coastal resources such as fish and shellfish, as has been suggested elsewhere. "It also suggests that the knowledge of landscapes, particularly plants, was such that they could readily adapt to the new plants and animals they encountered," Dr Field says.
PNG pandanus research
Sampling at Kosipe, Papua New Guinea.
The first Sahul residents were able to harvest foods to meet their requirements: "Harvesting and storage of foods permitted overnight trips to higher elevations to perhaps undertake pioneering exploration."
This, she says, speaks of planning for these forays into the highlands. The movement to higher altitudes may have been targeted at harvesting Pandanus – an Old World tropical palm-like plant with leaves and roots that provide dietary protein, fats and starch.
"The dating of Pandanus fruit tells us these were an important part of the diet over 30,000 years ago," she says.
Botanist Michael Lovave inspecting wild pandanus fruit.
In the course of reproducing, all flowering plants produce pollen. The shape and size of pollen grains reveals their family and species history and so extracting them from soils and counting and identifying their types allows scientists to reconstruct past environments.
"Importantly, it opens for us a window on the distant past, revealing what the environment was like thousands of years ago," Dr Field says.
The fossilised plant remains were found in and around the Kosipe Mission in the Ivane Valley, roughly 100 kilometres north of Port Moresby. The valley sits at 2000 metres above sea level in the spectacular Owen Stanley Ranges. The base of the valley is dominated by a large swamp, 28 square kilometres in size. The Ivane Valley is blessed with relatively mild and constant temperatures throughout the year.
(The valley's undulating topography contrasts with the adjacent and perilously steep Kokoda Valley, which is constantly reworked by river systems.)
The dense tropical rainforests of the ruggedly beautiful mountainous nation, located on one of the world's largest islands, is home to remote tribes, some of whom have had little contact with the outside world.
"The tiny fossils were derived from plants and are often well preserved in the soil deposits of swamps, lakes and caves for tens of thousands of years," Dr Field explains.
When humans first arrived in Sahul, between 45,000 and 50,000 years ago, the climate was characterised by big swings in temperature and moisture, sometimes leading to increasing dryness.
Much of the evidence comes not just from studies of pollen but also isotopes obtained from Antarctic ice drill cores. Other studies have dated fluctuating lake levels at Lake Eyre and Lake Frome in South Australia, as well as the formation of sand dunes during the peak of the ice age in central Australia. "Environmental records from the Ivane Valley reflect changes in conditions at continental and local scales," Dr Field says.
Professor Geoff Hope from the Australian National University analysed cores for pollen and charcoal from the swamp and some small alpine lakes. When people first arrived in the Ivane Valley temperatures were 4 to 6 degrees lower on average – with forests dominated by beech trees.
"There are no forests like that in the area today," Dr Field says. Evidence of human activities in the form of microscopic charcoal deposits from fires become apparent between 38,000 and 41,000 years ago.
Scientists know from pollen records that during that period, the environment was very different, with a highly variable climate. "It's not surprising then that human visits to the Ivane Valley 45,000 to 50,000 years ago were probably brief and targeted," she says.
Weighing the evidence
Why is this research important for understanding how humans colonised Sahul? "The microscopic and macroscopic evidence provides important clues to the ability of the first colonisers to plan and quickly adapt – and then move through a completely new environment," Dr Field says.
"It also provides us with hints as to the pathways people may have taken to enter the Australian continental land mass."
The first terrestrial plants appeared more than half a billion years ago.
Among the earliest to emerge were liverworts that flourished about 475 million years ago. They were followed by mosses, horsetails, ferns, cycads, ginkgos and conifers. Roughly 145 million years ago, flowering plants blossomed forth.
Not all these plants evolved from one another. Liverworts, for instance, seem to have existed more or less continuously while other plant species evolved more recently from ancient ancestors, many of which are now extinct.
Plant scientists know all this from fossil records and from the subtle changes they have measured in plant proteins over time.
"Plants are the cornerstone of human existence and, as such, are intimately linked to the movement of people across the globe," says Dr Field.
In conjunction with sophisticated dating techniques, the development of microscopic studies of plant remains from archaeological excavations has permitted the mechanics of human movements and arrivals to become clearer.
"The excellent preservation of both plant remains and residues on the stone tools provides detailed information on food choices which has not been recorded in sites of this age from this region and provides new insights into the life of human arrivals in New Guinea," Dr Field says. "The degree of preservation of starch residues on the surface of the stone tools exceeds anything else we have studied."
The evidence was compiled from campsites buried by volcanic ash. The people who lived there, more than 40,000 years ago, made and used stone tools, hunted small animals, gathered the tropical nuts of palm-like Pandanus trees and processed the starchy tubers of yams in conditions much colder than today.
Yams only grow at altitudes below those of the Ivane Valley. "The discovery of yam starches on stone tools, coupled with the presence of stone not known from the Ivane Valley, clearly shows that people were living across several altitudinal zones," she explains.
"We can confidently say that humans were actively exploiting both Pandanus and yams at the time of earliest settlement."
Key to colonisation
Social scientists know that the Aborigines colonised Sahul relatively quickly.
The earliest occupation dates from Tasmania, with other areas, bar the Australian rainforests, colonised more than 30,000 years ago.
"Permanent settlement of the Australian rainforests is a recent phenomenon and we think it was linked to access, and the technology, to process toxic starchy foods," says Dr Field.
"Until now, we have had little detail about who these people were or the skills and knowledge they brought with them. It's a pretty big change from south-east Asia. These important skills and broad knowledge of landscapes must have been critical to such rapid dispersal across Sahul."
The role of archaeobotany, the study of fossil plants, has benefited from refinements in investigative techniques.
Dr Andy Fairbairn of Queensland University identified the Pandanus nut from plant remains using the technique of scanning electron microscopy. This relies on a beam of electrons instead of light to reveal the intricate details of the smallest of objects.
An object, such as the carbonised remains of Pandanus, is placed on a small stand. An electron gun fires a beam of electrons at the object. As the beam scans across it, a detailed image of the surface appears on a high-resolution monitor.
The technology can identify a range of plant types from tiny grains, some as small as a few thousandths of a millimetre across. "Their size and shape can be used to identify a genus or species of plant," Dr Field says.
Find out more about the first Australians at: www.nma.gov.au/exhibitions/first_australians/home
Check out the ancient world of fossilised plants at: www.fossilplants.com/site/index.cfm?action=what_is
Discover the work of Dr Field at: www.bees.unsw.edu.au/judith-field
AusVELS Science; Biological sciences: ausvels.vcaa.vic.edu.au/Science/Curriculum/F-10
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