Indigenous cave paintings

Aboriginal rock painting

Aboriginal and Torres Strait Islander people have lived in Australia for more than 65,000 years, and over this time have recorded and passed down the history of the land through their culture. One method of documentation used by Indigenous Australians is rock painting, the art form for displaying stories of the land from thousands of years ago. Rock art in Kakadu has been dated to show that paintings are approximately 20,000 years old, making the paintings the longest historical records kept by humans. Some examples of rock art are thought to depict extinct species of Australian animals, such as megafauna, consolidating evidence for when exactly these animals roamed the Earth.

In addition, by dating the rock paintings and analyzing the style of art, scientists and anthropologists have been able to connect significant geological events to changes in the organizations of populations. Scientists from the University of New England have shown, through dating methods, that the Wanjina period of indigenous history began 5000 years ago, rather than the previously assumed 4000 years ago. This period was the beginning of the organization of groups linked to particular territories of land. From the research, and by understanding stories told in the rock paintings, anthropologists suggest that this organization occurred as a result of sea level stabilization, which caused a loss of land in the Kimberley region.

Indigenous paintings of people and kangaroos

Examples of the three rock types

Banded iron formations

Preserved fossils in rock formations

Rock structure and formation:

Fossils preserved in rocks provide a wealth of information about past ecosystems. Preserved flora and fauna can give us an idea not only of what organisms inhabited the Earth in the past, but what environmental conditions they would have needed to sustain life. Fossils are generally found in sedimentary rocks. Sedimentary rocks generally form in water as layers of sediment as it is deposited. Igneous rocks form from lava and their heat destroys biological materials. Metamorphic rocks similarly destroy fossils.

• Mineralized remains such as molds and casts, petrified wood and opalized remains

• Organic remains (e.g. preservation of soft and hard tissues in ice, amber, bogs and dry caves, as well as carbonized remains)

• Impressions - the shape of the external organism recorded in sediment

• Trace fossils, which are the remnants of organic molecules or other signature atoms associated only with life. These are also known as 'geochemical remains' and require advanced technology to detect.

Rocks are not, however, only useful as preservers of fossils, but can also provide information about past environments, and therefore what ecosystems would have thrive in them. By observing rock structure and formation, we can gain information about what natural events occurred millions of years ago.

There are three types of rock structures, sedimentary rocks, igneous rocks, and metamorphic rocks.

• Sedimentary rocks: are created by deposition (placing) of material, and often have a sandy texture. These rock structures are formed when substances are moved by wind, fast-moving water, glaciers, or the sea (by erosion). They can therefore tell us about how ecosystems looked in the past based on weather patterns. For example, the formation we know as the Grand Canyon was formed by rivers, which carved the valleys we see today out of the stone. From this information, we know that the landscape did not always look that way, and millions of years ago, they dry and arid Arizona was actually a wetter environment.

• Igneous rocks: are formed from lava spilling either onto land or into ocean. Presence of these rocks in stratigraphic layers will indicate a volcanic event in past ecosystem. This could show that areas which are now mountains were once active volcanoes, and provide information about how ecosystems would have looked when this was case.

• Metamorphic rocks: are formed by metamorphisms; the process of changing sedimentary or igneous sediments into a new rock form by extreme heat and pressure. This changes the arrangement and composition of minerals within rocks. As these rocks are formed deep below the Earth's surface, their presence in current landforms can give us information about how the earth has changed. Mountains containing quartz or marble would have moved from below the sea. The presence of slate indicates that an area was once a river or lake.

Banded iron formations are a form of geochemical evidence found in Australia and other continents. Scientists think that Earth's atmosphere has undergone many changes, with the change from anaerobic to aerobic around 1.8-2.5 billion years ago being one of the most notable. Banded iron formations are geological formations consisting of alternating bands of iron-rich and iron-poor sediments.

In addition to the types of rocks, formation of these materials can also provide information about how landscapes and ecosystems used to look. Canyons may been eroded by rivers. Mountains may have been pushed up from the seabed. Using strati-graphic tools, geologists can begin to piece together Earth's history, and imagine how continents have moved and changed. This provides useful knowledge also for understanding evolution, and how different organisms have changed as their environments changed too.

Ice core drilling samples

Ice core drilling

Process of radioactive decay

Radiometric dating

Radiometric dating is a technology which is used to date geological materials, such as fossils and rocks. Dating fossils and rocks is a useful tool for understanding evolutionary relationships, as it can indicate the order of organisms' existence, and therefore define ancestry. Radiometric dating can also be used to show how the Earth has changed over thousands of years, through a technique called geochronology. Geochronology is the practice of finding the ages of sediments, and therefore allowing scientists to determine a time-scale for geological phenomena. Additionally, many organisms are preserved within rocks, so an understanding of what the Earth may have looked like in terms of flora and fauna can be established. From this, rate and methods of evolutionary change can be established. Understanding the composition and age of rocks also allows scientists to make hypotheses about other factors, such as atmospheric composition, climate, and radiation levels, at different time periods in Earth's history. By building chronology of what rocks existed when, we can place organisms within this timeline, and see how they may have changed in the past.

So how does radiometric dating work? Well, it rests on the principle of radioactive decay. Known isotopes have quantifiable rates of decay (i.e. we know how long it takes for a certain amount of an element to disappear), and using this information we can determine how old a substance is, based on how much of the original element and how much of the decay product exist. Therefore, by measuring the number of radioisotopes and their decay products in a rock layer or a fossil, we can determine the age of that rock layer, and where they fit in the geochronology of Earth. Some common radioisotopes used for radiometric dating include uranium, potassium, and carbon.

Rainbow Lorikeet and a flowering gum tree

Evolution of Australian Biota

Due to Australia's long history of isolation, Australian ecosystems consist of a unique array of flora and fauna. Fossil evidence gives scientists clues to the slow, progressive changes that have taken place in Australian species over the last 30 million years since Antarctica separated from Australia. Australia's climate has alternated between warm/wet cycles and cold/dry cycles. This in turn has influenced the pattern of vegetation, which has gone from tropical rainforests with broad-leafed plants to predominantly open grassland and desert with sclerophyll plants as the dominant plant life.

The first mammals appeared on Earth around 240 mya and evolved from reptiles. The three groups, monotremes, marsupials and placental mammals, gradually evolved as branches from the common ancestor. The oldest mammals appear in Australia's fossil record around 125-110 mya. By around 110 mya, flowering plants were establishing in Australia and mammals had appeared. The mammals shared the land with dinosaurs, which consisted of Australia, Antarctica, New Zealand and South America. Eventually at 30 mya these links were severed, starting the period of isolation.

Parallel evolution of marsupials and placentals

Evolutionary tree of monotremes, marsupials and placental (eutherian) animals

Evolution of early mammals

Evidence of evolution in small mammals

Australia has been isolated as a continent for about 40 million years, meaning that during this time populations have been able to adapt and evolve into an array of unique animals. The Australian mammal population is mostly composed of marsupials (young carried in pouch), and also includes monotremes (egg-laying mammals) and placental mammals (carry young in utero until they are able to survive outside the body). Marsupials occupy a majority of ecological niches normally filled by mammals elsewhere in the world. This demonstrates the principle of convergent evolution, how very different animals with no recent common ancestors can evolve similar traits due to exposure to similar selection pressures.

Early evidence of monotremes in Australia are found as fossils, dating back to the Early Cretaceous period (145-99 million years ago). Teinolophos trusleri was a primitive platypus, whose remains were found in flat Rocks, Victoria. The platypus family, Ornithorhynchidae, first appeared on the Australian fossil record about 25 million years ago, followed by echidnas.

Early platypus

The marsupial is thought to have arisen in North America, spreading through South America to Australia while the landmasses were still connected. The first undoubted marsupial fossils in Australia are dated back to the Oligocene era 25 million years ago, although they may have appeared as early as 65 million years ago. During the Miocene era (23-5 million years ago) the Australian climate became drier, with rainforests becoming sparser and shrub-like. During this period, marsupials diversified to suit the new ecological niches, evolving to become more similar to modern marsupials.

The first placental mammals are theorized to have arisen 120 million years ago, from marsupial ancestors. It is thought that placental mammals came to Australia during the Miocene era, during which the continent drifted closer to Indonesia, and animals (such as the dingo) were able to migrate. The oldest bat fossil in the world was found in Murgon, Queensland, and dated back to 55 million years ago.

Sclerophyll forest

Evidence of evolution in Sclerophyll plants

Modern Australian ecosystems

Changing Australian ecosystems

The Permian-Triassic extinction event

Permian-Triassic extinction event

Bleaching of corals

Bleaching of the Great Barrier Reef

Tree's recolonizing the Fantastic Lava Beds

Reasons for ecosystem change

Causes of change to an ecosystem are called ecological disturbances. These are events which disrupt ecosystems and change resource availabilities, forcing organisms to adapt to new conditions. Ecological disturbances can be either abiotic (storms, floods, fires, droughts) or biotic (deforestation, plagues, tunneling by other organisms, grazing patterns of organisms).

Bushfires cause ecological disturbances

Invasive species are a major cause of ecological change. These organisms insert themselves into ecological niches, displacing other organisms within the environment by out competing them. This has a cascading effect on all organisms in the ecosystem, due to disruption of the food chain. An example of an invasive species is cane toads or rabbits in Australia.

Rabbits are an invasive species in Australia

Otters control sea urchin populations

Another type of ecological disturbance is the disappearance of a keystone species. Keystone species are those that play a unique and central role in ecosystem function. When they are removed, there can be a significant change in how the ecosystem works. For example, sea otters play an important role in maintaining Pacific kelp forests. Otters eat sea urchins, which eat the kelp. When otters were removed from this ecosystem due to hunting, sea urchins were left unregulated, decimating the kelp forests, and removing homes for fish and other sea life. The removal of the otters had widespread effects on a number of organisms in the system.

For fun: Otters go to war

Ecological disturbances also occur when resources are changed. This might be adding an excess or new resource, or removing existing resources. This will shift the ecological niches. New ones might emerge, or old ones might disappear, and this changes the sizes of populations within the system. A example of this is Eutrophication. This occurs in lakes or ponds when excess amounts of phosphates are added (typically from agricultural run-off) to the ecosystem. This encourages algae to grow in excess, which eventually covers the pond, blocks off oxygen to the rest of the ecosystem, and causes all other aerobic organisms within the pond to die.

Giant wombat - Diprotodon optatum

Changing flora and fauna

Marsupial lion - Thylacoleo carnifex