Discovering Australia's evolutionary past

Key text

Deep within the rainforests of tropical North Queensland grows one of the world's most spectacular flowering plants. The red silky oak, or rainforest waratah, grows in only a few kilometres of the Atherton Tableland and attracts nectar-feeding birds that flock to its brilliant scarlet flowers. Meanwhile, 14,500 km away on the slopes below the ancient Inca stronghold of Machu Picchu, in the Peruvian Andes, grows a shrub whose leathery leaves and bright pink flowers are uncannily similar to those of Australia's rainforest waratah. Could there possibly be a connection between these two flowering plants?

It turns out the rainforest waratah and the South American shrub are close relatives. The two trans-Pacific cousins belong to a family of flowering plants known as the Proteaceae. Varieties of the family include the king protea of South Africa, the Sydney waratah and the old man banksia. Others are also found in New Caledonia, South East Asia, New Guinea, New Zealand, Fiji and Madagascar. So how did they end up at opposite sides of the Pacific and Indian Oceans?

The old in with the new

Scientists have been able to fill in details of the travel diary of the globe trotting Proteaceae by using a combination of old and new techniques. Traditionally, relationships between organisms are established by studying the morphology or the physical form and structure of the living organisms.

Comparisons are then made between the structures of the living organisms to establish relationships. Their fossilised relatives can also be included in these analyses to identify the relationships. But this method is sometimes prone to classification errors, for example when traits inherited from a shared ancestor are mistaken for traits arising through convergent evolution. Such a classification method may therefore be unreliable and its results should be tested using evidence that is independent of morphology. No source of evidence is sufficiently fool-proof to be used exclusively in determining the interconnections between organisms.

One of the more recently developed techniques used to establish relationships between organisms is molecular phylogenetics. Scientists determine the evolutionary relationships between groups of organisms by studying their DNA sequences. More specifically, scientists are interested in establishing the rate and the pattern in which changes occurred in the DNA sequences. A distinct advantage of the DNA studies is that they typically rely on genes that contain very large numbers of variable traits and are independent of morphology.

Reconstructing the Proteaceae family history

Studying fossilised pollen grains provides 'snapshots' into the Proteaceae's family history. The pollen grains tell us that certain species were once present in a region and also an idea of their age. The oldest pollen grain belonging to the family dates back to 94 million years ago and has been found in both Brazil and western Africa, hinting the two continents had a much closer relationship than what is seen today.

With the information obtained from studying the DNA of the living members of the family and their fossilised relatives, scientists can construct a phylogenetic tree or a dendrogram.

In the simplified Proteaceae family tree shown, the common ancestor is represented by the tree trunk and each descendant is placed at the tip of a branch. The greater the number of similarities in the DNA between two family members, the shorter the distance between the two branches. Family members with more differences between their DNA are located on branches that are separated by longer distances.

Going with the continental flow

What scientists have discovered is that virtually all of the areas in which members of the Proteaceae family are found today were once part of the southern supercontinent of Gondwana. The supercontinent was made up of what is now Africa, South America, New Zealand, Australia, Madagascar, Antarctica and India.

Related site: Gondwana and the South Pole Image of the continents that once made up the supercontinent Gondwana. (National Geophysical Data Center, USA)

In many – but not all – cases the distribution of the Proteaceae matches the geological sequence of events in the formation of the Gondwanic continents. This suggests that the movements of the continents can be used to explain the evolutionary history of the Proteaceae.

The continental jigsaw puzzle

Plate tectonics is commonly used by scientists to describe the interactions and movements of the continents on the Earth's surface (Box 1: Plate tectonics). Up until about 165 million years ago, the continents of the Earth were all connected and formed a giant supercontinent known as Pangaea. Then Pangaea began to divide into the northern and southern group of continents of Laurasia and Gondwana.

Related site: Plate tectonics – continental drift Provides a timeline and maps to the sequence of events in the break up of the supercontinent Pangaea. (University of Texas at Austin, USA)

During this time, Australia was part of Gondwana but had begun its slow journey away from the South Pole towards its present day location. The Australian continent and Tasmania are currently moving north towards Asia at a rate of 6 to 7 centimetres a year.

Gondwanic ties

Before Gondwana split apart, Antarctica lay between the landmasses of Australia and South America, and the three continents shared common types of plants. This was confirmed by the abundance of Proteaceae pollen found in ice cores from Antarctica's continental shelf and the fossilised pollen found in Brazil and western Africa. It is possible that the plants that produced this pollen found their way from Australia all the way to South America via Antarctica by dispersing over land, as during this time Antarctica did not have an icecap.

In countries on the southern edge of the Pacific Rim, red- and pink-flowering plants which bear striking resemblance to each other are found flourishing. These include the Australian waratahs and tree waratahs, as well as their cousins found in South America. The similarities among these plants suggest that their features were inherited from a common ancestor that grew in Gondwana when Australia, Antarctica and South America were linked by land, some 50 to 70 million years ago.

Fire and Ice

By about 30 million years ago South America and Australia were no longer connected by land. Australia had 'un-zipped' from Antarctica and was heading towards the equator, while Antarctica was travelling in the opposite direction towards the South Pole. The northward drift of Australia explains why the Proteaceae continue to thrive here while none have survived the icy temperatures of Antarctica.

Since Australia separated from Antarctica, our climate has become drier and more prone to fires – the enemy of plants and animals that evolved in wetter times. As a result, many of the species related to the proteas found today became extinct. While the distributions of many others contracted into rainforest refuges between the eastern coast and the Great Dividing Range of Australia.

In contrast, half way across the globe in the cool temperate forests of Chile, a number of living and extinct relatives to the Proteaceae growing in Australia can be found, including the Chilean firebush which closely resembles the Australian waratahs. Fossilised pollen from the firebush have also been found within the sediments from the Murray-Darling and Otway basins of south-eastern Australia, confirming the South America–Australia connection.

The marsupial connection

The events in the break up of Gondwana can also explain the distribution pattern of marsupials. Palaeontologists believe that Australia's marsupials – kangaroos, possums and wombats, and the monito del monte, a tiny possum-like marsupial found today in the Chilean forests – had a common ancestor which journeyed from South America to Australia through the Antarctic land bridge around 60 million years ago.

As scientists continue to uncover evermore information about the evolutionary trek of the Proteaceae family, the techniques used to obtain the information and the knowledge gained may be used to piece together other bits of evolutionary history (Box 2: The name game – Australia's eucalypts and acacias). And maybe, in the not too distant future, we will be able to view a realistic portrait of not only Australia's biogeographical history but that of the world.

Box 1: Plate tectonics

The Earth is a rocky planet 12,700 kilometres in diameter. Deep in the centre of the Earth lies the core, which has a diameter of about 6900 kilometres. The core probably consists mostly of iron and nickel. The temperature in the inner part is estimated to be about 4000°C. The pressure is intense since the weight of the rest of the planet is pushing down on it.

The core is surrounded by a rocky layer called the mantle, about 2900 kilometres thick, which constitutes about 80 per cent of the planet's volume. Due to the heat and pressure, the minerals in the mantle can move slowly, rather like thick putty. In some places the top part of the mantle is partly molten. Above the mantle lies the lithosphere, the outermost shell of the Earth. The lithosphere is about 100 kilometres thick and is rigid and strong. The upper part of the lithosphere is the Earth's crust.

There are two types of crust: oceanic and continental. The oceanic crust consists of lava flows that are about 5 kilometres thick. They form at the mid-ocean ridges where the lava wells up from the interior. The crust spreads away from the ridges as it forms and dives back into the mantle when it collides with a continent or another plate. As it sinks back down, the pressure converts the minerals in the rock to denser ones, like garnet. This makes the slab of rock heavier, so that it sinks deeply into the mantle. This seems to be the main driving force for plate tectonics. The sea floor spreads away from the mid-ocean ridges because it is dragged back down into the interior, pulled rather than pushed.

As the thin oceanic crust dives down beneath the continents, the high temperatures 100 kilometres below the surface cause parts of it to melt. The mixture of molten rock (magma) and water that is formed erupt at the surface as spectacular explosive volcanoes, like the Rabaul volcano in Papua New Guinea that erupted on 19 September 1994. These volcanoes not only add ore deposits but also contribute to the thick continental crust. In contrast to the dense oceanic crust, the continents are typically about 40 kilometres thick and are made up of lighter rocks like granite. The lighter, thicker continental crust elevates the continents above the level of the ocean basins.

The Earth's surface is made up of moving plates

Although it feels solid enough, our planet's rocky surface, on land and under the sea, is a restless jigsaw of slowly moving pieces.

The fact that the lithosphere is rigid, and that the mantle can move a little, is important in explaining this. During the 1960s geologists came to realise that parts of the lithosphere are in constant motion relative to one another and that they carry the continents with them. These moving parts are called plates.

Each plate is about 100 kilometres thick. The plates move extremely slowly, creeping along at a rate of about 1-12 centimetres per year. Although slow, such movements are driven by great forces and dramatic events occur when two plates are pushed together or pulled apart.

The movements and collisions of plates account for the existence of folded mountain ranges, earthquakes, volcanoes and continental drift. Over millions of years, the movement of plates can make entire continents split, come together or drift apart. When looking at a map of the world, you might have noticed that the outlines of some continents suggest that they once could have fitted together.

Why the plates move

The Earth's plates move because of the heat inside it. Within the mantle, convection currents circulate, slowly mixing its material. The slow rising and falling of these currents goes on continuously. In the process, parts of the lithosphere are moved apart by the sideways movement of the currents underneath. The convection currents in the mantle also bring heat to the surface.

Earthquakes

When plates try to slide past each other, friction between them stops their movement at first. Tremendous strains then build up. Eventually, the friction is overcome and the plates suddenly snap past each other, moving by a few metres at a time. Earthquakes are the result.

Mountain building

Where the movement of plates has caused land masses to collide (although in slow motion), huge mountain ranges are pushed up over millions of years, like wrinkles on a tablecloth. The Himalayas were formed in this way as the plate carrying India slowly pushed into the one carrying Asia. These plates are still moving together, at the rate of about 11 centimetres per year, causing the surface to buckle and the ridges of the Himalayas to continue rising.

Related sites

• When the Earth moves: Seafloor spreading and plate tectonics (Beyond Discovery, National Academy of Sciences, USA) (A PDF file of the complete article is available.)

• Layers of the Earth (Volcano World, USA)

• Plate tectonics (Wheeling Jesuit University, USA)

• This dynamic Earth: Historical perspective (US Geological Survey)

Box 2: The name game – Australia's eucalypts and acacias

In 1979, Australian botanists Barbara Briggs and Lawrie Johnson proposed that the Eucalyptus genus be divided into eight genera, based mainly on distinctive floral and bud structures, but this re-classification was never formally implemented. In 1995 Lawrie Johnson and Ken Hill did split the Eucalyptus genus up, but only into two genera, Eucalyptus and Corymbia, on the grounds that Corymbia (the bloodwoods) were more closely related to members of the Angophora genus than to other eucalypts. This proposal was also supported by data collected from DNA analyses of the eucalypts done in 1995, the first for any major group in the Australian flora.

Most botanists and native plant enthusiasts accept the Johnson–Hill scheme of three genera – Angophora, Corymbia and Eucalyptus.

For example, some of Australia's most familiar gum trees like the scarlet flowering gum became Corymbia ficifolia, while the lemon-scented gum became Corymbia citriodora.

The gum trees are however not the only group of Australian plants subject to a name change.

Comparative DNA and morphological analyses by Australian researchers supported the view that Australian acacias – the generic name for Australia's wattles – formed a distinct group of plants to the acacias found in other parts of the world, justifying their assignment to a new genus. But the daunting task of renaming all the species within the Acacia genus delayed such action.

In 2005, delegates at the International Botanical Congress in Vienna voted to retain Acacia as the generic name for the wattles of Australia. In doing so, delegates avoided the task of renaming some 950 species of the Acacia genus.

Related sites

• Australian plants online (Australia)
  • Eucalypts but not Eucalyptus
  • Corymbia, Corymbia...wherefore art thou Corymbia?

• The name Acacia retained for Australian species (Centre for Plant Biodiversity Research, Australia)

Activities

• Plate tectonics (Volcano World, University of North Dakota, USA)
  • Plate tectonics – at a glance – students learn about the layers of the earth and the concepts of continental drift, sea-floor spreading and plate tectonics.
  • Activities and teaching suggestions for plate tectonics – provides a number of activities on plate tectonics, including 'Illustrating the layers of the earth' and 'Plate tectonics and the scientific method'.

• Science by email (CSIRO, Australia)
  • Try this: Pancake peaks – edible earth science – activity that demonstrates the basic concepts of plate tectonics.

• Daily lesson plans (New York Times, USA)
  • Fantastic fossil – students learn and discuss about the impact of a transitional animal on the evolution theory.

• Science NetLinks (American Association for the Advancement of Science, USA)
  • Fossils and geologic time – students are introduced to the major time periods in Earth's history, and the use of fossils to help with understanding this history.
  • The history of evolutionary theory – students are introduced to the concept of evolution and examine the arguments in support of this theory.
  • Comparing theories: Lamarck and Darwin – students compare Lamarck's and Darwin's evolution theories.

• Evolution and the Nature of Science Institutes (USA)
  • Making Cladograms – provides students with an introduction to building cladograms.

Further reading

Useful sites

Provides an introduction to the Proteaceae family.
http://www.amonline.net.au/factsheets/proteaceae.htm

Proteaceae illustrations (Australian National Botanic Gardens, Australian Government Department of the Environment and Water Resources)

Contains a series of illustrations of plants in the Proteaceae family.
http://www.anbg.gov.au/proteaceae/illustrations.html

Drifting proteas or continents? Historical biogeography of the Proteaceae (Public lecture series 2006–07, Australian Academy of Science)

Transcript of a lecture given by Dr Peter Weston. Discusses the biogeographical history of the Proteaceae.
http://www.science.org.au/events/publiclectures/os/weston.htm

The story of Gondwana (Australian Plants Society, Australia)

Provides a geologic timeline of Earth and Gondwana.
http://www.apstas.com/gondwanastory.htm

Continental drift and evolution (Biology at University of Cincinnati Clermont College, USA)

Contains information on the evolution of species and continental drift.
http://biology.clc.uc.edu/Courses/bio303/contdrift.htm

Continental drift animations (Paleomap Project USA)

Provides links to interactive animations and descriptions of continental drift.
http://www.scotese.com/newpage13.htm

Understanding evolution (University of California Museum of Paleontology, USA)

• Welcome to Evolution 101!
  Provides information on the patterns and mechanisms of evolution.
  http://evolution.berkeley.edu/evolibrary/article/evo_01

• Phylogenetic systematics, a.k.a. evolutionary trees
  Provides information on the use and construction of evolutionary trees.
  http://evolution.berkeley.edu/evolibrary/article/phylogenetics_01

Bone diggers (Nova, USA)

• Evolution down under
  Answers the question 'How did Australia come to be marsupial heaven?'
  http://www.pbs.org/wgbh/nova/bonediggers/evolution.html

• The extinction enigma
  Discusses reasons behind the extinction of 'megafauna' in Australia.
  http://www.pbs.org/wgbh/nova/bonediggers/extinction.html

• Australia's vanished beasts
  Contains illustrations of 'megafauna' that lived in Australia during the Pleistocene Epoch.
  http://www.pbs.org/wgbh/nova/bonediggers/vanished.html

Australian Broadcasting Corporation

• Oldest fossil vertebrate embryo (The Science Show, 31 May 2008)
  Announces the Australian discovery of the oldest fossilised vertebrate embryo
  http://www.abc.net.au/rn/scienceshow/stories/2008/2260805.htm

• OZ fossils
  Explores the evolution of life in Australia using fossils.
  http://www.abc.net.au/science/ozfossil/

• Fossils from the final frontier (Features, 16 August 2007)
  http://www.abc.net.au/science/features/antarcticfossils/

• Found! World's oldest tree (News in Science, 19 April 2007)
  http://www.abc.net.au/science/news/stories/2007/1901623.htm

• Ediacara – Australia'a own geological period (Catalyst, 9 November 2006)
  http://www.abc.net.au/catalyst/stories/s1785092.htm

• Evolution River (Catalyst, 2 November 2006)
  http://www.abc.net.au/catalyst/stories/s1778907.htm

• Proteas hitched a ride as Gondwana split (News in Science, 20 September 2006)
  http://www.abc.net.au/science/news/stories/2006/1744327.htm

• Tropics the hot spot for speedy evolution (News in Science, 2 May 2006)
  http://www.abc.net.au/science/news/stories/2006/1625917.htm

• The pine that time forgot (Features, 19 May 2005)
  http://www.abc.net.au/science/features/wollemi/default.htm

• Turning of the Fagus (Scribbly Gum, 1 April 2000)
  http://abc.net.au/science/scribblygum/April2000/default.htm

Basic botany (Royal Botanic Gardens at Kew, UK)

• Studying plant diversity – plant classification
  Provides information on the classification of plants.
  http://www.kew.org/ksheets/pdfs/b1classification.pdf

• Plant names
  Explains the naming of plants.
  http://www.kew.org/ksheets/pdfs/b2names.pdf

• Plant structure – leaves, stems and roots
  Provides diagrams of the leaves, roots and stems systems of plants.
  http://www.kew.org/ksheets/pdfs/b3plant.pdf

• Flower structure and variations
  Contains information about the functions and structures of flowers.
  http://www.kew.org/ksheets/pdfs/b4flower.pdf

Glossary

convergent evolution. The development of similar functions and structures in unrelated or distantly-related organisms.

dendrogram. A diagram which shows the interrelationships between a group of organisms, as well as estimates of when the organisms evolved and separated into different species.

DNA (deoxyribonucleic acid). The nucleic acid forming the genetic material of all organisms with the exception of some viruses which have RNA. DNA is present in the nucleus and other organelles such as mitochondria and chloroplasts.

genus. A group of organisms which may contain one or more species that exhibit similar characteristics.

ice cores. Cylinders of ice drilled from glaciers and icesheets that are used to provide information about the earth's climate history.

molecular phylogenetics. The study of molecular structures to establish the evolutionary relationships of organisms.

morphology. A branch of biology that deals with the shape and form of organisms.

Pacific Rim. A region which includes countries bordering the Pacific Ocean.

palaeontologists. Scientists who study prehistoric lifeforms by examining plant and animal fossils.

plate tectonics. The theory that the Earth's surface is made up of huge plates that have moved very slowly during geological history, and continue to move, thus changing the position of continent and oceans. The plates are about 100 kilometres thick and move at a rate of about 1-12 centimetres per year.

Proteaceae. An ancient family of flowering plants found mostly in the southern hemisphere.