Discovering Australia's evolutionary past
Key text
This topic is sponsored by the Australian Academy of Science Flora Fund.
With the help of modern technology, scientists are unearthing more about the continent's biogeographical past.
A bridge too far
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
Image of the continents that once made up the supercontinent Gondwana.
(National Geophysical Data Center, USA)
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.
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
Provides a timeline and maps to the sequence of events in the break up of the supercontinent Pangaea.
(University of Texas at Austin, USA)
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.
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.
Posted October 2007.