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Origins of an Icon
By Stephen Jackson
Allen & Unwin Copyright © 2007 Stephen Jackson
All rights reserved.
FROM THE (UNDER) GROUND UP?
Evolution and relationships
Our slow moving koala is a triumph of evolution, able to survive and thrive on a diet of gum leaves, not the most nutritious plants. Its solution to the problem of handling such food made the koala a success story of the Australian forests.
The koala is the largest arboreal, or tree-climbing, mammal on the island continent of Australia. It is instantly recognisable by its short, stocky build, lack of tail, large fluffy ears and spoon-shaped nose. The koala can vary considerably in size and colour throughout its distribution. A Queensland koala has light grey fur and weighs from 7 to 9 kilograms, while a Victorian koala has chocolate brown fur and weighs between 10 and 14 kilograms. Throughout the range of the koala, adult males are up to 50 per cent larger than adult females. The following chapters will, I hope, show readers why the koala can be considered a 'triumph of evolution', but every story must have a starting point and to find the koala's we must travel far back in time.
Many Australian schoolchildren would be able to describe the koala as a 'marsupial' because it has a pouch, but what is a marsupial, and how do they differ from other mammals? The subclass Marsupialia consists of pouched mammals which are found throughout North America and South America (known as opossums) and the Australasian region — Australia, New Guinea and the surrounding islands. The name 'Marsupialia' was coined by Caroli Illiger in 1811, in his Prodromus Systematis Mammalium. The word comes from the Greek word marsupion, 'little purse', and refers to the abdominal pouch present in most marsupials that encloses the mammary glands and in which the animals' offspring complete their development.
Along with their abdominal pouch, the main difference between marsupials and other mammals is in their reproductive systems. Marsupials are odd creatures. Not only do the males of most species have twin penises, but the females have three vaginas: two lateral vaginas through which the semen travels after mating and a medial (middle) vagina through which the young travels during birth. The female marsupial also has two uteri and two oviducts, whereas other mammals have a single vagina, cervix and uterus, and only the oviducts are paired. The French anatomist and zoologist Henri de Blainville examined these distinctive features in his Prodrome d'une nouvelle distribution systématique du règne animal, published in 1816, and named the marsupials Didelphia, or 'two uteri', and other mammals Monodelphia, or 'one uterus'. Some time later, de Blainville realised that the reproductive system of the monotremes — the platypus and the echidna — had more in common with that of birds than either marsupials or placental mammals and so named them Ornithodelphia or 'bird uterus'.
Unlike placental mammals, marsupials have a very short gestation period, between 12 and 35 days, depending on the species. At birth, the young are still embryonic in form. These tiny creatures are called neonates and range in size from as little as four milligrams (the honey possum) to only 830 milligrams for the eastern grey and red kangaroos.
This brief gestation period and the lack of development of the neonates may be one of the reasons that, throughout the 19th century, the marsupials and monotremes were considered to be 'inferior' to the placental mammals. When Darwin published his theory of evolution in 1859, de Blainville's names for the various groups appeared to make sense. The argument for evolutionary progression from the supposedly 'primitive' monotremes to the 'advanced' placental mammals was developed by Darwin's most fervent supporter, Thomas Huxley, who is still remembered as Darwin's 'bulldog'. Huxley suggested the terms Prototheria (prototype or early mammals) for the monotremes, Metatheria (intermediate improved mammals) for the marsupials, and Eutheria (complete mammals) for the placental mammals, and the different groups are usually referred to as monotremes, marsupials and placental or eutherian mammals. The terms 'eutherian' and 'placental' are, strictly speaking, inappropriate, as they were proposed at a time when it was thought that the marsupial foetus was not nourished by a placenta. Also, we now recognise that rather than being primitive, intermediate or advanced, each group of mammals is actually superbly adapted for its particular environment, and the terms Prototheria and Metatheria have fallen out of favour. Stephen Wroe and Michael Archer speak for many biologists when they say that Huxley's definitions 'implicitly support the heresy that marsupials are intrinsically inferior'.
What can we learn about the evolution of marsupials from the fossil record? Did they follow a similar evolution pattern to that of placental mammals? The earliest known mammal-like or proto-mammalian fossil remains were unearthed in Texas and date from 225 million years ago. They comprise the partial skull of a creature named Adelobasileus cromptoni. The fossil remains of marsupials were found in the same area, and for many years it was assumed that marsupials must have evolved in North America before dispersing through North and South America (which was still attached to Australia via Antarctica), Asia, Europe and Africa. In 2003, however, the origin of marsupials was turned on its head when a near-complete skeleton of the chipmunk-sized marsupial Sinodelphys szalayi was shown to the world. This amazing specimen was dug up from 125-million-year-old shales in China's Liaoning Province, 200 kilometres north-east of Beijing.
Given that the most obvious differences between marsupial and placental mammals are in their reproductive systems, how do palaeontologists know if a fossil specimen is from a marsupial or a placental mammal? The teeth and skulls of marsupials and placental mammals are superficially similar but have a number of important differences that distinguish 'most' fossils into the two groups. While most marsupial features can be found in a handful of placental mammal fossils or living animals, there are two features that are found only in marsupial mammals. These are the non-replacement of the first two premolars and a reduction in the maximum number of premolars from four to three.
In 2002, the team that discovered Sinodelphys szalayi also discovered the world's oldest placental mammal, Eomaia scansoria. Eomaia scansoria is also 125 million years old, and these two finds give a powerful boost to the theory that placental mammals and marsupials both evolved in Eurasia, and then spread to the rest of the world. The debate continues, however, given that Adelobasileus cromptoni predates the Chinese fossils by 100 million years.
At the time of these earliest mammals, some 125 million years ago, Australia and New Guinea were still joined to Antarctica and South America as part of the great southern supercontinent known as Gondwana. The dinosaurs reigned supreme, and would do so until their dramatic disappearance some 40 million years later. This 'great extinction' seems to have been the result of a catastrophic collision between the Earth and a meteor some ten kilometres in diameter that hit the ground near Chicxulub on Mexico's Yucatan Peninsula. It was at this time that South America, Australia and Antarctica separated from Africa and India.
The fossil record allows us to estimate that marsupials arrived in Australia some 55 million years ago, when Australia was still connected to South America and Antarctica, which was not yet covered in ice. At this time in Australia, broad-leafed rainforests were beginning to replace the earlier conifer forests. Australia finally broke away from Antarctica and South America some 45–35 million years ago, long after the extinction of the dinosaurs, leaving this land to be ruled by mammals. Ever since, the island continent of Australia has been an evolutionary ark, moving northward at the leisurely rate of about 5–7 centimetres per year, or bit less than the rate our hair grows!
Australia's oldest marsupial fossils to date were found in the Tingamarra deposits in Murgon in south-eastern Queensland. The fossil remains are of a yet-to-be-named marsupial whose closest living relative appears to be the South American Colo-colo or Monito del Monte. The Colocolo is the sole survivor of the family Microbiotheridae and is found only in Chile and just over the border into Argentina.
During the Oligocene epoch (that is, 34–24 million years ago), South America and Antarctica finally separated, creating the Drake Passage. This final rupture between the land masses resulted in the Antarctic circumpolar current that makes it difficult for the warmer southward ocean currents to enter the region, and Antarctica grew steadily cooler and cooler until the southern ice cap was not only permanent, but began expanding approximately 15 million years ago.
Antarctica's changing climate had no immediate effects on Australia, however, as the Miocene epoch (23–15 million years ago) was one of Australia's lushest greenhouse periods. Northern Australia was covered in verdant rainforest with a diversity of plant and animal life equal to that found today in the rainforests of Borneo and Brazil's Amazon basin. Mammal diversity was high with many archaic groups co-existing with more modern groups, as has been revealed in the world-famous fossil deposits of Riversleigh. These fossil deposits cover an area of 10 000 hectares in north-west Queensland, and include the southern section of Lawn Hill National Park.
Ultimately, of course, Australia's climate did change. As rainfall decreased and temperatures steadily dropped, Australia entered a severe icehouse phase some 15 million years ago. The northern and central rainforests collapsed and the land's increasing aridity saw the emergence of new groups of plants better able to tolerate drought conditions. These included the sclerophyllous or hard-leaved drought tolerant plants of the family Myrtaceae, in particular the eucalypts, which would ultimately contribute to the evolution of the koala. Changing climates throughout the world meant that many forests were being replaced by grasslands, and the marsupials of North America, Asia, Europe and Africa were disappearing.
As Australia's grasslands spread, the first megafauna emerged — giant kangaroos, enormous wombat-like animals, marsupial lions and koalas. At the same time hoofed ungulates were roaming the plains of Africa, Europe, Asia and North America. Australia's rainforests had now all but disappeared, being replaced by the drought-tolerant forests of Eucalyptus and Acacia (or wattles) that would become home to so many of Australia's marsupial species, and ultimately the deserts we see today.
Today's koala is the only surviving member of its family and has evolved into a specialised tree-dweller that feeds almost exclusively on the leaves of various species of Eucalyptus. However, the koala's evolutionary history dates back at least 30 million years, possibly even longer, and the animal bears a remarkable resemblance to its prehistoric ancestors. Over the past 30 years, a number of koala species have been uncovered in fossil deposits, especially in Queensland and South Australia. To date, some 18 species within six genera have been discovered, though a number of these are still unnamed. One genus, Koobor, is considered of uncertain status within the suborder Vombatiformes (which includes the families that contain koalas and wombats) until its position can be further resolved through the discovery of more complete fossil material.
Given the number of different species in the koala fossil record, a quick glance at the following table would appear to suggest an alarming decline in diversity. Closer inspection, however, shows us that since the late Oligocene (some 24 million years ago) only two species of koala at any one location have co-existed at any one time. In terms of abundance, the scarcity of fossil evidence suggests that despite the decline in numbers through the 19th and 20th centuries, koala populations are larger now than at any point in their distant history. It is thought that before the ancestors of modern eucalypts evolved (some 24 million years ago), the koala lived in rainforests. During the mid- to late Miocene (10 to 5 million years ago), the development of a drier climate saw eucalypt-dominated woodlands spreading at the expense of rainforest, thus allowing the koala to expand its distribution and become more abundant.
The fossil record also shows a considerable difference in size between the various genera of koala. Madakoala and Perikoala were similar in size to the modern species, while Nimiokoala and Litokoala were only half to two-thirds the size of a modern koala. Phascolarctos yorkensis, once placed in the genus Cundo-koala but now included in the same genus as the modern koala, was twice the size of the koalas we see today.
The differences between the modern koala and its fossil ancestors can be seen in the size and shape of the teeth, skulls and other bones. Professor Michael Archer and his team from the University of New South Wales in Sydney have found thousands of fossils in the Riversleigh World Heritage area, including one species of Nimiokoala, two species of Litokoala, and there appears to have been at least two other koalas or koala-like animals at various times during the Oligocene and Miocene periods.
The fossil record suggests that koalas once enjoyed a considerably wider distribution than they do today. Fossils of the modern koala genus Phascolarctos have been found through eastern Australia, across southern Australia to south-west Western Australia and even into central Australia. Pleistocene fossils of Phascolarctos have been discovered in eastern South Australia near the Victorian border, and fossil remains from the late Pleistocene have been found north of Perth at Koala Cave near Yanchep, and south of the city at Mammoth Cave near Margaret River. Devil's Lair near Boranup has Phascolarctos fossils from the main excavation, with other fossils also known from Labyrinth Cave near Augusta. Koala fossil deposits have also been found in the Madura Cave on the southern edge of the Nullarbor Plain. There are still plenty of viable eucalypt plantations in south-east Western Australia, so it is difficult to establish why there are no koalas. One hypothesis is that the koala became extinct in Western Australia after the arrival of the Aborigines.
The koala family Phascolarctidae appears to have evolved from a group of marsupials now known as the diprotodonts, literally, 'two front teeth'. These included the enormous ancestor of today's wombat, Diprotodon australis, a creature the size of a modern-day rhinoceros, weighing about two tonnes. Although it is not a close relationship, the koala's nearest relatives are the wombats of the family Vombatidae. The two families went their separate ways in the late Eocene, approximately 42 million years ago.
We do not know how large the common ancestor of the koala and the wombat was, but it is possible that it was a burrower who began climbing trees to access a different food source. The koala's backward-facing pouch is an interesting evolutionary legacy of this upwards movement into the trees. The wombat's pouch opens backward to protect the young from flying dirt raised by a digging adult. The koala joey needs no such protection, but now has what must be a pretty vertiginous view down to the ground.
Excerpted from Koala by Stephen Jackson. Copyright © 2007 Stephen Jackson. Excerpted by permission of Allen & Unwin.
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