once distributed more or less uniformly were reworked, concentrated, or removed from the scene. In some instances this led to localized lodes of gold, bauxite, and uranium, but overall the processes only encouraged the generic pauperization of soils, the loss of phosphorus and such critical trace elements as molybdenum, copper, cobalt, and boron. Without renewal by major tectonic forces, without intervention by some process like glaciation that could overturn or scrape away the surface debris or recharge lowlands with new minerals derived from mountain catchments, there was little chance to reverse the inexorable tread toward an entropy of emptiness. The process might be slowed, even locally defied, but it could not be resisted. In Western Australia, at Mount Narryer, weathering exposed zircon some 4.1–4.2 billion years old, incorporated into rocks formed 3.6 billion years ago. New landscapes emerged from the successive exfoliation, by erosion, of old landscapes. Australia’s biologic renaissance had to rise out of a geologic decadence.2
THE BREAKUP OF GONDWANA freed Greater Australia to pursue a separate destiny. That burden fell to its biota: its curious flora and fauna would proclaim the unique character of the island continent. Old Australia’s geology and its biology coexisted in weird counterpoint. Where one degraded and removed, the other exploded into a biotic efflorescence, a proliferation of species unlike those found anywhere else on Earth. Instead of much devolving into less, a relatively small Gondwanic inheritance evolved into more. Some components of green Gondwana not only accepted the geologic legacy—soil depletion and geographic isolation—but turned them to advantage.3
Its originating biota was one Old Australia shared with most of Gondwana. The ancestral forest was dominated by the gymnosperms—the southern conifers, the araucarias and podocarps—but just as Gondwana began to break up, and perhaps in partial response to the stimulus of that profound dislocation, the angiosperms—the flowering plants—proliferated. From a projected point of origin where Africa joined South America, the angiosperms spread throughout Gondwana. Their migration was selective, and the resulting geographies of the conifers and the flowering plants varied. Gondwana was too large, too unbroken a landmass for a single climate to characterize it everywhere, and as the giant continent fragmented, separate lines of biotic advance and exchange danced in slow choreography with geologic cratons. This meant that a single pan-Gondwanic biota did not exist everywhere in equal composition. What part of the ancestral forest the different continental fragments took with them depended on their relationship to Greater Gondwana and on the sequence of their fissioning, all played against the larger drama in which the angiosperms invaded and claimed greater proportions of the supercontinent.
It appears that Old Australia embarked with a solid complement of the ancestral rainforest, a Gondwanic ark. Among the dominant conifers were the araucarias and the podocarps; among the angiosperms, the dominant genus was Nothofagus, the Antarctic beech. Minor families included the myrtles, the grasses, casuarinas, chenopods, and xanthorrhoeas; important genera included Banksia, Hakea, Melaleuca, Eucalyptus. Much the same paleoflora characterized large portions of Antarctica, South America, and New Zealand. In all these lands the Gondwanic rainforest was sustained by a persistent, year-round moisture regime. The relentless rains leached and degraded soils, but the process occurred so slowly that the biota kept pace and adapted. The minor flora claimed special niches; many probably scavenged along the margins of rainforest, better adapted to disturbances, occasional dryness, a more fractured biotic environment.
When Old Australia broke away, about 30 million years ago, the rupture was remarkably final. There would be some late contamination along the north from Indo-Malayan biotas, though these would be restricted by the high mountains thrown up along the New Guinea border and by the deep waters, between islands, that only select species could cross. Together mountain and sea presented an effective biological filter. Greater Australia differ entiated into the Australian mainland, New Guinea, and Tasmania, to be reconnected and sundered from time to time with the geologic tides of a rising and falling global ocean. Not until historical times would there be a further, significant contamination of the biota.
The move to the tropics, while slow, induced climatic change, a new biotic force. Aridity did to the Gondwanic rainforest what tectonic stress did to the Gondwanic supercontinent. The ancestral rainforest fractured and multiplied, cleaving along biotic planes of weakness that divided those species that required uniform moisture from those that could accommodate dryness and change. The onset of aridity did not simply replace one enduring condition with another; it made regular and sporadic change a fundamental part of the biological calendar.
Australian aridity was seasonal, episodic, and chronic. It became, in places, part of an annual cycle of wet and dry seasons. In the tropics, the seasons followed the monsoonal winds, wet in the summer and dry in the winter. In the Mediterranean-like climates of the southeast and southwest, aridity took the form of a prolonged, parching summer, with moisture mostly a product of winter storms. Elsewhere aridity manifested itself as drought, extending regionally over several years. In the enormous center of the continent, aridity became a relentless presence, crowding moisture regimes to the coastal fringe and assaulting the littoral with desiccating winds. The southeast trades and the Great Dividing Range combined to raise moisture along the eastern seaboard, but the interior deserts, like a stony ice sheet expanding and contracting, defined the frontier. At times, like a red giant exploding among the stars, the desert core threatened to engulf the continent.
This transformation—the Great Upheaval—occurred over the course of the Tertiary period. It commenced with Greater Australia’s segregation from Gondwana during the Eocene epoch, and acquired a signature rhythm, long but emphatic, during the Oligocene. The Earth cooled and, overall, dried; Antarctica acquired an ice sheet; Australia continued its tread toward the equator; new circulation patterns established around the Southern Ocean and within and around the Australian continent; global changes in sea level catastrophically flooded then reexposed vast portions of the continental plains, reshaping the interactive meteorology of ocean and continent. By the Pliocene and Pleistocene epochs—over the past 5 million years—the trend became all but inevitable. Aridity became the norm and humidity the exception. The area of stony desert came roughly to equal the area of true forest. During the last glacial epochs, the transfiguration could be called irreversible. Australia’s low latitudes and low relief confined glaciation to Tasmania; there were no loess plains blown downwind, no fresh rocks exposed to weathering, no transfer of nutrients and sediments from mountain to plain. Where rainforest taxa reemerged, they bloomed like ephemerals after a desert storm. The Great Upheaval ended with a Great Inversion of the Australian biota.
In this biotic revolution once-minor constituents, now hardened and shaped by drought and disturbance, became dominant. The survivors evolved into scleromorphs (or sclerophylls)—literally, “hard leaves,” referring to the small, tough evergreen leaves that hoarded nutrients and resisted the transpiration of precious water. The scleromorphs adapted not only to soil impoverishment but to aridity—and, in fact, to disturbances of many kinds. By the mid-Miocene epoch (c. 15 million years ago), as the continental interior acquired its imperishable dryness, the relatively homogeneous biota of ancestral Australia began to differentiate.
What had been a more or less uniform cover of closed Gondwanic rainforest splintered into new, peculiarly Australian biotas. The hermetic forest became open; woodland surrendered to savanna, shrub and heathland, grasses, or outright sand and stony desert. The ancestral rainforest dominated by Nothofagus and Podocarpus gradually retreated before aridity like leaves before a blower. In its place emerged a scleroforest. Casuarinas succeeded araucarias. Tough grasses and scrubby scleromorphs seized understories formerly softened by fern, moss, and fungi. About 34 million years ago the eucalypts appeared, quiet and unannounced. Sometime around 25 million years ago acacias arrived, probably by sea from elsewhere in Gondwana. Thereafter the biotic isolation of the island continent was nearly total. By the time of European discovery the ancestral rainforest had retreated to minor enclaves in the Great Divide, where they occupied probably less than 1 percent of the total land surface of Australia.
The Great Upheaval had all but replaced a pan-Gondwanic biota with a marvelously endemic suite of biotas. About
