How did this happen? Why did Australia get a preponderance of pouched and egg-laying mammals? And, at the same time, precious few of the kind of mammal that dominates every other land in the world?
The story is a long one—say, 100 million years or more—and for decades was missing key sections. Only in recent years have paleontologists succeeded in filling in some of those gaps to their satisfaction, enabling them to draw a reasonably detailed portrait of Australia's unique evolutionary history.
Tooth Be Told
Two long-standing questions were answered, remarkably enough, by a single fossil tooth. Or two single fossil teeth actually, unearthed on opposite sides of the globe, both in 1992.
Until sometime in the Cretaceous Period (146 to 65 million years ago), Australia, Antarctica, and South America all abutted one another in the southern supercontinent Gondwana. While they were attached, experts believe a single belt of forest likely stretched from southeastern Australia, through Antarctica, and into southern South America, and they know that early versions of all three mammal models existed at the time. Yet today no monotremes exist outside of Australia (and New Guinea), and no placental mammals that didn't fly or swim there—for example, bats or dugongs—exist in Australia except for rodents (which arrived only about five million years ago) and mammals that were introduced by people (who arrived by 60,000 years ago).
Why didn't monotremes use the connection to leave Australia? And why didn't placentals use it to enter Australia?
One of the teeth, uncovered in Argentina from deposits 63 to 61 million years old, answered the first question. Monotremes had lived elsewhere, for paleontologists determined that the tooth belonged to a platypus, an extinct species now known as the Patagonian platypus. (Which way the monotremes originally crossed the Antarctic bridge—from or to Australia—remains a mystery.) The second tooth, meanwhile, answered the second question. Placentals had come to Australia, for some experts believe the tooth, dug out of Queensland deposits radiometrically dated to at least 55 million years ago, belonged to a primitive, nonflying placental known as a condylarth. More recent discoveries hint that other early placentals lived in Australia, even before marsupials turn up in the fossil record.
But the monotremes in South America, and the placentals in Australia, didn't last.
Another nagging mystery involved the marsupials. While Australia is their capital, marsupials also occur in the Americas. South America, in particular, has a host of species, from the dwarf fat-tailed mouse opposum to the yapok, an aquatic marsupial. Paleontologists long wondered, When did the marsupials cross between Australia and the Americas, and which way did they go? That is, where did they originate?
The big animals Down Under walked an ecological tightrope.
It turns out the most "primitive" known marsupials come from early Cretaceous deposits in China (Sinodelphys genus), implying that the pouched mammals arose there. Recent studies suggest, however, that all or most of Australia's marsupials derive from an order of early North American marsupials of which today just a single representative exists, a sprightly, mouse-sized critter native to southern Chile known as the monito del monte, or "small monkey of the mountains." Paleontologists believe that the common ancestor that gave rise to the monito del monte, and the kangaroo and most or all other Australian marsupials, scurried from South America to Australia (through the intervening Antarctica) sometime before about 60 million years ago.
The Australian mammalogist and paleontologist Tim Flannery likens the evolutionary history of the three mammal varieties in Australia and South America to a race. All three began the race in both places. Monotremes, which have the lowest metabolisms and energy needs of the three, thrived in Australia but lost the race in South America. Placentals, which have the highest metabolisms and energy needs of the three, thrived in South America but lost the race in Australia. Marsupials, whose metabolic rate and energy requirements lie between those of the other two, didn't lose on either continent but clearly prevailed in Australia.
Why? Focusing solely on Australia now, what accounts for its singular evolutionary path?
The answer has everything to do with what happened to Australia after it broke away from Antarctica—its final tie to Gondwana—between about 45 and 38 million years ago. And what didn't happen to it.
First, what happened. After its severance, Australia began a long, slow drift north, which continues today. During its drift, the continent's climate changed repeatedly, from warm, wet "greenhouse" conditions to cold, dry "icehouse" conditions and back again. Between about 23 and 15 million years ago, the continent witnessed one of its lushest greenhouse phases, and marsupial diversity exploded, as did that of flamingoes and thunderbirds—giant, flightless herbivores that competed with the "elephants" and "rhinos" of the marsupial world, the big-bodied, wombat-like Diprotodon species. After 15 million years ago, rainfall and temperatures began to drop, a trend that has continued to the present day. Over this time, Australia dried out significantly, transforming the continent's great northern and central forests into semi-arid grasslands.
Now for what didn't happen. During its drift north over those roughly 40 million years, Australia eased into tropical waters even as the globe began a general cooling. One result was that the climate in Australia, notwithstanding those flips between greenhouse and icehouse, remained relatively stable during those 40 million years. Antarctica, meanwhile, traveled south and froze solid, and North America and Europe suffered repeated ice ages.
In fact, during its drift Australia kept just north of latitudes where glaciers form, so it was never glaciated in a substantial way. On top of that, over the past 60 million years the continent has enjoyed a degree of geologic restfulness not experienced by any other continent. Without glaciers, active volcanoes, and earthquakes stirring everything up, Australia's soils didn't get renewed, leaving it with the poorest-quality earth of any of the large landmasses.
Walking A Tightrope
All these things that happened and didn't happen, particularly in the past 15 million years, had a significant effect on Australia's wildlife. Because of their lower resting metabolic rate, marsupials could survive using less energy than similar-sized placental mammals, and they flourished. (Other creatures, including the thunderbirds and flamingoes, suffered extinctions as their lakes dried up and the luxuriant vegetation in the continent's midriff became grasslands.) Eventually the survivors began to grow in size, probably because larger-bodied animals were able to eat more of the progressively lower-quality vegetation. By the time humans arrived around 60,000 years ago, an impressive suite of megafauna roamed the landscape.
Even as marsupials succeeded in Australia, however, they faced restrictions forced on them by the harsh environment. One concerned size. Despite their name, the megafauna in Australia never boasted any extremely large members. The biggest marsupial that ever lived, a Diprotodon, weighed about 4,400 pounds, or only about a third as much as an elephant. The largest kangaroo, now extinct, may have reached 440 pounds; the largest antelope, its closest counterpart in Africa, can weigh 2,200 pounds or more.
All told, one-quarter of all native Australian mammals have disappeared since Homo sapiens first came ashore.
Another restriction was in the diversity of top-level carnivores. Australia had only one cat-like meat-eater, Thylacoleo, the marsupial lion, and one dog-like meat-eater, Thylacine, the Tasmanian tiger. There was also a carnivorous kangaroo. Three top-order mammalian carnivores out of 60 or so large mammal species before humans turned up is paltry compared to the number on all other continents (save the usual exclusion, Antarctica). The paucity of mammalian meat-eaters left room for the rise of reptilian carnivores, including land crocodiles and the Komodo dragon-like giant goannas—all of which eventually disappeared.
Indeed, if the extinct megafauna shared one characteristic, it was vulnerability. Ultimately because of the poor soils, animals evolved low rates of reproduction and the ability not only to live on nutrient-poor flora but to opportunistically exploit the good times and withstand the bad. Australia is the only continent whose climate is largely dictated by the El Niño phenomenon, a non-annual event, and scientists see no reason why this wasn't the case when the megafauna were alive. The big animals Down Under, in short, walked an ecological tightrope.
Gone and Here
Sadly, they fell off that tightrope not long after the initial arrival of the smartest placental ever to walk the Earth—us. Sometime between 50,000 and 45,000 years ago, most megafaunal species, whether carnivore or herbivore, went extinct. The giant rat kangaroo, the thunderbirds, the giant goannas—all gone. These animals had ridden the Australian ark successfully through millions of years of natural change, but the change brought about by people was more than they could handle, and they succumbed. All told, one-quarter of all native Australian mammals, for one, have disappeared since Homo sapiens first came ashore on the continent.
Fortunately, some megafaunal species survived, including the largest living marsupial, the red kangaroo, and Australia remains marsupial and monotreme central. The trick now is to figure out how to ensure that those that pulled through the last mass extinction—wombats and bandicoots, dingos and emus—pull through the current one we humans have triggered worldwide.