Living World / Sex & Reproduction

Gondwanaland was the largest landmass of its day—40 percent larger than Eurasia is now—and it spawned other major animal groups, including songbirds and the forebears of pythons and boa constrictors. Why not placental mammals? If the earliest placental mammals originated in Gondwanaland, they could have been heading north throughout the entire period from about 150 million years ago to 50 million years ago. The theory is plausible because of documented geologic changes. During much of Gondwanaland’s existence, a hot current was rising from deep in Earth’s mantle, repeatedly fracturing Gondwanaland’s continental plate and causing fragments of land to drift north. The largest fragments included what were destined to become India, Myanmar (Burma), and other parts of modern-day Southeast Asia. Placental mammals could have been carried north on the fragments or on Africa when it broke off from Gondwanaland and eventually docked with Europe and Asia.

Gondwanaland continued to break up, with Australia and South America splitting off from Antarctica between 30 and 60 million years ago. Antarctica’s isolation allowed a mighty ocean current to flow around it, unimpeded by any landmass. That forever changed the climate worldwide. The current locked Antarctica into perpetual frost and plunged Australia into colder and drier weather. “It was an event of global proportions,” says Liz Truswell, a paleontologist at the Australian National University in Canberra who specializes in pollen studies. “I think Australia would have been a challenging place once you started to break up the vegetation covers, break up the rain forests.”

Today Australia brims with plants and animals seldom found elsewhere on Earth. The most well known examples are marsupials. When European biologists first encountered them during the 18th century, they viewed them as ridiculous-looking, oddball, and hopelessly antiquated. The squishy pink marsupial offspring looked barely alive when they were born. Such underdeveloped young seemed a sure sign that the marsupial uterus was too primitive to grow the sort of developed fetus that a placental mammal can deliver. They seemed to lie halfway between hot-blooded, live-bearing placental mammals and cold-blooded, egg-laying reptiles.

The story of marsupial evolution, however, proves to be much more complex. Paleontologists now know that marsupials did not evolve in Australia. Fossil finds show that they originated in Laurasia. North America holds the most traces, but some recent discoveries also point to Asia. Marsupials radiated into South America, possibly through the chain of islands in the Caribbean Sea, and then migrated across South America and Antarctica into Australia. Biologists have surmised that marsupials abound in Australia simply because their main competitors elsewhere, the placentals, had not migrated to the Australian continent.

Rich’s finds suggest an entirely different pattern of colonization. If early mammals thrived in the southern landmasses—and Rich believes the jawbone came from an early placental mammal (see caption, page 70, on the jawbone’s dental features)—the marsupial hegemony in Australia is no accident of geographic isolation.

Marilyn Renfree, a biologist at the University of Melbourne and a specialist in marsupial reproduction, contends that marsupials are well adapted to surviving challenging conditions. “Marsupials aren’t physiologically inferior in any way,” she says.

Marsupials have tailored the basic mammalian trait of breast-feeding to suit a specific set of survival skills. Nourishing offspring outside the womb permits more flexibility for mothers facing a fickle environment, says Renfree. If drought decimates the food supply, a red kangaroo can simply halt milk production and let her baby die—another will soon be on the way. Fertile female kangaroos keep one or two embryos queued up in suspended animation, and once a baby dies, another embryo begins development and will be born four weeks later. By contrast, a pregnant moose, say, must nourish the fetus for eight months until birth, regardless of how conditions change. Under harsh conditions, extra energy demands can endanger the mother.

Marsupial metabolism holds other surprises. Biologists used to think the marsupial metabolic rate, which is 30 percent lower at rest than the placental rate, represented a primitive form of mammalian metabolism. “But the marsupial’s capacity to increase metabolic activity during exercise or to keep warm is superior to placentals’,” says Terence Dawson, a comparative physiologist at the University of New South Wales in Sydney. “Your average marsupial has got more horsepower in its metabolic machine.” This greater range of metabolic rates is powered by a marsupial heart that’s 25 percent larger than the placental heart, and as with elite athletes, that heart beats more slowly during rest.