A joey, or infant kangaroo, takes a breather in its mother’s pouch. Joeys nurse in their mother’s external pouch until they are about a year old.
Between my thumb and forefinger I hold a cork. Driven into that cork is a tiny pin. And glued delicately to the tip of that pin is a jawbone the size of a fingernail clipping—all that remains of a small mammal that scurried beneath ferns and fallen logs 115 million years ago in what is now Australia.
I am sitting with paleontologist Tom Rich in a laboratory at Monash University in Melbourne. Lining the walls are row upon row of wooden drawers filled with the petrified remains of turtle shells, fish bones, dinosaur toes, and the like. But this jaw—and the teeth lining it—is the star attraction. Rich is explaining the teeth’s intricate facets, referring to areas on an enlarged dental diagram.
“Here you have the slicing trigonid,” he says, pointing to the spiky front half of a lower molar. “There in the back, you have the basin where the upper tooth lands and grinds the food.” The closely interlocking arrangement is a distinctively mammalian innovation and a crucial advance in evolution. Each pair of upper and lower teeth can simultaneously slice food and grind it like a mortar and pestle. Animals equipped with these teeth can choose from a wide variety of foods—woody seeds, fruit, leaves, insects, or small animals.
“It doesn’t look like it’s 115 million years old,” says Rich. “It really looks like a mammal that you wouldn’t expect to see until 50 million years ago.”
It especially doesn’t look like an animal that would have lived in Australia, because the jaw and teeth show features of animals that nourish their young inside the mother’s uterus. Australia is the land of weird mammals like kangaroos and koalas that nourish their young in external pouches after a brief gestation. Biologists call the first group placental mammals and the second group marsupial mammals. They have assumed that marsupials could prosper only on a backwater continent like Australia, where they were insulated from competition with placental mammals. But if Rich is correct, and his jawbone means what it appears to mean, placental mammals not only lived in Australia eons earlier than ever imagined but could also have competed with marsupials and lost. That scenario upends a long-standing theory about where some of the earliest mammals originated and how they colonized the world.
About 115 million years ago, dinosaurs ruled. The only mammal-like critters were usually smaller than rats, and they scuttled through the underbrush chasing insects. There were only two giant continents. Geologists call the northern one Laurasia and the southern one Gondwanaland. Both landmasses were breaking apart. Laurasia would become North America, Europe, and parts of Asia. Gondwanaland became South America, Africa, Australia, Antarctica, Madagascar, and the southern parts of Asia.
Most paleontologists say fossils show that both placental and marsupial mammals originated in the northern continent more than 110 million years ago. About 80 million years ago, when the two landmasses touched, both groups supposedly spread into the southern continent. The dinosaurs also died out around this time, allowing mammals to expand and diversify. Placental mammals came to dominate in most places; marsupials thrived only in Australia and parts of South America.
But if Rich’s suspicions are correct, the first and second acts of this evolutionary drama are radically different: Not only did placental mammals live in Australia eons ago, they originated on the southern part of the vast first continent and spread to the northern landmasses more than 100 million years ago, during Gondwanaland’s breakup. Rich and his colleagues call this new theory the Garden of Eden hypothesis.
Lesley Kool, a member of Rich’s team, sits with me in a car overlooking the cliffs that line Flat Rocks, a coastal site in southeastern Australia. Rain falls and waves crash on the beach below. Some 115 million years ago, she tells me, this shoreline was a flat-bottomed valley where Australia and Antarctica touched. Broad, pebbly rivers meandered through that valley, and their ancient sediments can still be found in these cliffs.
We zip on our parkas and head down a stairway to the bottom of the cliff. After a few yards, Kool kneels and points to blotches of stone that seem to drip like a bad paint job into the cliff’s thin horizontal layers of stone. She says the drips hint at what the ancient climate and terrain were like. Some 115 million years ago, when it was much colder here than it is today, mud pushed down into the permafrost layers of soil below, forming these blotches.