Pouch or no Pouch

Whether 'tis wiser to nourish a fetus in an external flap or grow it in a womb has shaped a new mystery of evolution

By Douglas S. Fox, Martin Mischkulnig|Sunday, July 25, 2004
RELATED TAGS: SEX & REPRODUCTION

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.

Geologists say this valley may once have been as far south as 75 degrees south latitude—only 15 degrees from the South Pole. The average annual temperature would have been about freezing—possibly rising as high as 70 degrees Fahrenheit in summer and plummeting to less than –10°F in the winter.

Despite cold, snow, and three months of darkness every winter, life thrived here, as it does today in parts of Alaska with a similar climate. Forests of tree ferns, conifers, cycads, and ginkgos with trunks three feet across covered the valley floor. Hundreds of bones found here reveal that wallaby-size hypsilophodontid dinosaurs nibbled on undergrowth, as did club-tailed ankylosaurs, which reached the size of small cars. Large, two-legged meat eaters stalked through snow looking for small prey, or they hibernated. Egg-laying monotreme mammals related to modern-day platypuses scurried through the bushes. Pterosaurs and primitive birds glided above, and rivers teemed with turtles, fish, and fish-eating plesiosaurs.

Tom Rich and his colleagues believe the half-inch-long jawbone belonged to a 115-million-year-old placental mammal that lived in what is now Australia. Teeth are crucial for fossil identification because they are both durable and closely regulated by genes. Reptilian teeth, for example, are simple pointed cones; mammalian teeth are distinctive for the basins and cusps that form complex interlocking upper and lower molars. The inset at right shows 15 features on the jawbone’s molars. One reason Rich believes the jawbone belonged to a placental mammal is that it has three molars and one smaller, somewhat similar tooth called a premolar. Marsupial mammals have four molars.

Kool stops to point out black-flecked sandstone at the base of the cliff. The flecks are bits of carbonized wood that accumulated in a riverbed. “It looks like some annual flooding event,” she says, noting five layers in the speckled rock. “Each layer is deposited directly above the previous one. It’s quite possible they were laid down in five consecutive years.”

The layers slope downward into the waves. Farther into the waves lies a spot where hundreds of bones settled, perhaps because a sandbar slowed the ancient river’s current. Since the first jawbone was found here in 1997, Rich’s team has chipped at least 28 more from the rock. Many of the jaws come from two creatures Rich has deemed placental mammals—Ausktribosphenos nyktos and Bishops whitmorei.

No other type of bone from a mammal has turned up here. Rich says his hunch is that “we have a Samson effect. Samson slew a thousand Philistines with the jawbone of an ass. It’s the toughest bone in the body.” For mammals, jawbones are the remains most likely to last millennia.

Rich’s trove of jawbones are not the only clues that placental mammals could have originated in Gondwanaland. In 1999 a team from the Field Museum of Natural History in Chicago and the University of California at Santa Barbara found a 170-million-year-old jaw in Madagascar that looked as if it belonged to a predecessor of both placental and marsupial mammals. And in 2001, a team from the Museo Paleontológico Egidio Feruglio in Argentina unearthed an intriguing jaw from 160-million-year-old rocks in Chubut, Argentina. Although the Argentine team suspects that the bone came from an egg-laying mammal, other paleontologists believe it shows traits of a placental mammal. These fossils are every bit as assumption-shattering as Rich’s. They turned up at sites on landmasses that once belonged to Gondwanaland—sites where they should not be if placental mammals arose in the northern landmass of Laurasia.

Molecular studies also turn up results that don’t fit with a Laurasian origin for placental mammals. When geneticists compare DNA sequences of the world’s placental mammals, the results cluster into a family tree with four major groups. By studying variations within the DNA of each group, researchers approximate how long it took the groups to diverge and develop as distinct entities. Their calculations suggest that the oldest two groups emerged around 100 million years ago, predating the other two groups by 10 million years. Surprisingly, these two older groups are made up of mammals that are found in South America and Africa. They include animals such as elephants and sloths.

No clear-cut fossil evidence of these older South American and African groups has been found in Laurasia. And even if these older groups once had predecessors on Laurasia that have not yet been found, it would have been impossible for the creatures to migrate to the southern landmasses that long ago because the continents of Gondwanaland and Laurasia made contact only about 80 million years ago.

These inconsistencies can be resolved by the simple hypothesis that placental mammals originated in Gondwanaland, not Laurasia, says mammalogist Tim Flannery, director of the South Australia Museum in Adelaide.

“The earliest branches are in the Southern Hemisphere,” he says, “so that’s where you’d start looking for the origin of placentals. And just coincidentally, here we have these fossils in the south that look like placental mammals.”

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.

YOUR CHANGING PLANET

The fossil record was supposed to show that placental mammals evolved in the Northern Hemisphere more than 110 million years ago and began migrating into the southern landmasses 80 million years ago. A controversial theory that draws on geologic events and fossil evidence proposes that placental mammals may have originated in the southern landmasses and spread throughout the world as the first two continents—Laurasia and Gondwanaland—were breaking apart more than 100 million years ago.

(Graphic by Don Foley)

The world 240 million years ago

In the traditional theory, placental mammals began spreading south

only 80 million years ago.

In the Garden of Eden theory, placental mammals migrated from the south as the continents

were breaking up.

 The world today

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.

Slowing metabolism while resting “is tremendously efficient,” says Hugh Tyndale-Biscoe, a comparative physiologist with the Commonwealth Scientific and Industrial Research Organization in Canberra. “When conditions are bad, you just shut your metabolism down and you don’t have to eat so much, but when conditions are good, you can wind it up to six times the rate. There’s no placental mammal that can do that.”

MARSUPIALS

The kangaroo, quoll, wombat, and cuscus are marsupials that inhabit ecological niches in Australia equivalent to those of the deer, rat, marmot, and flying squirrel of North America. Nursing young in a pouch reduces the need for extensive maternal investment—a good strategy in an uncertain environment. One disadvantage: Marsupials grow more slowly and thus have smaller brains than placental mammals. Marsupials do not show the spectacular diversity of placental mammals. Australia has around 140 species of marsupials, many extinct or endangered. The only placental mammals native to Australia are rats and bats. South America has three types of marsupials, including 72 species of opossum. North America has only one marsupial species: the Virginia opossum. 

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