Welcome to You Ask Andy

Tommy Mastroni, age 10, of Bridgeport, Connecticut, for his question:

Why do kangaroos live only in Australia?

For a long time, scientists thought they knew why the kangaroo is isolated on the huge island of Australia, way out there in the wide and lonely ocean. Surely his ancestors must have been carried off ages ago, when their island drifted like a raft, away from the other large continents. This was a good theory, but nobody could prove it true or false. Now some amazing new evidence is coming to light. Actually it is not about kangaroos, but indirectly it promises to explain how they came to survive and thrive only in Australia. At last, we almost know the answer.

This story takes us around the globe and many millions of years back into the earth's history. So let's begin it close to home and allow one of our own animals to lead us along in easy stages. Our timid little opossum has a built

in pouch, where she nurses her helpless babes. She is a marsupial mammal. However, the kangaroo and all her other marsupial kinfolk live in faraway Australia and on several nearby islands.

Obviously the marsupial family got separated. Perhaps our opossum got left behind when his kinfolk went on a global journey. The answer lies in the remote past, perhaps 200 million years ago. This was when the first simple little mammals arrived in a world of fishes and frogs, insects and lizards. Many experts suspect that the early mammals gave birth to helpless, unformed babes and nursed them in the mothers’ pouches.

Animals change through the ages and so does the earth. Right now, earth scientists are tracing the map as it was 200 million years ago. Several of our separate continents then were huddled together, like jig

saw pieces separated by narrow seas. Antarctica was nestled close to Africa, India and Australia. Perhaps there were dry bridges across the narrow seas and the four footed animals wandered all over the dry land. In the past two years, researchers have found fossils of a mammal type reptile who lived in Antarctica, Africa and India.

He shared his world with the early mammals, and no doubt many of the marsupial types lived in the jigsaw piece that became Australia. Slowly, very slowly their island raft inched them far out into the Pacific Ocean. Through the ages they improved, but they kept their pouches. Meantime on other continents, pouches went out of style.  Mammal babies developed much longer and more fully inside the bodies of their mothers. Only our little opossum gave birth to immature babies and nursed them in her pouch.

The mystery is why the kangaroos and other Australian marsupials did not change with the times. Some experts suspect that life was harder on other continents. Fast meat eaters preyed on the gentle vegetarians and toting around helpless infants became cumbersome. Perhaps the mammals just had to give birth to babies that could be hidden and left for a while. Certainly our opossum, with her load of infants, leads a risky life. But how her ancestors managed to survive when pouches went out of style is another story.

 

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