It appears that nature was predisposed to handle nurture after reproduction by three very diverse alternatives.
#1 eggs which contain a fertilized ova and all the growth nutrients in a yolk sack = birds and dinasaurs [Note: Scientists now believe that Tyrannosaurus Rex had feathers= ]
#2 maturation in the womb due to an ambilical cord which supplies nutrients and oxygen = placentals
#3 maturation in the stomach pouch equipped with milk-producing teats = marsupials
One line of thinking is that marsupial diversity is greater in Australia than in South America because there were no terrestrial placental mammals to compete with marsupials in ancient Australia.
A second line of thinking is that terrestrial placental mammals once thrived in Australia along with the marsupials. Recent discoveries have found the remains and fossil evidence of a large number of terrestrial placental mammals existing in Australia in ancient times and it is now thought they they all became extinct because they could not adapt to the unique and very challenging Australian environment we see today which started to affect existing Flora and Fauna millions of years ago. As Australia began to get dryer and aridity increased, the marsupial animals (as well as eucalyptus trees that survived and thrived above all other trees) adapted to the new harsher weather conditions leading to their continued survival in a land that has to endure droughts that can last years and then suffer from floods that can inundate vast areas with meters of water that can last months. This feast to famine weather pattern that affects the Australian environment to this day favored the marsupial reproductive system which can take advantage of these hard times /good times far better than placentals. This increased advantage for marsupials led to an island continent [Australia] dominated by this type of mammal.
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