What caused early mammals to stop laying eggs and to begin giving birth to their live young? Why did it happen?

This is from a web site, http://www.bobpickett.org/evolution_of_mammals.htm/. About 120 million years ago, the mammalian line ceased laying eggs and began bearing live young. These forms of mammals were the first marsupials, who bore their young at a very early stage in their development and transferred them to a... show more This is from a web site, http://www.bobpickett.org/evolution_of_m...
About 120 million years ago, the mammalian line ceased laying eggs and began bearing live young. These forms of mammals were the first marsupials, who bore their young at a very early stage in their development and transferred them to a pouch where modified sweat glands secreted milk. It is generally accepted that the first marsupials arose in North America and spread to South America, then to Antarctica and Australia some time before the breakup of Pangaea near the end of the Cretaceous period, some 70 million years ago. Others argue that a southern continental origin is more probable.
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