evrim

OK, Viviparity (live-bearing) evolved from Oviparity (egg-laying) to times via egg retention and eggshell thickness due to colder climates. How Oviparity ended and Viviparity just begun the very first time? Did the off-spring decided to stop Oviparity, and begins to Viviparity immediately for the next generations to come? If this is an adaptation to colder climates evolves by transition, there must be a later or final step where the very thin egg is laid vulnerable to cold. (Maybe mother protects it by incubating like birds, or young is ready for cold before laying?) I cannot find simple explanations, as well as videos and images. Thank you!