How did mammals evolve to create Porpoises (Whales, Dolphins, Orcas)?

Whales evolved from an artiodactyl or even toed ungulate mammal. The closest relative of all whales is the hippo. Artiodatcys evolved from shrews after the dinosaurs were wiped out by a giant meteor the size of Mt. Everest striking the earth 65 million years ago. Before then, mammals were unable to evolve into large land animals because the dinosaurs were dominant. Nevertheless, there were some medium sized marsupials in North America during the age of dinosaurs, but they too were wiped out by the giant meteor, because the impact created oven-like temperatures on earth and it simply baked all the land animals alive.

Only animals that spent a lot of time underground (shrews, lizards, snakes, salamanders toads, invertebrates) survived because heat rises. Temperature underground was cool enough for these animals to survive. Another safe place to be was in or near the water, where shorebirds, fish, turtles, crocodilians, and frogs were able to escape the high temperature since water takes a lot of energy to heat up just one degree.

After the dinosaurs were gone, shrew-like mammals in Eurasia evolved into a variety, including primates, rabbits, rodents, artiodactyls, perissodactyls, carnivores, and of course whales. In Africa, the shrew-like mammals there evolved into elephants, elephant shrews, manatees, aardvarks and others. In North America the shrew like mammals evolved into sloths and armadillos. Because the duck-billed platypus spent a lot of time in the water, it too survived in Australia. In South America the marsupial shrews survived and gave rise to a lot of different marsupials, some of them later migrated to Australia via Antartica, which did not freeze over until about 35 million years ago.

The surviving shorebirds also evolved into the variety we see today. During the age of dinosaurs, there were no passerine birds, penguins, parrots, hawks, ducks, chickens, ostriches, pelicans or geese.

You are wrong. In theory, mammals evolved in the sea first.

The answer is only a click away.

https://evolution.berkeley.edu/evolibrary/article/...

http://www.whalefacts.org/evolution-of-whales/

As a rule life evolves to fill any available niche.

There were a few intermediate steps.

The first mammals were actually micro organisms... All of which appeared in the ocean, near the heat vents. Some evolved into land animals, others evolved into the fish we have in the ocean.