Short answers, pulled from the story.
The transition from land to sea began approximately 49 million years ago with the Ambulocetus. This animal had leg-like limbs but was already fully aquatic, indicating that a switch to a lifestyle independent from land happened extraordinarily quickly. The transition from land to water was completed in about 10 million years.
Molecular biological evidence suggests that hippos are the closest living relatives of Cetacean. Cetaceans are deeply nested within the artiodactyls, and the two groups together form a clade known as Cetartiodactyla. Common anatomical features include similarities in the morphology of the posterior molars and the bony ring on the temporal bone known as the bulla.
The blue whale reaches a maximum confirmed length of 29.9 meters and a weight of 173 tonnes, making it the largest animal known to have existed. This species is part of the Mysticeti or baleen whales, which contains 15 species of large whales. Most mysticetes prefer the food-rich colder waters around the poles of the Northern and Southern Hemispheres.
Since the Industrial Revolution, Cetacean have been extensively hunted by humans for their meat, blubber and oil by commercial whaling operations. In the 18th and 19th centuries, baleen whales especially were hunted for their baleen, which was used as a replacement for wood. In the 1930s, 30,000 whales were killed, and this increased to over 40,000 animals per year up to the 1960s.
In the wild, females who survive infancy live 46 years on average and up to 70 to 80 years, while wild males who survive infancy live 31 years on average and can reach 50 to 60 years. Captive whales have reduced life expectancy, on average only living into their 20s, although some live longer, including several over 30 years old and two, Corky II and Lolita, in their mid-40s.
Underwater noise from military sonar and vessel traffic can disrupt Cetacean behavior such as their use of biosonar for orientation and communication. Severe instances can panic them, driving them to the surface, which leads to bubbles in blood gases and can cause decompression sickness. Since 2000, whale strandings frequently occurred following military sonar testing, with the US Navy admitting partial responsibility for the beaching and deaths of several marine mammals in March 2000.