Cetaceans are completely aquatic mammals, and some mainly rely on echolocation to find food and avoid enemies. They generally feed on mollusks, fish and zooplankton, and some species can also prey on seals and seals. You must take a deep breath every once in a while.
From the anatomical point of view, many characteristics of cetaceans show a high degree of adaptation to marine life. However, anatomical evidence is not enough to answer many questions about the origin of cetaceans.
However, from the biochemical and genetic point of view, cetaceans are closely related to ungulates, and the fossil record (about 50 million years ago) also supports that cetaceans evolved from ungulates.
Extended data:
Biological sonar
Sonar is not a patent of human science and technology. Many animals have their own sonar. Marine mammals such as dolphins and whales have "underwater sonar", which can generate very definite signals to explore food and communicate with each other.
Insects such as moths also have "passive sonar", which can clearly hear the ultrasonic waves of bats 40 meters away, so they often avoid attacks. But some bats can use high-frequency ultrasound or low-frequency ultrasound that insects can't hear.
Many kinds of whales use sound to detect and communicate, the frequency is much lower than that of dolphins, and the range is much farther. Other marine mammals, such as seals and sea lions, also send out sonar signals for detection.
References:
Baidu Encyclopedia-Cetaceans