Scientists have discovered that once a mammal has become fully aquatic, it passes a threshold that makes a return to terrestrial landscapes almost impossible.
Holocene extinction: currently ongoing. Extinctions have occurred at over 1000 times the background extinction rate since 1900, and the rate is increasing. The mass extinction is a result of human activity, driven by population growth and overconsumption of the earth’s natural resources. The 2019 global biodiversity assessment by IPBES asserts that out of an estimated 8 million species, 1 million plant and animal species are currently threatened with extinction. In late 2021, WWF Germany suggested that over a million species could go extinct within a decade in the "largest mass extinction event since the end of the dinosaurs.
source: https://en.wikipedia.org/wiki/Extinction_event?wprov=sfla1