While reading this thread about birds & dinosaurs, I wondered mammalian evolution.
IIRC, the first mammals were little shrew-like critters who were “niched” into that position. When other niches opened up, they were filled like gangbusters.
So what “persuaded” cetacean ancestors into going back to the sea?
A great book to read if you’re interested in this type of thing is The Ancestor’s Tale, by Richard Dawkins. I’m reading it right now.
Anyway, he notes in the book that the closest living relatives to whales and dolphins are hippos! He posits that a hippo-type ancestor simply adapted more and more to living in the water all of the time instead of most of the time.
Also, I heard on NPR recently that while the common wisdom has held that the little shrew-like mammals that rapidly filled up the niches left behind by the dinosaurs ultimately led to us and the other mammals alive today, the latest research now indicates that most of these mammal lineages actually died out within 10-15 million years. The mammal lineages that led to virtually all of the mammalian species alive today actually starting diverging from the other mammal lineages some 10 million years before the K-T event (which itself occurred about 65 million years ago). Those lineages did not branch out dramatically until about 10-15 million years after the impact, when those first lines of mammals died out.
Thinking on whale evolution changes from time to time as new fossils are discovered, but the current thinking is that whales (all Cetaceans) are most closely related to the Artiodactyls - which includes Hippopotamuses, deer, pigs, goats, etc.
The most recent common ancestor of cetaceans and artiodactyls probably lived in the Paleocene or Eocene eras 50-60 million years ago, and was most likely a Mesonychid - Mesonychidae - Wikipedia - a large carnivorous ungulate. The development of cetaceans probably began in the Tethys Sea (near the current Mediterranean and Black Seas as well as stretching through central asia all the way to the present Himalayas), and dramatic changes brought about by the closure of the Tethys around that time may have encouraged a large diversification of whales and dolphins. The fossil record for cetaceans in Pakistan, the Calvert formation of Maryland, and California’s central valley gives us a much better look at the diversification of cetaceans in the Neogene 20-30 million years ago than it does into the origin of whales much earlier.
As to what “persuaded” them: There was room to make a living! I’d like to avoid anthropomorphizing the process, but good incentives may have included the availability of food in the ocean, lack of sufficiently large oceanic predators, lack of competition for that food, and presence of safe calving sites. It’s really hard to know which factors might have been important (or it could be all of them) knowing as little as we do about Cetacean evolution.
Evolution of cetaceans - Wikipedia (I’ve only skimmed the article, but it seems to have some good photos, at least)
Remember that the Cretaceous extinction didn’t just kill off the dinosaurs, it killed off all sorts of marine reptiles…Ichthyosaurs, Plesiosaurs, marine lizards, marine crocodiles, marine turtles, as well as the Ammonites. So a freshwater hippo-like creature that moved to the ocean would find wide open spaces.
I always find it amazing and amusing, given the colloquial image of early life crawling up out of the sea, that having found themselves on land these animals saw fit to slither back into the water.
That designer MUST have been intelligent to plan such a circuitous trip!
Minor nitpick: ichthyosaurs died out about 90 million years ago - about 25 million years before the K-T extinction.