Originally posted on the Sketchy Science website.
It is tempting to think of evolution as a linear process. You start with some primitive creature like an amoeba or a slug and over time you get rid of the slime, add some legs, maybe a bit of fur and you’re on your way to something more advanced. Unfortunately this is utterly and completely false. Natural history is full of examples of animals that just don’t change no matter how much time you give them. Crocodiles and sharks are two well-known examples but the clad of stubborn animals also includes horseshoe crabs, dragonflies, coelacanths and nautiluses, just to name a few. Other animals oppose the idea of straight-line evolution in an entirely different way: they change, and then change their minds about changing.
It is strange to think that over millions of years a line of animals can commit to a new way of life, slowly accumulate the necessary genetic changes to become better and better at it, and then totally cut their losses and try to go back to what they used to be, but it happens all the time. The best example of the evolutionary yo-yo effect comes from everyone’s favourite paradoxes of the natural world, marine mammals.
The story of how we got whales, dolphins, seals and the like is a supremely interesting one that spans the entire history of life on earth. Actually, “interesting” is sort of in the eye of the beholder on this one because things kick off with the evolution of single-celled life around 3.6 billion years ago and then not much happens for about 3 billion years. After that, things got downright exciting, though. Multicellular marine life came onto the scene around 555 million years ago; the ancestors of scorpions, spiders; and millipedes took to the land 100 million years after that; land plants soon followed around 420 million years ago; and then, fashionably late to the land-dwelling party, four-limbed vertebrates crawled out of the ocean around 360 million years ago.
This last group is the one we are interested in. The animals that evolved from these air-breathing pseudo-fish became reptiles, dinosaurs, and all the mammals that have ever lived, including us (humans) and – perplexingly – marine mammals. Mammals as a group really came into our own in the dust of a global catastrophe around 65 million years ago. A massive object from space had just paid an unwelcome visit to Earth and wiped out the dominant creatures of the time, who we now know as dinosaurs. From the rubble of that world crawled a small, shrew-like animal that is the last common relative of everything from bats, to humans, to whales.
Lineages diverged and new species evolved for around 15 million years until around 50 million years ago a rebellious creature known to scientists by the name of Pakicetus decided it wanted to change things up. Pakicetus was an interesting animal. Imagine a wolf with an elongated head and hooves and you aren’t far off. We believe that they hunted animals both in an out of the water on the edges of oceans, because over time Pakicetus gave rise to other species that were more well-equipped for life in the water. Gradually the offspring of Pakicetus developed a longer tail and more hydrodynamic shape along with limbs better able to propel it through the water.
Over many generations these slight changes became more and more dramatic. The nasal passages moved up to the top of the head to allow for easier breathing at the surface, the tail became the primary means of propulsion to the extent that hind legs were only in the way and eventually shrunk down to basically nothing. These adaptations were incredibly successful and allowed this line of animals to grow into the largest animals that have ever lived on this planet: modern whales – as well as their smaller cousins, dolphins and porpoises.
Surprisingly, it only took about 10 million years to turn Pakicetus into the first recognizable whale (Dorudon), demonstrating that abandoning one way of life for another isn’t all that challenging for natural selection. If you want a snapshot of the midway point along this evolutionary line, take a look at seals, sea lions and otters.
Obviously, evolution’s work is never done. In spite of how well adapted these animals now are for life in the ocean, they still need to come to the surface to breath. Over time (a lot of time) that might change, or maybe they will transition back onto land as turtles and tortoises have done a number of times over the past many millions of years. All we know for sure, is that evolution plays by it’s own rules.