An explanation of how long it took for fish to evolve into mammals, and why this will never happen again.
How long would it take a fish to evolve into a dog/mammal?
The first fish evolved around 530 million years ago, while mammals are thought to have evolved between 225-167 million years ago (depending on how you define mammal). Canine creatures are around 40 million years old, and dogs are thought to have become distinct from wolves around 40,000 years ago. While this might seem to imply that fish could evolve into mammals within 205 million years, or a dog within 530 million years, the realistic answer is that this will never happen again.
To understand how unlikely the sequence of mutations that led from a fish to a dog is, imagine the genome of a fish as a long novel. In fact, as a series of novels. The genome of the dogfish shark, for example, contains around 6 billion bases. Each page of a book contains around 1,000- 1,500 letters per page, meaning that the series we’re imagining would be 4.8 million pages long. This is not 4.8 million pages of gibberish either – the whole thing makes sense, from the sentence structure to the plot, and the individual characters in it. Now imagine a mammal genome as a similarly sized novel series, but with a different story. In a different genre. And a different writing style.
You’re now going to try to make changes to the dogfish shark’s genome to slowly turn it into the mammal genome, but you can only do it by tiny steps at a time. Humans have a mutation rate of around 42 mutations per generation, meaning that each step in the changing novel is different by 42 letters. You have to be careful though – although many small changes could be made to a dogfish shark’s genome without causing any problems, there are many changes we could make which would result in a genome that could not possibly support a living creature. In our analogy, this means that every step between the beginning novel and the ending novel needs to be a work of literary brilliance (or at least competence). Imagine how many billions of brilliant intermediates you will need to go through to achieve this.
However, evolution doesn’t have it this easy. It has to make its changes blindly.