The concept of speculative evolution is probably about as old as the recognition of natural selection itself. In the first edition of On The Origin of Species in 1859, Charles Darwin speculated about swimming bears eventually becoming whale-like animals as a hypothetical example of the evolutionary process in action:
“In North America the black bear was seen by Hearne swimming for hours with widely open mouth, thus catching, like a whale, insects in the water. Even in so extreme a case as this, if the supply of insects were constant, and if better adapted competitors did not already exist in the country, I can see no difficulty in a race of bears being rendered, by natural selection, more and more aquatic in their structure and habits, with larger and larger mouths, till a creature was produced as monstrous as a whale.”
(This passage was widely ridiculed at the time, and most of it was removed from later editions, but compared to many modern spec projects a whale-bear doesn’t actually seem that weird!)
But it was the science fiction genre that really got things going, with H.G. Wells’ 1895 novel The Time Machine and its depiction of a future time when humans have diverged into two new species: the docile pampered Eloi and the underground-dwelling predatory Morlocks. A later chapter in the book also gives a glimpse of an even further future populated by huge terrestrial crabs – and, in some editions, small rabbit-like animals that are the distant descendants of the Eloi.
Various other works experimented with similar fictional creatures over the next few decades, both Earth-based and alien, although usually on a small scale with only a handful of species. Edgar Rice Burrough’s Barsoom and Pellucidar series featured slightly more developed speculative faunas inhabiting Mars and the interior of a hollow Earth, and Olaf Stapledon’s Last and First Men detailed a future history of humans across eighteen different species and two billion years of time.
Then, in the late 1950s came the publication of a book with some much more rigorous evolutionary worldbuilding… where we’ll continue tomorrow.