Evolution is a strange, unpredictable thing.
Surviving species either adapt, become extinct, or are extremely lucky. Historical evidence seems to indicate several ‘bottle-neck’ events have occurred before. Between 69,000 and 77,000 years ago, one such event nearly destroyed the human race.
A big ass volcano blew itself to hell. Sound familiar?
The Toba supereruption is recognized as one of the earth's largest known eruptions. The related catastrophe theory holds that this supervolcanic event plunged the planet into a 6-to-10-year volcanic winter, which resulted in the world's human population being reduced to 10,000 or even a mere 1,000 breeding pairs, creating a bottleneck in human evolution. Some researchers argue that the Toba eruption produced not only a catastrophic volcanic winter but also an additional 1,000-year cooling episode.
Supporters of the Toba catastrophe theory suggest that the eruption resulted in a global ecological disaster with extreme phenomena, such as worldwide vegetation destruction, and severe drought in the tropical rainforest belt and in monsoonal regions. Τhis massive environmental change created population bottlenecks in species that existed at the time, including hominids; this in turn accelerated differentiation of the reduced human population. Therefore, Toba may have caused modern races to differentiate abruptly only 70,000 years ago, rather than gradually over one million years.
Good thing humans had canned food this time around.
As populations become smaller, genetic drift plays a bigger role in specialization. Land animals like the brown bear may find themselves locally reduced to a few dozen pairs on an Arctic island. That likely happened as the last Ice Age came to an end, and the Bering land bridge receded into the sea. In that circumstance, a beneficial trait appearing in an alpha male or two may change the color, size, swimming ability, cold resistance, or aggressiveness of the group in just a few generations.
They didn’t have to deal with zombies though.
Or did they?
Nearly every culture has its tales and legends about the undead. From China’s jiang shi, or hopping zombies, to the more widely known practice of West African bokor using Vodou to animate the dead, mythology surrounding reanimated corpses is as old as recorded history. Zombies. Vampires. Ghouls. Revenants. The storytelling mediums and methods changed as the centuries passed, even if the nightmares they portrayed remained mostly the same.
Which begs the question; Can undead evolve?
One could argue that while the infected hosts do not mate, and are therefore unable to pass along beneficial traits, the microorganisms, or catalyst of the zombie condition, may be able to adapt. Evolve. Alter changes to the host in order to better sustain itself. A zombie only able to eat humans for sustenance would eventually starve as their food source grew scarce. Unless they became better hunters or learned to eat something else.
In Canada, stories among survivors began to circulate which described extremely hairy zombies able to lay dormant for months. Until food passed nearby.
One group in Florida encountered a pack of undead who were able to inflate their stomachs using gases generated from tissue decay. Those gasses allow the zombies to move more quickly through the Everglades. Necrosis of internal organs ceased after feeding, resulting in dispersion of the gasses. The zombies would then sink back into the waters until enough gas was generated to begin the cycle again.
Green-skinned zombies in Greenland. Withered, mummy-like zombies in Egypt. Polish zombies who relied on acute hearing rather than a strong sense of smell. African zombies seen carrying spears and sticks. Norwegian zombies that burrowed underground and popped up to attack like trapdoor spiders.
Evolution is a strange, unpredictable, weird fucking thing.