Could they ever evolve in nature?
First, we need to define what is a Zombie and for this, there are some possible theories:
- Human-being infected with some kind of virus, fungus or bacteria that kills the host and raise a new being, the Zombie. (Most of my answers will be based on this variant theory)
- Variation of the Human race that can infect their similar species and transform us into one of them.
Then, I will answer it by the end of my text after some very important context information explained on the following questions.
Are their bodies in decomposition?
No, this is a common mistake associated with the decomposition odor they emanate. The real answer is that the parasite (virus, fungus or bacteria) is actively controlling the body by providing the electrical impulses that activate the muscles created by a chemical reaction that smells like decomposing bodies.
This is important because the Zombies does not always respond to a single centralized Brain, like for example, a limb cut off will still move for a while before the parasite loses the energy material for the reaction that causes the electrical impulse and the limb will enter in hibernation.
The problem with that is if a limb is stitched to another Zombie, it will be fully operational once the parasite can link both parts.
But if so, why destroying the Brain is the fastest way to "kill" a Zombie?
The Brain is the perfect habitat for the parasite, it has all the nerve system at hand and the skull provide the best protection they could ask for the initial phase of infection.
Disabling it will eliminate the Control Center of the parasite and the body will start to decay, killing slowly the parasite.
What is their "set of abilities"?
How strong are they compared to humans?
About the strength and fatigue the fact the zombies feel no pain, an excess of lactic acid will not bother them nor make them stop using their muscles.
Basically, a regular person will stop overusing their muscles as a reaction either to the muscular pain or the censorial pain related to the pressure applied to their fingers when pressing an object as an example. None of these will stop the zombie.
They will use the original body strength without restriction, what would make them "stronger" than before, also the more they use it, stronger they become and more hunger they will have.
Finally, this hunger will press the adrenal glands to produce adrenaline or epinephrine that will increase the blood flow to muscles increasing the muscular capacity to incredible levels not possible to the Human host before getting infected.
Can they run?
The other aspect is that there are some "exercises" that are aerobic (uses oxygen) and others anaerobic (uses no oxygen).
Zombies will not be able to run a marathon, but they can flex their muscles until its rupture.
Since there are two different types of Muscular fiber in a Human body, the red and the white ones.
The White is used for long-lasting movements like marathons, while the Red is for a fast reaction like 100 meters sprint. A Zombie will exceed in Red fibers and they will be able to run very fast but only for a small amount of time (few seconds) and then they will need to feed.
This characteristic is based on the predator and prey nature model for carnivores, where an animal like a Jaguar can outrun their prey but not for very long.
Do they have some degree of intelligence?
The Zombies will not have Intelligence on their own, but the parasite will be connected to a hive mind, like Bees or Ants and they rely on the smell to send and receive messages and to identify other infected beings.
While having a human covered with blood and gore from a zombie will mask the human smell and make them identify the human as one of them, the decay of the parts will kill the parasite and the characteristic odor will diminish making it possible to differentiate the human smell after some time.
An unconfirmed theory seems to point that Zombies having a hive mind, responds to a Queen-type entity with a higher degree of infection capable of some basic thinking on top of the basic predator instinct.
What other characteristics are important to mention?
When a Zombie needs the extra energy, they will produce adrenaline as mentioned before, this will cause pupil dilation and with that, they will have an increased light sensitivity and better nocturnal vision.
This makes them very active during the night time or really dark ambient like caves or sewers and very slow reactions during the daytime.
What is their diet like?
Zombies eat bloody meat and Brains, but why?
- They do not convert aliments into energy like regular Humans;
- Due to their unstable state, they need to keep feeding without a break or they will either perish or enter in hibernation
Since they need to keep eating, they will always seek for the meal that acquires the best resources they need the most.
To regain the used chemical elements, their hunger for bloody meat and brains is the fast way to absorb it from another being that is chemically similar to them.
Roughly they see us a powerful electrolyte and blood meal.
What happens if they do not absorb the chemical elements that they need?
Missing calcium: The low blood calcium level, aka hypocalcemia, causes muscle irritability. This is the cause of muscle cramps and twitching of your muscles.
Based on it, the characteristic twitching muscles and erratic movement of the zombie can be defined by their super low diet of calcium.
If they fail to acquire the necessary amount of meat, blood, and brains, they will enter a critical state that can put them in hibernation.
Please consider this part of my answer as possible outcomes, none of this has been proved so far.
To answer the main question, now that we have the basics, it is possible to understand how they can evolve:
The parasite will duplicate in a fast rate depending on the ingestion of more organic material, this will cause it to either jump to another host (infection) or add more organic material to the original Zombie.
When they infect another host, the same parasite will occupy more than one host, what is, in fact, a single organism spread in multiple "Zombies". This allows the hive mind to operate.
To add organic material, the parasite needs to have a established control of the host, this means months or years of infection and the degree of infection will raise.
That effect can create a growth on some parts of the host, creating a new category of Zombie.
So far the common type is the "Worker" like in an Ant society. Then some more specialized variations will appear, the "Carriers", the "Soldier" and "Matrons" with some variants.
Workers will just search for more organic material and hunt for preys (Humans are the preferred meal)
Carriers are the heavy lifters, they are super strong and can carry very heavy weights, they are powerful, can break strong materials but they are very slow, because of that is common to see the Soldier type protecting their carry activity from other parasite hives.
Soldiers are fast and strong, some limbs got a shell type cover that they use as weapons, they usually protect the Carriers when they are moving big loads of organic materials for the nurseries.
A variant version of the Soldiers is called Elite soldiers, they have the duty of protecting the Matrons and the big difference is the body armor made of the same shell cover than the limbs. They are a clear evolution of the infection phase. They also rely on other forms of communication for faster response (some kind of sound frequency).
Matrons are the ones that produce the organic material that transform the soldiers into Elite Soldiers and Workers in Matrons, they are a big larva-like creature that ingests big quantities of organic material and transforms to smaller and super-concentrate orbs of energy (a human in contact with this product can be infected just by touching it) they are the workers of the nurseries.
Another possibility is the Queen-type Zombie, that orchestrate the work of all the others and can explain the origin of the Host Zero.