Note that I really dislike the term pure human. I didn't care much for Gamma World's pure strain human but at least that made a bit more sense.

In Mutant Future, pure humans are the unmutated decendants of the wealthy who survived the cataclysm. How can I tell they were wealthy? Easy, they could afford a process that made them and their bloodlines immune to mutation (ala the Gamma World article in Dragon 106). Everyone else's kids turned into mutants. One question I have played around with for a few years is what happens when a pure human has kids with a mutant? Are the parents sterile with each other, will the kids be pure humans or will they be mutants? Different answers work for different settings.

But what made pure humans different from everyone else (including us)? I have a few answers that I have used in various settings. The Elder People, those who lived before the apocalypse, had themselves engineered and everyone had (and has) at least 25 pairs of chromosomes (so 50 total). The extra 4 chromosomes were sockets of a sort that allowed people to upgrade their bodies and their kids. In some places people had to take drugs to give their extra chromosomes to their kids and in other places they needed drugs to void those chromosomes and allow better versions to be implanted. The wealthy had as many as 27 pairs and used them to gain all kinds of advantages. Any defrosted Elder Person would have all the mutations that start with increased (senses, physical attributes, etc.) as well as an intellectual affinity and anything else the GM deems appropriate for that character. But they needed the drugs to pass along these advantages. When the cataclysm occured, the result were some people who were a bit healthier but lost almost everything that made them special. For mutants, the extra 2 pairs of chromosomes are partially the reason they are so heavily mutated.

For those who like parasite mutations, some of them could be decendants of implants that did the same as the extra chromosomes mentioned above. Instead of using drugs to pass along genes, they used them to pass along symbiont embryos.When the drugs weren't available, the symbionts were forced to survive by becoming parasitic and passing between species (pure humans, mutant humans and animals) as their hosts become locally extinct (something all to common in the wastes).

And then there were the Elder People who didn't use soft biotechnology at all. They were cyborgs. Metal and plastic enhanced their bodies and gave them long lives. Though the implants could be taken out of those who had died and placed into others, they were not designed for that. A cybernetic eye that has had 10 users will be a lot more buggy than a new one even with a good power source and little wear and tear as the software had to deal with 10 different minds and bodies. Eventually they will not be used at all as they will tend to kill their users too often.

To make pure humans a bit more interesting to play, here are some suggestions:

Their hit die changes from d8 to d4+4 or even 3d4. This makes them signficantly more durable than a mutant (with a d6 hit die) and may actually survive more than 2-3 levels. And that makes sense- the process that keeps mutation at bay also altered humans so they would live longer, healthier lives.

Or one could also make a few genetic changes that represented species wide engineering but are not on the level of mutations. These can include better health (+1 to saves versus disease), more energy (increase movement rate by 30' (10')), a climb speed equal to 1/2 their land rate (though it requires bare feet), reduced food requirements (eats half that of mutants), slower aging (lives to 150), no senility, heals faster (add one hit point per day), longer time to suffocate, control over sweat content (draws less insects and other predators) and better senses (less chance to be suprised).

One advantage that could be almost worthless or huge depending on how the Mutant Lord uses it is disease resistance or immunity. Ancients made themselves immune to all natural diseases and were only vulnerable to bioweapons made specifically to kill/incapacitate them.

Pure humans could be resistant to psionic powers. Their minds are very different from mutant humans and those differences could make all the difference.

Or they could have control over their emotions. They would be immune to empathic attacks (including fear) and react quicker in some situations (initative bonus for the first round of combat).

They have genetic locks to certain powerful technologies. These locks limit what technology mutants can use as they are missing the key sequences of DNA. This is from the 5th edition of Gamma World (the Alternity version) and gives them a slight to significant advantage versus those with mutations (depending on what technology they have exclusive use of and how common it is). This takes a bit of paperwork but it encourages players to consider having at least one pure human in the party.

A variant of the above is to allow cybernetic implants only work for pure humans. They are still buggy but they allow a more even playing field.

The Mutant Lord could allow artifact medicine to work perfectly for pure humans and have a chance of having a reduced effect, or even be dangerous, to mutants.

Give them one mutation as a residual of their genetic legacy. I would restrict it to those I mentioned above (increased X or intellectual affinity) and not allow things like killing sphere, wings or energy ray. The latter are just have more of a mutant vibe for some reason.

Give them a bonus to learning artifacts and possibly to using them. Histories passed down in pure human enclaves of Elder People machines could give them a leg up in this.

Give them a species trait that comes from real world humans. Extended endurance, heat tolerance, family interaction (bonuses to defend them), etc.

It turns out that 8% of the human genome are former viruses. Some could be reactivated and engineered to do something constructive. What I would do with this is give pure humans the ability to alter plants and animals so that they are edible. They can still be toxic or noxious but they can not be useless as a food source. Basically it reverts Earth organisms that have drawbacks like altered chemistry and xenochemistry. This takes time and the food has to be exposed to pure human spit or blood a week before being eaten. This could also make pure humans dangerous as food for many mutant critters. They eat the humans and slowly lose some of their mutations.

And then there is the chance that humans have heritable nanite factories. This could be a strength or a weakness. As a strength, pure humans would be immune to disease and heal faster (2 points per day). As a weakness, pure humans are normal but vulnerable to EMP effects. If their nanites are destroyed, such pure humans would die from the toxic environment within a matter of days.

With just a little work, pure humans can be made much more interesting for players.