Are "Pure" Humans Extinct In Warhammer 40,000?

Warhammer 40,000's Imperium of Man was built on lies. At least half of it was, or with certain truths being omitted to protect the general public. Some were valid, others required to fulfill the Emperor's plan, and some were done in order to more swiftly grab power. It's always been an interesting point about the setting as a whole, as you have to ask yourself if it really was all worth it at the end of the day.

However, the idea that is going to be discussed today isn't focused upon lies in general so much as one possible lie. The Emperor prized defending and protecting the "pure" humans who had not suffered Chaotic mutations, the nightmares of technology or abominable self-enhancements. This was, for better and worse, passed onto his sons and then the Imperium as a whole. However, given the state of the galaxy there is a question which needs to be asked:

What if there is no such thing as a genetically unmodified human in Warhammer 40,000 anymore?

This is a fan theory, something that I'm considering but will freely say up front that it has limited support in the overall canon. With that said, it's one which seems as if it might fit into the setting overall given what has happened over the last thirteen thousand years.

For starters, consider what humanity's Empire was prior to the Age of Strife. While it is known as the Dark Age of Technology for a reason, it produced no shortage of wonders along with just as many horrors, and achieved things that makes even Star Trek's Federation or Star Wars' various powers seem primitive by comparison. From completely re-ordering worlds to mass terraforming to giving ships both time manipulation technology and the power to fire weaponized black holes, the galaxy was humanity's plaything. In many cases, genetics were equally something malleable and could be reshaped or reworked as needed.

We already know that various factions of humans encountered by the Great Crusade were modified to an extreme extent. Some were wiped out due to this experimentation, while others were kept such as the tribes of Fenris. Given how easy it was to do that, it is not out of the question to think that such modifications might have become widespread. While most would have likely kept a typical human guise (two arms, four limbs, the nose in the right place etc) certain baseline modifications could have easily been written into a person's genetic code and then passed on through hereditary means.

The reason that I bring this up is twofold. Firstly, the Emperor was willing to accept and ignore certain mutants when it suited his means. Even if you ignore the Fenrisians due to their association with Russ, or the Navigators due to their usefulness, there were still others atop of this. Abhumans, despite significant evolutionary divergence, were accepted and a multitude of human sub-species were deemed safe enough to blend into the Imperium. The Emperor was more concerned with those who were dangerous to humanity as a whole, and those that devastated the past empire were those afflicted by Chaos. Were it not for that direct link, and their genetic instability, I wouldn't be surprised if the Emperor would have happily ignored them.

The second reason is simple: Humans even in modern M42 have this habit of surviving things which should outright kill them. Think about it, they can thrive in places where populations should not be possible. Hive worlds, just for one, are so frequently polluted beyond all reason that they are almost barren. Their lower levels are filled with smog, the remaining ecosystem is heavily mutated and it is outright poisonous. The very fact that their bodies are capable of filtering such toxins to an incredible extent is eyebrow-raising.

Even if that is not worth fully considering, also take into account the level of punishment that certain humans can take within the setting. Yarrick had his arm ripped off, but remained upright not only long enough to take down the Warboss who de-limbed him, but gave a full speech to an Ork WAAAGH! and drove it into retreat with his sheer force of will. The various members of the Tanith First and Only have endured so much that should have outright killed them at various point that it can be ridiculous, from Feygor's multiple throat wounds to Gaunt surviving extreme torture. Even minor characters can benefit from this, with some taking hours to die from extreme blood loss or living through having a Carnifex stab them through the chest.

You can argue that the case with the wounds is extreme - and that is true for many here - but when you consider how many characters can shake off lasbolts, bullets and even the odd stab wound, it's not out of the question. Perhaps some simply inherited a stronger batch of these possible upgrades than others, or they were further enhanced by humanity's odd relationship with the Warp itself. Or, more likely, they could have simply been written to play upon the satirical extremism of the setting and thematic elements at the time.

Still, when you consider the sorts of people churned out by the likes of Catachan, Tallarn, Valhalla, and Posul, most clearly have abilities which are beyond human. Some can simply live by shrugging off plagues, problems, and failings which would have otherwise killed them or triggered severe devolutions among the population. You could even argue that the fact humans can so easily accept bionic implants without any risk of rejection (save for one or two extremely rare exceptions) could even be a sign of this genetic tampering.

Again, this is a theory, but it fits in with much of the setting and adds one more hypocrisy to the Imperium while adding another legacy to the Age of Strife. It's just worth considering as a possible way to view the lore and where humanity has gone in the last ten thousand years.