Another way to look at this is to see the machines as enmeshed as a symbol machine along with ourselves, the humans, as symbol machines, and thus to maintain stability, the system will regress to the mean so that the machines appear more human like and humans appear more machine like. [ed: At least this is my take on the concept not having read the book. -- SunirShah]
Hayles suggests rather pessimistically that we are becoming somewhat "less human," (hence "PostHuman") but that's not entirely clear. It may simply be that we remain human, but our environment changes around us, so we react and adapt differently to it. Certainly if you believe humankind has never been more self-actualized than in the present day, then it would be hard to reconcile this with PostHumanism.
Hayles, N. Katherine. (1999). How we became posthuman: Virtual bodies in cybernetics, literature, and informatics. Chicago: University of Chicago Press.
In earlier days of the Internet, many people thought that the new medium would create much more than a GlobalVillage, but a separate CyberSpace. This abstract land was the new frontier where the mores and rules of conventional reality were no longer significant. In fact, cyberspace was infinitely malleable, allowing those empowered with its authorship the ability to create their own realities as they saw fit. It seemed only natural that since a person's representation, or projection, in cyberspace was merely data subject to its arbitrary rules, that a person's identity was similarly arbitrary. A person's sense of self was constructable, reducible, destructable, and reconfigurable.
This idea wasn't new. In fact, it was influenced greatly by earlier social theories that already sought to demonstrate traditional essentialist ideas of category were merely figments of social convention. The belief that one could arbitrarily change their gendre roles, and even their gendre, for instance led many people to question just how solid their perception of sex was. Coupled with ideas that one need not be attracted to the opposite sex, that in fact there was no fixed concept of an opposite sex, broke barriers of expectation down. On another axis, many began thinking that since biology doesn't support the subcategorization of species by race, and that traditional race classificationss in human culture weren't actually identified by genetic principles but instead by cultural beliefs, that the notion of race was really a notion of ethnicity, a very fluid and constantly changing idea. Many people also began arguing that DisabledPeople?, or differently abled people, had a newfound opportunity once the world shifted more towards a knowledge-based society. Some even began trumpeting the new era of the cyborg as the natural progression of a humankind stuck at the end of a too slow biological evolution.
The notion fundamental to these ideas that a person's self was a purely mental construction, both of him or herself and the society in which he or she is embedded, led many to consider that the purely abstract cyberspace was a perfect place to reconstruct what constitutes humanity. On the Internet, no one knows you are a dog, said the famous cartoon. The Internet would provide individuals the opportunity to define themselves to the rest of the world on their terms.
Meanwhile, back in the separate MeatSpace, medical science was working diligently on freeing the other half of the mind/body duality. Surgeries had already made the outward appearance of physical gendre a choice, and reproductive technologies were improving to make this even less of a divide. Social values were changing laws grounded in an older conception of humanity, such as gay rights legislation, and the continued and expanding civil rights movement for raced minorities.
Ultimately, many felt (and still feel today), that humanity was moving towards something different, something PostHuman. A somehow homogeneously extreme heterogeny, where every individual will be free to define themselves how they like without external faux essentialism. We would all be free creatures of spirit and mind. Some have called this TheSingularity.
As strongly alluring as this is to many, it may not be consistent with how people see themselves. Contrary to many popular ideas, the mass marketing throughout the Twentieth Century did not in fact create a homogeneous Anglo society in North America. It in fact did quite the opposite, giving consumers much more choice in defining who they are simply by ShoppingForIdentity. Feminism turned into post-feminism, where feminine sexuality was re-elevated to importance.
See also CarnalArt.