Let me have another go at it…

We are the only species that has virtually managed to counter the law of natural selection, reason why I might have used the word ‘fittest’ as a short cut.
A way to put it:
- the human story has been a cultural journey that took hominids out of Africa to the modern age of Information Technology. They started to use primitive tools, discovered the use of fire, developed their technological skills, improved their communication, produced art, etc, etc…
- Ultimately we – Homo sapiens – are the only species on Earth that has managed to develop a form of intelligence, which has allowed us to conceptualise the world we live in (yes… I know other animals are quite ‘clever’. However none of them has managed to write the Principia Mathematica yet…).
- This has allowed us to develop language, cultural codes, etc. We have been able to capture this conceptual intelligence, to pass it on and to relentlessly organise it from generations to generations: from the 1st natural philosophers to your PhD thesis…
- Large chunks of this conceptual intelligence have devolved into practical ‘technologies’, which have contributed to human development.
- … which leads to the point: Science has underpinned ‘progress’ in fields such as hygiene, medicine, biology, agriculture, which has enabled humans to develop their understanding of, and to influence the way they interact with the natural environment… and clearly to counter the natural selection process. The word ‘Natural’ meaning ‘whatever Nature imposed on us 10,000 years ago’; whatever ‘state of nature’ – to paraphrase Rousseau (!?) – they would find themselves in without the entire surrounding Infrastructure.

We are in a unique position as a species where we have developed ways & means to turn the table on Nature. This is precisely the point:
We might be ‘fit’ “in the evolutionary sense, meaning just the capability of an organism to pass its genes to subsequent generations”.
However we have been achieving this ‘fitness’ by writing our own rules, and at times have divorced ourselves from ‘fitting’ in the overall ecosystem. There is absolutely no problem with that: we just to remind ourselves that this comes with a duty to pause and think about it from time to time.

BTW, As I was writing this answer I was thinking of another way of looking at it. It is going on a bit of a tangent but since we started a conversation… :

Still considering the notion of ‘fitness’ “in the evolutionary sense, meaning just the capability of an organism to pass its genes to subsequent generations”: let’s think of childbirth in humans…

Childbirth in humans is an extremely difficult and dangerous process that is a result of human ‘bipedalism’ and ‘encephalization’.
- ‘Bipedalism’: The evolutionary modifications of the human pelvis that enabled hominids to walk upright necessitate that most infants exit the birth canal with the back of their heads against the pubic bones. Resulting in an awkward delivery position that requires assistance.
- ‘Encephalization’: During the evolutionary process, humans evolved large brain sizes and as a result, infants have an extraordinarily large head to body proportion. Encephalization makes it extremely difficult for the infant to fit through the birth canal, therefore must make a series of twists and turns.

So… the high mortality rate with unassisted human births almost outweighs the benefits of encephalization and bipedalism: the species would die out if it could not successfully reproduce. It was therefore evolutionarily necessary for birthing mothers to seek assistance during childbirth. The biological evolution has led to sociocultural evolution: Birthing difficulties forced cooperation among humans, resulting in the formation of social interactions and the beginning of culture among early humans.

-> Provide all the right environment and assistance, the birth goes well, the individuals are considered ‘fit’: we’re happy…

-> Now… suddenly remove the science and medical assistance behind it, and consider that the average pelvic opening in women today is around 13cm to 10cm, that average infant head is ~10cm, and the shoulders are ~12cm across. (would have to double check those numbers…)

Are we still fit?

Fitness, in the evolutionary sense, means just the capability of an organism to pass its genes to subsequent generations. Thus, an individual of a species that might be perceived to be ‘weak’ is more fit than an individual of the same species perceived to be ’strong’ if the former is able to reproduce and pass on more of its genes to the next generation.

So, by definition, humans that survive to pass on their genes because of medical intervention are just as ‘fit’ as individuals who pass on their genes without intervention.

However – I’d encourage everyone to divorce the notion of ‘fitness’ from the notion of ‘moral’, as to do so is to open up a hairy can of worms.