I don’t share the prohibition against the supernatural, but I recognize that it’s a boo-word. And also the Victorian connotations of seances and spirit mediums
. So it is not a word I like (although worth noting in passing that ‘metaphysical’ is the Greek equivalent of the Latin ‘supernatural’).
It is here Buddhism can provide an alternative lexicon. The corresponding Buddhist term is ‘lokuttara’ which is customarily translated as ‘trans-mundane’, although you shouldn’t need an English degree to spot the convergence.
There has been an effort to ‘naturalize’ Buddhism (e.g. Owen Flanagan) but it fails to recognize that Buddhism is a religion - although that word means something else in the Buddhist contex. But we tend to bring a set of assumptions to it based on the connotations that it has in our culture.
But this is why I tend to look towards Buddhism rather than existentialism when it comes to the positive content of philosophical praxis. Husserl’s criticism of naturalism and the epochē seem to have some convergences with Buddhism (Husserl himself said so) but it lacks the ‘cultural infrastructure’ that exists in the Buddhist world, so I don’t know if it really ever took root (although I think it does explain why it is so easy for cross-cultural philosophies such as enactivism to link phenomenology and Buddhism. There’s also quite a lot of literature on the possible influences and cross-overs between Tao, Buddhism and Heidegger — subject of much commentary by the Kyoto School.)
And Buddhism does provide a reason for existence - but it’s a very different to the Christian.
But the use of Darwinian theory to justify reason is a just-so story . ‘Hey, fire and tool use! Good! Chemical and nuclear weapons! Bad!’ It extends the rationale of evolutionary fitness far beyond its domain of applicability. (An essay on this by Antony Gottlieb, It Ain’t Necessarily So.)
No, the real issue is the one given by Jacques Monod in the first quote in the OP:
The universe was not pregnant with life, nor the biosphere with man… Man at last knows that he is alone in the unfeeling immensity of the universe, out of which he emerged only by chance.
Doesn’t the notion that ‘life begins by chance’ vitiate reason? From those premisses, the ‘darwinian algorithm’ kicks in so as to maximize survival, through the physical processses of cognition, adapatation and reproduction. Hence Jonas’ charge, that in the physicalist ontology, inert matter is the norm, and life an anomaly that has to be explained. And also the attempt to re-introduce intentionality in the earliest stirrings of organic life - no, this is not simply ‘the collocation of atoms’, but a directed activity acting with a purpose in mind, namely, to persist, to carry on existing. That accounts for something essential to organic life, although in itself it doesn’t provide an explanatory framework for rationality.