After reading Kant's Idea For a Universal History, I've become intrigued by Kant's notion of Nature guiding the dealings of men towards a "perfectly rightful civil constitution." Although the relationship between man and Nature remains clear, what would Kant say is the meaning for any human existence independent of Nature? Perhaps there is some overlap between the meaning of human life simpliciter and the meaning of human life in relation with Nature?
I long ago found Keith Ward's work suggestive and even explicit about the purposiveness and teleology in which human life is involved. I would not normally quote so much but your question merits an answer, I'm under pressure of time, and Ward expresses what I would say anyway. As an answer it is perhaps better than no answer.
REASON AND NATURE
It is Nature which sets before man certain ends which, negatively, may not be violated, and, positively, need to be furthered by personal effort. The Metaphysic of Morals makes it clear that Reason must preserve and perfect the purposes of Nature. Moreover, it must finally transcend those purposes, in realizing its own infinite dignity.Legislative Reason promulgates, not a senseless set of absolute imperatives, but the rational condition of a harmonious system of ends, the greatest possible harmony of human purposes which are compatible with, yet transcend, the purposes of Nature. So Reason must take account of and seek to perfect the purposes of Nature; in doing so it freely determines Nature in accordance with Nature's own inner purposes; thus freedom is for the purpose of fulfilment, and to use it otherwise is to contradict its essential character. It is this process of freely preserving and fulfilling the ends of Nature, against the opposition of inclinations, which gives human life its value, for Kant; " the end for which man is destined is to achieve his fullest perfection through his own freedom ".15 Moral effort is of supreme worth; but it would be senseless, on Kant's view, if it were not aimed at the realization of one's natural perfections in a harmonious community. This is the doctrine implicit in the Fundamental Principles; and it is unfortunate that Kant's attempt to provide just one supreme principle of morality, and a purely formal one at that, has helped to conceal it from those critics who have failed to take into account the Metaphysic of Morals, to which the Fundamental Principles was meant to be a propaedeutic. (Keith Ward, 'Kant's Teleological Ethics', The Philosophical Quarterly (1950-), Vol. 21, No. 85 (Oct., 1971), p.341.)
REASON AND HISTORY
Kant elaborates on this teleological view of nature in the third Critique and various essays on history, written between 1784-1795, in which he out lines an evolutionary, purposive view of history, developing through wars and struggle to the foundation of a world-federation of states and " culture " -the development of natural, intellectual and artistic tendencies and of the conditions for moral freedom. " Nature has willed that man . .. should partake of no other happiness or perfection than that which he himself . . . has created by his own reason ".16 Nature, by means of social antagonism and even war, aims at developing all human talents to a stage at which man is released " from the womb of nature " as a freely choosing moral agent, finding in himself the final end of the world, and henceforth impelled to struggle to seek active, self-wrought perfection and happiness. The historical destiny of Nature is thus to form a being which must be fashioned through discord and strife to transcend its animal condition and realize its essential superiority to Nature, through its moral freedom. (Ward, p.342.)
REASON AND HAPPINESS
He affirms that duty must be done because it is duty; but also that " The moral law . .. defines for us a final end, and does so a priori, and makes it obligatory upon us to strive towards its attainment. This end is the summum bonum ". And he defines the summum bonum in this way: " The highest good means the whole, the perfect good, wherein virtue is always the supreme good .. .while happiness . . . presupposes conduct in accordance with the moral law as its condition ". These are clear statements that there is a supreme end of human action, which is determined a priori by the moral law; and it may be defined as " happiness in accordance with virtue."