When John Stuart Mill claims that something is “the chief interest of the history of mankind,” it is worth taking notice. In On Liberty, Chapter III: “Of Individuality, as One of the Elements of Well-Being,” paragraph 17, he writes:

The despotism of custom is everywhere the standing hindrance to human advancement, being in unceasing antagonism to that disposition to aim at something better than customary, which is called, according to circumstances, the spirit of liberty, or that of progress or improvement. The spirit of improvement is not always a spirit of liberty, for it may aim at forcing improvements on an unwilling people; and the spirit of liberty, in so far as it resists such attempts, may ally itself locally and temporarily with the opponents of improvement; but the only unfailing and permanent source of improvement is liberty, since by it there are as many possible independent centres of improvement as there are individuals. The progressive principle, however, in either shape, whether as the love of liberty or of improvement, is antagonistic to the sway of Custom, involving at least emancipation from that yoke; and the contest between the two constitutes the chief interest of the history of mankind.

To a surprising degree, custom vs. liberty and improvement is not only the chief interest of the story of mankind, but the chief interest of the story of life itself, since natural selection operates by amplifying the small fraction of random variations in the genetic code that lead to improvement in survival and reproduction:

• The genetic code establishes the customary order. Without a large measure of this order, life would fall apart.
• Given what one can expect from basic chemistry, random variations in the genetic code are the DNA or RNA chemistry equivalent of liberty.
• Natural selection among those variations is the closest counterpart in chemistry to improvement.