You have to look at it in context. Don't take a snippet from the middle and complain that it doesn't make sense.
What is the SUBJECT of the DECLARATION OF INDEPENDENCE?
The subject is GOVERNMENT, not biology. And so it begins,
"The unanimous Declaration of the thirteen united States of America,
When in the Course of human events, it becomes necessary for one people to dissolve the political bands which have connected them with another, and to assume among the powers of the earth, the separate and equal station to which the Laws of Nature and of Nature's God entitle them, a decent respect to the opinions of mankind requires that they should declare the causes which impel them to the separation..."
In a world which at that time held that there are different CLASSES of people, and that a person could not exceed his class, the founders of America took a far more noble view; a view that offered opportunity to each individual regardless of who his parents were or which country his parents came from.
That, in brief, is it.
...and it is one of the GREATEST documents of humanity, the product of rational human minds, not some invisible imaginary friend.