St. Thomas and St. Augustine knew that life itself began at conception. They simply didn't believe it had advanced to the point of having a soul until it had developed to the point of movement. Nor was there any scientific evidence that changed the mind of the Church. It was "decreed" in the 1850s that life and soul began at conception. It was thought by some to be as much a political decision by the Pope as a theological one.
St Augustines views on early development of the fetus were not what one would call biologically apt.
Life begins at conception. God says He knew us before we were in the womb. God deals in souls. Ergo, ensoulment and conception go hand in hand.
I'm not entirely sure of that, although I may be mistaken. It was a fairly common view that life could come from non-living matter. The best known instance is Nile mud, which was thought capable of yearly giving birth to all sorts of living creatures. That one was still current in the Renaissance.
I suppose it depends what you mean by soul. It was commonly thought, from ancient times through medieval to the Renaissance, that plants had a single soul, which governed growth and reproduction; animals had a second soul, which included the senses and locomotion, and humans had a third soul, which involved reason, will, and memory. Some also speak of a higher spiritual soul, but this is not a firmly established idea.
But etymology, both Hebrew and Greek, suggest that life, breath, wind, and spirit are related concepts (ruach and pneuma). God breathes into Adam to give him life, and when the breath leaves the body we die.
Further, the soul is not only spiritual. The Catechism of the Catholic Church still includes a formulation that goes back to Aristotle, that the soul is the form of the body. The body lives only when it has a soul. So, I would think this was a development of doctrine, not a change of doctrine.