An infant form is not an adult form, that's all you're saying when you say that an acorn isn't an oak. In the same way, a brand new human being formed by the fertilization of a human egg and sperm is everything that a human being is in its substance or essence. That would include being soulish. Fertilization is not an arbitrary point because that's when a new being comes into being by biological definition. This distinction between a property and a substance is very important because properties are what substances have.
Properties can grow and change, but that's different than the essence itself.
Any particular thing has all those things which define it as the particular thing because if it didn't have those things it wouldn't be that particular thing. A square always has four sides. If a human didn't have a soul it wouldn't be a human being because that's what human beings are. You can't say that one being gains a property that changes it from one being to another. All beings have the same profile of what they are and they gain and lose and develop properties over time.