That’s not a rational approach towards conception. Fertilization of an ovum is a far more reasonable line between “cell” and “developing human”.
By the time implantation would occur, a unique composite genome has already formed, and many cellular divisions have already taken place, effectively creating a new and unique organism. Quite frankly, that’s life.
That’s life far more complex than you’re going to see in any prokaryote. Implantation is merely a a phase of that organism’s life, one it may complete or not (such as gestation, infancy, childhood, college, etc.).
Any dividing line drawn significantly after conception is really unsupportable. Some dishonest or easily influenced individuals may point to the high rates of human miscarriage, and the significant number pregnancies that end without implantation successfully taking place, but that’s a truly illogical argument with no comparable basis in fact - the fact that a child may suffer from SIDS, or that historically the MAJORITY of children born never reached adulthood, does not imply that a unique individual person did not exist prior to leaving infancy or childhood.
The rate of miscarriage or failure to implant is utterly immaterial to the argument of whether or not life has begun. Just as the historical childhood mortality rate is immaterial to the argument of whether or not a child is a human being.
The question of whether it has a spirit, a soul or a mind may not be possible to answer with science but science does answer, without equivocation, that it is an individual living human being from the moment of conception or at least very shortly after when the two half-strands of DNA combine.
You cannot point at *any* event and say that is the start of life, because there is no such point. Life began billions of years ago, and we all exist on the continuum of that original life. The sperm and egg are alive before they fuse to become a zygote. As a scientist, I have grown millions, maybe billions of human cells for experimentation in the lab. They are alive. They are human. They are not, and will never be, human beings. In some cases, they even form higher level structures very much like tissues in an intact organism. But this low level of differentiation is not sufficient to make those cells into a person.
I see a number of things that people almost worship, which are hardly even significant. They worship the fusion of sperm and egg, as if cell fusion is some magical event--it is not. Any two cells can fuse. Also, it is possible to take stem cells and induce them to start embryogenesis--without any kind of fusion event taking place. A person resulting from that process would still be as human as anyone else. They worship the new mixture of DNA, as if it is something really special--it is not. Unique configurations of DNA are all around us, and mean quite little as far as personhood is concerned. If a thousand people were cloned from identical cells and all had identical DNA, they would still be separate people regardless of their lack of uniqueness.
I can go on and on, because I have observed first hand that human + alive does *not* equal human being. The defining attribute of a human being is that it has a nervous system that enables it to sense and interact with the world around it. The time that occurs has been thoroughly observed and documented. The nervous system starts to form with the folding of the neural tube at 3 weeks after conception, and sometime between 3 and 5 weeks, that nervous system takes over control of the body and begins sensing and responding to its environment. At that time, what was merely human and alive and not significantly different than the millions of cells I have grown for research becomes a human being.