Let me explain: In science, we have these things called the 7 Characteristics of Life. If a thing exhibits all 7 of these characteristics, it is considered a living organism (Side note: the reason viruses are not considered living things is because they do not exhibit all 7.)

So what are they?

1. Living Things are Composed of Cells.

Is a just-fertilized egg composed of cells? Yes it is; it is one cell. Check.

2. Living Things Have Different Levels of Organization.

Does a just-fertilized egg have different levels of organization? Yes, it has a nucleus, genetic material, mitochondrion, endoplasmic reitculum, and more! Check.

3. Living Things Use Energy.

Does a just-fertilized egg use energy? Yes, as mentioned before, they contain mitochondrion and in these mitochondrion, energy is created in the form of Adenosine-TriPhosphate (more commonly known as ATP) which is transported throughout the cell where its stored chemical energy can be harnessed. Check.

4. Living Things Respond To Their Environment.

Does a just-fertilized egg respond to its environment? Yes, although it would be unethical to try (though I’m sure it has been done), I can guarantee that it would respond to environmental changes, such as changes to pH, salinity, or temperature. (Ask me how if you’re really interested, I am). Check.

5. Living Things Grow.

Does a just-fertilized egg grow? Duh. It grows, divides, and multiplies and will start to look like me and you. Check.

6. Living Things Reproduce.

Could a just-fertilized egg reproduce? Oh snap, you think you’ve got me here, don’t you? Well, the question is asking if at some point in the life cycle of this thing, would it be able to reproduce? You wouldn’t consider a tadpole not a living creature because in its current state, it cannot reproduce, would you? Are kids or old people not living? Of course they are! Aside from abnormalities, the general answer is, if allowed to grow and flourish, this newly-conceived form would most likely be able to one day reproduce. (And note, this question isn’t asking about one specific fertilized egg, it’s asking about all eggs, and that organism in all its forms, such as its reproductively-capable later form.) Check.

7. Living Things Adapt To Their Environment.

Lastly, does a fertilized-egg adapt to its environment? Yes, just as I mentioned when it responds to its environment, it can also adapt accordingly, for example, it can change the composition of its plasma membrane in response to changes in temperature. Check.

So there you have it. Even at its newly-fertilized, single cell form, it is a life form. But is it human?

Of course it is, but how do we tell? It doesn’t look human. It doesn’t sound human. But if you look at the number of chromosomes in the cell, (most of the time) it has 46, 23 from its mother and 23 from its father. Why, that’s how many you and I have! Furthermore, if you were to take the genetic material contained and decode it, it would be unmistakably human.

Ergo, at conception, you have a human life form.