There isn't another line in life except the evolutionary imposed one

For centuries people have always asked themselves what is it to be human, and for centuries they have always been getting it wrong because of not having any good education in biology, so let's get out all the wrong definitions first.

Wrong
1. A human is a cell after fertilisation from human parents
Well, first of all, a human is not just made of solely cells, also present are various chemicals, as well as proteins, antibodies, water, and lots of many other components. Also when considering what a cell really is, what happens when a cell divides and reproduces 2 cells? Is each one a separate human? Or are they together considered one human? If we were to split up the two cells, they can become two humans can't they (identical twins)? How do we draw the line between one human or many humans? All these questions would perhaps never be answered, not because biology can't answer them, but because biology describes the process, but doesn't distinguish anything along the way.

2. A human is a being shaped like a human
Obviously this definition have a huge problem, not considering robots and all other fanciful things we can make up, in a life cycle of a person, the person takes on many different shapes and sizes. While on the other hand a person can die of permanent vegetative state, and we wouldn't want to call that human. No, a human must carry the essence of what it means to be alive.

3. A human is whatever has the potential to be a human
This is perhaps the most ridiculous definition that I have ever came across. First of which, almost anything has the potential to be human, including cells of any species of mammals, through a process of transplanting the nucleus. Perhaps the worst way to argue this is to say that a cell has the potential to become a great person, and so we can't kill it. Well, many other cells have that same potential and they die by the trillions, perhaps even magnitude more, each day. Anyway, doesn't all the cells have the same potential to be a, hmm, lion or tiger?

4. A human is a person capable of reproducing with a human
This is the standard biological/taxonomical definition of what a species is. To biologists, Homo sapient as a species is just a linage that have branched out from chimps and early Homo ancestors hundreds of thousands of years ago and have the property to interbreed with one another, but not with other animals. One of the great drawbacks of this method is that any person have to first reach reproductive age and then attempt reproduction for the standard to be ascertained.

One thing that the above definitions all share is there is no standard to compare what a human is in the first place. They assume that there have never been a first human, in effect comparing humans with, well humans. For all purposes, the term human that we should use, should be one that can be practically put into use in game theory, that means discarding the notion of biological appearances and description, and work at a whole different level of biology.

Therefore, the new candidates for qualities of humans as far as I can come up with are:

1. Able to hold responsibility
Responsibility is perhaps one of the greatest contributing factor to living in society, a quality even chimps understand, but not foetuses. Any animal taking at least a minimal personal responsibility can be considered human, at that point in time. The title of human however is not permanent and can be easily lost any time.

2. Able to reciprocate to an action made by anyone else
Another property of a society is that individuals can have a certain developed way of depending on others, necessary for survival. A human must be able to have the intention of repaying or retaliating to either a friendly or hostile response made by, not just individuals in one's own species, but by any other thing.

So evolution have forced us to draw lines to judge what qualifies to be human, in the end all these lines are just arbitrary. In the eyes of nature, no divisions exist between any species of life, but due to the need for survival, the products of nature are required by the mechanical forces of nature, namely survival, to draw up the lines for themselves.