This was my comment.I think there is a trap when we try to answer this question. Actually it is the same trap that makes a lot of philosophical question endless, without any progress, or even oppressive.
The trap consists of looking for a conclusive answer, something stable, reasoning with a mentality oriented towards static concepts. Since we are immersed in history, which means a lot of epochs and a lot of places, any answer is easily exposed to be demolished, criticized or, as I said, it becomes just a sterile endless discussion.
We know that, over time and according to different places of our planet, a lot of different ideas, even opposite, conflicting and oppressive ideas have been kept as stable about what means to be human, or a person, but the question can be extended to everything: what is truth, what is freedom, and so on.
So, I think the best answer to your question is a methodology of work, with some criteria like the following ones:
1) as I said, try not to fall into traps of static thinking
2) which means: let’s work on provisional answers, and then work again, and then again and again;
3) in this work let’s use the best resources we have: dialogue, space for opposite perspectives, welcoming criticism, research, space for science and for criticism of science;
4) let's make decisions, but they must be always considered provisional, temporary decisions.
So, today I would say: what is essential to being human being is to be perceived as human by other humans. It seems circular, but I think it is not: I think that, as a starting point, everybody assumes they are humans, so, let’s consider humans those you think are humans, by using your sensitivity, history, culture, science.
The question involves the huge debates about abortion: by considering this we can realize, again, how sterile it is to look for definite conclusions.
My answer is aimed at opening discussion by helping towards perspectives, not to be a conclusion.
I would add that we need to make a good use of subjectivity and objectivity in order to work well on it.
I would like to add here a reference to the recent attention that Google's episode about Lamda and sacking the engineer Lemoine for saying that Lamda is sentient. My comment about this, half seriously, is that Lamda is so intelligent that it already shows the same flaws of today’s philosophers, that is, a mentality strongly based on static concepts.