A lot depends on the answer to this question. The conventional answer, probably shared by most of academia and the media and much of the modern world, is that a human being is an animal that has evolved on this planet, and probably only on this planet. According to this view, life itself began by chance, and then we evolved over millions of years, through many stages, from the most primitive life-form, through fish, land based-vertebrates and primates. We are top of the evolutionary tree and – this is the controversial part – we are the most intelligent species on this planet. The same view also believes that we do not exist before conception, nor after the death of our body. There is more to the answer, but I think I have covered the essentials. If this is what you believe, then you almost certainly subscribe to the materialist worldview, in which the physical reality is the only possible reality. It is significant that chance and absence of deeper meaning play a large part in this worldview.
There is another, very different, answer to the question. At its heart is the belief that we are primarily spiritual beings, and that we incarnate on to the physical plane for particular purposes. There is nothing meaningless about this, and chance plays no part. It follows from this that birth is incarnation and death is “decarnation”, i.e. shedding the physical body. But there is much more within this answer! For example, because we are spiritual beings, we have spiritual attributes and we are connected to spiritual planes. Perhaps it is more accurate to say that we inhabit a Cosmos that is both spiritual and material, and that we exhibit, and experience, aspects of both.
One major implication of the second answer is that so-called “paranormal” phenomena and “spiritual” experiences are alien to those who believe in the first answer. It is because they subscribe to the materialist worldview, and to the science that flows from this, that they believe paranormal and spiritual phenomena are impossible and should be ignored. Yes, it is true that they seem impossible, but only if your beliefs are rooted in the metaparadigm that the Cosmos is physical, and only physical. If, however, you really do experience yourself as a spiritual being and feel that you are living in a world that is both material and spiritual, then much more is possible, including the “paranormal”.
I should add that there is some significant overlap between the two answers, between the two versions of the human being. There is much evidence that our physical bodies from primitive life forms. That said, there are some important missing links in the chain of human evolution. As for science, it has many useful things to say about the physical aspects of the Cosmos, but absolutely nothing to say about the spiritual aspects. The mistake scientists make is to believe that the physical Cosmos is one and the same as the whole Cosmos. However, this is changing, on two fronts – the bizarre findings of quantum physics (such as “entanglement” and “non-locality”); and the emerging science of consciousness. Although most people are unaware of this, science is going through a major transformation, and there is a small, but powerful movement (e.g. the Galileo Commission) to “expand” science, so that it can accommodate all those phenomena that it currently ignores or tries to explain away. A “science of the whole” is emerging, in which the physical and spiritual aspects of the world and the human being will be seen, and explored, as integral parts of a unified whole. There will come a time, I believe, when most of the world will believe in the second answer to the question: what is a human being?