There are experiences that humans have that no other animals have on Earth. We are capable of abstract thought, building monuments, going into space and humans can live on any continent. We love, hate, argue, hope, fear and have faith in our own abilities. It is possible to define what it means to be human, it's just that there are 7 billion different definitions.
Yes, you can define what it is to be human because humans have characteristics and can be identified by their characteristics. All you have to do is say what the characteristics of the human are and they will be identified for who they are. Humans have specific characteristics that you can identify them by.
There are well-defined scientific definitions that separate one species from another. While there may be moral or ethical questions regarding humanitarian characteristics and things of that sort, the strict definition of a human, or homo sapien, is easily established.
This definition is based not on race, sex, creed, creativity, potential, etc, but rather on biological physiology. A species definition is cut and dry, you are either a human being or you are not.
What it means to be human may never be truly definable because as the time passes so will what that definition. We as humans have always been sporadic, one minute advocating peace and rights for all and the next war and genocide. Being human is generally defined as the ability to reason but as humans we usually let our emotions get the best of us and lead our opinions. So, what is being human is it really something so simple as being apart from as a species or something else deeper and meaningful the world will NEVER agree.
I do not believe it is possible to define what it means to be human. Due to different personality traits, everyone is "human" in their own way. With everyones differences in opinion, how can you possibly say someone is more human than the other? Because of this, I don't think it can ever truly be defined.
We can define as a human what we think or feel our purpose is to be here or what we think or feel it means to be human but the truth is there is so much more to being human I do not think we could ever define it. Even if we begin to come close if it upsets any of the religious cultures we have to back away from the idea and move on. Until we all have a desire to learn more about where we came from we will never be able to define what it means to be human.