It seems like a no-brainer, that we are humans by birth and until death. But perhaps we can turn out to be a whole variety of things. Take the Oxana Malaya for example.
Oxana was abandoned at 3 years old and was raised by wild dogs until she was 8 years old. In the video, she is actually 22 years old, and is showing what she "learned" and how she lived. She is what psychologists call a feral child, "one who, from a very young age, has lived in isolation from human contact, unaware of human social behaviour and unexposed to language," (Grice, 2006). It's easy to see why these feral children are an important part of the nature versus nurture debate. At the time the article cited above was written, Oxana was living in a community living home for the mentally disabled. She had to re-learn how to walk, eat, and speak.
Do you consider Oxana a human being? In my opinion, she isn't completely human. And when she was found, I don't think she could even be considered human. She has been so far removed from the psychological norm for humans, that I don't think she can ever be human like us. While nature is definitely a strong factor in creating us as individuals, the power of nurture should not be underestimated.
If the case of Oxana Malaya interests you, I suggest reading the article I have linked. And if you are interested in other cases of feral children, there is an entire National Geographic special on youtube. Part 1 is found here.