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If you have a car and throughout the life of that car you gradually replace bits of it as they breakdown until you get to a point 20 years down the track when you have replaced everything except the radiator cap, is it still the same car? In one sense yes, but in another sense no. The identity of the car is based upon our perception of continuity and same is true for being human. If you were able to replace every part of the body throughout one's lifetime there would still be a perceived sense of continuity by the self and by the observers and so still regarded as human.
The lay concept of human is to a large degree based upon perception rather than scientific analysis. However, if one uses a scientific definition of human, the argument then shifts to whether the one remains the same "person" when every part is replaced. Again this is more about perceived continuity.
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