According to Buddhist teachings consciousness enters at the moment of conception. So from that moment a human being has been created.
As to the ending of life, we have to be really careful not to take on science's ideas of the ending of life. They have this idea of being "brain-dead" therefore the being is dead, but according to Buddhism, the person may not be dead at that time.
The consciousness may leave the body if there is a wish to become and then it seeks another body. However the consciousness may still be around the body. That is, the person may be brain-dead but they may not have let go of their attachment to the body at that time. The consciousness may be significantly affected by the surroundings and what people are saying about the body. They may be even stuck in that realm at that particular moment not wanting to leave the body, not going on to the next life. So if this is true, we have to be very careful about what we do around a body that is supposedly dying or dead, as it may not be dead.
I am not an organ donor for this reason, I would prefer that if I am still around the body, that the medical professionals don't start cutting it up. As it may actually agitate me. I don't know what level of non-attachment to the body I may have at that time, so I would prefer that they don't interfere with the body in any way. In fact, in some Thai ceremonies, they won't let people interfere with the body for 100 days or so. They will often perform a lot of chanting of Buddhist texts just in case the consciousness is still there, hanging around the body. Also people will carry out a lot of merit making activities to remind the person of their good Kamma, good deeds and good thoughts to generate good feelings to possibly reduce the level of suffering.. So I would prefer to go with the Thai traditional approach and be very careful about what you do with the body, showing more respect to the body until time for cremation.
There have been stories about Thai teachers who have psychic powers, who can actually see if a being is stuck in a ghost realm, unable to get on to the next realm because they have too much attachment to this one.
One of our Thai teachers told us this story to illustrate the importance of letting go of attachment and using wise reflection at the moment of death. There was a monk who had been doing a lot of concentration type meditation when he was younger and had developed the capacity to see invisible beings or Devas. He was staying at a monastery close to a small village and in the village was an old man in his late seventies, who was not very well. The old man decided that he wanted to make some merit before he died so he came to help build a new hut at the monastery for the monk to live in.
It gave the man a lot of joy and on completion, they decided that it would be good to build a toilet to go with the hut. However, when the toilet was only partially built, the old man got very ill and unfortunately died. Well the monk thought, this old man has been a good Buddhist, always kept his precepts, he made merit by building a monk's hut, so he's probably been reborn in a heaven realm or a good human birth. So the monk decided to go into meditation and use his concentration powers to see where the old man had been reborn.
That night he concentrated his mind, developed the capacity to see invisible beings and he was shocked! He could see that the old man was a ghost, hanging around the unfinished toilet, looking very upset. The monk said, "What are you doing hanging around the toilet, you should be in a heaven realm?" The old man said, "But the toilet isn't finished yet".
The teacher lost his concentration, he was so shocked, and came out of meditation. The next day the monk went around to see the old man's relatives in the village and told them what he had seen. Obviously the relatives were quite upset that the old man was a ghost hanging around a toilet. So they wanted to help him. So they got all the able people in the village to go to the monastery and finish building the toilet that day. At the end of the day, they asked the monk to go in meditation to see if it helped the old man. So the monk meditated that night, went into a high concentrated state, developed the power to see invisible beings and again he was shocked. The old man was still a ghost hanging around the toilet! However, this time he had a big smile on his face admiring the toilet. So the monk said, "What are you doing, reflect on your good Kamma and go on to your next life". With that the man started to disappear and went to his next life.
The teachings say that if we are terribly attached to the body, just after death, we may be hanging around it and wondering what doctors, etc. are doing with it. So it would be helpful to be able let go of our attachment to our body, just in case our body happens to get into the hands of people that don't know our wishes.