You are right, 8 cells doesn't qualify as human, too bad by the time you can tell if a woman is pregnant or not, it's a helluva lot more than 8 cells.
So... what constitutes as a human being? (my opinion in parenthesis)
Tzygote of Homo sapiens? (no)
8 weeks old embryo of Homo sapiens? (no)
12 weeks old fetus? (no. I don't think there's enough strucural ability to be human being here yet.)
24 weeks old? (getting on the limits here. This is the fuzzy ground, because technically a child born at this time could still survive and when it's born it automatically becomes a person, though physically it's the same when it's in utero. At the risk of ruining my own argument I say that yes, at this point it is possible that the fetus could be considered human being already.
A recently born Rattus norvegicus? (no, for obvious reasons)
A recently born Homo sapiens? (yes, newborn babies have a personality and some level of sapience, and they gain these abilities at some point during the last trimester of the pregnancy (my estimation). Empathy too, as they react to the feelings of people around them - maybe instinctively, but then what couldn't be called instinctual.)
Three years old Homo sapiens? (yes...)
Adult Pan troglodytes? (yes, as they have much the same mental, emotional and cognitive capabilities as a three-year old child.)
Adult Gorilla gorilla? (yes, same as above.)
By the way, yes, I do know that defining humanity with abilities can become a slippery slope. However, I do consider ability to sentience, personality and empathy pretty much the defining features of being a human being. Many (most mammals and birds at least) animals have personality (pet owners or wild life researchers would be quick to agree), but their sentience is questionable, as is their ability to empathy in most cases. Pack animal behaviour can perhaps be explained by instincts in cases like canines or bovines, or horses or lambs or such critters.
However, with species like great apes, their behaviour leads me to conclude that yes, they are "human beings" in the sense that they are sentient, have personality and ability to feel empathy (among other similar abilities like communication, using tools, abstract thought etc. etc.). Same applies to some cetacean species. I would consider shooting a chimpanzee a crime comparative to killing a two or three-year-old child. On a hand-to-hand fight though, chimpanzee would win... so it's just manslaughter. You couldn't, though. A grown healthy chimpanzee would probably rip right through Alexandr Karelin, Bruce Lee and Chuck Norris if it wanted to.
An embryo, and a fetus up to certain level, has none of these abilities. It's only ability for quite long is just to grow; then it starts to gain motoric functions, and at some point it obviously crosses a certain threshold where it's nervous system is capable of generating personality and some level of sentience. But like said, drawing an arbitrary line where the fetus can be considered human being is difficult and I would hate to be in a position of making such decisions, because whatever you decide you're always wrong.
