The moment where two haploid gametes come together and form a diploid zygote.
Some people would have this be where life
starts. In actuality, life started 4 billion years ago.
Beleiving a haploid cell to be less human than a newborn infant has made a big mess out of abortion issues.
This moment is of interest, because it is when two sets of genetic information are merged into a new reproductive unit. At this point the physical and genetic aspects of a individual are for the most part fixed. The personality may not start to develop until around birth.