Genomic sleuthing spots early divergence After fertilization, the human zygote divides into two cells. Fasching et al. used genomic analysis from cellular samples taken much later in development to back-calculate the cell division trees that went before. Although the first cell division in human development looks symmetrical from the outside, the fates followed by daughter cells from each of those first two blastomeres are anything but the same. As much as 90% of blood cells are derived from just one of the first two blastomeres. Science , this issue p. 1245