4 potential (totipotent → pluripotent → multipotent → unipotent cells) accompanied with an increase in cell specialization. Certain types of cells can be generated following more than one path from the fertilized egg. This process generates some closed loops of edges in the network. The NHCD comprises 529 branches of different lengths with each branch ending when the cell types do not undergo further differentiation. Note, however, that the most recent compilation of cell types in normal, healthy, human adults done in [6] reports only 407 cell types. Therefore, not all branch endpoints correspond to cell types in born humans. Thus, not all 873 cell types are present in a human being. Among those absent are the placenta cells that are generated from the fertilized egg during embryo development, as well as other somatic cell types that are important to control embryo and fetus development. The cell types that survive in a human are denoted by filled circles, while non-surviving ones are indicated by empty circles. The complete collected data is listed in the Supplementary Information, including an enumeration of cells and links between the cell types, their time of appearance in days after fecundation ( T$_{a}$ ), and the reference to the publications reporting each link. To the best of our knowledge, the structure identified here provides the most complete schematic diagram of the human differentiation process to date. It is visually apparent from Fig. 1 that the NHCD has a prominent modular structure. The continuous differentiation of cells into more specialized functions naturally leads to the formation of dense isolated clusters in the NHCD. As a first approach to understand this modular structure we cluster cell types in the network of Fig. 1 according to their known functions; different colors indicate 19 functional modules extracted from the literature ( C 1 − C 19) (See SI-Table I and references therein. The largest communities were extracted from Refs. [1–6, 14–28]). There is, however, a certain degree of arbitrariness in this modular structure as the separation of the nodes into communities in our dataset is not unique. For instance, community C 12, the neural lineage, could be divided into two sub-communities, representing the neural and the supporting (glial) cells [1–3, 5, 6, 14, 15]. On the other hand, the neural system module could be merged with the eye system module [1, 5, 6, 14] on a larger scale, since they have a common ancestral cell type. Therefore, a finer or coarser community structure can be extracted from the data when we look at the whole network at different scales of observation; a novel module-detection algorithm is needed to identify these communities in a systematic way.