 The third discovery was the elucidation of gene regulatory networks (GRNs). A GRN is based on paracrine factors, signal transduction cascades, and transcription factors. While ignoring post-transcriptional gene regulation, this concept attempts to explain how initial conditions (RNAs and proteins within the oocyte, position of the embryo within the uterus, etc.) could create the conditions whereby cell types differ, even though their genomes are identical. Spearheaded by Eric Davidson (2001, 2006), this approach uses systems theory concepts such as modularity and dissociability to explain how the genes interact in a hierarchical manner to produce different cell types (Levine &amp; Davidson 2005) and how the cell types in related species could differ by the recruitment (co-option) of a particular GRN by altering transcription factor binding. More generally, the discovery of GRNs has enabled the integration of developmental biology with paleontology (Jablonski 2017; Hinman et al. 2003; Hinman &amp; Cheatle Jarvela 2014) and may also be extended into areas of symbiosis and niche construction (Laubichler &amp; Renn 2015).