We provide a novel, unifying physical interpretation on the origin, the average shape, the scatter and the cosmic evolution for the main sequence of star forming galaxies and AGNs at high redshift (z>1). We achieve this goal in a model independent way, starting from redshift-dependent SFR functions based on the latest UV/ far-IR data from HST/Herschel and related statistics of strong gravitationally lensed sources (see for details Fig. 1 in Mancuso, Lapi et al. 2016a, ApJ 823,128). We also used deterministic evolutionary tracks for the history of star formation and black hole accretion, inspired by the in situ co-evolution scenario (see Fig. 2), and gauged on a wealth of multiwavelength observations including the observed Eddington ratio distribution. We validated our ingredients against many observables (see Mancuso, Lapi et al. 2016A, ApJ 823,128). The resulting Main Sequences are in excellent agreement with current observational data. We also provided a new interpretation for off-main sequence galaxies and AGNs in terms of the in situ coevolution scenario.