The Natural Science Underlying Big History
  Nature's many varied complex systems (including galaxies, stars, planets, life, and society) are islands of order within the increasingly disordered universe. All organized systems are subject to physical, biological or cultural evolution, which together comprise the grander interdisciplinary subject of cosmic evolution. This is global history greatly extended, big history with a scientific basis, and natural history broadly portrayed across 14 billion years of time. Such evolution writ large has significant potential to unify the natural sciences into a holistic understanding of who we are and whence we came. No new science (beyond frontier, non-equilibrium thermodynamics) is needed to describe cosmic evolution's major milestones at a deep and empirical level. Quantitative models and experimental tests imply that a remarkable simplicity underlies the emergence and growth of complexity for a wide spectrum of known and diverse systems. In particular, energy rate density is an objective metric suitable to gauge relative degrees of complexity along a hierarchy of widely assorted systems observed throughout the material universe. 
Eric J. Chaisson
physics.pop-ph astro-ph.CO q-bio.OT