that brought the world into the modern era What will happen in the future and predicting what will be the next threshold in our history Goldilocks conditions A theme in Big History is what has been termed Goldilocks conditions or the Goldilocks principle, which describes how "circumstances must be right for any type of complexity to form or continue to exist," as emphasized by Spier in his recent book. For humans, bodily temperatures can neither be too hot nor too cold; for life to form on a planet, it can neither have too much nor too little energy from sunlight. Stars require sufficient quantities of hydrogen, sufficiently packed together under tremendous gravity, to cause nuclear fusion. Christian suggests that complexity arises when these Goldilocks conditions are met, that is, when things are not too hot or cold, not too fast or slow. For example, life began not in solids (molecules are stuck together, preventing the right kinds of associations) or gases (molecules move too fast to enable favorable associations) but in liquids such as water that permitted the right kinds of interactions at the right speeds. Somewhat in contrast, Chaisson has maintained for well more than a decade that "organizational complexity is mostly governed by the optimum use of energy—not too little as to starve a system, yet not too much as to destroy it". Neither maximum energy principles nor minimum entropy states are likely relevant to appreciate the emergence of complexity in Nature writ large. Other themes Advances in particular sciences such as archaeology, gene mapping, and evolutionary ecology have enabled historians to gain new insights into the early origins of humans, despite the lack of written sources. One account suggested that proponents of Big History were trying to "upend" the conventional practice in historiography of relying on written records. Big History proponents suggest that humans have been affecting climate change throughout history, by such methods as slash-and-burn agriculture, although past modifications have been on a lesser scale than in recent years during the Industrial Revolution. A book by Daniel Lord Smail in 2008 suggested that history was a continuing process of humans learning to self-modify our mental states by using stimulants such as coffee and tobacco, as well as other means such as religious rites or romance novels. His view is that culture and biology are highly intertwined, such that cultural practices may cause human brains to be