In thermodynamics, the first law of thermodynamics is the principle of the conservation of energy applied to any thermodynamic system. [1] In short, the first law defines the equivalent relationship, between the heat input Q to a thermodynamics system to that of the change in internal energy U of that system and the work output W produced by the system. The original first law, introduced by German physicist Rudolf Clausius in 1850, states that when, according to Boerhaave’s law, a unit of heat is added to a body, it will cause the body to expand, where the expansion can be quantified as the body pushing out a region of new volume V against the weight or pressure P of the atoms and molecules of the surrounding atmosphere of the earth, and that the change in the energy of the body dU will be amount of heat added less the work one:

This is always the basic starting point of the first law. Other terms are added on as system analysis becomes more detailed or involved. Knowing, for instance, that W is pressure-volume work PdV, with substitution we have:

To note, after circa 1875, through the work of German mathematician Carl Neumann, differentials which are inexact, or path-dependent, such as heat Q or work W, began to be signified using the small Greek delta δ or the d-hat đ symbol, among others.

Carnot cycle model of Earth
Each day, due to the configuration of the solar system, biospheric portions of the earth's surface, during its rotation, are put in contact diurnally with a hot body (the sun) and cold body (the night sky) on an alternating basis, according to which heat Q flows through various partitioned off human social systems, e.g. one small city, that each function as "working bodies", i.e. any partitioned off system through which heat may flow, of molecular species (e.g. a set of human species). In the human point of view, during each cycle, work-output is produced cyclically through the operation of economic, socially-mediated, substrate-attached, human molecular interactions in the form of multiple coupled social heat engines. [2]

Clausius notes
In the words of German physicist Rudolf Clausius, "whenever an indefinitely small quantity of heat dQ is imparted to this body, the question arises what becomes of it, and what effect it produces?" [3] It may serve in part, according to Clausius, "to increase the amount of heat actually existing in the body; in part also, if in consequence of the imparting of this heat the body changes its condition, and that change includes the overcoming of some force, it may be absorbed in the work done thereby." If we denote, according to Clausius, "the total heat existing in the body, or more briefly the quantity of heat in the body, by H, and the indefinitely small increment of this quantity by dH, and if we put dL for the indefinitely small quantity of work done (by the body)", then we can write:

dQ = dH + dL

The forces against which the work is done by the working body (e.g. a system of people, who each may be considered as individual human molecules), according to Clausius, "may be divided into two classes: (1) those which the molecules of the body exert among themselves, and which are therefore dependent on the nature of the body itself, and (2) those which arise from external influences, to which the body is subjected. According to these two classes of forces, which have to be overcome, the work is divided into internal and external work." If we denote these two quantities by dJ and dW, we may put:

dL = dJ + dW

and the foregoingequation becomes:

dQ = dH + dJ + dW

This outlines the essential first law analysis for any working body, in the words of Clausius, which thus serves as the starting basis for the thermodynamic analysis of human life.

References
1. 30+ Variations of the First Law of Thermodynamics - Institute of Human Thermodynamics, Chicago
2. Thims, Libb. (2007). Human Chemistry (Volume Two). Morrisville, NC: LuLu.
3. Clausius, Rudolf. (1879). The Mechanical Theory of Heat, London: Macmillan & Co. (second edition), original.