The magic of life and death
Life has to meet a number of criteria such as mobility, metabolism, growth, independent reproduction (requiring information-carrying molecules like DNA or RNA), integration and regulation. Although the last two properties are also present in single-celled creatures, they reach the pinnacle of their development in the evolution of the nerve cell.
Separately, no single criterion specifies life. Flowing water moves, rusting iron metabolises, a crystal grows, and nowadays many young people decide that they can have a better life without reproduction. Integration and regulation can also be programmed in a computer. To define life, the combination of all these criteria must be present.
At the other end of the scale, doctors have been establishing death for centuries by the absence of a heartbeat and respiration, and by calculating that these functions will not return.
We have always been taught that nerve cells are extremely susceptible to a lack of oxygen: irreparable brain damage occurs after just four to five minutes without oxygen. This is true, but apparently it is not because nerve cells are so sensitive to a shortage of oxygen.