Footnote on Fat
* more on the topic: Fat!
Fat is incredibly important for life of all forms: it prevents us from mixing with our environment and thus enables “life”. But some passage must be possible: oxygen and nutrients need to be taken up and salt levels adjusted. Therefore the permeability of the membrane, which separates inside and outside, is adjustable.
In humans membrane permeability is regulated by cholesterol. James showed that bacteria, who lack cholesterol, use hopanoid as a functional analog of cholesterol. And they might have already done so since 3.5 billion years of earth history!
Read more in his publications, for example his open-access paper in PNAS here.