Cross reference: Steroids
"Lipids are a broad group of naturally occurring
molecules which includes fats, waxes, sterols, fat-soluble vitamins (such as vitamins A, D, E and K), monoglycerides, diglycerides, phospholipids, and others. The main biological functions of lipids include energy storage, as structural components of cell membranes, and as important signaling molecules.
Lipids may be broadly defined as hydrophobic or amphiphilic small molecules... Although the term lipid is sometimes used as a synonym for fats, fats are a subgroup of lipids called triglycerides. Lipids also encompass molecules such as fatty acids and their derivatives (including tri-, di-, and monoglycerides and phospholipids), as well as other sterol-containing metabolites such as cholesterol. "
"Amphiphile (from the Greek αμφις, amphis: both and φιλíα, philia: love, friendship) is a term describing
a chemical compound possessing both hydrophilic (water-loving) and lipophilic (fat-loving) properties. Such a compound is called amphiphilic or amphipathic."
In spite of the fact that both the inner and the outer surfaces of the cell membrane are hydrophilic (see: Membrane Interface), which should repel hydrophobic molecules such as lipids, lipids seem to have access to the hydrophobic inner layer. This allows them to migrate across the membrane without help from any sort of receptor. This suggests that the first molecules by which cells communicated with one another were lipids.
So, are the quorum sensing (see: Prokaryote Colonies) molecules lipids? If so, perhaps they originally evolved as metabolic energy storing molecules which diffused across the membrane and thereby served the secondary function of lubricating the cells' motion. As the concentration of these molecules built up outside the cell, the molecules still being produced inside the cell would have had more difficulty diffusing across the membrane because of the reduced concentration gradient. This build-up of the lipid quorum sensing molecules inside the cell could then have had an effect on the cells' metabolism without the necessity of an intervening receptor.
I'm going to look into this.