TITLE:  Origin of the First Cell Membrane?
      AUTHOR:  MADDOX, JOHN
     JOURNAL:  Nature
    CITATION:  September 8, 1994, 371(6493): 101.
        YEAR:  1994
    PUB TYPE:  Article
 IDENTIFIERS:  EVOLUTION; CELL MEMBRANES; BIOCHEMICAL CONSERVATION; 
               BIOCHEMISTRY; ORIGIN OF LIFE; TERPENOIDS; FOSSILS
    ABSTRACT:       Questions regarding the origin of life can be broken 
               into several parts. One required component for life is the 
               presence of self-replicating molecules of some kind. Life as 
               we know it, however, involves a blend of distinctive chemical 
               components that are separated from their surroundings by an 
               external membrane. There have recently been some intriguing 
               experiments showing that such systems can be fashioned from 
               quite simple materials.
                    Information regarding what existed on Earth more than 
               three billion years ago may be gleaned to some extent by 
               determining what elements of biomolecules have been strongly 
               conserved across species. Scientists Guy Ourisson and Yoichi 
               Nakatani, from the CNRS laboratory for the organic chemistry 
               of natural products, contend that they have been able to 
               infer, from the constituents of the membranes of extant 
               cells, that terpenoids were plentiful among the constituents 
               of the first cell membranes.
                    Cell membranes are typically described as phospholipid 
               bilayers which are reinforced by other polar molecules, 
               usually cholesterol, incorporated in the structure. This is 
               not the case, however, in bacteria and archaebacteria. In the 
               former, cholesterol is replaced by hopanoids, and, in the 
               latter, the whole phospohlipid bilayer is replaced by 
               molecules on which phosphate groups are anchored to the two 
               ends of a branched hydrocarbon structure through ether bonds 
               and glycerol residues, so that (with the two hydrophilic ends 
               being pulled in opposite directions) no reinforcement is 
               needed. Traces of these molecules are common among the 
               compounds of high molecular weight recovered from sediments 
               and from petroleum.
                    Ourisson and Nakatani constructed a tentative 
               evolutionary hierarchy for the terpenoids in their fossil 
               record, beginning with the making of and then polymerization 
               of isopentenol units (possibly attached at one end to a solid 
               surface through a phosphate group). These polymers might then 
               get long enough to form a piece of membrane and enclose 
               nearby materials which would inevitably include chemical 
               components required to synthesize the isopentenol units. The 
               two scientists are designing experiments to test this theory.
                    But the manufacture of ingredients of early membranes is 
               not the only goal of the scientists. The polymerized 
               isopentenol units provide just the kinds of structures needed 
               to form the polycyclic terpenes. So there is a prospect of 
               being able to use the mechanical reinforcing molecules in the 
               membranes of bacteria eukaryotes as a means of telling when, 
               in the course of evolution, the synthetic pathways required 
               for making more complicated reinforcements were recruited to 
               the biochemical repertoire.