Origins of Life: Biblical and Evolutionary Models Face Off by Hugh Ross

Author:Hugh Ross [Ross, Hugh]
Language: eng
Format: epub
Tags: Christian
Publisher: RTB Press
Published: 2014-08-31T00:00:00+00:00

Contemporary Formation

At some point in the various naturalistic origin-of-life scenarios, cell membranes composed of phospholipids must emerge. This necessity is real whether the pathway that leads to the first contemporary cell membranes begins with primitive membranes comprised of simple amphiphiles, or whether the initial cell membranes appeared anew as phospholipid bilayers. Naturalistic scenarios that attempt to explain the formation of the first phospholipid-containing biological membranes face enormous difficulties.

Robust chemical pathways leading to phospholipids have not yet been identified. Plus, the first phospholipids likely possessed a tendency to form nonbilayer aggregates that could not perform as a barrier.

One can reasonably assume that the first phospholipid species on early Earth, and hence the first contemporary cell membranes, consisted of phosphatidylethanolamines (PEs) and phosphatidylglycerol (PGs). These two phospholipid classes have ethanolamine and glycerol as headgroups, respectively (see figure 11.2). PEs and PGs are the primary phospholipid classes found in bacterial membranes, and bacteria were among the first life-forms to appear on Earth.35

Because PEs and PGs stand as the dominant phospholipid species in bacterial membranes, origin-of-life scenarios must specifically account for the formation of biological membranes comprised of these two phospholipid types. Herein lies the problem. While phospholipids do self-assemble into bilayers, they also form nonbilayer structures. Phospholipids display rich and complex phase behavior. They tend to form specific aggregate types based on the head-group structure (see figure 11.2). Head-group characteristics determine a specific phospholipid’s overall molecular shape, which in turn dictates the type of aggregates it forms.

PEs tend to form nonbilayer phases, and in the presence of calcium, PGs also form nonbilayer structures.36 Cardiolipin (a derivative of PGs also present in bacterial membranes) likewise readily forms nonbilayer states. Finally, PGs’ presence in PE aggregates increases their tendency to form nonbilayer aggregates.37

Biochemists are uncertain of the biological significance of these nonbilayer structures, but they agree that if the nonbilayer structures do play a role in cell membrane processes, they must quickly pass in and out of existence. These nonbilayer phases compromise the cell membrane’s structural integrity and also its barrier function.38 Permanent or long-lived nonbilayer phases would rapidly lead to cell death.

In the early 1980s, Swedish researchers conducted experiments that highlighted the relationship between bilayer stability, lipid composition, and lipid shape.39 These workers showed that the bacterium Acholeplasma laidlawii adjusted its membrane’s lipid composition as environmental conditions changed. These changes preserved the proper lipid shape, thereby maintaining bilayer stability. If the lipid composition was not altered as environmental conditions changed, the bacterium’s cell membrane adopted a nonbilayer structure that led to cell death.

With respect to the origin of life and the emergence of the first protocell, calcium’s presence in early Earth’s environment and the tendency of PEs and PE/PG mixtures to form nonbilayer aggregates means that these phospholipids may not have readily formed bilayers, frustrating the pathway leading to the first cell membranes. Moreover, fluctuating environmental conditions and altered bilayer composition would have given early bilayers comprised of PE and PG the potential to transition to nonbilayer phases. This transition would have inhibited the origin-of-life process.

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