The origins of bacteria

The primitive oceans had the consistency of a diluted broth, and it is likely that their organic molecules were used by the first cells as nutrients. Even such a large food store, however, was inevitably destined to become extinguished, and this created the conditions for the appearance of two very different survival strategies. Some cells adapted their metabolism to smaller and smaller starting molecules, and eventually learned to perform all metabolic reactions from inorganic compounds. In this way they ceased to be consumers, and became producers of organic matter (and when this happened, the 

There were, in other words, two possible reactions to the alimentary crisis produced by dwindling organic sources, and the descendants of the first cells explored both of them, thus giving rise to two divergent evolutionary lines. The interesting thing is that we can reach this conclusion even by a totally different route. 

Let us put aside for a moment the food problem, and let us assume that the descendants of the first cells had to cope with a transformation of the ancient potassium seas into sodium-dominated oceans. In this case the threat was represented by the osmotic effects of sodium, and cells could counterbalance it in two ways either by developing a rigid external wall that prevented the cell membrane from swelling, or by building an internal net that could bind the plasma membrane from within. Each solution, however, could be realised only under certain conditions. 

A rigid external wall would let only small molecules through, and could be adopted only by cells which managed to survive with small metabolites. An internal net of filaments that tied the plasma membrane was a more complex solution, but those cells that were incapable of surviving with small metabolites had no other choice. They could not adopt the external wall solution, and could counteract the sodium osmotic pressure only by anchoring the cell membrane from within, with a network of filaments. 

As we can see, the evolution of two divergent cell lines could have been provoked either by a food crisis or by an osmotic danger, or even by both factors. We conclude therefore that the first dichotomy 

---

Gaucher, E. A., Thomson, J.M., Burgan, M.F., and Benner, S.A. 2003. Inferring the paleoenvironment during the origins of bacteria based on resurrected ancestral proteins. Nature 425 285-288. 

---