The cell, initial metabolism, and minimum mass

Let the origin of life be replicating molecules with no internal metabolism, practically no mass, and a replication that is driven directly by an extrinsic source of energy. This replicator may evolve into a small self-replicating cell by the evolution of an internal metabolism that is able to generate the essential net energy that is required for self-replication.

To show this we note that the initial metabolism of the self-replicator is dependent upon the mass of the molecules in the metabolic pathways, and on the mass of the heritable code that codes for the self-replicator and its metabolic pathways. An organised metabolism is also dependent on a compartment - like a cell – where the metabolic molecules can concentrate. Hence, when comparing across variants with similar mass specific metabolism (w_|β) we find that the quality-quantity trade-off

∂ r / ∂ ln w_|β = - 1

is selecting a partial attractor (w_β^*) that has a minimum cell with the minimum mass that is required to sustain the given mass specific metabolism (β).

When looking along these partial attractors with minimum masses we find a rate of mass specific metabolism that increases with an increase in minimum mass

∂ ln β / ∂ ln w_β^* = β_β^•

with the degree of increase in metabolism (β_β^•) declining from an initial value (β_β,0^•) towards zero as larger cells have more fully developed metabolic pathways.

Now with fitness given as r = ln [p α β τ / w], and ∂ r / ∂ ln β = 1, we may combine the dependence of metabolism on minimum mass with the selection of the quality-quantity trade-off

∂ r / ∂ ln w_β^* = [ ∂ r / ∂ ln β ] [ ∂ ln β / ∂ ln w_β^* ] + ∂ r / ∂ ln w_β^*

and obtain an overall selection of mass

∂ r / ∂ ln w_β^* = β_β^• - 1

Consider now a line of replicating molecules where the initial dependence of mass specific metabolism on mass is stronger than linear [ β_β,0^• > 1 ]. The selection will then proceed towards an equilibrium [Fig. 1, ∂ r / ∂ ln w_β^* = 0 ], where the dependence of mass specific metabolism on mass has declined to a linear dependence [ β_β^• = 1 ]. These selected self-replicating cells are so small that they will have an underdeveloped metabolism in the sense that it is biochemically possible to increase the metabolic efficiency per unit mass with an increase in mass.

Evolution will take a different direction in a line of replicating molecules that have a biochemical constellation where the initial mass specific metabolism is increasing sub-linearly with mass [ β_β,0^• < 1 ]. There will then be no selection of a metabolism, mass, and cell; but only a sustained selection for the evolutionary maintenance of replicating molecules with no internal metabolism and practically no mass.

Fig. 1 An initial dependence of mass specific metabolism on mass that is stronger than linear will generate selection for a self-replicating cell with a small body mass. This selection is frequency independent, which implies that the intra-specific fitness landscapes (left) resemble the evolutionary attractors of the selection integrals (right). From Witting (2017).

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References

• Witting, L. 2017. The natural selection of metabolism and mass selects lifeforms from viruses to multicellular animals. Ecology and Evolution 7:9098--9118, https://dx.doi.org/10.1002/ece3.3432.