The problemIn many models of abiogenesis which support primitive replicators the smallest organisms have a significant advantage and rapidly come to dominate the system.
The effect can be seen in artificial chemistry simulations, cellular automata and self replicating programs - as well as in real systems.
A classic example of this "survival of the smallest" effect in a biological system is the [Spiegelman Monster] - a short RNA molecule capable of self replication in an artificial environment. Seeding the environment with longer RNA molecules results in a rapid reduction in the length of their offspring until they closely resemble the original Spiegelman monster.
Strong selection for small organisms has a negative effect on evolution - since it prevents larger and more complex organisms from developing. Small organisms tyically have simple phenotypes, and have limited scope for genetic variation. This effectively prevents most kinds of interesting evolutionary development.
The reason that small organisms have an advantage is fairly obvious: they need fewer resources to construct their offspring - and can perform the task in a shorter time.
In order for any kind of open-ended evolutionary development to happen, larger organisms need to come into existence. These must be able to compete effectively with smaller organisms.
The problem is discussed briefly here.
Natural solutionsIn nature there are a number of selection pressures which can favour larger organisms.
For example among photosynthesising plants, victory often goes to those that succeed in intercepting the sun's rays before their competitors can reach them. This has led to the invention of cellulose - and the development of fibrous stems capable of lifting plant leaves high into the air.
Similarly in a fight between two organisms, the larger of the two organisms often has an advantage.
However most of the selection pressures responsible for selection for large size are not realistic in a primitive environment.
In order for non-trivial evolution to get off the ground, some sort of selection for large size must have been present from an early era - and the selection must have been strong enough to overcome the considerable advantage of that naturally favours organisms of small size.
GravityOne selective force seems far more likely to be involved than any other - this is the force of gravity.
Gravity has a chance to select for large size because surface area to volume ratio is a "visible" trait - which varies considerably with size.
Consider a falling object:
The force of gravity exerts on an object is proportional to its mass - which in turn depends on its volume. Another force acts on falling objects: friction. The magnitude of frictional forces depends mainly on their surface area.
Gravity's ability to "see" the mass of objects directly is not unique. In theory inertia could play a similar role. However gravity is ubiquitous - whereas measuring inertia seems likely to require equipment - such as a centrifugue.
While there are other ways of applying a strong selection favouring large size, gravity seems to be by far the simplest and most obvious candidate.
The effect of the difference in volume and surface area causes large objects to be more affected by gravity and less affected by surface area - presenting a "size" phenotype easily accessible to selection.
DiagramsA diagram is the easiest way in to illustrate a scenario in which large objects could be selectively favoured.
Gravity can select for large size
The diagram shows self-replicating organisms suspended in liquid flowing up through a tube.
The black arrows show gravitational forces. The green arrows indicate the direction of fluid flow.
The motion of the liquid causes small objects to rise upwards.
By contrast, very heavy objects fall out of the bottom.
However objects of an intermediate size would experience a downward force (due to gravity) and an upward force (due to convection) that were in balance. Such objects would neither rise nor fall. Instead they would remain balanced some distance above the bottom of the system.
Gravity can select for large size
The diagram shows self-replicating organisms suspended in liquid above a heat source.
The arrows have a similar meaning: the green arrows indicate the direction of convective forces caused by the heat source.
Notice how the fact that the up-draught gets less intense with height causes objects of different sizes find stable points at different heights in the system.
If large objects were destroyed by heat - and small objects were pushed off the top of the diagram - then only objects of an intermediate size would survive within the system.
The considerations discussed on this page seem to lead in the direction of a specific scenario for the origin of life.