[If you are not sure what these are, I recommend you follow this link before reading further]

It attempts to describe the ways in which these can vary.

Here, the large, yellow regions represent phenotypes, and G1 and G2 are the primary and second genetic substrates respectively.

While it is conventional to interpret this diagram as representing organisms containing a mix of genetic substrates, it is also possible to view it as representing ecosystems.

This is perhaps better explained with reference to a new diagram:

This diagram shows yellow and red organisms existing within a green ecosystem.

To summarize the difference between the two diagrams, in the second diagram, the second genetic material is no longer arising directly within the bodies of the ancestral population - but instead has bodies of its own - reducing the possibilities for symbiosis.

There are many similarities between the two scenarios - in both:

• there is the origin of a new information-storage technology;

• this cab be developed by the primary organisms to store inherited information;

• there can be a period of co-existence during which information can flow between the genetic substrates.

A naive look at the second diagram might see the creation of a new species, which wipes out its predecessors. However, there are still excellent reasons to view the primary organisms as the ancestors of the secondary ones:

• Without the primary organisms the secondary organisms would probably have been effectively incapable of forming from nothing;

• There is a period of co-existence - during which the two species can form symbiotic relationships pass information between the species.

The two diagrams are not really mutually exclusive. Because the definition of a "body" is vague - and because an organism's phenotype may extend well beyond what appear to be its physical boundaries, there is a continuum of possibilities - from the secondary organisms forming within the bodies of the primary ones - to the secondary organisms forming from the waste products of the primary ones.

A sceptic might ask - if a second organism can form from scratch - what need is there to hypothesize a previous species of organism whose only role appears to have been being wiped out by later organisms?

However, it appears that the secondary organisms may have been incapable of forming without being under the protection of the primary organisms - and their origin may prove incomprehensible without their consideration.

As to how the secondary organisms could arise in the presence of the primary organisms - without being eaten by them - there are various possibilities:

• The secondary organisms might occupy niches unoccupied by the primary ones - and thus initially avoid direct competition with them;

• The secondary organisms might be deliberately created by the primary ones - for their utility as slaves, for their information-storage capabilities, their abilities at fighting - or possibly for other reasons;

It appears to me that the most probable candidate for such a takeover appears to be between the hypothesised "naked gene"-based ecosystem - and one in which the organisms are surrounded by membrane bubbles.

It seems likely that any "naked gene" ecosystem would have been most likely to have been based on mineral crystals. These are likely to operate best in an environment which is maintained in a slightly super-saturated solution by crystallization processes. This mechanism depends on diffusion between the medium surrounding the organism and the rest of the environment.

Membranes are useful for holding an organism together so its component parts don't diffuse away. It also offers physical protection from perturbations in the environment.

However crystals hold themselves together with inter-molecular forces - and rely on perturbations from the environment to reproduce.

Membrane boundaries seem unlikely to have helped crystal-based organisms directly - it appears that the main function of an enclosing membrane would have been be to get in the way.

While it may be possible to hypothesize intermediate forms, it seems quite possible to me that organisms that held themselves together with membranes arose outside the immediate bodies of the organisms that synthesized their component parts - from either waste products, as tools or from decaying body parts.

There appear to be essentially four possibilities for our form of life in the future:

1. The human race and its technology are destroyed - perhaps by war, perhaps by the impact of the earth with a meteorite, or perhaps by the earth being invaded by an alien ecosystem - and no future descendants of our ecosystem succeed in establishing themselves.

2. The humans engineer their own genomes and those of ther companions fast enough that their capacities are not rapidly dwarfed by those of machines.

3. We form close symbiotic relationships with machines - which eventually come to replace our genetic material.

4. We form more distant symbiotic relationships with machines - and something like companies producing robots evolve into the dominant form of life.

Option 1) is a possibility - but it is hard to evaluate the probabilities involved.

Option 2) seems to me tremendously unlikely in the long term, due to technical problems with DNA as a storage medium. I certainly don't think mankind should aim towards this - since I think limiting our descendants to the use of DNA would prove to be short-sighted.

I think that options 3) and 4) represent our most probable long-term future.

While it seems likely that we will form close symbiotic relationships with machines, there do exist scenarios in which significant living systems can form without much in the say of co-operation or symbiosis with us:

			Computer viruses
			These show an unpleasant glimpse of how self-replicating entities can arise as a by-product of mankind's waste products - rather than from a mutually beneficial symbiotic relationship with us. Some would dismiss computer viruses as potential forms of life - since they are isolated in cyber-space and can't gain access to material phenotypes. However this is a fundamental misconception - they have material phenotypes - like any other self-reproducing system - and no-doubt will come to aspire to occupy a wider range of niches. I mention computer viruses since - if their descendants established themselves - they would represent a modern takeover of the second type - where the new organisms arise initially largely outside the bodies of the existing ones.
			Grey goo
			Another candidate for this type of scenario is the nanotechnology "grey goo" idea - where a very efficient self-replicating agent is manufactured - and rapidly comes to out-reproduce all existing living systems. While I think it is probably hubris to think that our technology stands much chance of developing something so much better than evolution has managed over billions of years in the short term, in the long term such possibilities can't be completely discounted.

It seems likely that future potential genetic substrates will arise outside our physical bodies - but within our extended phenotype. In order to best avoid valuable information being lost to our long-term descendants, it appears that it will be desirable for the modern genetic takeover to be engineered in such a way that the two systems in question form a close and extended symbiotic relationship.