We have moved from an organism that responds to its environment to organisms that become significant to one another.
But there is another possibility.
What if two formerly independent living systems do not merely interact?
What if they become parts of the same organisation?
That is the remarkable story of eukaryotic cells.
The endosymbiotic revolution
Modern eukaryotic cells contain mitochondria, descendants of bacteria that became endosymbiotic partners within an ancestral host cell. The precise identity of the host, the bacterial partner and the sequence of events remain subjects of active research, but the endosymbiotic origin of mitochondria is one of the central and robust features of current accounts of eukaryogenesis.
Plastids, including chloroplasts, have a corresponding origin from cyanobacterial ancestors.
The astonishing thing is not merely that one organism lived inside another.
It is that the relationship became integrated into a new biological organisation.
From partners to components
The ancestral bacterium was once an independently organised organism.
Over evolutionary time, the relationship changed.
Mitochondria retain traces of their bacterial ancestry, including their own genomes, but they have lost many genes and become deeply dependent upon the host cell; most mitochondrial proteins are now encoded by nuclear genes.
The former partner became an organelle.
That is a remarkable transformation in individuation.
The question is no longer simply:
How do two organisms affect one another?
It becomes:
When does one organism become part of another organism's organisation?
The topology changes
Our previous post described relational significance as the point at which another organism becomes consequential to an organism's own organisation.
Endosymbiosis goes further.
The other organism is no longer merely outside.
It becomes part of the system's internal organisation.
The boundary of the larger organism has effectively been redrawn.
What had once been:
organism A ↔ organism B
becomes something more like:
organism A + integrated former organism B → a new organisation
The topology has changed at the level of individuation itself.
What happened to the bacterium's value?
This gives us an intriguing question.
The ancestral bacterium had its own biological value.
It maintained its own organisation.
It responded to its environment.
After integration, the mitochondrion still performs functions that are essential to the larger cell, while its own organisation is deeply constrained by the host.
Its individuality has not simply vanished.
It has been transformed.
The mitochondrion remains a distinct biological lineage and compartment, but it no longer lives as the autonomous organism it once was.
So what happens when one value-organised system becomes a component of another?
Perhaps the answer is:
its value becomes nested within a larger organisation of value.
Nested value
This is a different phenomenon from social coordination.
Two bacteria can signal to one another while remaining independent organisms.
A mitochondrion and its host are different.
Their relationship has become constitutive of the larger organism.
The host's organisation depends upon the mitochondrion.
The mitochondrion's organisation depends upon the host.
The relation has become structural interdependence.
Neither can simply be understood in isolation.
A new individual emerges
This is why the origin of the eukaryotic cell is so important for our project.
The eukaryotic cell was not simply a larger bacterium.
It represented a new level of organisation assembled through an evolutionary merger involving previously distinct lineages. Current research describes eukaryogenesis as a major transition in biological complexity, while emphasising that the precise sequence remains unresolved.
A new individual emerged from a history of interaction between individuals.
That complicates any simple distinction between:
individual
and:
collective.
Sometimes a collective becomes an individual.
Integration rather than aggregation
The distinction is crucial.
A colony of bacteria can contain many organisms.
A eukaryotic cell contains many internal processes and descendants of once-independent organisms.
But the latter is not merely an aggregation.
Its components are integrated into a common organisation.
Their reproduction, metabolism and regulation become increasingly coordinated through the larger system. The evolutionary transition involved extensive gene transfer from the endosymbiont to the host nucleus and increasing host control over the former symbiont.
The result is something new:
organised unity without complete homogeneity.
Individuation is therefore not simple separation
We often imagine an individual as something bounded against everything else.
Endosymbiosis suggests a different picture.
An individual can be constituted through incorporated difference.
The eukaryotic cell became what it is partly by taking another lineage into itself.
Its individuality is therefore relational in a surprisingly literal sense.
This may eventually prove important when we return to multicellular organisms.
From interaction to incorporation
We can now extend our emerging sequence:
biological value
↓
relational significance
↓
signalling and coordination
↓
integration
↓
nested organisation
The important point is that integration is not simply "more cooperation".
It is a transformation in the level at which organisation is individuated.
The host changes too
The transformation was not one-sided.
The host cell had to change as well.
Housing and controlling a formerly independent symbiont created new evolutionary pressures and new forms of cellular organisation. Current research continues to debate exactly how those changes unfolded, but there is broad agreement that mitochondrial acquisition was central to the emergence of the complex eukaryotic cell.
The symbiont changed the host.
The host changed the symbiont.
The resulting system was different from either.
That is perhaps the deepest lesson of endosymbiosis:
a relationship can transform both participants by creating a new level of organisation.
A new kind of mattering
We should still avoid calling this social mattering.
The relationship is not social in the sense we are developing.
But it gives us another form of relational significance.
The former symbiont is consequential to the host.
The host is consequential to the symbiont.
Their continued organisation becomes intertwined.
This is relational significance becoming constitutive of individuation.
That is a major step beyond signalling.
And then there were more
The same broad principle appears again in plastids.
A cyanobacterial lineage became incorporated into eukaryotic cells, eventually producing chloroplasts and related plastids. These organelles too retain bacterial ancestry while functioning as deeply integrated components of their host cells.
Life therefore repeatedly found a way to turn relationship into organisation.
A former partner becomes a component.
A component becomes indispensable.
A new individual emerges.
The next problem
We have now encountered something more radical than organisms influencing one another.
We have seen organisms becoming parts of other organisms.
That leaves us with a question that will recur throughout this series:
When does a collection of living systems become a new individual, and what happens to the value of the components when it does?
The next transition will take us in another direction.
Instead of one organism incorporating another, we will look at living systems that remain distinct yet produce remarkably organised collective behaviour.
The Collective Without a Centre
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