From Molecules to Metabolism — II. When Reactions Began to Form a Cycle

A chemical reaction happens.

Then another.

Then another.

There is no shortage of reactions in the universe. But metabolism requires something more than chemical activity. It requires organisation among reactions.

One process produces something another process can use.

That process produces something that feeds another.

Eventually, the products and conditions of the network begin to support processes that contribute to the network itself.

A cycle has appeared.

A cycle is different from a sequence.

In a sequence, events simply follow one another.

In a cycle, what happens later can help sustain the conditions for what happens earlier.

The system acquires a kind of closure.

Not complete closure. Matter and energy must still pass through it. But some of the relations within the system become mutually sustaining.

This changes the significance of an individual reaction.

A reaction that would otherwise be chemically unremarkable may become crucial because it occupies a particular place in the network. Its products may enable another process; that process may contribute to conditions required for the first.

The parts begin to acquire their roles from the organisation of the whole.

This is an important step towards metabolism.

We do not yet need a cell, a genome, or even a sharply defined boundary.

We need only a network in which chemical processes become sufficiently interdependent that the network begins to participate in its own continuation.

And this introduces a new kind of possibility.

The chemistry is no longer merely happening.

Some of what happens helps determine what can happen next.

A reaction becomes part of a history.

Its consequences alter the conditions for subsequent reactions, whose consequences alter the conditions again.

The cycle carries its own past forward.

Perhaps this is one of the first hints of life: not a molecule that is alive, but a network in which processes have begun to make one another possible.

Chemistry has started to acquire a history of its own.

And once a history can contribute to its own continuation, the question of life is no longer simply:

What reactions are occurring?

It becomes:

What organisation makes these reactions continue to occur?