First of all, you may have notied something a little different here. I am now a member of Field of Science, a new network of science blogs. You can investigate the other blogs in this network through the links at the top and bottom of this page. All the content from the original blog has been imported here (including your lovely comments), and the original address redirects here, so no need to update your links. At the moment, we're looking a little plain in the visual department, but some massive redecoration plans are in the works.
On to today's topic: life. How exactly did a collection of random chemicals give rise (eventually) to sentient beings? Where did it start?
This question supposes that at some point, a collection of chemicals that were "not alive" found a way to organize themselves into an entity that was "alive." But we immediately run into another, more fundamental issue: what, exactly, is life? How can we distinguish life from whatever came just before it?
Some researchers (see this Wikipedia article, for instance) consider chemicals such as DNA or RNA to be the distinguishing feature of life, and reduce the question of life's origin to looking at how these chemicals were synthesized.
But this perspective, in my opinion, misses the point. Life is a process, not a chemical. The distinguishing feature of life is not what it's made of but what it can do: namely, it can reproduce itself. More precisely, we can define the process of life by this picture:
Or in words:
An entity is alive if it can produce copies of itself using the free energy and materials that are available in its environment.
There are other caveats we may want to add, such as that living entities can tolerate a certain amount of mutation or environmental change without losing their reproductive ability. But self-replication is a good starting point.
Now the question becomes, how could such a process have arisen? Scientists have managed to synthesize a few self-replicating molecules, but the sponaneous formation of such molecules from inorganic matter seems highly unlikely.
On the other hand, nature is full of chemicals that do this:
In this diagram, A plays the role of a catalyst, helping to synthesize B from other chemicals in the environment. The ubiquity of catalysts led Stuart Kauffman to hypothesize that life may not have started with a single self-replicating molecule, but with a collection of catalysts, each catalyzing another in a cycle:
or in a more complex network:
Kauffman called such collections autocatalytic sets. If such a set of chemicals were able to surround themselves with a membrane, and eventually produce enough of themselves so that the membrane would split in two, we could have our very first example of a living cell.
This idea has a number of interesting implications, which I intend to explore in the very near future. In the meantime, enjoy the new site!
Macrocycles, flexibility and biological activity: A tortuous pairing
16 hours ago in The Curious Wavefunction