[LUCID LIVING #5] The Secret of Life: Reliable Systems from Unreliable Parts
By Piet Hut
The question of the origin of life on Earth, and possibly elsewhere in the Universe, is a fascinating topic of study. In order to ask how life first appeared in a non-living environment, obviously it would help to know what exactly life is, and what sets it apart from non-living forms of matter and energy.
Interestingly, nobody has come up with a good definition of life. Apart from problems posed by bacteria and virusses, at least recognizing plants and animals as being alive wouldn't seem that hard.
Many definitions have been attempted. Eating and procreating are natural candidate features for life forms. However, the ability to procreate is no longer present in a mule, while a phenomenon like fire can consume fuel and propagate quite efficiently; yet we call a mule alive and a fire not. In the literature we can find lists of dozens of definitions, not a single of which is generally accepted by biologists.
At first sight it seems surprising that we can't find a definition of something so obvious to us as to whether something is alive or not. What is this secret of life that is so elusive, and so hard to catch? In days past many people, including scientists, postulated the presence of some kind of life force, but nowadays such an ad hoc assumption has lost its popularity. Instead we talk about life as an emergent property. But what kind of property or coherent set of properties? What forms the basis of life, its basic characteristics?
I have argued that we cannot construct science from the ground up, by starting with a few well defined ideas. As a case in point, even in something as fundamental as classical mechanics, force and mass need to be defined in a circular way, each concept needing the other. What is true in physics must hold even more true in the far more complex field of biology, where myriad of interconnections are ubiquitous, from the intricate workings of many parts inside a single living cell, all the way to the countless interrelationships within the biosphere as a whole.
On this largest scale, we witness a circular relationship between life and its environment. Organisms not only adapt to their surroundings, they also sculpt it in turn. Worms plow the ground, creating new niches that allow plants to take root much more easily. Elephants clear paths through dense forests, which allow for greater mobility of other animals, that in turn can carry seeds. The ecosphere, the joint system of biosphere together with its physical environment, shows niches everywhere, as the result of interactions between living organisms and non-living forms of matter and energy.
It is not surprising, then, that nobody has ever been able to provide a definition of life -- just like nobody has ever come up with a definition of force, or a definition of mass, in isolation. If we were to define the biosphere in a narrow way as the sum total of all that lives, that is the whole biomass on Earth, we deal with a useless abstraction. Without air, or soil and water, most living beings would perish almost instantly. While some of the hardiest single cell organisms may survive even in the vacuum of outer space, they still need an uptake of matter and energy in order to move and procreate.
So it seems that the best we can do is to highlight a feature of life that we consider the most essential, the most characteristic of life. One popular attempt to come close to such a definition is the NASA Astrobiology Institute's version in which life is seen as "a self-sustaining chemical system capable of Darwinian evolution." While this seems to cover life on Earth, at least to a large extent, there is no obvious reason that life elsewhere in the universe should be chemical, nor that Darwinian evolution is necessarily the only way that life persists.
My own proposal is more general. I consider the most salient feature of life its ability to form reliable systems from unreliable parts. When a house falls down in an earthquake, its bricks are still intact, and similarly a house of cards can fall down without damaging the cards. In non-living constructions, typically the building blocks are stronger than the whole structure. Atoms are more robust than molecules that they form, and in turn the atomic nucleus is far harder to break apart the atom it resides in. But in a plant or animal, its cells are replaced continuously, with the whole organism far more resilient and reliable than the parts it is built up from.
This description does not specify whether the underlying material constructions are chemical, electronic, or any other form for that matter. It also does not specify how it will be conserved in time. Darwinian evolution is one form, highly successful in our biosphere, but who knows how many other ones there may be? Rather than specifying what does the persisting, it seems more modest to specify that there is persistence, through the stipulation of reliability over time.
An interesting implication of such a description is that it covers not only biological organisms, but also societal structures such as human organizations, from tribes to cities and corporations. In my last post, towards the end, I mentioned an example, noting that science, as a human institutionalized activity, turns out to be far more reliable than individual scientists are.
In addition, the description covers modern technological structures such as the internet, which has been constructed with the specific aim to be resilient. Even when many servers, or connections between servers, fail, there is typically enough redundancy to prevent complete failure.
The internet can be seen as the backbone of the technological sphere, built up in recent memory, a product of the human cultural sphere, which itself is a product of the biosphere. More about these three spheres, which all share remarkably similar life-like phenomena, in a future post.
Piet Hut is President of YHouse, Professor of Astrophysics and Head of the Program in Interdisciplinary Studies at the Institute for Advanced Study in Princeton, and one of the founders of YHouse.