Roughly 3.5 billion years ago, life on Earth began from molecular origins and over time evolved into the impressive array of creatures we know today. This is the modern line of reasoning. But we still do not have a clear definition of life. For example, is the virus alive? Or a whole forest ecosystem? After all, many aspects of the ecosystem are as dependent on each other as the organs within the body.
Therefore, biologist Chris Kempes and complex systems researcher David Krakauer of the Santa Fe Institute in New Mexico put forward the idea that our focus on evolution as the driving force of life may have "blinded us to additional general principles of life."
To explore this issue, the researchers expanded the definition of "life" to combine two energetic and informational processes that can encode and transmit adaptive information forward in time.
The use of this definition greatly expands the concept of "life" to include concepts such as culture, forests and economics. A more traditional definition might view them as products of life rather than life itself.
"Human culture lives on the material of the mind, just as multicellular organisms live on the material of unicellular organisms," explains Kempes.
Based on their new definition, the researchers argue that life has arisen on Earth many times, and that we actually coexist with many forms of life today.
The system proposed by Kempes and Krakauer has three hierarchical levels of constraints on what life implies, as shown below.
At the first level, life is limited by the possible materials from which it can be formed (for example, molecules). At the second level, life is limited by the limitations of the surrounding Universe (for example, gravity), and at the third level, life is optimized by adaptive processes (for example, natural selection).
Within this hierarchy, concepts are considered that unite the worlds of physics and biology. For example, life uses many options for producing energy using the first level constraints, but all of them must adhere to the second level constraints of the law of thermodynamics.
"No cell will contain more internal structure than can be accounted for by the total free energy available from the environment," the team writes in their article.
We expect that many rich biological concepts will be defined by a "strange tangle" of three levels, because these three levels will inevitably co-evolve."
This theory fully perceives life as a spectrum and not as a discrete phenomenon, for example, what makes us individuals? Only cells born from the same DNA, or is our microbiome the same? Not to mention the smooth connections between environmental energy, cellular physiology, and evolutionary processes.
While all of this can be as mind-bogglingly theoretical as much of quantum physics, it's a fascinating attempt at looking at an old concept from a new perspective. In complex systems, such as life and the consequences of our lives, sometimes the expansion of our thinking can trigger different ideas that will lead to new understanding.
The authors hope that such a broader view can lead to an understanding of what exactly we mean by life, help create devices for its creation or search, and also understand what level of life we observe - even if it is radically different from the usual life on Earth. …
Defining them will certainly put an end to the debate about whether viruses are really alive, since they fit well with Kempes and Krakauer's theory of life.
While this diagram is certainly detailed and gives us a lot of food for thought, changing definitions is hard work. And when most of us think about life, we will most likely only talk about biological life - at least until we meet aliens who challenge biology as we know it, or until our artificial intelligences not evolved enough to form their own minds.
"We claim that we can say that we have a new theory of life, when it can reveal to us many origins and many types of life," they write.
This theory was described in the Journal of Molecular Evolution.