Since I’ve been featured in the 40th Carnival of Evolution, I guess I’d better get on the stick and finish this book review.
Approaches to the Definitions of Life
This chapter takes on two topics. What is the definition of life and how to take that definition and apply it. Most of the ‘definitions’ of life are really descriptions of life. I’ll probably continue to use ‘definition’.
Life is like pornography. We might not be able to define it, but we know it when we see it. (Or do we? But more on that later.)
Why should we define life. If we know it when we see it, then what’s the point. I’ll answer that with another often asked question.
Go find a biochemist, a paleontologist, a cell biologist, and a zoologist in a bar and ask them to define ‘evolution’. I’m willing to bet you will get at least 4 answers, probably 6 or 7. The reason is simple, they are looking at evolution from a different perspective. Evolution, just like life, is such a broad and all encompassing concept that being able to define it in just a single sentence is extraordinarily difficult.
Luisi attempts to do so in this chapter and he says he refines this definition in later chapters, but we’ll start with what he has.
First let’s look at a few definitions of life as presented in the book.
- Life is the existence form of proteic structures, and this existence form consists essentially in the constant self-renewal of the chemical components of these structures. F. Engels 1894
- Life is a potentially self-perpetuating system of linked organic reactions, catalyzed stepwise and almost isothermally by complex and specific organic catalysts which are themselves produced by the system. Perret 1950s
- Life is a self-sustained chemical system capable of undergoing Darwinian evolution. Horowitz and Miller 1962
- Life appears as a population of RNA molecules (a quasi-species) which is able to self-replicate and to evolve in the process. Luisi 1998
But these all have some flaws. In the first there is the requirement for proteins. Are proteins required for ALL life (not just life as we know it). What if we found an organism that had protein-like structures, but they were not amino acid based, would it still be alive?
So, Luisi takes us on a little journey. Let’s say a little green alien comes to Earth and is perplexed by the variety of living and non-living things. He approaches an average human and gives them a list and asks which is life and which is not.
It would be child’s play for almost any human on the planet to separate the living from the non-living things. Even most young children could do it.
But how do you know? What qualities and attributes discriminates the living from the non-living?
Reproduction? Mules don’t reproduce, neither do babies.
Movement and growth? Trees and corals don’t move much and their growth is limited to long time frames.
Reaction to stimuli? You can poke a tree with a needle all day long and nothing will happen. Poke the inside of a computer with a needle and you might get pretty strong reaction.
So what is it. There are two obvious things here. The first is cells. All the living things are made of cells. But I really don’t like doing that. Cells are a structure, not a process or attribute. It is easy to think that limiting our discussion of life to things with cells will really confound us if we find something alive that it not made of cells. This is especially true when we consider the origin of life.
The second is the basis for Luisi’ first definition of life in this book:
a system can be said to be living if it is able to transform external matter/energy into an internal process of self-maintenance and production of its own components.
I like this definition/description of life. It’s robust and it covers a great many systems that I can think of that aren’t alive, but may well become that way (computer programs).
Well, how does this affect our approach to the origin of life? Can this definition of life help us when it comes to origins of life research? We shall see.
However, Luisi takes a brief detour here to examine the various models of origins of life research. Not all of them. A serious review of the literature will reveal a large variety of OOL models, hypotheses, and notions. [For a brief non-technical review go here.]
The ones that Luisi focuses on here are the RNA-World, the cells first approach, and the metabolism first approach.
I’ll be brief, there’s LOTS of work done on these. I would suggest you pick up Luisi’s book or click on the Origins of Life topic on the sidebar here for lots of links and research.
Basically, the RNA world hypothesis goes like this. RNA –> Ribozymes –> Proteic enzymes –> DNA
There is a great deal of research to support this. It is probably the most popular model currently being explored. That’s not to say it doesn’t have it’s problems. Though I believe that some of the issues Luisi mentions have been solved since the writing of the book.
Anyway, the premise is that it is possible to develop self-replicating RNAs from pre-biotic compounds. Once you get to self-replication, everything else falls into place. (Again, this isn’t meant to be an exhaustive treatment, if you want one, let me know and I’ll put it on my list to write about.)
Basically, the cells form and concentrate all the cool things that life needs. This allows for greatly increased reaction rates because everything is contained in a much smaller area than (for example) the entire ocean.
This, as a basic concept, has a lot going for it. It is trivial to watch totally non-living systems form complex vesicles (cells).
On the other hand, it’s a little more difficult to get the stuff you need in the vesicle and the stuff you don’t need out of the vesicle. (Please forgive the anthropomorphic language here.)
Personally, I think that this concept is not mutually exclusive with either of the other two models under discussion.
This model (actually many, many models) tracks the energy rather than the information system. Instead of needing self-replicating RNAs, these models use non-living catalysts to replicate long chain organic molecules. In this way you get replication, even mutation (as not every substrate would be exactly the same), and some thermodynamically valid models of energy flow. You also get a stable environment in which these prebiotic systems can grow.
The issue is finding a model that can go from simple long chain molecules to self-reproducing molecules and maintain the chemical and energy balances needed for this to work.
Each of these models have advantages and disadvantages and I would be willing to bet that the final model that does work will incorporate elements from all of these models. It is entirely possible that life began many times on Earth and in many different ways. Again, we will never know exactly how it happened.
The research being done can show us plausible paths that prebiotic chemistry took to get to life. The research can also show us when it is chemically or physically impossible for something to go from one step to another. And that’s OK too. At least we know at that point.
However, let me add in closing, I’ve been studying this topic for a while now and I have seen no research that would lead me to believe that any part of the non-life to life process requires an outside influence of any kind.