What is Life?

1. Classification: Carolus Linnaeus (Swedish biologist, 1707 - 1778, born Carl von Linne, but changed his name to the Latin version) developed the Linnaen classification system to place different species in a hierarchical, structured way to avoid confusion when naming species. A unique binomial (i.e. two-part) name is given to every species, consisting of the Genus (first letter capitalized) and the Species name (all lower case) in Latin. The basic divisions used today [Abelard.org] are as follows (there are more super- and sub- layers).

• Kingdom:
1. Monera: bacteria and blue-green bacteria (cyanobacteria)
2. Protista: single celled creatures other than bacteria, including unicellular algae
3. Fungi: mushrooms, yeast, molds, etc.
4. Plantae: plants, e.g. mosses, ferns, etc., including multi-cellular algae
5. Animalia: animals

• N.B. this does not include viruses, mainly because viruses are on the borderline of life and do not have a cell. But they are parasitic on cells, as they need cells in order to reproduce. Bacteria are single celled creatures, but without a nucleus. All the other kingdoms have cells with a nucleus.

• Phylum (or Division in the plant kingdom)
• Class
• Order
• Family
• Genus
• Species

For example, backboned animals are Kingdom - Animalia, Phylum - Chordata (backboned animals), and Chordata has two sub-layers:

• Sub-phylum Vertebrata, classes:
• Mammalia (Mammals)
• Aves (Birds)
• Reptilia (Reptiles)
• Amphibia (Amphibians)
• And there are four classes of fish: Actinistia (Coelacanths), Actinopterygii (Bony Fish), Chondrichthyes (Cartilaginous Fishes), Placodermi (Jointed Neck Fish)
• Sub-phylum Urochordata, classified as Chordata because of the presence of a larval notochord, classes:
• Ascidiacea (Tunicates - types of jelly-fish)
• Thaliacea (Salps)

And modern man (Homo sapiens) is classified as Kingdom - Animalia, Phylum - Chordata, Class - Mammalia, Order - Primata, Family - Hominidae, Genus - Homo, Species - sapiens

2. Definition: a traditional definition of life based on the classification above, admittedly a very biased point of view since it is based solely on life that arose naturally on planet Earth, is that the candidate must satisfy most of the following criteria to be considered alive:

• Motion: does it move independently?
• Consumption: does it consume raw materials?
• Reproduction: does it produce new instances of itself, either sexually or asexually?
• Growth: does it grow from a young to a mature specimen?
• Response to stimulus: does it respond to an external stimulus?

Note that 'reproduction' and 'growth' in the criteria above both imply 'consumption', since some resource must be consumed to create new material. Most plants do not move independently, relying on wind, water or animals to scatter their seed so that they can spread - hence 'motion' is not a necessary attribute of life. Also a worker bee is definitely alive, but does not reproduce either sexually or asexually (although she helps, indeed is essential, in the upbringing of the next generation) - hence we should modify the definition of 'reproduction'. From the above and tightening up the criteria, we can come up with these revised criteria:

• Consumption: does it consume raw materials to maintain its existence?
• Reproduction: can the population increase in a suitable environment by consuming raw materials to create new instances?
• Growth: does it consume raw materials to grow from a young to a mature specimen?
• Response to stimulus: does it respond to an external stimulus?

Of course, given the revised criteria for reproduction, an individual alone may not be sufficient to determine whether it is alive or not, and pathological exceptions can always be found, e.g. a person in a persistent vegetative state or a population of 'senseless' bacteria could perhaps be engineered. But these revised criteria now cover plants and bees well. Let us test them further by applying them to three cases which are commonly taken to be not alive:

1. A virus is not independent as it relies on hijacking a living cell in order to reproduce, but many parasites which are most definitely alive (e.g. a cuckoo) also rely on co-opting or highjacking things which are alive, and the whole point of an eco-system is that no creature is independent of other living things. A virus does not really grow or consume resources itself, but is a pattern or template for the highjacked cell to assemble more examples. It does respond to external stimuli in order to invade a target cell. It fails to meet the definition of being alive on 'consumption' and on 'growth'. It passes on 'reproduction' and 'response to stimulus'.
2. A self-powered robot, e.g. the Mars surveyors Spirit and Opportunity, have some limited ability to respond to external stimuli and they generate power from their solar cells in an analogous way to how plants photosynthesize for energy. However, they do not consume raw materials to grow, and, although there are two of them on Mars, they do not reproduce. Hence they fail to meet the definition of being alive on 'consumption', 'reproduction' and 'growth'
3. Crystals, although they will grow in solution, fail to meet the definition of being alive on 'consumption' and 'reproduction' (although crystals grow they do not form new independent instances). It could be argued that the way they grow, depending on concentration gradients, temperature, etc. means they pass on 'response to stimulus'.

However, it has been proposed that the best way to colonize other planets would be to send an autonomous factory unit that would land on the target planet, start to generate power, mine resources, build other units to exploit the environment, and eventually launch a new factory unit to search for the next planet or star system to colonize. Indeed it is estimated that the entire Milky Way galaxy could be colonized in this way in < 50 million years. Such factory ships would pass on 'consumption', 'reproduction' and 'response to stimulus', failing only on 'growth'.

Primitive life (and possibly the autonomous factory unit) has an imperative to grow and reproduce until all resources are consumed; it will be stopped by other life from achieving that goal, but that is its goal.

By analogy, virus is the term used to define a fragment of a computer program which needs some other program to transport and replicate itself (given some user intervention). Thinking of computer programs, would consumption be needed in a virtual world? In John Conway's cellular automata game of 'Life', the new cells do not draw on any resources from the game world to be born. There are other computer games where growth is limited by lack of resources, but there is no real resource being rationed, rather an arbitrary level is set to make the games more interesting. The real resources of a computer are CPU time, network connection bandwidth, and memory. Another example of computer malware is a bacteria, which is a program which, once it is running, spawns further examples of itself. These instances spawn more examples, until the exponential growth in the number of these programs results in the consumption of all the resources of the target computer. This is an example of a Denial of Service (DoS) attack, where the aim is to make some aspect of the target system unavailable to legitimate users. Again, a computer bacteria program does not need to grow as part of its reproduction, so it is not alive.

2.1 Summary

Revised criteria for natural life:

• Consumption: does it consume raw materials to maintain its existence?
• Reproduction: can the population increase in a suitable environment by consuming raw materials to create new instances?
• Growth: does it consume raw materials to grow from a young to a mature specimen?
• Response to stimulus: does it respond to an external stimulus?

Notes:

1. The emphasis on consumption in the criteria implies that artificial life, i.e. computer life, cannot satisfy these criteria - hence these criteria are specifically for natural life.
2. Although the whole point of a computer bacteria is to consume resources, it does not need to consume resources to maintain its existence.

3. Alternative classification: we can also classify life as prokaryotic (which can be sub-divided into either eubacteria or archaebacteria), eukaryotic, or neither (e.g. viruses, prions).

• prokaryotes: single cell creatures without a nucleus, i.e. the kingdom Monera
• eubacateria
• archaebacteria: examples of these creatures today are able to tolerate high salt, high acid or alkaline conditions, extremes of temperature, etc., i.e. they are extremophiles
• eukaryotes: all creatures with cells containing a nucleus, which includes all multi-cellular organisms, i.e. the other four kingdoms
• non cell: viruses, prions

Prokaryotes were the first life on Earth, as archaebacteria, from which evolved both eubacteria and eukaryotes, first the protista, then fungi and plants. The total mass of prokaryotic life on Earth is about the same as the total mass of eukaryotic life [Van Gooch, Palaeos, Bacteria Museum].This table from Van Gooch.

Fast forward to early man to expand the last two million year table entry.

4 Autopoiesis. The theory of autopoiesis is another alternative attempt to define life which was introduced by Varela, Maturana and Uribe in 1974. Hall et al (2005) summarizes the critria listed by Varela et al which were deemed necessary and sufficient for an entity to be considered autopoietic or living:

• Self-identifiably bounded (demarcated from the environment by membranes, the entity's components are identifiably tagged)
• Identifiable components within the boundary (complex)
• Mechanistic (i.e., driven by cybernetically regulated energy fluxes or metabolic processes)
• System boundaries internally determined (self referential)
• System intrinsically produces own components
• Self-produced components are necessary and sufficient to produce the system (autonomy)

Hall also notes that there is no consideration of time here, and that a further criteria should be:

• An autopoietic entity must be able to maintain its state of autopoiesis over a period of time

I think Varela et al were trying to provide a definition of life which fitted our intuitive feeling for what is alive, as Hall says "[to list] the minimum set of properties for complex biological entities to be considered living". Hall pointed out that some economic organizations met the criteria to be autopoietic, and so could be considered biologically alive (e.g. as alive as a worker bee). He considers how autopoietic theory could apply to an economic entity:

• Self-identifiably bounded. People know what organisations they belong to. For the benefit of other individuals and organisations, members are variously tagged with ID badges, bear membership cards, wear uniforms displaying the company logo, etc.
• Identifiable components within the boundary. Members are individually unique, recognize one another as members, and are identified as such within the organization; also machines, property, bank accounts, etc. are identified with tags, catalogued in property registers, etc.
• Mechanistic. Individuals receive rewards and benefits to belong to the organization, and are involved in processes, routines, procedures etc. that the organization needs for its survival.
• System boundaries internally determined. Rules of association, voluntary allegiance to organizational goals, etc. are defined within the organization itself and determine what people, property, etc. belongs to the organization.
• System intrinsically produces own components. Members are recruited from the environment, and inducted, trained, monitored, and managed, etc. Other property and assets are procured and variously integrated into the overall functioning of the organization.
• Self-produced components are necessary and sufficient to produce the system. Most organisations outlive the association of particular individuals and are readily able to replace plant and equipment as it wears out.

I think an economic entity, such as a firm, that meets the criteria above can be treated as if it is alive, and then some theories which apply to biological entities (such as the biological theory of evolution) can be legitimately applied to the economic entity. However, I do not think an economic entity is alive in the same sense that a worker bee is alive. Given the criteria: consumption, reproduction, growth, and response to stimulus, a firm could conceivably reproduce another firm as a new instance of itself, that new firm would need to grow as an essential part of reproduction, and economic entities certainly have to respond to external stimuli. It is only on consumption being necessary to maintain existence that it would fail.

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