Complexity_ A Guided Tour - Melanie Mitchell [54]
Simon contends that evolution can design complex systems in nature only if they can be put together like building blocks—that is, only if they are hierachical and nearly decomposible; a cell can evolve and then become a building block for a higher-level organ, which itself can become a building block for an even higher-level organ, and so forth. Simon suggests that what the study of complex systems needs is “a theory of hierarchy.”
Many others have explored the notion of hierarchy as a possible way to measure complexity. As one example, the evolutionary biologist Daniel McShea, who has long been trying to make sense of the notion that the complexity of organisms increases over evolutionary time, has proposed a hierarchy scale that can be used to measure the degree of hierarchy of biological organisms. McShea’s scale is defined in terms of levels of nestedness: a higher-level entity contains as parts entities from the next lower level. McShea proposes the following biological example of nestedness:
Level 1: Prokaryotic cells (the simplest cells, such as bacteria)
Level 2: Aggregates of level 1 organisms, such as eukaryotic cells (more complex cells whose evolutionary ancestors originated from the fusion of prokaryotic cells)
Level 3: Aggregates of level 2 organisms, namely all multicellular organisms
Level 4: Aggregates of level 3 organisms, such as insect colonies and “colonial organisms” such as the Portuguese man o’ war.
Each level can be said to be more complex than the previous level, at least as far as nestedness goes. Of course, as McShea points out, nestedness only describes the structure of an organism, not any of its functions.
McShea used data both from fossils and modern organisms to show that the maximum hierarchy seen in organisms increases over evolutionary time. Thus this is one way in which complexity seems to have quantifiably increased with evolution, although measuring the degree of hierarchy in actual organisms can involve some subjectivity in determining what counts as a “part” or even a “level.”
There are many other measures of complexity that I don’t have space to cover here. Each of these measures captures something about our notion of complexity but all have both theoretical and practical limitations, and have so far rarely been useful for characterizing any real-world system. The diversity of measures that have been proposed indicates that the notions of complexity that we’re trying to get at have many different interacting dimensions and probably can’t be captured by a single measurement scale.
PART II
Life and Evolution in Computers
Nature proceeds little by little from things lifeless to animal life in such a way that it is impossible to determine the exact line of demarcation.
—Aristotle, History of Animals
[W]e all know intuitively what life is: it is edible, lovable, or lethal.
—James Lovelock, The Ages of Gaia
CHAPTER 8
Self-Reproducing Computer Programs
What Is Life?
CHAPTER 5 DESCRIBED SOME OF THE HISTORY of ideas about how life has evolved. But a couple of things were missing, such as, how did life originate in the first place? And what exactly constitutes being alive? As you can imagine, both questions are highly contentious in the scientific world, and no one yet has definitive answers. Although I do not address the first question here, there has been some fascinating research on it in the complex systems community.
The second question—what is life, exactly?—has been on the minds of people probably for as long as “people” have existed. There is still no good agreement among either scientists or the general public on the definition of life. Questions such as “When does life begin?” or “What form could life take on other planets?” are still the subject of lively, and