Darwin viewed evolution as a branching tree in which "descent with modification" is the cause of a 'horizontal' divergence of species. Because of contingency in the modification processes, the 'tape of evolution' cannot be payed twice with the same result. This gives the impression that the course of evolution cannot be predicted. However, the tree of life also harbours 'vertical' aspects of evolution. For example, bacteria must cooperate during the construction of endosymbiontic cells. And cells must cooperate for the construction of a multicellular life form. In any replay of the tape of evolution, these -or similar- steps are obligatory for the making of more complex types of organisms. Accordingly, such steps are independent of the exact course of evolution. By focusing on such steps, one obtains a more fundamental analysis of evolution, in which next steps no longer depend on contingency, but on stringent types of organisation.

A branching tree

In many cases, the offspring of an organism do not possess exactly the same genes as the parent. The genes of the offspring may differ due to simple changes in a single DNA base-pair, or may result from parts of DNA that 'jump' to another position. Other causes of change over generations are the recombination of genes as the result of sex, or the doubling of an entire genome. Generation after generation, such changes accumulate. When the accumulation of change in a group of individuals causes large enough differences to block mating with individuals of another group (either genetically or behaviorally), a new species has taken shape. Due to this process, a few first cells have evolved over millions of generations into a magnificent tree of life with hundreds of thousands of species 'growing' at the tips of all the branches. Branches need not grow for ever. Of all the species that once lived, less than 1% is estimated to stil live on the earth.

Recognizing levels in the tree of life

Figure 1: A 'leveled' tree of life
The 'branching' of species represents just one aspect of evolution. Another aspect is represented by transitions in structure that depend on interactions (Figure 1). Compared to genetics, the viewpoint of transitions based on interactions emphasises the changes in the overall body plan of the organisms. And a transition in structure only happens after many small genetical changes. For example, it requires many genetic adaptations before two bacteria can integrate and become 'host cell' and 'endosymbiont'. Meanwhile an host with endosymbiont represents a new type of organism. Similarly, it requires many genetic adaptations before single cells can cooperate and form a multicellular. The attachment of the cells and the formation of the plasma strands signify the multicellular state, which again represents a new type of organism.

An additional level: the memon

The operator theory suggests that the tree of life contains an additional level above multicellularity: the level of the multicellular organism with neural network, or 'memon', which inimally requires hypercyclic neural interactions and a sensory interface. The neurons offer the memon the capacity to store representations in the brain. If an organism has a sufficiently complex neural network it can additionally exchange abstractions of representations in a process of imitation/communication. Units of information that are stored in brains and that are transferred via immitation/communication have been given many names, the most popular seems to have become the 'meme' (as was introduced by Dawkins and later discussed by Blackmore).

Using levels to define dimensions

The tree of life is strictly and only based on organisms. And because the tree focuses on reproduction, every branch grows from parental organisms to their offpring, and not from parents to population, or from parents to organs. The result is a very 'pure' ranking. The complexity levels in the tree of life now represent a type-based perspective on the construction of organisms. In nature one can apply two additional perspectives, namely: 1. the perspective of how organisms interact, and 2. the perspective of how organisms are organised internally. In the next page, it is expliained how populations and the internal organisation in organisms can be viewed as two additional dimensions for organisation in nature. This three-dimensional way of looking at the organisation of nature forms the topic of section 1.3