In which we consider how organisms, including many unicellular organisms, have evolved to cooperate with one another, leading to the formation of multicellular organisms composed of distinct cell types. Similar evolutionary mechanisms have produced a range of cooperative (social) behaviors. One particularly important such behavior is sexual reproduction and we consider its effects on the behaviors and morphologies of organisms.
The naturalist Ernst Mayr stressed the difference in thinking in biology compared to physics and chemistry. The history of an electron, an atom, or a molecule is totally irrelevant to its physical and chemical properties. Each carbon isotope atom, for example, is identical to all others - one could be replaced by another and you could never, in theory or in practice, be able to tell the difference. In contrast, each organism, how it is built, how it behaves, how it interacts with other organisms, and the future evolution of its descendants is the result of a continuous evolutionary process involving both selective and adaptive and non-selective and non-adaptive processes stretching back ~3.5 billion years. This history encompasses an unimaginable number of random events (mutations, accidents, environmental disasters, isolated and merging populations). Because of its molecular and cellular complexity and distinct history, each organism is unique and distinguishable from all others.
In biology, we normally talk about organisms, but this may be too simplistic. When does an organism begin? What are its boundaries? The answers can seem obvious, but then again, perhaps not. When a single-celled organism reproduces it goes through some form of cell division, and when division is complete, one of the two organisms present is considered a new organism and the other the old (preexisting) one, but generally it is not clear which is which. In fact, both are old, both reflect a continuous history stretching back to the origin of life.
When an organism reproduces sexually, the new organism arises from the fusion of pre-existing cells and it itself produces cells that fuse to form the next generation. But if we trace the steps backward from any modern organism, we find no clear line between the different types (that is, species) of organisms. When did humans (Homo sapiens) appear from pre-humans, or birds from their dinosaurian progenitors? The answer is necessarily arbitrary, since cellular (and organismic) continuity is never interrupted. In a similar manner, we typically define the boundaries of an organism in physical terms, but organisms interact with one another, often in remarkably close and complex ways. A dramatic example of this behavior are the eusocial organisms. While many of us are familiar with ants and bees, fewer (we suspect) are aware of the naked (Heterocephalus glaber) and the Damaraland (Cryptomys damarensis) mole rats. In these organisms reproduction occurs at the group level; only selected individuals, termed queens (because they tend to be large and female) produce offspring. Most members of the group are (often effectively sterile) female workers, along with a few males to inseminate the queen111. So what, exactly, is the organism, the social group or the individuals that make it up? From an evolutionary perspective, selection is occurring at a social level as well as the organismic level. Similarly, consider yourself and other multicellular organisms (animals and plants). Most of the cells in your body, known as somatic cells, do not directly contribute to the next generation, rather they cooperate to insure that a subset of cells, known as germ line cells, have a chance to form a new organism. In a real sense, the somatic cells are sacrificing themselves so that the germ line cells can produce a new organism. They are the sterile workers to the germ line’s queen.
We find examples of social behavior at the level of unicellular organisms as well. For example, think about a unicellular organism that divides but in which the offspring of that division stick together. As this process continues, we get what we might term a colony. Is it one or many organisms? If all of the cells within the group can produce new colonies, we could consider it a colony of organisms. So where does a colony of organisms turn into a colonial organism? The distinction is certainly not unambiguous, but we can adopt a set of guidelines or rules of thumb112. One criterion would be that a colony becomes an organism when it displays traits that are more than just sticking together or failure to separate, that is, when it acts more like an individual or a coordinated group. This involves the differentiation of cells, one from the other, so that certain cells within the group become specialized to carry out specific roles. Reproducing the next generation is one such specialized cellular role. Other cells may become specialized for feeding or defense. This differentiation of cells from one another has moved a colony of organisms to a multicellular organism. What is tricky about this process is that originally reproductively competent cells have given up their ability to reproduce, and are now acting, in essence, to defend or support the cells that do reproduce. This is a social event and is similar (analogous) to the behavior of naked mole rats. Given that natural selection acts on reproductive success, one might expect that the evolution of this type of cellular and organismic behavior would be strongly selected against or simply impossible to produce, yet multicellularity and social interactions have arisen independently dozens (or more likely millions) of times during the history of life on earth113. Is this a violation of evolutionary theory or do we have to get a little more sophisticated in our thinking?
111 An Introduction to Eusociality: http://www.nature.com/scitable/knowl...ality-15788128
112 A twelve-step program for evolving multicellularity and a division of labor: http://onlinelibrary.wiley.com/doi/10.1002/bies.
113 The Origins of Multicellularity: https://bcrc.bio.umass.edu/courses/f...default/files/