Title: Noah and the spaceship: Evolution for 21st Century Christians
Ellen Clarke, University of Bristol
Philosophy Department, University of Bristol, 9 Woodland Road, Bristol, BS8 1TB, UK
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Email: firstname.lastname@example.org Review of “How Life Began – Evolution’s Three Geneses” by Alexandre Meinesz, translated by Daniel Simberloff.
Evolution has increasingly become a topic of conflict between scientists and Christians, but Alexandre Meinesz’s recent book How Life Began aims to provide a reconciliation between the two. Here I review his somewhat unorthodox perspective on major transitions, alien origins and the meaning of life, with a critical focus on his account of the generation of multicellularity.
Christianity; major transitions; panspermia
Meinesz is already known as a popular science author for his work Killer Algae(1999) about the dangerous spread of non-native algae. As a phycologist he is a little further away from his home territory in this book which attempts the much grander task of assimilating biology, history and philosophy into the sort of text that wouldn’t look out of place on the bookshelf at Sunday school. This is evolution for the religious, the book of Genesis rewritten for the modern world. It combines personal reflections on art with summaries of the latest discoveries in molecular biology and paleoecology to offer a uniquely spiritual perspective on cutting edge science.
Meinesz claims there have been three distinct geneses or creations in evolutionary history. One is the origin of bacteria (about which he says surprisingly little, save that it didn’t happen on earth). One is the origin by symbiosis of the eukaryotes - which is actually a rather heterogeneous collection of separate transitions, including the origins of the nucleus and of sex, supposedly unified by the common mechanism of symbiosis. The third and last is the origin of multicellularity. It is not entirely clear what these three ‘events’ have in common, that could distinguish them from many other candidate transitions such as the origin of life, the origin of chromosomes, the origin of cell walls, the origin of language, or the origin of superorganisms or colonies. He distinguishes four fundamental forces or motors of evolution – mutation, sexual recombination, natural selection by the environment and mass extinctions or cataclysms. He emphasizes contingency in evolution, as well as union – the alliances and aggregations that have made the evolution of complex life possible. Like Gould (and McShea) his emphasis is on a long term paleontological view of evolution which emphasizes the success and longevity of bacteria, in contrast to a more progressive or adaptationist view.
Some highlights are worth mentioning - chapter three is devoted to bacteria, in fact it’s a tribute to them. We learn about their astonishing super-powers, their incredible diversity and resilience, their unsurpassed dominance in terms of numbers and longevity on the planet, as well as their indispensability to all other forms of life. Chapter six looks at controversies in the history of biology regarding large scale trends and the tempo of evolution with an impressively accessible treatment of the epistemological problems that paleoecologists face. He explains how different sources of evidence are collected and combined to try to reconstruct the history of life, and how the limitations of these sources of evidence constrain and possibly bias that picture. There is lots of science here – real photos of specimens and details about dates, combined in a way that lets the reader feel what it must be like to be at the forefront of science. Chapter nine contains the high point of the book – the issue of the so-called sixth mass extinction. Here he manages the difficult task of stirring your passion on an issue that is so well-rehearsed in these sorts of books that it is difficult to write about without sounding like you’ve simply copied it all down from some ecological holy book. Yet Meinesz manages to breathe new life into the topic.
How Life Began would be called a work of popular science, although it is as much history of science as science, with a good deal of philosophy thrown in as well, which is a huge amount of ground to cover in just one book. He covers most of the major debates in recent evolutionary biology and parts of the work are dense and rich, but other times things are rushed over so fast that I wonder what the lay reader would really have gleaned from it. The book opens with a motif that runs throughout the work – a description of a painting of Antoni van Leeuwenhoek by Vermeer, which Meinesz uses as an illustration of science as he claims it ought to be done – with one’s attention turned to the infinite and the mysterious but with one’s feet placed firmly on the ground. All chapters open with some sort of concrete setting, a personal anecdote or a distant memory recalled, demonstrating the author’s determination that he doesn’t sound like a distant inaccessible scholar lecturing from his ivory tower. Meinesz is keen to reassure the reader that his great intellect does not preclude him from reaching out to mere mortals.
Largely Meinesz’ efforts to inject his story with spiritual and artistic flourishes left me cold. I found them clumsy and patronising at best. Maybe there are issues of translation here, or maybe just turns of phrase that only a philosopher would object to – for example, he claims that the origin of life entails the origin of the first cells (Meinesz 2008, 23). At his worst he is arrogant and chauvinistic – why does he want to alienate half his readership by interrupting his chapter on bacteria to muse on the thought of a “beautiful woman with bare breasts” (Meinesz 2008, 65)? Some chapters feel more like random collections of essays than coherent pieces of a larger story, and some of it is downright repetitive although his writing is at its strongest in the most scientific parts, where you feel that he benefits from letting go of his stiflingly self-conscious desire to sound profound.
Three features of this book make it stand out from the crowd of other books on the history of life, such as Dawkins’ Ancestor’s Tale or Maynard Smith & Szathmary’s Major Transitions in Evolution (which Meinesz conspicuously fails to mention.) Firstly, panspermia. Although this book is entitled ‘How life began’ it does not actually treat the origins of life at all, it merely discusses the arrival of life on earth. Secondly, the emphasis on union – symbioses and endosymbioses. Meinesz claims that these kinds of relationships represent a revolution, a schism, under-represented in evolutionary theory and a departure from Darwinian evolution. Thirdly, and most conspicuously, religion. This is a self-consciously spiritual work of popular science which some might find jarring. You cannot ignore the religious content in this book, nor easily separate it from the scientific- in fact a discussion of the relationship between evolution and Christianity comprises the heart of several chapters. I’ll discuss all these departures, as well as carrying out a critical examination of his treatment of one of the more conventional topics – the transition to multicellularity.
Panspermia (or more properly, exogenesis).
Meinesz devotes a whole chapter to defending the theory that life originated on an alien planet before seeding earth and it is evidently one of his favourite axes to grind (although I struggled to find further work on it by him). It is a passionate defence, using various plausibility arguments as well as giving an impressively clear account of some fairly convoluted evidence. Meinesz presents evidence from Friedmann showing that the meteorite ALH84001 dated to around 4.5 bya (and originating on Mars) contains traces of compounds (magnetite chains) usually only formed by bacteria. Supporting evidence suggests that there is a very low possibility that such compounds were formed in abiotic reactions such as mineralization especially since they were found aligned into perfect ‘necklaces’ just as in our cells. He further argues against the possibility of contamination of the meteor or samples. Meinesz acknowledges widespread controversy as to the veracity of these claims, but attributes it to personal jealousy and conservatism. What mainstream science would have to gain by sidelining such evidence is not spelt out. This bit reads as a fascinating insight into the lives (and political battles) of scientists and it would add up into a reasonable hypothesis if it wasn’t so obviously one-sided and if he showed at least an awareness of the typical response of most evolutionists to panspermia theory. Meinesz offers no answer to the ‘So what?’ problem. What difference does it make? Most evolutionists find panspermia a hypothesis with limited appeal, simply because it seems to want to side step the mysteries that really get evolutionists going, by removing them to a more distant location. Panspermia per se does not solve the problem of how life originated, it simply extends the available time frame and environment. Meinesz chooses not, after all, to discuss hypotheses about how life began at all and, like Will Wright’s computer game ‘Spore,’ simply starts at bacteria.
Meinesz intends his work to emphasize the power of an oft-neglected force in evolution– symbiosis. French biology since Portier has tended to pay more attention to unions in biology than have the anglo-american traditions (see Sapp 1994 for a history), with their greater attention to individuals and to competition, and that tradition is continued in this most patriotic of books. We are presented with the Elysia sea slug, hero of Meinesz’ previous book Killer Algae, to illustrate the manner in which lineages can borrow traits from one another by symbiosis. The slug apparently preys on Caulerpa, a tropical alga, and ingests the alga’s cytoplasm without digesting its chloroplasts. It then deposits the chloroplasts under the surface of its skin where it uses them to produce energy just like a plant ordinarily does. It is a slug that photosynthesizes. Meinesz tells the tale well and inserts it within a larger piece on symbioses and the role they have played in major transitions in evolution. He draws a direct analogy between the sea slug using stolen chloroplasts, and prokaryotes using engulfed mitochondria to become the first eukaryotes in Margulis’ endosymbiosis theory.
It is great that Meinesz puts so much emphasis on this defining kind of union in evolutionary history, and he is honest here in presenting competing hypotheses and emphasizing the speculative nature of some of the claims. The somewhat dense material is also aided tremendously by his cheerful little cartoon storyboards. Yet the details don’t all come through crystal clear, and at times this chapter is muddled, partly because there is just too much in it. For example, the role of symbioses more generally in evolution is left unclear, so that it seems indistinguishable from co-evolution, and the text gets muddied up by non-precise use of terms like ‘individual’ and ‘partner’ which is common but damaging to a discussion of endosymbiosis. I also take issue with the extent to which Meinesz wants to claim that symbiosis is a process different in kind from Darwinian evolution. He declares it a revolution and a new genesis. But these are not equivalent. Evolutionary transitions in individuality such as the origin of eukaryotes and of multicellulars do indeed comprise new geneses in that they create objects at new higher levels of selection, but they are generally perceived as occurring due to standard Darwinian processes of variation and selection. Symbiosis may count as a different source of variation than mutation, especially if we insist that not all new behaviours have genetic mutations underpinning them, but so does lateral gene transfer, polyploidy and sex. It is true that symbiosis has often not been given a sufficiently important place in the history of life, but the optimal way to redress this balance is not to cry revolution.
Many evolutionary histories restrict their examination of aggregations to the rather prejudicially named ‘problem’ of altruism. The free-rider problem sees no mention at all within this book. Instead alliances are depicted as unproblematically synergistic relationships, illustrated with cartoon amoeba smiling even as they ingest one another.
It is true that even after Margulis’ initially astonishing hypotheses have been incorporated into mainstream orthodoxy, many biologists still view symbiosis as the exception to the norm. But would moving symbiosis closer to the spotlight in evolutionary writing really constitute the revolution that Meinesz heralds? Is symbiosis at odds with the neo-Darwinian synthesis? As is common what we have here is a difference of emphasis, presented as a difference in kind. It is true that mainstream accounts of evolution focus on the accumulation and natural selection of mutations. The question we must ask, however, is whether symbiosis constitutes a phenomenon that contradicts, extends, or fits neatly within, this description. Most authors would say that while symbiosis may constitute an evolutionary mechanism of organism construction in addition to the selection of mutation, it does not undermine or contradict traditional Darwinian selection. The problem is that the theoretical battle fought between adherents of cooperation and mutualism and of competition and survival of the fittest, is that the two sides were often misrepresented as sentimental utopians, on one side, and hard-headed realists on the other. The mundane truth acceptable to all is that natural selection will favour alliance whenever it offers synergistic benefits that cannot be acquired alone. The war of appropriate emphasis then turns on the empirical question of how often such benefits exist, and the answer increasingly looks to be: a lot. To this extent then Meinesz can be applauded for seeking to secure a more central position for symbiosis within evolutionary theory, but I fall short of calling such a change a ‘revolution.’
You can’t escape the religion in this book. Not only does Meinesz constantly refer to it, but several chapters are specifically dedicated to evaluating science in the light of belief and vice versa. One might accuse Meinesz of wanting to rewrite The Ancestor’s Tale for theists. Like Dawkins he wants to make the deep of history of life accessible and exciting for non-scientists, but without the atheistic vitriol for which Dawkins has become famous. It is not a bad ambition, as I’m sure Dawkins’ name on the cover is enough to prevent many people who would benefit from it from even opening The Ancestor’s Tale. Meinesz, a Catholic, wants to present evolutionary theory as something compatible with religion and spiritualism, without shrinking from the actual hard science. Unfortunately, he lacks Dawkins’ effortless capacity for bringing science to vivid and colourful life in the mind of the reader. I am happy to allow that I am not the believing layman at whom this work is aimed but I still feel it is a shame that scientists feel the need to muddy their work by associating it with all this metaphysical stuff on which they are not qualified to pronounce.
Meinesz sets the tone of the book in chapter one when he talks about a friend’s determination to retain ownership of land in which some of the oldest prehistoric cave paintings are found. It is a metaphor perhaps for our common ownership of our past, for Meinesz goes on to discuss the various creation myths and tries to emphasize that whatever your beliefs, our past is a shared truth to which all of us remain connected. He mixes palaeontology with bible stories in a way designed to emphasize our fascination with our origins. He mostly presents the bible stories as just stories, while the palaeontology is fact, but he demurs sufficiently to leave room for people not to feel contradicted. He even presents the standard Catholic line about God inserting the soul at some critical moment in evolution, by saying “Present-day knowledge would surely have led the authors of Genesis to reserve for God alone the impulse to create the soul.” (Meinesz 2008, 17) It is unsubtle word-weaselry. He doesn’t call it science, but he mixes his religion in with his science sufficiently closely that only prior knowledge allows the reader to easily tell them apart. Meinesz spends a long time evaluating evidence for a large scale flood that could have served as the inspiration for the biblical story of Noah and the Ark, about which I was tempted to say ‘who cares’ leaving the reader to fill in the obvious answer. But maybe that’s mean, maybe even ‘non-believers’ can find interest in the capacity that science gives us to explore the origins of these undeniably important old myths.
Contingency is the issue that Meinesz keeps coming back to because he seems to view it as presenting the biggest threat to a religious thought. He settles for Gould’s line that if we reran the tape of evolution, we would see a radically different outcome, and asks how much this non-teleological worldview threatens the way in which theists view the ‘meaning of life.’ Meinesz also adopts Gould’s other position about science and religion being NOMA (non overlapping distinct magisteria) (Meinesz 2008, 116) and he states that scientists should stick to their half of the two ‘Magisteria’ and keep out of metaphysical debates that are best left to theists (even if he fails to take this particular piece of advice himself). So while the scientific Magisteria rule that evolution is fundamentally contingent and not goal-directed, this has no bearing on the separate kinds of arguments that the spiritual magisterial are going to offer regarding mankind’s purpose in life, or what we can expect to happen after we die. He criticises intelligent design hypotheses for failing to distinguish these domains, for letting the scientific picture be dictated to by religion. He claims that the religious or spiritual domain ought to let science proceed by its own lights without intruding on or feeling threatened by its discoveries.
Most people simply reject this proposed bifurcation. Atheists reject the idea that the moral domain is only accessible by believers, while theists probably resent the idea of having to survive on science’s leftovers. Meinesz puts the religious domain into a subservient relationship with science that many theists would, I imagine, find hard to accept. The onus is on religious leaders to adjust their views as science makes new discoveries, not vice versa. Science and religion are not non-overlapping domains because we don’t know what science will discover in the future so we don’t yet have a properly delineated scientific domain. If this is the case then it is impossible for religion to be sure it does not pronounce on something which science will later contradict. Religion is condemned to play second fiddle, playing catch up.
Meinesz certainly is no apologist for creationists, and emphasizes that the bible is just plain wrong about lots of things and that life really is just “the result of a long, pitiless, random struggle for survival in the face of incessant arbitrary decapitations.” (Meinesz 2008, 193) Yet he doesn’t think this necessitates conversion to atheism, just reinterpretation of scripture. “Ancient, divine messages can be adapted to modern knowledge.” I’m really one of those atheists who would prefer ‘the faithful’ to be proper full blooded believers or not at all. Talking about the need to reinterpret religious texts while simultaneously asserting their truth and divinity seems like a textbook recipe for trouble: at least literalists have a limited number of excuses at their disposal. However Meinesz doesn’t present his religious views as justifying or excusing anything. Ultimately he is trying to show that believing in evolution does not force a person to become a non-believer. And he is probably right. Nonetheless, his religiosity is bound to raise the hackles of anyone accustomed to finding wonderment in nature without having to overlay it with a greasy coat of magical realism.
Origins of multicellularity
The move to multicellularity is a new genesis because, like Lego pieces, it provides a new and unlimited way of constructing novel organisms by the addition of different combinations of pieces in different ways. The creative power of multicellularity lies in its modularity. Organisms can be created in cumulative stages, where each stage is robust and can be added to without limit. Meinesz claims that the move to colonial living was presaged by a change in life style – some organisms left the surface waters of the oceans and settled on narrow ledges of continental shelf, where the water is shallow enough that plenty of sunlight filters through. Here they faced a whole different set of ecological challenges – no longer required to subfloat, they in fact secured advantage by anchoring themselves to the floor of their hospitable new home.
Much of this chapter is spent debating a single question – was the transition to multicellularity a simple case of responding adaptively to a changed environmental circumstances – i.e. to living on a shallow ledge (the “convergent evolution hypothesis”). Or did the move from open water simply allow a pre-existing capacity to develop and thrive where the previous environment did not favour it (the “shared software hypothesis”). Meinesz places a lot of importance on comparing these hypotheses. But what turns on whether those mutations happened before of after the change in habitat? Even if multicellularity is a new life history trait that appeared in response to a change of habitat, that trait was made possible by an underlying genetic architecture (combined of course with various other epigenetic and environmental conditions.) Probably a mutation, or a series of mutations, had to happen to that architecture before multicellularity was available as a strategy. There are a few reasons why Meinesz thinks it is important to distinguish between the rival hypotheses.
Firstly, Meinesz sees the existence of multicellularity in multiple distinct lineages, but not all lineages, as a fact in need of explanation. A trait such as flying is present across multiple lineages, including birds and mammals. We say that this trait is analogous, or has appeared by convergent evolution, because the evolution of flight in bats took place long after the bat lineage separated from the bird lineage. On the other hand, we say that possession of a vertebra is a homologous trait across vertebrates because all vertebrates descend from a common ancestor that had a vertebra. Meinesz thinks the existence of underlying homologous genetic architecture provides this explanation, but analogy does not, because if multicellularity was an analogous trait in distinct lineages then we should expect to find it in all lineages that have sessile lifestyles. Only the ‘shared software’ hypothesis can explain why some lineages haven’t made the transition to multicellularity. This is too strong, because many things that are evolutionary possibilities fail to happen. Dolphins and sharks have a convergently evolved aquiline body form that helps them swim efficiently in water, yet there are no fully aquatic marsupials. However, if the various lineages diverged before any of them had acquired the mutation necessary for multicellularity, then we have to suppose that the multicellular lineages all acquired the necessary mutation independently, which Meinesz seems to think is less attractive on the grounds of parsimony.
Another reason Meinesz has for advocating the shared software hypothesis is that offers him a route to coherence with his preferred explanation for the appearance of life on earth – panspermia. If some but not all organismal lineages possess some sort of necessary genetic precursor to multicellularity, then Meinesz can say that those lineages descended from different strains of alien bacteria.
Lastly, it seems that Meinesz prefers the shared software hypothesis because it allows him to give a scientific explanation that is compatible with a theistic need for the evolution of man to be inevitable. Meinesz claims that if multicellularity is the result of some shared software, then “organisms were pre-programmed to become multicellular when they became sedentary” (164) and multicellularity is a deterministic phenomenon.
Meinesz wants to believe that multicellularity evolved simultaneously across all lineages and only after the move to sedentary living, and that examples of multicellular organisms achieved their multicellular status via a single common mutation or set of mutations. Yet the most up to date evidence suggests that multicellularity appeared early and repeatedly, because of a confluence of environmental, ecological and genetic factors. In a recent review, Rokas 2008 claims that multicellularity is a heterogeneous trait across different lineages but that it first appeared in filamentous cyanobacteria, appearing in the fossil record 2.5-2.1 bya. Multicellular eukaryotes appeared soon after the appearance of eukaryotes, around 1.2 bya, with complex forms appearing 1.0-0.4 bya. Volvocine algae represent the most recent invention of multicellularity, around 0.05 bya. It is obvious therefore that complex multicellularity appeared neither rapidly nor simultaneously an all phyla. Furthermore, Rokas says that “Not all instantiations of multicellularity are the same, and they do differ in important details.” (Rokas 2008, 239) For example, multicellularity in volvox likely evolved after incomplete separation after cell division, whereas in Dictyostelium it is a result of aggregation. “Thus any expectation that gene families participating in cell adhesion in the two lineages would show similar trends would likely be unfounded.” (Rokas 2008, 239) It is now known that Dictyostelium achieve multicellularity using a distinct array of genetic software from the fungi, plants and animals (Williams et al 2005).
Research done on Volvocine algae also shows that volvocale multicellularity differs from metazoan multicellularity precisely because they are underpinned by distinct genotype-phenotype map structures. V. carteri achieve multicellularity using a gene RegA to conditionally inhibit chloroplast production in some of their cells. These cells are then prevented from growing to the size which triggers mitosis, and so are restricted to somatic functions throughout the lifetime of the group. This rather crude way of achieving a division of labour prevents V. carteri from developing multiple cell types and is offered as an explanation for why the volvocale transition to multicellularity has not been followed by an explosion of diversity, as in metazoan lineages (Nedelcu & Michod 2003, 2006). Other metazoans have achieved multicellularity using a more complex series of mutations at the cellular level so that different sizes of cell can be produced, and mitosis can be controlled by independent factors such as cell signalling. The key to the hypothesis is an explanation for why volvox ran up against these constraints, when other phylas did not. The answer lies in a peculiarity of volvocine mitosis. While most metazoan cells divide by binary fission, with one cell splitting to produce two, volvocales have multiple fission. Binary fission allows you to incrementally increase cell size, for example. Multiple fission means that mitosis of an adult cell reproduces a whole multicellular individual.
We can surely imagine similar sorts of constraints might block the possibility of multicellularity altogether in some lineages, refuting Meinesz’ conjecture that only a shared software hypothesis could explain the absence of multicellularity in these lineages. On the other hand it has been found that most of the genetic toolkit necessary for multicellularity in metazoan lineages is also present in unicellular ancestors. Genes have mostly been co-opted rather than gained anew, though they have often dramatically increased in number or gained new functions. Some components however do seem to be genuinely novel innovations. The main genetic changes concern genes responsible for regulating cell differentiation, cell-cell signalling pathways and cell adhesion. In the evolution of animal multicellularity, “gene machinery predated but was co-opted for multicellularity in the time antecedent to the transition.” (Rokas 2008, 246) In fact if we look to the literature we find the most up to date consensus is that the whole bilaterian clade - i.e. all the different animal phyla that evolved from a common sponge or cnidarian ancestor - share a common genetic toolkit, including ancient hox gene clusters. Geneticists think that duplications and modifications (tinkering) of these very old genes underpin all of the modern body plans. The closest relatives of the bilaterians, the cnidarians and sponges, show intermediate forms of multicellularity/coloniality and a range of sessile and motile lifestyles.
Multicellularity is a homologous trait in some respects, and an analogous trait in others. It is interesting to ask what was the relative contribution of extrinsic (ecological and environmental) and intrinsic (genetic) factors in the origin of animal multicellularity, but this does not amount to the dichotomy that Meinesz portrays. The evidence therefore says that to some extent the software did predate the transition (within this lineage) but to some extent new mutations were needed, as well as much gene duplication and cooption of function. Meinesz may be overplaying the importance of analogy versus homology here - the matter turns on common ancestry, which is a matter of degree (Griffiths 2007 denies this, but on the alternative developmental view of homology then multicellularity probably is not a candidate homologue at all). All phyla have a common ancestor (probably, because they all use the same DNA code) and homology and homoplasy are not sides of a dichotomy but ends of a continuum, separated by varying degrees of modification, reflecting deep or more recent ancestry (see Hall 2007). The final question then is why Meinesz or anyone else should believe that securing one end or other of this continuum as the explanation for a trait has any bearing at all on the meaning of life? Homology looks a long way away from the kind of inevitability that theists really seek.
The epilogue that ends How Life Began serves primarily as a call to arms. Scientists, Meinesz declares, must leave their ivory towers and face the responsibilities of dissemination and communication of knowledge. Biologists, in particular, have a duty to guarantee a widespread appreciation of the beauty and fragility of the world we live in. Whether or not I believe in life after death, I agree that biologists have a special part to play in ensuring that there is life after tomorrow.
Gould, S. J.: 2000, The Lying Stones of Marrakesh: penultimate reflections in natural history, Random House.
Griffiths, P. E.: 2007, The Phenomena of Homology, Biology and Philosophy22: 643-658.
Hall, B. K.: 2007, Homoplasy and homology: Dichotomy or continuum?,
Journal of human evolution.
Nedelcu, A. and Michod, R. E.: 2003, Evolvability, modularity, and individuality during the transition to multicellularity in volvocalean green algae, in G.Schlosser and G. Wagner, (eds.) Modularity in development and evolution, Univ. Chicago Press, Chicago.
Nedelcu, A. & Michod, R. E.: 2006, The Evolutionary Origin of an Altruistic Gene, Molecular Biology and Evolution.
Rokas, A.: 2008, The Origins of Multicellularity and the Early History of the Genetic Toolkit for Animal Development, Ann. Rev. Genet. 42, 235-51.
Sapp, J.: 1994, Evolution by Association – A History of Symbiosis, Oxford University Press, Oxford.
Williams, J., Noegel, A. & Eichinger, L.: 2005, Manifestations of multicellularity – Dictyostelium reports in, Trends in Genetics 21, 392-398.