ESSAY

NATURE|Vol 435|26 May 2005

The great chain of being Our persistence in placing ourselves at the top of the Great Chain of Being suggests we have some deep psychological need to see ourselves as the culmination of creation. Changes in ocean ecosystems wrought by Bacteria and Archaea contributed to the deposition of the ocean sediments, an For centuries the ‘great chain of being’ held event of enormous significance: these seda central place in Western thought. This iments became the habitat for bacteria that view saw the Universe as ordered in a linnow constitute about one-third of the total ear sequence starting from the inanimate living biomass today. (A side-effect of the world of rocks. Plants came next, then anideposition is the oxygenation of the mals, men, angels and, finally, God. It was atmosphere by photosynthetic bacteria.) very detailed with, for example, a ranking Evolution continued for bilof human races; humans themlions of years, with many remarkselves ranked above apes above Human able innovations stimulated by reptiles above amphibians above both cooperation and conflict. fish. This view even predicted a Ape For example, Bacteria evolved the world of invisible life in between capacity to communicate chemithe inanimate and the visible, livReptile cally to coordinate attacks on othing world, long before Antonie ers, and a willingness to commit van Leeuwenhoek’s discoveries. Amphibian suicide for the greater good of the Although advocates of evolution community. Around a billion may have stripped it of its superFish years ago, a great experiment natural summit, this view is with occurred: Bacteria and Archaea us still. Amphibian came together in a fusion event to Common presentations of evosynthesize a whole new domain lution mirror the great chain by Reptile of life, the Eukarya. Sadly, the outviewing the process as progrescome was rather uninteresting: sive. For example, in their book Ape the resulting organisms displayed The Major Transitions in Evolua very limited metabolic repertion, John Maynard Smith and Human toire and much restricted habitat Eors Szathmáry take us from the requirements. origin of life, through to the origin Fish Over the past 600 million years of eukaryotic cells, multicellularthe Bacteria, Archaea and microity, human societies and, finally, of language. They explicitly point Although both representations are equally valid, we instinctively bial Eukarya have continued to out that evolution does not neces- position ourselves at the top of phylogenetic trees (upper panel). evolve into brand new niches. As it happens, a few branches of sarily lead to progress, and even refer to the great chain by its Latin name, about it in this way, suggesting, instead, Eukarya —plants and animals — grew scala naturae. But it is impossible to over- that we have some deep psychological need freakishly huge bodies. They also created look the fact that the ‘major’ evolutionary to see ourselves as the culmination of cre- both new substances for bacteria to transitions lead inexorably, step by step, to ation. Illustrating this, when we represent exploit, such as plant lignins, and new us. Similarly, in their recent essay in Nature, the relationships between species, includ- environments for microbes to inhabit, ‘Climbing the co-evolution ladder’ (431, ing ourselves, in a family tree, we automat- such as feathers and urinary tracts. Indeed, 913; 2004), Lenton and colleagues illustrate ically construct it so that the column of some of the richest and most interesting their summary of life–environment inter- species’ names forms a chain with us as the ecologies on Earth can be found inside the actions through the ages with a ladder top, as in the first of the trees pictured. But animal gut. One of the huge species, Homo sapiens, whose rungs progress through microbes, the other construction is equally valid. Here is another view of evolution, but got remarkably self-important. But when, plants, and, at the top, large animals. In his recent book The Ancestor’s Tale, this time from the point of view of microbes to his surprise, a virus wiped him out, most Richard Dawkins reverses the usual tempo- — the main form of life on our planet. From of life on Earth took no notice at all. ■ ral perspective and looks progressively fur- the mists of time, nearly 4 billion years Sean Nee is at the Institute of Evolutionary ther back in time to find our ancestors. Like ago, three great domains of life emerged: Biology, School of Biological Sciences, Maynard Smith and Szathmáry, he cautions Bacteria, Archaea, and the molecular University of Edinburgh, West Mains Road, us against thinking that evolution is pro- parasites of these, such as viruses. Over Edinburgh EH9 3JT, UK. gressive, culminating with us. He empha- hundreds of millions of years the Bacteria FURTHER READING sizes that with whatever organism we begin evolved an extraordinary variety of bio- Lovejoy, A. O. The Great Chain of Being (Harper and the pilgrimage back through time, we all are chemical capabilities, including the ability Row, New York, 1965). reunited at the origin of life. But by begin- to generate light, and to ‘eat’ and ‘breathe’ Gee, H. Nature 420, 611 (2002). ning the journey with us and looking metals. The Archaea also evolved remark- Maynard Smith, J. & Szathmáry, E. The Major Transitions Evolution (W. H. Freeman & Co., Oxford, 1995). backwards along our ancestry, Dawkins able capacities to thrive in every environ- of Dawkins, R. The Ancestor’s Tale (Weidenfeld & generates a sequence of chapter titles ment available, including superheated, Nicolson, New York, 2004). Nee, S. Nature 429, 804–805 (2004). that would read like a typical chain to a pressurized water deep in the oceans.

Sean Nee

medieval theologian, albeit with some novelties and the startling omission of God. By starting with us, Dawkins regenerates the chain because species that are more closely related to us are more similar as well, and such similarity was an important criterion in determining the rankings in the classical chain. But there is nothing about the world that compels us to think

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