The Great Chain of Being

26.5 essay 429 MH 23/5/05 9:59 AM Page 429

NATURE|Vol 435|26 May 2005 ESSAY 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.

Sean Nee medieval theologian, albeit with some nov- Changes in ocean ecosystems wrought elties and the startling omission of God. by Bacteria and Archaea contributed to For centuries the ‘great chain of being’ held By starting with us, Dawkins regenerates the deposition of the ocean sediments, an a central place in Western thought. This the chain because species that are more event of enormous significance: these sed- view saw the Universe as ordered in a lin- closely related to us are more similar as iments became the habitat for bacteria that ear sequence starting from the inanimate well, and such similarity was an important now constitute about one-third of the total world of rocks. Plants came next, then ani- criterion in determining the rankings in living biomass today. (A side-effect of the mals, men, angels and, finally, God. It was the classical chain. But there is nothing deposition is the oxygenation of the very detailed with, for example, a ranking about the world that compels us to think atmosphere by photosynthetic bacteria.) of human races; humans them- Evolution continued for bil- selves ranked above apes above lions of years, with many remark- reptiles above amphibians above Human able innovations stimulated by fish. This view even predicted a both cooperation and conflict. world of invisible life in between Ape For example, Bacteria evolved the the inanimate and the visible, liv- capacity to communicate chemi- ing world, long before Antonie Reptile cally to coordinate attacks on oth- van Leeuwenhoek’s discoveries. ers, and a willingness to commit Amphibian Although advocates of evolution suicide for the greater good of the may have stripped it of its super- community. Around a billion natural summit, this view is with Fish years ago, a great experiment us still. occurred: Bacteria and Archaea Common presentations of evo- Amphibian came together in a fusion event to lution mirror the great chain by synthesize a whole new domain viewing the process as progres- Reptile of life, the Eukarya. Sadly, the out- sive. For example, in their book come was rather uninteresting: The Major Transitions in Evolu- Ape the resulting organisms displayed tion, John Maynard Smith and a very limited metabolic reper- Eors Szathmáry take us from the Human toire and much restricted habitat origin of life, through to the origin requirements. of eukaryotic cells, multicellular- Fish Over the past 600 million years ity, human societies and, finally, of the Bacteria, Archaea and micro- 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 sarily lead to progress, and even it happens, a few branches of 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. whose rungs progress through microbes, the other construction is equally valid. One of the huge species, Homo sapiens, plants, and, at the top, large animals. Here is another view of evolution, but got remarkably self-important. But when, 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 of Evolution (W. H. Freeman & Co., Oxford, 1995). backwards along our ancestry, Dawkins able capacities to thrive in every environ- Dawkins, R. The Ancestor’s Tale (Weidenfeld & generates a sequence of chapter titles ment available, including superheated, Nicolson, New York, 2004). that would read like a typical chain to a pressurized water deep in the oceans. Nee, S. Nature 429, 804–805 (2004).