Coevolutionary History

Edmund Russell is the Joyce and Elizabeth Hall Professor of United States History at the University of Kansas, where he has taught since 2013. He is the author of War and Nature: Fighting Humans and Insects with Chemicals from World War I to Silent Spring (Cambridge University Press, 2001) and Evolutionary History: Uniting History and Biology to Understand Life on Earth (Cambridge University Press, 2011). He co-edited Natural Enemy, Natural Ally: Toward an Environmental History of Warfare (Oregon State University Press, 2004) with Richard Tucker and Guide to U.S. Environmental Policy (CQ Press, 2014) with Sally K. Fairfax. His research interests include environmental history, history of technology, U.S. history, and global history. Coevolution has been one of the most important processes in history. Yet despite its ability to lead to new interpretations of well-studied events, it has also been one of the least appreciated. Its low profile is due partly to the disciplinary divide between human history and natural science, partly to historians' reliance on sources that did not recognize it, and partly to the ease with which one can take its products for granted.1 But look at some of its effects. By ushering in the Agricultural Revolution, it was responsible for the transition from prehistory to history (traditionally defined). It was the primary means of increasing physical power for almost all of history. It helped spark, and sustained, the Industrial Revolution. It helped human numbers to soar from 954 million to 7.1 billion in the past two hundred years. It was responsible for the food people ate, the way they made their living, the diseases they suffered, and the technology they developed. Not a trivial list of accomplishments.2 Coevolution is the process in which populations of different species evolve repeatedly in response to each other. The key ideas are reciprocity and continual change. Population A leads population B to evolve, the new version of population B leads population A to evolve, the new version of population A leads population B to evolve again, and so on through time. The idea of coevolution was first developed to explain why flowers seemed perfectly designed for the specific species of insects that pollinated them. Most likely, the traits of populations of plants and the traits of populations of their insect pollinators changed repeatedly in response to each other. Other examples of coevolution include fleet predators and prey (when one became faster, the other had to become faster, too, to survive) and the development of leguminous plants with nitrogen-fixing bacteria that inhabit their roots.3 Historians would have nothing to study without coevolution, because human beings probably would not exist. We might think of our bodies as entirely human, but it would be more accurate to think of them as porous ecosystems swarming with bacteria, fungi, protozoa, and viruses. Symbionts in our guts, hair, skin, and mouths help us survive by digesting food and protecting us from disease. They make up 90 percent of the cells in our bodies. Human cells are larger than bacterial and fungal cells, so our bodies are more human than not when it comes to volume, but our bodies are more bacterial than human when it comes to numbers. We have a lot to learn about human microbiota, the extent of which has only recently been documented, but evidence suggests that coevolution has adapted us to our microbiota and vice versa.4 (For more on this topic, see Julia Adeney Thomas's essay in this roundtable.) Coevolution in history occurred when populations of people and of non-human species repeatedly shaped each other's traits over time. Coevolution is an evolutionary process, and the idea that people can prompt evolution is unfamiliar to many scholars outside biology, so it is important to clarify the meaning of evolution before returning to coevolution. A popular definition equates evolution with the development of new species over millions of years through natural selection. If accurate, this definition would disqualify people as evolutionary actors. Few of us can identify a species that people created. Our species could not affect other species over millions of years because Homo sapiens developed perhaps 195,000 years ago. If “nature” refers to the non-human world, then natural selection would seem to exclude human actions. The problem with the popular definition is that it is too narrow. All development of new species over millions of years through natural selection is evolution, but not all evolution is development of new species over millions of years through natural selection.5 Biologists define “evolution” as change in the frequency of inherited traits in populations over generations. This definition contrasts with the popular definition in at least six ways. First, the entities that evolve are populations, not necessarily entire species. Populations consist of members of a species that live in a given place and usually do not interbreed with members of other populations. Sometimes all populations of a species evolve, in which case the species evolves, but local populations can evolve without affecting other populations of the same species. Second, any degree of change qualifies as evolution. Sometimes changes are so radical that populations become new species, but most evolution involves smaller changes within populations. All changes in the frequency of inherited traits of populations over generations are evolution, even if those changes are temporary and later reversed. Third, the time required for evolution is only two generations, not a specific or large number of years. Species with short generation times, such as bacteria, can evolve in hours. Fourth, natural selection and evolution are different processes. Natural selection is a mechanism that leads to evolution. It is the differential survival of individuals due to differences in traits. Selection acts within generations. Fifth, evolution does not require natural selection; there are other evolutionary mechanisms as well. Sixth, evolution is defined by a pattern (change in inherited traits), not by the cause of the pattern. People are as capable as any other species of affecting evolution.6 An example from Africa shows how evolution can take place in historical time as a result of human action. In some populations of elephants, the frequency of an inherited trait (tusklessness) increased over the twentieth century. Two mechanisms other than natural selection (if taken to exclude human actions) were responsible. One was human selection. Poachers killed elephants for their tusks, which they sold into an international ivory market. Poachers had no reason to kill tuskless individuals, which survived and reproduced at a higher rate than tusked individuals. Another mechanism probably was drift, which means differences in reproduction rates of individuals due to chance. Once elephant populations became small, tuskless individuals apparently reproduced more often than tusked individuals by chance, which led tusklessness to increase even in the absence of poaching.7 In addition to situating evolution in (not outside of) history, this example illustrates the need for historians and biologists to join forces to understand the way life has changed over time. The traditional tools of historians do an excellent job of explaining the social factors that led to selection for tusklessness. Art historians can explain the development and appeal of ivory carving, economic historians can analyze the development of the international ivory trade, political historians can explain why some African countries lacked the capacity or desire to enforce laws against elephant hunting, and social and economic historians can analyze the enduring poverty that created a strong incentive for poaching. But the traditional tools of history cannot explain why killing tusked elephants encouraged tusklessness over generations. The tools of biology can. Genetics explains how elephants inherited tusklessness from their parents. Evolutionary biology explains why tusks evolved (they aided survival and reproduction) and why they became less common (hunting made the risks of tusk-bearing outweigh the benefits). Reducing populations to a few individuals increased the odds that chance differences in reproduction would affect the frequency of traits in populations. Another example illustrates the life-and-death consequences of coevolution in historical time. The earth sustains more than 7 billion people today only because of coevolution that resulted in highly productive domestic plants and animals and people who knew how to tend them. But coevolution also has sent millions to their graves. The end of one of the earth's great killers, malaria, hove into sight after World War II when new insecticides (to kill malaria-carrying mosquitoes) and drugs (to kill malaria plasmodia) became common. The worldwide malaria-eradication project made stunning progress until mosquitoes and plasmodia evolved resistance to insecticides and drugs. The end of malaria receded back over the horizon, and the World Health Organization abandoned the eradication project. By 2000, about 2 million people were dying yearly of malaria, and the disease infected 300–500 million more. of life and of other examples of coevolution that changed the of human and the of members of non-human species, in large and Human beings have the evolution of populations of other species through many two of them. selection is the process in which people affect the traits of populations without to do usually by individuals with traits survive more often than those with other traits. selection by people as for natural selection in the for tusklessness is an selection is the process in which people affect the traits of populations through or by reproduction to individuals. and are A mechanism is drift, in which the frequency of traits in a population changes as a result of chance differences in reproduction of individuals with those traits. a has a way to the traits of populations, by from one to and in the or to from and and and plants to the of a human The of human beings on evolution has led an evolutionary to that are an By the so that scholars have that live in a new the have changed the in which all evolution led to coevolution when changes in traits of non-human populations back to change traits of human populations. A example is the coevolution of human populations with that The U.S. and first the of a of from bacteria, in the had changed a trait of a bacterial and this new trait in a bacterial population back to change the frequency of in a human The same can be about of other human diseases that have due to coevolution with non-human populations. By the U.S. and and the had the of products of of them from populations of a species of bacteria, so human populations were with at least populations of Other products have from populations of an and Historians have that the Agricultural was the most important in history. The development of led to populations, large social social outside the who populations, and increased disease up from domestic The Agricultural the for historians have and historians have one of its of to the end of prehistory and the of Historians other have the Agricultural to it one of the most important processes in history. Most of the or to life in with and to the of This definition has key most of them First, is an evolutionary process changes in traits of populations to them to a human Second, changes non-human The definition does not human but does it require Third, the of to The definition does not to non-human but does it require Fourth, might be a The definition does not continual change after but does it require Fifth, the on it to that people the or historians have and in with often as a and process that people and of years a way of about and history. the Agricultural to as usually is partly People did change the traits of and populations in that human beings of years ago. This process was for the Agricultural Revolution, but it was not The traits of human populations also had to change the process It is probably more accurate to think of as a between populations of two species, than as a into which one the other. it, of such a new and pattern of is the or in the is to and human and non-human populations had to to up to the The process has not been or but it has been often to but selection to have an even People for in the have the traits of non-human and human populations to result in One of the of this in by the K. He by for only one which he defined as to human generations, percent of the with They to their to and their It is to that people and each other by a to have more food than their them an in survival and which might have led to domestic populations One of the from was that selection for could also physical traits in domestic had traits in such as and and If need that could need only look to the have domestic with and insects and other species of and no one has them with A two other in the traditional definition of One is the that people the We could as that non-human populations the the example of could have and human human could have of and and to new where people could them as could have encouraged human to by them or We can about people probably helped to from by large But it would be accurate to that the process by large which encouraged human beings to and them. than us to one or the other as as the common of coevolution us to on the of in a Another is the that only people from They but so did the that people helped to survive and It might seem that changes in the frequency of in human populations can be as changes in the frequency of inherited traits. of us are to of traits as physical But are traits of individuals and populations, too, and they can be as for survival and think and if recognize as it might be to see how they could evolve, because evolution of traits. We that a means of physical traits. traits also such as the of and are the of can affect if Other have no the ability to the We can for a new when to a new but cannot explain the new the same The to this in that many species, have at least two means of and have to many and other scholars it to ideas about how to do and the same They all to for for the meaning of the of their The key is that and for traits or so in can in ways. It for traits from to for a trait in traits might result from a but result from the of might be or also from and in generations to is so it can change within an which traits to evolve more than traits. be as in or it be as when an can be outside bodies in a as as in bodies People have no on In many species, their how to other Human and non-human populations in at least ways. First, and probably most human populations changed and non-human populations changed People changed their for and for traits they in other as large of Second, human and non-human populations changed plants and animals evolved in that increased the of such as and the frequency of that helped people those Third, people evolved and populations of other species evolved human populations evolved of in animals to and to from to in the Fourth, human and non-human populations evolved People to and them in and to be Another of a for list is its on continual change. The first plants and animals would have had low so had to for traits to increase of plants and animals would populations, so had to that them. would evolve to a given to new and so on in a and animals to new to new and One of on history has been its to Historians in many the often without Two are physical power and social define physical power as the rate at which is to for power include and This definition our to two key that to and the sources of It is important to study physical power not only for its but also because it has to social power meaning other people to do one power might have such as to but nothing social without some physical at by human bodies. physical power also leads to social as and political have the for almost all of history, people on as the primary for to people in and do the of more than the of power that of the Industrial to The of to explain to how their could so they in of their In their the were so defined one as the a could or a in a time. This to the of the same with or with Coevolution was responsible for and the sources that them. Coevolution shaped the and of animals to make them in and in in the but and from and In their to were not them to but to designed for the same or The for from products of coevolution, such as and of plants for human also could from such as One of the of was their ability to domestic and in on such as and and on plants and and other animals were not the only The and of history were human People their to do and they or other people to for and physical power each other when some people from the of In all human were that food to the Agricultural Revolution, most of the food that human from domestic those that in such as and domestic most and in the of In from domestic animals also human The in from domestic and Coevolution was responsible for the domestic plants that to human and It was also responsible for the domestic animals that domestic and plants into that human The coevolution that led to animals human power and human power by of numbers could the of many and of could a of than a It between a hundred and two hundred for a to a of for a and a more than to do so with a a animals human power by increasing of domestic more food for people and for They did this with their for which helped more and by from with their guts, by into the primary for before in the twentieth century. on coevolution of domestic animals with non-human species the animals them could not of in their did it for power was not for but for the by other of history. a few domestic animals and the that the great of of and the and into and the and of on the of and social Coevolution was one of the the important means of and development in history. that people to do human are and all tools are time human domestic with new populations, they new of time they adapted a or to a place or they developed the We recognize and development in history, and processes with the and after the Industrial Revolution. But many of us are not to the power of and In addition to the Agricultural coevolution a key in the Industrial Revolution. This idea to a of in which biology as the in history. In coevolution made the Industrial in at least ways. were not for but they were First, coevolution to where the Industrial was was a of who such as and that helped the The their ability to on and development to who more food than they The of from their coevolution with domestic plants and This seem an and it but it is a to take the for If to explain how that need to about the of in the that the It would be to a and the of food in the that and social need to Second, coevolution the Industrial by the bodies of Most of the research on in the Industrial has on in the and the of and for sources the for was an of All the in the would have been without to the that on The of the Industrial this could not food to its so helped lead the to the and One that to for was the United The of partly on and partly on coevolution that adapted to increased and despite coevolution with that would have sent and the of a adapted by Third, coevolution with traits to Coevolution was important for but it was also important for of has been a in the Industrial Revolution. The first of developed to into developed to into The years apparently too short to into at less by have been for as in Human selection probably for first by and then by and this and did not in which new on the of did not the because of their They their to to an created for them by coevolution between populations and human populations The coevolution years and with development of and of populations to the of A explain the and of in that other have Historians have to such as with of and but they have a time this to explain why the at a specific time in a of the traits were common in other of the We can this if think of not as a as economic historians are to but as the of populations with different traits. from different species in the World and the World than World was because it the of into into strong might short into for but too often for it to be to less to as on of from the it to from the World into in the which partly explains the and of from the World to as of the trade, and the of of from the and the did not the World more than World but it because they had no In example of coevolution, the United States became a only after a population of a species to in the and other of the became important only after they World species with populations of World species that they adapted to local The only because populations of and of to This example shows how between populations with traits can lead to radical change in history. In this the a human population in with traits and a population with traits was a of chance. in the and to the economic and other traits evolved in other and to traits were but for If had been would have World If had been would have in the The new of a human population with traits and populations with traits created an (not a to that helped the The of the of history. It is not or as some historians might from an that on natural By it the of historical and By the of non-human populations, it leads to a more of and consequences than that to human actions. By and within populations, it the that historians By chance between populations and it is more than that historical change to human A up the of coevolution for history (traditionally defined). The of human such as and are the of a The only because it on a that has for of years. The in the are between populations of people and populations of domestic and people to the they more with new and adapted to new to a food became more they up other people to take up other and the of the and so created the who the first of the which has about as as the who the over the past of or The and The can be to why evolution has been to in history. the and to more and more time their food to the and many the so they had idea of the of human and non-human If they they often on the same and for often for It became to take the for and to think that the because of people the of and they their If on their to history, their If it to see that historical change is from changes in populations of non-human history is not disciplinary by natural It is a that historians and biologists to by ideas that Historians who the in biologists who our and By on the of history and biology, history can prompt new and We can by about social human and for differences in traits of populations of other differences in traits of non-human populations back to the way human populations have with each different economic shaped populations of a non-human species in different but changes in non-human populations back to human populations in non-human populations developed new traits that or change in human new traits or social If any of can on to the two most why and of the are to on of this I the other in the the the and at the University of Kansas, and the for the of this essay on ideas in Edmund Evolutionary History: Uniting History and Biology to Understand Life on Earth which was by a from the University of and a from the or in this are those of the author and do not necessarily the of the I those sources of The on power on ideas in Edmund Russell of the History of and Environmental and 2 I and for their to