Forest City
Ben Denham on forest fitness
What makes an organism fit? How can we measure its fitness? What are the unfit bits?
This essay is part of a new Sydney Review of Books essay series devoted to nature writing titled the New Nature. We’ve asked critics, essayists, poets, artists and scholars to reflect on nature in the twenty-first century and to grapple with the literary conventions of writing nature. Read the other essays in the New Nature series here.
When health insurance companies in the United States started offering discounts to customers who logged a certain number of steps on their personal activity tracker, artists Tega Brain and Surya Mattu decided people should be given an alternative to doing actual exercise. The solution was Unfit Bits, a series of simple hacks — like strapping a Fitbit to a metronome or a dog — that allow users to log physical activity without actually having to do it. Unfit Bits promised a free lunch to those seeking a discount on their health insurance.
Discounts for activity tracking data is an extreme example of efforts to quantify the fitness of an individual and translate it into dollars. The relationship between such devices and insurance companies is part of a larger logic of monitoring and surveillance in which the idea of individual responsibility meets a coercive technical and social apparatus. The broader set of social and interspecies relations that have been shown to contribute to human wellbeing is ignored in favor of a single measure of fitness which becomes a commodity for both the individual who uses their data for an insurance discount, and for the corporation that mines that data for commercial purposes.
Unfit Bits can help us to think about how quantifying biological fitness more generally is not simply a case of gathering the data. It always has an ideological bent. Conceptions of fitness, how we measure them, and why we focus on particular species all reflect broader ideological and ethical questions regarding our relationship with one another and the organisms that surround us and make us what we are.
This is true partly because there are so many agents operating in complex ecosystems, each in a set of complex relations that produce the many different forms of fitness that we find in the biological world. We can only ever tell a small part of this story. The bits of the story we choose to tell inevitably reveal something about how we see the world and our place in it. Ever since Darwin’s time evolutionary theory has found its way into social and political discourse. The phrase ‘survival of the fittest’ was coined by Darwin’s contemporary Herbert Spencer to draw parallels between Darwin’s work and his economic theories. Contemporary economists continue this dubious tradition of aligning their theories with evolutionary thought.
To say that fitness is a matter of life and death is both a simple statement of fact and an invitation to think through the many social and ethical problems posed by really interrogating our biological relations.
What makes an organism fit? How can we measure its fitness? What are the unfit bits?
When Richard Dawkins spoke to Radiolab co-host Robert Krulwich he articulated a view of biological fitness that would be familiar to Darwin, Herbert Spencer – and to pretty much anyone who has thought about this subject since the mid-nineteenth century.
Dawkins talks about the paradigmatically fit cheetah — what could be fitter than the fastest land animal? The cheetah appears to be ‘beautifully designed’ for catching gazelles, and gazelles for escaping from cheetahs, but, he notes, with suitable solemnity, these animals are ‘the end products of a sort of evolutionary arms race in which thousands, millions, of animals have died’.
The carving of the shape of a cheetah or a gazelle has come about through millions of unsuccessful gazelles being caught and the successful ones making it through, only to be caught later probably, but after reproducing and passing on the genes that help them to escape. So the shear number of deaths that lie behind the sculpting of these beautiful creatures is horrifying and at the same time it’s got a kind of savage beauty.
There are plenty of things we might object to in this cartoon version of biological fitness but we should first acknowledge what Dawkins gets right. Survival and, by extension, reproduction are what matters most in biological fitness. In fact, as microbiologist Lynn Margulis and others have noted (to Herbert Spencer’s likely chagrin), survival is fitness.
While Dawkins is right to connect survival and fitness it's also where his descriptions of the gazelle and the cheetah are misleading. By associating biological fitness with the kind of fitness that allows the cheetah and the gazelle to run fast, hunt or escape, Dawkins privileges a heroic, athletic form of individual animal fitness over the myriad survival strategies that we find in all kingdoms and domains of life. For every creature like the cheetah or gazelle who could log the required number of steps on a pedometer to qualify for an insurance discount there are many more whose fitness doesn’t depend on physical strength, speed or agility.
Biological fitness isn’t just about being able to run fast. If it were cheetahs wouldn’t be facing extinction. All extant life has found a strategy that works. The cheetah isn’t a great model for biological fitness. It has had trouble adapting to the changes caused by the human primates that evolved alongside it and the metabolism that allows it to run fast puts it right at the edge of its ability to feed that metabolism. Even the act of walking, let alone running, burns large amounts of energy that must be constantly replenished through successful hunting. The cheetah’s apparent fitness — its extreme speed and agility — may also be a vulnerability.
Once we look beyond the simplistic formulation of an evolutionary arms race between two species to see how those species relate to a broader ecology we start to see that there are many nested relationships that contribute to the fitness of any given species. We might call these nested relationships forest fitness. Here’s a fairly literal example of what I mean by this term.
The black howler monkey (Alouatta pigra) lives in the tropical forests between Guatemala, Mexico and Belize. It has long black fur, deep set eyes, and a large frowning mouth. It eats a diet of leaves, fruits and flowers. As these foods pass through the monkey’s digestive system they meet of menagerie of bacteria that help it to extract the nutrients it needs. Up to 31 per cent of the monkey’s energy comes from the microbes that break down what would otherwise be the indigestible parts of plants. These microbes ramp up their energy production for the monkey when seasonal variations mean that food quality and dietary energy intake are at their lowest.
In their 2013 study of habitat degradation Katherine R Amato and her colleagues have found that black howlers living in undisturbed rainforest environments eat a wide variety of plant species and have as many as 7000 bacterial phyla (or ‘species’, more on the scare quotes later) in their guts. In contrast, monkeys living in captivity and in degraded habitats eat fewer plant species and can harbour as few as a thousand bacterial phyla.
Low bacterial diversity may contribute to the premature death of black howler monkeys in captivity. Key fermenting and energy producing bacterial species are more prevalent in the guts of monkeys in undisturbed environments; harmful sulfate-reducing bacteria are found in greater numbers in the guts of captive monkeys and those found in disturbed environments.
It’s not just black howlers that benefit from bacterial diversity in the GI tract. It has become a proxy for the health and fitness in other mammal and primate species including humans.
The fitness of the black howler monkey depends on eating the wide variety of plant foods that is available in undisturbed forests. This diverse diet is reflected in diverse microbial communities in the gut of the monkey, diversity that can in turn be read as a proxy for the monkey’s general health and wellbeing. Unlike the one to one relationship between the gazelle and the cheetah, here a culture of relation to tens of plant species produces the diverse culture of thousands of bacterial phyla in the digestive tract, with all of these species and more combining to produce the forest fitness of the black howler monkey.
We can also find this culture of relation in the way the black howler monkey contributes to the fitness of particular plants by distributing their seeds through defecation. One such species is the breadnut tree (Brosimum alicastrum). It produces a highly nutritious seed that the black howlers love. The monkeys are agents that change the composition of the forest: their favourite foods sprout more frequently, particularly around their sleeping sites where they shit the most.
The black howler monkey is embedded in an ecological network that includes intimate relationships to the plants they eat and the gut bacteria that help to sustain them. This network of relation operates partly through different forms of chemical sensing and communication. Comprehending the full complexity of this sensing and communication between domains of life in diverse ecosystems is beyond the scope of reductive science. We can only imagine this diversity of relation. The task of imagining these networks takes us from science to art – and to the idea that we might think of relation in these ecosystems as the cosmopolitan structure of forest cities.
This idea comes from Peruvian author César Calvo’s novel Las Tres Mitades de Ino Moxo y otros Brujos de la Amazonía (The Three Halves of Ino Moxo and other Witch Doctors of the Amazon). This novel, first published in 1981, describes a series of encounters between the narrator, César Soriano, and witch doctors who all have some association with Ino Moxo, the witch doctor at the centre of the novel. These encounters are part of a journey through the Amazon that eventually leads César Soriano to a meeting with Ino Moxo.
On the way Soriano meets the witch doctor Don Javier in the bar of the Hotel Tariri, in Pucallpa, a city in the western Peruvian Amazon. Don Javier is experienced in the use of ayahuasca; a psychedelic decoction made with two plants that is used in the Amazon for both sorcery and medicinal purposes. During this encounter Don Javier alludes to something of the unknown complexity of relation in the Amazon:
This earth is made of beauties that have never been told, or that have been told badly, which is worse. I'll give you an example; you've seen those drawings on the walls of Hotel Tariri.
The design of these drawings was produced by the Shipibo people of the Western Amazon. As Don Javier explains to Soriano, what might appear to be merely pretty pictures are actually highy significant:
The Shipibo drawings are maps, but of forest cities, cut through with impossible rivers, not avenues, labyrinths of winding paths, not regimented streets, lovers, ravines, sadness and swamps instead of cold parks, cinemas and boulevards! Maps of cities, more than portraits of souls! Houses that change place, just like the days of life in the jungle, just like the houses of the Ashaninka who move every year and burn their huts and their gardens and return everything to the tangle, and go somewhere else and start again building their shelter, sowing their seeds, their life, before burning it all again the following year and going elsewhere to be reborn!
The soul is a reflection of the forest city, a reflection of the many relations that sustain life in the Amazon. We find an allusion to the agricultural practices of the Ashaninka people. The burning of huts and gardens and leaving the land fallow are ways to maintain and improve fertility for food and medicinal plants. This cyclic process is part of a broader relationship between Amazonian people and the hundreds of plants they eat or consume medicinally. Don Javier’s description of Shipibo designs suggests that they are maps of these dynamic and sustaining relationships between diverse organisms.
In my reading of this section from the The Three Halves of Ino Moxo the forest city is not a metaphor. It is, rather, a way of perceiving the forest of the Amazon as a dynamic and yet highly structured environment in which agents of all kinds — including plants, humans and other animals, and spirits — all produce both the map and the terrain. The forest city articulates the perception of multiple agents as they co-create a particular ecology. This is partly a psychedelic insight, related to the ceremonial use of ayahuasca in the Amazon, a practice which can facilitate relationships with other plants and animals, and reveal the patterns that connect species through the material and spiritual fabric of the forest.
Of course the forest city, seen from a completely different perspective, is also something that might be built by humans. Stefano Boeri, ‘the architect famous for his plant-covered skyscrapers’ is proposing to do just that in China.
The forest fitness that emerges out of a diverse community of organisms has a long evolutionary history. It is evident in the oldest living fossils and what is perhaps the greatest evolutionary success story in the history of life on our planet; stromatolites. These structures, created by the aggregation of sediments by communities of microbes, initially in marine environments, appear in the fossil record something like 3.7 billion years ago (the exact dates are a matter of conjecture).
In Shark Bay, Western Australia, living stromatolites continue to aggregate material and build their structure only a metaphorical stone’s throw from the Pilbarra region where wanderers can find 3.4 billion-year-old stromatolite fossils. The ancient stromatolites are similar in many ways to their contemporary counterparts. These similarities have been used to demonstrate that fossil stromatolites from the Pilbarra are not just oddly stratified rock layers. If we think of stable microbial communities as an organism then nothing comes close to matching their longevity in the fossil record.
But does it make sense to think about stromatolites in this way? That’s a matter of perspective. All multicellular organisms, including cheetahs, gazelles and howler monkeys live in intimate symbiotic association with communities of microbes that are crucial to their fitness. And every cell of all plants, animals, and fungi are made up of microbial unions like those that produced the mitochondria in our own cells and chloroplasts in plants. All visible life both harbours and is the product of microbial communities and unions. Organisms have been strapping their fitbits to each other for as long as life has existed on this planet.
How can we think of the stromatolite as an example of forest fitness? There are many ways to answer this question, and one of them involves photosynthesis. A square metre of stromatolite matches the photosynthetic primary productivity of a square metre of tropical rainforest. It's hard to comprehend how a structure in which all of the photosynthesis occurs in a layer ten to twenty millimetres thick can match the primary productivity of a tropical rainforest whose canopy grows fifty metres or more above the forest floor. However, whereas much of the carbon that is fixed by photosynthesis in the forest is visible to us as plants and trees, the carbon fixed by cyanobacteria in stromatolites is released and becomes the substrate for the growth of other bacteria. Microbiologist Andrea Wieland and colleagues puts it in these terms:
High rates of photosynthesis are counterbalanced by rapid carbon mineralization, leading to a high carbon turnover in these dynamic ecosystems and to almost closed cycles of carbon, oxygen and sulfur.
The nutrient cycling abilities of stromatolites provide us with another point of comparison to intact tropical rainforests. Like stromatolites tropical rainforests have very little leakage of nutrients from the system (as forests are degraded or cut down nutrients wash away and overall productivity drops). The comparison between rainforests and stromatolites also holds for niche differentiation. The diversity and productivity of tropical forests is in part due to the species that occupy the available energetic and nutrient-based niches and ensure that energy and nutrients cycle through the system and are not lost. Similarly stromatolites house a diverse community of microbes capable of exploiting the vast array of energetic and nutrient niches.
In their book Into the Cool: Energy Flow Thermodynamics and Life Eric D. Schneider and Dorion Sagan argue that ecosystems tend towards complexity over time in order to better utilise and dissipate incoming solar radiation. They see this as a corrolary of the second law of thermodynamics; ‘nature abhors a gradient’. Differences in the availability of energy and nutrients along gradients will drive the diversification of species that are able to exploit the variety of niches. A simple way to measure the overall effect of gradient reduction in tropical forests is to take their temperature.
Thermometers attached to planes and weather satellites demonstrate that the richest, most complex ecosystems, such as those of the Amazon River basin, are the best reducers of the thermal gradient between Earth’s surface and outer space. The thermodynamically proficient ecosystems cool themselves mostly by evapotranspiration, that is, via water flow up through and evaporating off the leaves of trees. Measured from space during the hottest months, the combined cloud-ecosystems of Congo, Indonesia, Java and the Amazon are the temperature of Northern Canada during the dead of winter.
The diversity of plant species and their ability to make the most of the gradient produced by incoming solar radiation, from the top to bottom in the forest canopy, produces this profound cooling effect and reduces the temperature difference between the earth and space. This is a measure of forest fitness on the ecosystem scale.
How does this play out on the ground? Consider again the breadnut tree. It forms part of the canopy fifty metres above the ground. This canopy is the first layer in a gradient in light levels that descends to the forest floor where a plant like Ajo sacha (Mansoa alliacea) receives only around 5 per cent of the light that hits the canopy.
In stromatolites a drop in light levels roughly equivalent to the difference across the fifty metres from canopy to forest floor occurs over only three millimetres. As we go deeper into the stromatolite the light levels continue to drop below the levels required by plants for photosynthesis. Yet there is evidence that even in these dark regions, microbes are using different photosynthetic machinery. They are able to capture energy from the infrared part of the spectrum and they continue to photosynthesise up to 20 millimetres below the surface. Stromatolites create extreme gradients of light, and just as the rainforest produces variously light-adapted plant species, stromatolites produce a diversity of microbial phyla and ecotypes that are capable of exploiting various points along the light gradient.
While light, and the photosynthesis that it facilitates, are the foundation of the stromatolite community, nutrient gradients and cycling are also important in producing the diversity of bacterial species. The diversity of species that these niches produce is tricky to quantify. Bacterial phyla are commonly defined as having 97 per cent similarity in the 16S rRNA marker genes. This is the measurement used to calculate the 7000 odd bacterial ‘species’ living in the guts of rainforest dwelling black howler monkeys. By this measure there are around 4800 distinct phyla in the Shark Bay stromatolites.
This is a very conservative measure of microbial diversity. If we consider our own divergence from other primate species we only need a little over 1 per cent genetic divergence to differentiate us from chimpanzees and bonobos, a little more for gorillas and just over the magical 3 per cent divergence that we use to differentiate bacterial species to differentiate our kind from orangutans. By using this framework we may be overlooking significant diversity. In a paper published in 2014 microbiologists Mikhail Tikhonov, Robert Leach and Ned Wingreen found that up to 20 ecologically distinct bacterial populations within what was previously defined by the standard measure as being a single phyla.
For the purposes of this essay these numbers matter only to the extent that they help us to imagine the diversity and complexity of microbial communities. When we talk about 4800 distinct phyla in stromatolites this is a fairly crude snapshot of the much finer grain diversity and complexity that exists in the metabolic processes of the bacteria that make up the overall community.
Part of the forest fitness of stromatolites comes from the diversity of genetic information and metabolic processes they contain. This is certainly the case when we cast stromatolites in the light of what we know about bacteria more generally. Bacteria are able to pick up useful genetic information from their neighbors and the environment and incorporate it into their cellular machinery through a process called horizontal gene transfer. Bacteria are able to communicate with one another and coordinate their actions as they adapt to their environment.
Learning is part of this process of sensing and adapting. In a study in 2008 E-coli were trained to associate a drop in temperature with a drop in oxygen levels, in a form of paired stimulus similar to the classical conditioning that Ivan Pavlov practiced with his dogs. Lowering the temperature of the colony was then enough to trigger a switch in E-coli’s metabolism, evidence that it was anticipating a drop in oxygen levels much as Pavlov's dogs would salivate in anticipation when they picked up on the cues associated with food.
The density of bacterial populations in stromatolites likely boosts these social processes of learning, communication and genetic exchange between bacteria with the whole community reaping the benefits of this cosmopolitan and interconnected way of living. We might then think of the stromatolite as a densely populated city in which the inhabitants are fluent in several molecular languages and have diverse genetic and adaptive knowledge that can be called on to keep the city and its people healthy and functioning. We can anthropomorphise a little here and say that the chances that you’ll find someone in this sort of microbial city who has useful genetic information, or that you’ll be able to pick up on molecular cues that might help you to boost your fitness, are far greater than if you were floating around in the outback with few connections or lines of communication to other organisms.
With all the advantages of communication and genetic exchange within stromatolites, and the wide variety of available niches, we can imagine how these communities of microbes likely became sites for the evolution of novel metabolic and cellular processes, just as human cities can drive cultural evolution through communication and exchange. What we find in stromatolites is forest fitness; a kind of fitness that emerges out of the communications and connections between species.
The levels of photosynthesis in stromatolites, and their abundance in the fossil record has led scientists to think that stromatolites must have played an important role in the early stages of the earth’s first pollution crisis; the great oxygenation event. There’s evidence to suggest that cyanobacteria floating freely in the primordial oceans didn’t really crank up their oxygen production until around a billion years ago. Nutrients are scarce in the open ocean and the evolutionary adaptations that have allowed cyanobacteria to thrive there are relatively recent. In contrast the densely packed community of microbes found in stromatolites, in which different groups perform complementary metabolic functions, allow nutrients to rise to levels capable of supporting rainforest-level productivity.
Stromatolites set the course of evolution that led to oxygen metabolism and the endosymbiotic event that produced mitochondria and the complexity of multicellular life that we see around us. But stromatolites are also a blueprint for a thoroughly networked and interdependent forest fitness. The intimate association of diverse organisms has been a successful evolutionary strategy because it provides genetic flexibility and the capacity to adapt to a changing environment that goes beyond what is possible through the genetic mutations in a single organism, that might, for example, allow it to run faster.
An example from the human intestinal microbiota can help to illustrate this point. By eating seaweed Japanese people introduced new and nutritionally important genetic information into their microbiome. This information comes from marine bacteria that shared genes, via horizontal gene transfer, with resident gut bacteria. The upshot of this exchange was that resident bacteria were now able to process the polysaccharides in seaweed and turn them into essential enzymes that humans cannot produce. Here we can see the networked nature of human gut bacteria working in a way that is likely analogous to kinds of networks that exist in stromatolites. Both the human animal and the microbial ecosystem of the gut benefit from these networks by being able to derive nutrition from a wider variety of food sources.
The decline in stromatolite numbers in the fossil record from around a billion years ago is often attributed to the advent of grazing animals. These animals are thought to have nibbled away at cyanobacterial mats preventing them from developing into stromatolites. Shark Bay has high salinity levels that help to protect the stromatolites from grazing animals. Just as stromatolites offered a refuge for anaerobic bacteria, the hypersaline environment of Shark Bay now provides a refuge for stromatolites.
As both humans and black howler monkeys show, animals themselves became, in time, new ecosystems that can be colonised by communities of bacteria. As with the stromatolites, our primate digestive tracts continue to provide niches for descendants of the anaerobic bacteria that were forced to find ways to escape oxygen produced by cyanobacteria and their kin.
More recent evolutionary adaptations allow cyanobacteria to thrive in the low nutrient levels found the open ocean. These bacteria now produce around half of our atmospheric oxygen. This is why marine biologist Sallie Chisholm has called these cyanobacteria ‘the forests of the oceans’. Chisholm studies a particular bacterium: Prochlorococcus; 'the smallest and most abundant photosynthetic organism on Earth'. It has been estimated that this genus produces 20 per cent of the oxygen in our atmosphere. The amount of carbon fixed by Prochlorococcus is roughly equivalent to the net global productivity of our agricultural crops.
Individual Prochlorococcus cells are both physically small and have small genomes. Some isolates that have adapted to life on the surface of the ocean where nutrient levels are lowest have as few as 1700 genes. These small genomes are in part an adaptation to low nutrient environments. Making DNA requires nutrients. A smaller genome means that Prochlorococcus has to make less DNA and that cells can reproduce at lower nutrient concentrations than would otherwise be possible. Prochlorococcus now has the smallest genome of any organism that gets its energy from the sun. It is about twelve times smaller than the human genome.
By adapting to and growing in areas on the surface of the ocean where nutrient levels are already low Prochlorococcus has driven those nutrient levels down even further. This process has also driven ancestral strains of Prochlorococcus to greater depths in the ocean. This leads to a stratification in which we find evolutionarily newer strains with small genomes that have adapted to the low nutrient and high light levels, on the surface of the ocean, and older strains with larger genomes, that are adapted to high nutrient and low light levels, in the ocean’s depths.
While the various spectra of light penetrate the depths of the ocean differently, the difference in light levels between the surface of the ocean and at a depth of 50 metres is roughly equivalent to the 95 per cent difference across that same distance between the canopy and the forest floor in a tropical forest. This is just a quarter of the Prochlorococcus range. Low light strains are able to photosynthesise at greater depths in the ocean than any other photosynthetic organism, down to about 200 metres below the surface.
As is the case in stromatolites and tropical forests, adaptation to a gradient of diminishing light levels is a driver of genetic diversity. There is greater diversity among the ancestral, low light adapted strains. One explanation: surface waters are constantly being mixed by weather and waves leading to a relatively homogenous environment in relation to both nutrients and temperature. As we go deeper into the ocean we find stable and more complex gradients of light, temperature, pressure and nutrients. It is the stability of these gradients that allow for the diversification of the Prochlorococcus strains that are able to exploit particular niches.
Here we start to get a hint of what I'm calling the Prochlorococcus ocean cosmopolis. The evolutionary patterns created by drawing down of nutrients on the surface and the stable gradients of the deeper ocean are the arterial paths that make up the cosmopolitan structure of this organism's diversity.
Like the cyanobacteria in stromatolites Prochlorococcus is a carbon source for other bacteria. Prochlorococcus pumps out more carbon than most cyanobacteria as a means to balance the chemical reactions that drive its metabolism. This excreted carbon is picked up by the SAR11 bacteria that lives in intimate relation to Prochlorococcus. SAR11 in turn produces nutrients that are useful to Prochlorococcus. Sallie Chisholm and her colleagues have compared the exchange between Prochlorococcus and SAR11 to the metabolic exchange that occurs between chloroplasts and mitochondria in plants.
Prochlorococcus cells have a problem that doesn’t exist for the bacteria in the densely populated stromatolites cities — they float in an incredibly diffuse environment, (‘hundreds of cell diameters away from another cell of any type’). This means they must find other means of genetic communication so that they can continue to adapt and thrive.
Bacteriophage (viruses that infect bacteria) are one way that genes can move between Prochlorococcus cells. We don’t understand the full complexity of genetic exchange that’s going on between viruses and Prochlorococcus cells, but we can give a sketch of at least one scenario that might drive this process.
In this case Prochlorococcus cells defend themselves from a virus by shutting down photosynthesis. By doing this they cut the virus off from the source of energy it needs to reproduce. The virus responds by incorporating the genes underlying photosynthesis in Prochlorococcus into its genome so that it can use those genes to keep cellular processes going during infection. These genes for photosynthesis evolve in the virus through many replication cycles and eventually find their way back into Prochlorococcus cells bringing with them traits that might be advantageous to the organism.
The virus can be thought of as a kind of broadband network that bridges the distance between individual cells. Viruses are both an evolutionary pressure that produces new resistant strains and the carriers of genetic information that helps those strains to evolve. Viruses and bacteria coevolve not through simplistic predator-prey dynamics but through an intimate relationship in which short term costs to particular bacterial cells might lead to longer term benefits for the ‘species’ as a whole.
Professor of geography and eclectic thinker Jared Diamond and, more recently, palaeoanthropologist and archaeologist Darren Curnoe have argued that the development of large scale agriculture has had a tremendous impact on human health. Curnoe writes:
At archaeological sites like Abu Hereyra in Syria, for example, the changes in diet accompanying the move away from hunting and gathering are clearly recorded. The diet of Abu Hereyra’s occupants dropped from more than 150 wild plants consumed as hunter-gatherers to just a handful of crops as farmers.
Pre-agriculture, the number of plant species consumed in Abu Hereyra is equivalent to the number of species consumed by Amazonian groups for both food and medicinal purposes. However in the Amazonian context the distinction between hunter-gatherers and agricultural producers isn’t as clear as this quote from Curnoe might suggest. In their article ‘How People Domesticated the Amazonian Forests’ published in Frontiers in Ecology and Evolution this year Carolina Levis and her colleagues tell us that in pre-Columbian times ‘at least 85 tree and palm species were domesticated to some degree’. Instead of selecting just a handful of crops for cultivation Amazonian people established long term domesticating relationships with hundreds of plant species through agricultural and forest management strategies that continue to this day.
In contrast, in other parts of the world the switch to monocultures and eating just a handful of plants created dietary deficiencies and likely produced a drop in the diversity of our intestinal bacteria, similar to what we see in captive black howler monkeys. For we humans lack of diversity in intestinal bacteria correlates with inflammatory conditions including arthritis, obesity and irritable bowel syndrome. The domestication of animals also brought with it a new disease burden.
Today, around 75% of infectious diseases suffered by humans are zoonoses, ones obtained from or more often shared with domestic animals. Some common examples include influenza, the common cold, various parasites like tapeworms and highly infectious diseases that decimated millions of people in the past such as bubonic plague, tuberculosis, typhoid and measles.
Curnoe also points out that the genes involved in immune function have undergone rapid evolution since the advent of agriculture. It seems likely that this is an evolutionary response to the increased disease burden. Boosting the immune system in this way may have contributed to the current prevalence of the inflammatory and autoimmune conditions that I’ve already mentioned. As we moved from food forests to monoculture fields and animal pens we reduced the number and diversity of our intimate relations to the plants, animals and fungi that sustained us for millennia.
Biological fitness emerges out of intimate relationships. It’s true that, as with the gazelle and the cheetah, a key dynamic in these relationships is the question of who eats what and how, but, as we’ve seen with stromatolites, Prochlorococcus, and the black howler monkeys, healthy animals and microbial communities are produced through intimate relationships between thousands of species across all domains of life and into the almost living realm of viruses.
While one to one relationships between species can create strong selection pressures that drive the evolution of two species to extremes, like those we find in the cheetah and the gazelle, this is more an exception than a rule. In complex ecosystems it is increasingly difficult to see such clear causal relationships. The ability of an organism to live in an ecosystem with hundreds of plants and animal species, and thousands of fungi and microbes, depends on relationships of all kinds and not simply predator prey dynamics.
Diverse forests produce networks of sustaining relationships that are, on one level, a thoroughly cultural phenomena. By culture I mean the ongoing negotiation and transmission of a set of relations. Species from all domains of life can learn and respond dynamically to cues from one another in a way that produces this kind of culture.
This is a culture in which successful species are agents that alter their environments in profound ways, including changing the composition of the atmosphere, drawing down nutrients in the ocean, and altering forest ecosystems. The effects of these changes ripple through the intimate and sustaining relationships that form and are transmitted socially, genetically and physiologically. There is no fixed ‘natural’ order but rather a dynamic set of relationships that is better thought of in cultural terms.
The genetic code of any given organism contains information not only about the production and reproduction of its own body, but also about the relationship of that organism to other species and the environment into which it is born. The set of relations that are written into genes are a form of intergenerational social learning. In these terms society and ecosystem are synonymous. Social animals pass on the microbial communities that are important to their ongoing fitness and the relationships to plants, animals and fungi that sustain animal and microbe alike. This is a form of culture as record of ecological relation.
In this sense genetic material and the living organisms that it produces are cultural artefacts. They contain information about the past that can be acted on in the present through learning, gene transfer and epigenetic change. An organism’s sustaining relationships, those that allow it to survive in the present, also become a pattern through which new relationships can emerge that might be part of a suite of adaptive strategies that allow it to survive in the future. All of the relations between organisms, and the records they leave in genetic material, are together the shared culture of all living things. This is our most vital heritage.
The potential of organisms to enter into sustaining relationships with one another is greater in complex forest ecosystems for no other reason if not for the number of species present.
Consider again the breadnut tree. Not only is it one of the black howler monkey’s favorite foods it is also a staple in Mayan forest gardens. It produces a highly nutritious seed that can be roasted or ground into flour for making bread. Across its range in the tropical forests of the Americas the breadnut tree is one of hundreds of plants that are used by humans for food and medicinal purposes and that form part of the home gardens, and managed fallows and forests, that have sustained settlements in diverse tropical forest ecosystems of the Americas for thousands of years.
Out of these relationships with hundreds of species comes César Calvo's vision of the forest city. There is a stark contrast between our cities ‘chained to rust and habit’ and the dynamism of the forest city. In the forest city all organisms are agents that negotiate relationships with other organisms in a larger pattern that is beyond the comprehension of any individual or species.
The dynamism of the forest city produces a different mode of attention to our cities of rust and habit. This is restorative not only in relation to the way that our cities tax our attentional resources with their demands for singular focus but also because a diffuse form of attention is precisely what allows us to see ourselves within the larger patterns of the forest city. There are clear limits to the western scientific perspective in mapping the full complexity of the forest city. Yet even from this limited perspective we can perceive patterns that suggest to us the importance of maintaining our relationship to diverse species for our own fitness and the well-being that comes with seeing ourselves as part of the forest city.
Thank you to Maestro Enrrique Paredes Melendez of Santuario Huishtin, in the Peruvian Amazon, for his teaching. Thank you to Dr Brendan Burns from the Australian Centre for Astrobiology (UNSW) for discussing stromatolites with me. Any errors are my own.
We’re grateful to Create NSW for funding the New Nature project.