Wolves help forests grow, old ladies save bumblebees, and whales increase the fish population despite eating them. The connections between various species are subtle and often surprising. Even humans, at some point during evolution, merged with certain bacteria and viruses that are intimately assimilated with our bodies today.
Ozyorsk is a city in Chelyabinsk Oblast in Russia, where plutonium factories used to operate. Radioactive particles are still present in the water and earth to this day. The locals often complained about chronic pain, tiredness and problems with their circulation, digestion and immune system. However, doctors failed to find any explicit links between the ailments and radiation. They didn’t detect any cancerous changes typically caused by radioactivity. And because the symptoms didn't fit the diagnostic criteria, patients were sent away feeling neglected and betrayed.
Professor Kate Brown, whose research focuses on areas affected by radioactivity, spoke at a conference in Santa Cruz, where she told the story of the strange illness affecting the residents of Ozyorsk. The lecture was attended by microbiologist Margaret McFall-Ngai, who suddenly recognized all of the symptoms described by Brown. Each and every one of those indicators had appeared previously in her own research. The scientists joined forces and managed to solve the mystery together. It turned out that although the radiation dosage was too low to cause cancerous tumours in people, it was high enough to inflict mutations in the patients’ intestinal bacteria. The bacteria were ill, which caused the people – their hosts – to suffer.
This story is a beautiful example of cooperation between researchers from different areas of science, but also a brilliant illustration of cross-species symbiotic relations. Can you imagine a bond closer than suffering from another creature’s sickness?
The plant partnership
Back at school when we learned about symbiosis, the cardinal example was always lichens. These organisms are absolute masterpieces of the evolutionary entanglement of species. Their bodies (called thallus) are composed of algae or cyanophyte cells that grow among the fibres of fungi. The fungus cannot live without the algae, which feeds it through the process of photosynthesis, nor can the algae live without its host. Two worlds meet; two kingdoms of life join forces. Fungi are neither plants nor animals, although they are a little closer to the latter. Algae (plants) or cyanophyta (bacteria) are not as closely related to fungi as humans are. And yet over the course of centuries, these two species evolved not only to depend on each other, but also to create a composite organism that is more than just the sum of its components. Lichens can survive almost any conditions. In exchange for the products of photosynthesis, the fungus provides its partner with a structure to live on, as well as minerals drawn from the ground. Fungi enzymes can dissolve stone, allowing some lichens to survive even deep inside a rock, nestled among the pebbles. Usually, lichens are the pioneers of nature, the first settlers on bare rock whipped with polar or mountain winds; burnt with desert sun. Only when these organisms make their home in a new area, creating a surface layer vaguely reminiscent of new soil, can it be followed by moss and later, grass. But the lichens come first, thanks to their dual nature.
Fungi were not the only organisms to come up with the idea of making a permanent deal with plant cells. Some free-living flatworms and sea snails, as well as radiolaria and mussels, live with single-celled algae organisms underneath the surface of their bodies. These organisms, called zooxanthellae, can provide their hosts with extra nourishment thanks to their photosynthetic abilities. In return, they receive free transport and protection. The best-known symbiotic relationship among marine organisms is the one between zooxanthellae and corals.
Similar to other cases, here too the single-celled organisms provide their calcified partners with food and energy obtained through the process of photosynthesis. The coral pay it back by letting the algae live in the safety of their calcified shells. The algae also benefit from the nutritional substances corals release when feeding. The coral control the number of algae inhabiting their bodies. They can eat the algae cells or get rid of them altogether, which proves that they can survive without their symbiotic partner for a while. In the long run, however, the algae are necessary for the coral to survive. It is the removal of algae under stress that causes coral reefs to bleach. It is coral’s way of responding to high temperatures, pollution and overexposure to sunlight. Having gotten rid of zooxanthellae, coral begins to starve, but those that survive hostile conditions have a very good chance of healing, growing new algae and staying alive. The last global episode of coral bleaching lasted from 2014 to 2017. The reefs are slowly improving, but long-term prognoses are grim. Scientists believe that the Great Barrier Reef off the coast of Australia – the world’s largest single living structure – will be the first ecosystem irreversibly destroyed by human activity. All this because the relationship between two separate organisms has been disrupted.
The thinking virus
The roots of symbiotic co-dependencies are even more profound. In 1967, the Journal of Theoretical Biology published a paper written by a young faculty member from the Department of Biology at Boston University. The article, titled ‘On the Origin of Mitosing Cells’, was previously rejected by over a dozen other journals. What’s more, almost nobody paid attention to it for another decade. Now we know that it was one of the most important papers in 20th-century biology studies. The biology theorist who wrote it was called Lynn Margulis (although when the article was published, she was known as Lynn Sagan, after her first husband, Carl). Margulis was one of the most significant figures in the history of science, and the fact that she never received a Nobel Prize should be considered a disgrace to the Swedish Academy.
In her paper, Margulis laid out the foundations of her theory, claiming that once upon a time, some parts of the cells that make up our bodies did not belong to us. Or perhaps I should rather say that there was a time when they were not exactly part of the human organism.
Both animal and plant cells usually contain several fixed elements. In our cells (except for the red blood cells, which, in the case of mammals, do not have a nucleus), there is a cell nucleus in which our genetic material is stored, alongside cell organelle, including mitochondria. Mitochondria produce energy for our bodies and, just like the nucleus, they hold our genetic material in the form of DNA and various kinds of RNA. If we had the body of a plant or a plant-like protozoan, we would also store our nucleic acids in plastids – the best-known being chloroplasts in which photosynthesis takes place for most plants.
In her article, Margulis argued that mitochondria and plastids appeared in plant and animal cells as symbiotic bacteria which, over the course of evolution, have lost their independence and fused their existence with ours. The traces of their former autonomy can still be seen. It’s why mitochondria and plastids can still multiply independently within the cell, and their genomes contain their own genes that are closer to those of bacteria than to ours.
Let’s consider the meaning of these facts. It means that down in the deepest core of our being, we are not entirely alone! Yes, we are ourselves, but certainly not without company. I’m not just a human who carries bacteria in his gut or mites roaming on his skin. I am those creatures as well. My cells would not be what they are today if not for other species. The real me is a collective of various organisms; the same conclusion applies to each and every one of you.
I believe that three people in the history of the world really made us realize who we are. Copernicus told us that we are not the centre of the universe. Darwin dealt another blow to our pride, proving that we aren’t anything special even here on Earth – being related to all other organisms, we are subject to the same laws and mechanisms as the rest of the living world. And then Margulis revealed that the human body is, in fact, an assemblage of various species that make for a single, but not singular, unit: a holobiont. Margulis coined that term in 1991, using the ancient Greek root ὅλος (hólos), meaning ‘whole’.
But… here’s the thing. Even the genes present in our cells cannot be clearly labelled as those that are ours, human (in the nucleus); and those that are also ours, but of bacterial origin (inside mitochondria). The entanglement goes further.
The fact that I can write those words (and you can read them, several months later) is probably something we owe to viruses that have nestled in our DNA. In January 2018, two articles were published in Cell magazine. The authors describe the evolution of the Arc gene that releases proteins in the human brain, ensuring the proper functioning of synapses (the connections between nerve cells). Without delving into molecular details, it is enough to say that this gene and the protein it codes point to its virus origin. In other words, we owe our seemingly inherently human qualities – such as consciousness and long-term memory – to a virus that attacked our ancestors millions of years ago. What’s more, articles published in the same magazine just two years prior indicated that 40% to 80% of the human genome originated in a viral invasion. Therefore, viruses joined our DNA and then throughout the process of evolution were found useful for our nervous system, as well as for our immune system and early embryonic development. There really is no deeper connection one organism could establish with another (although we should note that viruses are not exactly organisms, but snippets of nucleic acids that keep on multiplying). This process can be very harmful to humans, although it often doesn’t affect us at all. But sometimes it can come in very handy.
Of wolves and rivers
Let’s now rise above the narrow spaces of our own nuclei, cells or even bodies; let’s take a breath and look at the bigger picture. We’ll soon realize that the correlation between ourselves and the rest of the living world reaches much further than we first thought. The whole world is entangled in a net of ecological connections that are much subtler than simple trophic structures of who eats whom.
In the original meaning of the word, ‘ecology’ is the science of researching the connections various organisms have with their environment (which includes other organisms). The name of this discipline comes from the Greek words οἶκος (oikos), meaning ‘home’ or ‘environment’, and -λογία (-logia), meaning ‘knowledge’ or ‘research’. When I studied it in the last decade of the previous century, it all seemed relatively simple. Trophic relations looked like this: if there are fewer acorns this year, there will be fewer forest rodents. Or to use the classic Canadian example: if there aren’t many rabbits one year, there won’t be enough prey for the bobcats to eat, which will impact their breeding, so come next year, there will be fewer bobcats eating rabbits. Come the following year, there will be lots of rabbits, so bobcats will have plenty of food to hunt. Come the year after that, there will be more bobcats so they will deplete the rabbit population, and so on. Population cycles seemed understandable and predictable enough, but even back then we realized that whenever scientists dig a little deeper, the number of variables expands and everything becomes much more complicated. Now that we know more, nobody uses such simple models of relations. We no longer talk of trophic chains; now we recognize them as trophic networks. Scientists have realized that all elements of various ecosystems are interconnected, just like ecosystems are linked to one another.
The ambiguity of such connections makes the foundation for the recently-popular concept of trophic cascades – that is, ecological processes that begin at the very top of the food pyramid and then roll all the way down its slope. One of the most famous and spectacular examples of this process is the changes that occurred in the Yellowstone National Park after the population of wolves was reintroduced. These predators were completely exterminated in the park in the early 20th century, and only in 1995 were several wolves let out into the park after 70 years of absence. The results were astonishing. When there were no top predators in the park, the elk population grew so large that it destroyed tree saplings, removing all vegetation from large swaths of the parkland. Naturally, the wolves started hunting the elk, which not only helped to control the population but also changed their behaviour – the elk began avoiding open spaces where they were easy prey. This way, empty patches of land grew thick with trees again.
A similar correlation, called the ecology of fear, was also noticed in Europe, for example, in the Białowieża Forest in Poland. In Yellowstone after reintroducing wolves, trees began to grow back immediately. Over the course of just six years, the average tree height in river valleys increased five times over, and bare hills turned into forests. This change has brought back songbirds, whose population in the park has proliferated. The return of wolves also helped increase the population of beavers, as they eat riverside trees. Out of all mammals, only humans reshape the environment more than beavers do. By building dams and creating marshes, these animals created a friendly habitat for otters, muskrats, fish, waterbirds, reptiles and amphibians, whose populations also soared. At the same time, wolves reduced the numbers of their smaller cousins – coyotes – which in turn created more rabbits and rodents. This prompted the increase in numbers of birds of prey, weasels and foxes. Ravens and bald eagles could now feast on leftovers from wolf dinners, which also led to an increase in their population. Bears could now feed on carcasses left by wolves, but they also benefited from the abundance of forest fruit growing in revived areas. Bears also hunted elk calves, helping wolves control the elk population.
And finally: wolves managed to improve the malnourished landscape by changing the course of rivers. Riverbanks, newly reinforced with trees and thickets, were now far less susceptible to erosion, which made rivers less meandering and more narrow. This, in turn, created new bodies of water in oxbow lakes and gave life to new habitats. Nobody expected that the introduction of just one critical species would change not only the ecosystem, but even the very geography of the park.
The Gaia principle
Let me give you another example of a subtle and seemingly vague connection between species; one that is especially close to my heart. Let’s talk about the influence of whales and other cetaceans on marine ecosystems. Industrial whaling in the 20th century has reduced their population drastically – the number of blue whales in Antarctic waters has shrunk a thousand times compared to the population from a century ago. When large-scale whaling was finally abandoned (mainly because it was no longer profitable), populations of large sea mammals started to increase slowly. At the same time, researchers found that the numbers of whale prey (not considering the influence of the fishing industry) were also going up, seemingly against logic. It was not until a few years ago that scientists turned their attention to the surprisingly beneficial effects of whale faeces for marine ecosystems. Everything became clear. Cetaceans feed in deep waters, but often defecate near the surface. Whales are enormous, and so is their poop. Moreover, the excrement is full of nutritious micro-elements that, thanks to whales, reach water surfaces, fertilizing phytoplankton that subsequently grows and feeds zooplankton, which is consumed by fish. Moreover, by swimming up and down, whales mix the water, helping drowning phytoplankton return to the surface, near the light, where it can reproduce even better. This works for minerals, too. Scientists estimate that cetacean movements in the water column are as effective in mixing minerals as all the waves and tides in the world. Therefore, despite appearances, more whales mean more plankton and fish.
There are many such examples, but I will only list the two that seem most relevant to our daily lives. One comes from the Norwegian salmon farming industry. Salmon monocultures were expected to be extremely productive, but they turned out to be the perfect environment for parasitic crustaceans. Their population – usually low – explodes in monocultural farms, affecting the fish negatively. At first, farmers tried to solve the problem with drugs and other chemicals, but the parasite soon became immune to those substances. Eventually, it became necessary to keep salmon together with wrasses that eat the offending crustaceans. Young wrasses, however, feed on copepods – zooplankton. Sourcing zooplankton from the ocean proved ineffective, so it also had to be farmed artificially. We have reached an ironic situation where a twisted idea of a monoculture depends on the biodiversity it was supposed to reject. Marine organisms need one another; they cannot exist on their own.
The last example comes from our human backyard. To be precise, from the United Kingdom. This is my all-time favourite illustration of an unexpected connection between organisms. In the mid-20th century, scientists found an odd correlation between the number of lonely elderly ladies in rural areas and the density of clover fields. And it’s not because lonely ladies enjoy gardening. They very well might, but they also like keeping cats. More elderly residents mean more feline companions present in the area. As we all know, cats enjoy hunting mice (and birds, which is why domestic cats should not be let outside, but that’s a different story). More cats means fewer rodents. Mice and rats dig burrows in the ground, and in doing so, they destroy bumblebee nests. Therefore, more cat-owning ladies equals fewer mice equals more bumblebees. Bumblebees are the only insects with tongues long enough to pollinate clovers… Et voilà!
Lynn Margulis didn’t research life just at the cellular level. In the 1970s, she formulated the famous Gaia principle together with James Lovelock. According to their hypothesis, our planet is a living organism whose physical and biological processes are closely connected to one another to in order to maintain a dynamic balance. The Gaia principle is a perfect illustration of the processes we’re still discovering. The life of our planet folds into one great mechanism on all levels of natural order. We’re so busy destroying other species and killing the Earth’s biodiversity; we fail to notice that in doing so, we are also exterminating ourselves. We cannot live without one another.
I consider the Gaia principle a metaphor. It would be difficult to prove that the Earth is indeed a self-sufficient organism with, say, its own immune system. Still, I can’t help but see some analogies. Just like the Ozyorsk residents suffer because their intestinal bacteria are unwell, the Earth is also suffering because of the disease plaguing humanity. We are sick with greed and overgrown ambitions. So far, the planet is responding with a bit of a fever.
Translated by Aga Zano