For the midterm exam of the UCSD/SDSU graduate course Integrative Microbiology that Doug Bartlett and I teach, we gave students the option of answering this Talmudic, open-ended-type question:
'How would you go about defending the statement that "all living things are connected to other living things" to an educated lay audience? What examples would you present to illustrate how pervasive this is in the living world?’
We liked the answers, so we decided to post them all (slightly edited). The students were, of course, enthusiastic. Interestingly, several used the term "living world" in the Gaia sense of "the world being alive," not as we had meant it, namely "the world of living things." No matter.
No. 1 – All From One
I reckon the beauty of our class exercise lies at the tail end of the essay question: "What examples would you present to illustrate how pervasive this is in the living world?"
Indeed it is a living world, at least according to some. Perhaps all living things comprise one biological entity, one large functioning ecosystem (life-force) with planet Earth as skeleton. Has been denied by many. But Lovelock thought so. Margulis supported it. Heck, maybe Lennon did too, "I am he as you are he as you are me. And we are all together" ("I Am the Walrus"). It really depends on one’s definition of "life", on what a discrete "species" or "individual" is, how you view "time". It is fine if you don’t want to believe in Gaia, but for the sake of this essay try to entertain it…or something similar.
OK, right, so Charles’s work suggests we all came from one common ancestor. And if evolution as we know it is true, then every living thing that was and currently is, is from the same "seed" or "stock" – in a way it’s the same organism. Contained molecules, appearing and behaving differently in space and time. Somehow, the prokaryote we (likely) evolved from is our great, great, great…grandmother.
1) Life is connected because it (likely) comes from the same, single prokaryotic origin. Information with function passed down and modified over time. It’s all one force. Each bit of it, interacting with other bits of it, to sustain it. You’ll notice the "tree of life" is composed of many branches but its still just "one tree." One Life.
Life doesn’t die (at least not yet); it falls apart (is digested). The organic matter is broken down, recycled into remineralized nutrients ("dust to dust"), and then rearranged by "life" as it evolves. So as each component ("individual" of a "species") falls apart, it gets recycled back into the system. It supports itself, regenerates itself, transfer pieces of itself around to other parts of itself.
2) Life is connected because the atoms that compose it are under constant rearrangement; shuffling through different components, or between what were "living things" to other "living things." Much of these rearrangements are made possible by microbial regeneration of nutrients.
Now look down at planet Earth from a far-away galaxy using a powerful telescope. What would you see? Perhaps a live "organism" consisting of many moving parts that function together to produce oxygen, carbon dioxide, and essential nutrients, moving about and constantly modifying (evolving) itself.
So to be fair, living things may not really be connected to other living things. There may be just one living thing, Life, with many working components.
No. 2 – A Cat, A Primate, A Protist
The perception of consciousness makes it exceedingly appealing to believe that we are autonomous individuals, isolated from the world around us by the boundaries of our bodies. However, modern science has blurred the lines of the individual by shedding light on how interdependent life is. To understand this, it helps to go back in time and consider how life started.
Life on earth likely emerged from a primordial soup, and was based on the foundation of building barriers. By creating a membrane, the first unicellular organism was able to control energy forces by creating chemical and physical gradients. These first simple organisms appeared around 4 billion years ago. They did not evolve in isolation, but rather competed daily for common resources (iron, oxygen, free nitrogen, etc.), creating alliances and waging wars on a scale above and beyond what Game of Thrones could ever approach.
A major turning point in these epic interactions began over 2 billion years ago, when one of these self-bound organisms engulfed another, but instead of dying, the engulfee not only survived, but prospered in a way that made the engulfing cell advantageous. This marked the rise of the eukaryotes, the group to which we belong. Over incredibly long amounts of time, the complexity of eukaryotes and of their interactions with other microbes grew, eventually leading to the dazzling alliance of the literally trillions of cells comprising our bodies. Therefore, it is simplistic to imagine ourselves as cleanly defined individuals, perpetually annoyed by microscopic bugs that should be whisked away with antimicrobial hand soaps. Rather, we are a complicated colony of eukaryotes which squeezed its way into a microbial world.
Because we cannot see them, it is easy to forget the extent to which our nutritional requirements, the plants and animal products we eat, are dependent on microbes. Coffee, bread, miso, alcoholic beverages, cheese, kimchi, and chocolate, just to name a few, are all examples of fermented foods. In other words, the primary metabolic step performed by yeast or bacteria must be performed in order to produce the food products we are familiar with. Humans are not an exception, a majority of nutritional interactions among larger organisms is mediated by microbes.
One of my favorite examples of how microscopic life connects to life on our scale involves a cat, a primate, and a protist. Toxoplasma gondii, a protists , in other words, a single celled eukaryote, is a parasite that infects a broad range of warm-blooded animals. In order to be able to complete its life cycle, which can only take place in cats, this protist alters the behavioral patterns in its host. More specifically, mice infected with T. gondii are no longer fearful, and in some cases even seek out, the smell of cat urine, making them easier to catch. Bad for the mice, good for the parasite. Humans infected with T. gondii have also been noted to have behavioral and psychomotor alterations, and recent research has demonstrated that these effects could be vestigial traits from a time when large cats hunted man.
Examples of life affecting life on a scale normally unacknowledged surround us. Recent microbial research has elucidated how pervasive this phenomenon is by allowing us to identify the dense populations of microorganisms continually surrounding us.
- Locey, Kenneth J., and Jay T. Lennon. Scaling laws predict global microbial diversity. No. e1808. PeerJ PrePrints, 2015.
- Poirotte, Clémence, et al. "Morbid attraction to leopard urine in Toxoplasma-infected chimpanzees." Current Biology26.3 (2016): R98-R99.
No. 3 – How An Elephant In Kenya Is Connected To A Prairie Vole In The Midwest
The concept that "all living things are connected to other living things" at first seems implausible in the natural environment given a "six degrees of Kevin Bacon" approach. For instance, an elephant in Kenya seemingly cannot possibly be connected to a prairie vole in the Midwest. Not only is there geographic separation between distant savannah and prairie ecosystems, but these organisms are on the same positon in the food chain, which makes it hard to distinguish them. We can still make this example work, however, if in a slightly more abstract way. Consider the grazing elephant, breathing in oxygen and giving off carbon dioxide that is readily distributed around the globe thanks to winds. Although unlikely, a molecule of CO2 produced by the elephant may reach our little prairie vole and thus connect the two.
If we move our thinking to the world of microbes, parasites, and viruses, suddenly connecting living things to one another gets easier. You are aware that diseases transmit between species. Voles have been found to harbor the cowpox virus (Crouch et al. 1995), and transmission from rodents to elephants (and then eventually to humans) has previously been documented (Kurth et al. 2008). More familiar examples are found in epidemics like swine and bird flu, which prompt cause for concern in humans (Holmes 2010).You may be surprised to hear that large proportion of the human genome is viral in nature, meaning that several different living things are likely to have have been attacked by the same viruses and be connected that way. Bacteria too are known to exchange genes between their various species and also with their animal hosts (Dunning Hotopp et al. 2007). Parasites also provide great examples of connectivity. Some, like tapeworms, are ubiquitous across vertebrates. Parasites make up a huge global biomass that connect living things (Hechinger 2015) because they cause such similar infections in different hosts.
Probing into the genomes across a wide biological spectrum should increasingly include looking for genes derived from microbial backgrounds, including parasites, which too are often not considered. To prove this claim of connection between species, we need to collect more empirical data and to analyze them with this conception in mind.
Crouch, A. C., D. Baxby, C. M. McCracken, R. M. Gaskell, and M. Bennett. 1995. Serological evidence for the reservoir hosts of cowpox virus in British wildlife. Epidemiol Infect 115:185-191.
Dunning Hotopp, J. C., M. E. Clark, D. C. Oliveira, J. M. Foster, P. Fischer, M. C. Munoz Torres, J. D. Giebel, N. Kumar, N. Ishmael, S. Wang, J. Ingram, R. V. Nene, J. Shepard, J. Tomkins, S. Richards, D. J. Spiro, E. Ghedin, B. E. Slatko, H. Tettelin, and J. H. Werren. 2007. Widespread lateral gene transfer from intracellular bacteria to multicellular eukaryotes. Science 317:1753-1756.
Holmes, E. C. 2010. The comparative genomics of viral emergence. Proc Natl Acad Sci U S A 107 Suppl 1:1742-1746.
Kurth, A., G. Wibbelt, H. P. Gerber, A. Petschaelis, G. Pauli, and A. Nitsche. 2008. Rat-to-elephant-to-human transmission of cowpox virus. Emerg Infect Dis 14:670-671.
No. 4 – No Man is an Island
"No man is an island." These words have been spoken in an effort to build up teams and inspire communities to look past their differences. John Donne’s words were written in a time when very little was known about microbes but his words ring quite true as we think about our place in this invisible world.
The human body seems to be one single organism from its appearance, but in fact, there are 10 times as many microbes living on and in us as there are human cells in our body. These microbial communities greatly influence our immediate environment and even our own faculties.
Take for instance your diet. Humans eat all variety of foods and rarely give a thought to how all these complex molecules are turned into fuel for your body. Although our own digestive enzymes are doing a large portion of the work, we also microbes that lend their hand in this process. They help our bodies digest foods that have not been digested by the stomach or small intestine. Lactobacillus, which you find on your yogurt label, helps breakdown sugars in dairy products and in the process, creates an inhospitable environment for unfriendly bacteria. Other bacteria found in the colon aid in the break down of proteins, sugars, and fats. By breaking our food into smaller components, our intestine can turn it into nutrients that we can absorb.
It goes beyond digestion: our welcome passengers assist our immune system as well. Colonizing an area of our skin with benign bacteria can prevent pathogenic ones from taking over that area and possibly attacking our system. Additionally, some of our friendly bacteria can produce anti-inflammatory compounds that ward off "bad" bacteria. Our immune system constantly monitors the bacteria in and around us to determine its next course of action.
As we interact with the world around us, whether with other humans, animals, orentirely different ecosystems from distant parts of the world, we do not act as a single being. Everywhere we go, we are leaving behind bacteria and picking up new ones. We are encountering fungi or sloughing off skin cells which feed nearby mites. Microbes are constantly being transmitted. We are familiar with this idea because we know the effects of viruses and pathogenic bacteria, sometimes spreading like wildfire through communities. Thinking through this paradigm, one can see how easily and quickly microbes are exchanged. This connects us to many other animals in the world as we can exchange microbes with them, giving us a seat in a intricate web of living interactions.
Our deep connection to these invisible living beings has even given rise to a project of national scope called the Human Microbiome Project. Their mission is to gather data in order to figure out how these trillions of organisms might influence our health and disease. We are only at the beginning stages of characterizing what is out there, but already we are seeing how extensive and complex our relationship is with other living beings.
No. 5 – My Uncle Joseph
Dear Uncle Joseph,
I hope all is well in Des Moines and all the kids enjoyed their winter vacation. It was great catching up with you over Christmas dinner. Since we were shouted down by Grandma for "talking politics" when you asked about evolution and climate change, I wanted to follow up with you since you seemed genuinely interested in better understanding these matters. The best way I’ve found to conceptualize these topics and why they’re important is falling back on a common theme in biology: that every living thing is connected, through both our ancestry and our environment. Let’s first use this concept with evolution.
We are connected to every living thing in today’s world through our common ancestors, in the same way you and I are connected through Grandma and Grandpa, our common ancestors. Our species, Homo sapiens, is connected to everything that is alive now and everything that has ever been alive, with all the variety of species coming from small, incremental changes in our DNA over billions of years. Obviously, by looking at the things alive today, we’re more similar to some species, like monkeys and apes, than we are to others, like dogs or fish or bacteria. You can compare these connections to generational ancestry in families. You would probably say you and I are more similar to each other than you are to a shepherd in Mongolia, and you’d be right. You and I had a common ancestor just one generation prior to yours, while your common ancestor with the Mongolian shepherd (maybe Genghis Khan?) lived many, many generations ago. Therefore, your DNA is more similar to mine than it is to the Mongolian shepherd. In the same way, our DNA as Homo sapiens is more similar to that of that of monkeys and apes than it is to dogs or bacteria because our common ancestor with primates lived many fewer generations ago than we that of those other forms of life. All the same, we still have a common ancestry with everything alive on Earth, separated by distance and time. Evolution is exactly this —the relationship between different forms of life through common ancestors. The changes in our DNA over many, many generations occur through a mechanism called "natural selection," which is essentially trial by error; changes in your DNA that are "good" you pass on to your descendants, while changes that are "bad" you don’t. What’s a "good" and what’s a "bad" change you might ask? Well that leads us to the next way all life is connected, and it’s a good opportunity to talk about climate change too.
The second way life is connected is through the environment. You might have also heard it called an ecosystem. You interact with everything in your ecosystem to varying degrees. You can have good interactions, like getting oxygen from trees and nutrients from plants and animals, and you can have bad ones, like getting mauled by a bear. If you are born with a random change in your DNA that makes you more efficient at getting nutrients from your food, or better at avoiding bear mauling, that’s obviously a good change. Since your change makes your better at surviving in your ecosystem than some other guy without the change, your descendants (who have that change) are going to survive better than that guy’s descendants. After many, many generations, your descendants are going to make up pretty much all of mankind in this ecosystem since they’re "more fit" to survive. So even though changes in DNA are random, those "good" changes stick around through future generations; this is what we call natural selection, and it’s another way all life is connected..
But remember, natural selection is slow—it takes many, many generations for the change in your DNA that made you more fit to become the DNA of every human in your ecosystem. And normally this is fine, because our ecosystems change slowly too. But what if we go back to our ecosystem example and we suddenly kill all the crops in your ecosystem, or suddenly there’s a lot more bears trying to maul you. Natural selection makes you incrementally better at adapting to changes to your environment, but you need to have an answer to no crops and lots more bears now, not thousands of years from now. This is the problem that global warming poses: a dramatically faster change than life can adapt to. Temperatures rise and fall slowly over thousands of years, and life can more or less adapt to these slow changes. But fast changes in temperature like we’re seeing now are things life can’t adapt to easily, and so instead of changing with their ecosystems, different species will start dying out. And remember, everything’s connected—when things start dying out, that’s humanity’s ecosystem changing rapidly too, in ways we to which we won’t be able to adapt.
Well, on that cheery note, let me know if you have any questions, and say hi to Aunt Mary and the kids for me!
No. 6 – The Gaia Hypothesis
James Lovelock in the 1960’s, while working for NASA on projects concerning detecting life on Mars, developed what he termed "The Gaia Hypothesis." Lovelock had been struck by the atmospheric differences between Earth and Mars, and thought that he could explain these differences by taking into account the biotic influences on Earth’s atmosphere. Microbiologist Lynn Margulis reached a parallel conclusion based on the microbial origin of atmospheric gases. Together, they developed a hypothesis that the biosphere, or all living organisms on Earth, act in concert to maintain the homeostatic conditions required for life1. This hypothesis was embraced by environmentalists, not surprising considering that it was brought about at the beginning of the modern environmental movement. Although that Earth acts as a single organism is considered controversial, it has provided a thought provoking foundation to examine the interconnectedness of all life.
Obviously, all life on this planet is intensely interconnected, largely based on the biochemistry performed by microbes. From its earliest beginnings, microbial life has hugely impacted geological chemistry. A key event was when the first photosynthetic oxygen producers turned the Earth’s environment from anoxic to oxic, thus determining the continuing course of the evolution.
Continuing today, photosynthetic algae and cyanobacteria are major players in the carbon cycle. We tend to think of rainforests as the key photosynthetic players that fix CO2 and create O2, but phytoplankton is responsible for half of the primary production of carbon compounds2. In fact, the "microbial loop" in the ocean is a great illustration of the unassuming but overwhelming effect microbes have upon large scale nutrient cycling3.
These are all examples of the interconnectedness of life by organisms as separate entities, no matter if micro- or macro-scopic. Perhaps a larger question is: are any organisms truly singular? All animals appear to have a microbiome: insects4, fish5, mammals6, reptiles, birds7, and plants8 as well. All life on earth is dependent and interconnected with other life, whether distantly through nutrient cycling, or directly, through symbiotic, commensal relationships.
- Lovelock, J. E. & Margulis, L. Atmospheric homeostasis by and for the biosphere: the gaia hypothesis. Tellus A 26, (1974).
- Field, C. B., Behrenfeld, M. J., Randerson, J. T. & Falkowski, P. Primary production of the biosphere: integrating terrestrial and oceanic components. Science 281, 237–240 (1998).
- Azam, F. F. T., Fenchel, T., Field, J. G. & Thingstad, F. The Ecological Role of Water-Column Microbes in the Sea. Mar. Ecol. Prog. Ser. 10(3), (1983).
- Runckel, C. et al. Temporal analysis of the honey bee microbiome reveals four novel viruses and seasonal prevalence of known viruses, Nosema, and Crithidia. PLoS One 6, e20656 (2011).
- Desai, A. R. et al. Effects of plant-based diets on the distal gut microbiome of rainbow trout (Oncorhynchus mykiss). Aquaculture 350–353, 134–142 (2012).
- Turnbaugh, P. J. et al. The human microbiome project. Nature 449, 804–810 (2007).
- Hanning, I. & Diaz-Sanchez, S. The functionality of the gastrointestinal microbiome in non-human animals. Microbiome 3, 51 (2015).
- Berendsen, R. L., Pieterse, C. M. J. & Bakker, P. A. H. M. The rhizosphere microbiome and plant health. Trends Plant Sci. 17, 478–486 (2012).
No. 7 – Skittles and Evolution
In essence, all life is connected to other life because we all exist in the same space. We share the air we breathe, the sources of food we eat (or make ourselves), the space we live in. It may sound like a paranoid thriller novel, but every action we take has an effect on others simply because we have a finite amount of resources that we all must share. For all forms of life, these effects accumulate over time and lead to evolution (perhaps better described as co-evolution).
Let’s use family as an analogy: it’s snack time, and there’s a small bowl of colored Skittles candies in the kitchen. Each color denotes a different flavor, but you and your four siblings all like the same flavor: orange. Your speedy and tough oldest brother gets to the bowl first, claiming all the orange candies for himself and his favorite youngest sister, who provides flower crowns in return. (These two siblings are comparable to mutually beneficial relationships seen in the natural world, such as clownfish and sea anemones.) What do the rest of you do? Perhaps your adventurous sister heads out of the house in search of new candies, the same way many animals migrate when food becomes scarce. Maybe you settle on the lemon yellow candies as the next best option, which become your new favorites over time. Without any competition for the yellow candies, you develop a new role in the sibling group, comparable to an ecological niche in an ecosystem.
In natural ecosystems, life lives in harmony by developing their own place in the system despite the competition for shared resources. Food, oxygen, water, space and shelter must be shared for the sake of survival. Evolution results as interactions between lives slowly push and pull each other, with no single lifeform living in complete isolation.
 Holbrook S.J., and Schmitt R. J. (2005), Growth, reproduction and survival of a tropical sea anemone (Actiniaria): benefits of hosting anemonefish. Coral reefs, 24(1): 67
No. 8 – The Butterfly Effect
It is hard to imagine what connections a human living in the middle of a concrete city may have to a bacterium living on a deep ocean vent or a parasite living on a Nepalese yak, but in reality all life forms on earth are connected by simply sharing a planet. Perhaps this is easiest to contextualize when considering the butterfly effect1, whereby even the smallest beat of a butterfly’s wings can influence the trajectory of a hurricane. The interactions of life on earth are a collection of butterfly effects. Consider, for example, a cow and grass she eats. Uneaten, blades of grass will die, providing bacteria and fungi with nutrients. If eaten, the grass is digested by a plethora of bacteria, protozoa, archaea and fungi living in the cow’s rumen2. The cow relies on these microbes to help digest the plant material and the microbes rely on the cow for, essentially room and board. Eating the grass also impacts the atmosphere, as it is no longer available to produce oxygen and sequester carbon dioxide. Additionally, fermentation of the grass in the rumen produces methane. Cows produce about 30% of the world’s methane, which contributes to trapping heat and global warming. This small bite of grass has not only impacted the cow and the local community, but the world at large through small atmospheric changes.
A more grandiose example of this is the "Great Oxygenation Event" 3, which occurred around 2.3 billion years ago. For about the first half of the earth’s existence, oxygen was not part of the atmosphere and life on earth consisted of anaerobic organisms. Cyanobacteria evolved the ability to produce oxygen through photosynthesis and slowly began to oxygenate the atmosphere3. An individual cyanobacterium had very little effect on the atmosphere, but they were able to organize into microbial mats4 and collectively change the entire composition of the earth’s atmosphere. This gradual change lead to a mass extinction of oxygen-intolerant anaerobes and the evolution of other aerobic organisms. All because one tiny cyanobacterium evolved the ability to produce oxygen!
It is easy to feel that we humans can control the connections we make with other living things. For example, many people have control over what they decide to eat, whether to put the spider outside or squish it, to plant an invasive species of blackberry, to dump raw sewage into the ocean, etc. However, in reality we are connected to all other living things regardless of our perceived control. Every interaction we make with the environment causes small changes that ripple out throughout the world and eventually affect all living organisms. Even within ourselves, we carry a diverse ecosystem of microorganisms that are impacted by our every move5. While we may be physically distant from the parasite on the Nepalese yak, we are inevitably connected to it through other living things and our environmental perturbations.
1 Lorenz, E.N. (1972). Predictability: does the flap of a butterfly's wings in Brazil set off a
tornado in Texas? 139th Annual Meeting of the American Association for the
Advancement of Science (29 Dec 1972), in Essence of Chaos (1995), Appendix 1, 181.
3 Holland HD. The oxygenation of the atmosphere and oceans. Philos Trans R Soc Lond B Biol Sci. 2006 Jun 29;361(1470):903-15.
4 Valentina Rossettia et al. The evolutionary path to terminal differentiation and division of labor in cyanobacteria
5 Ron Sender et al. Revised estimates for the number of human and bacteria cells in the body. bioRxiv doi: http://dx.doi.org/10.1101/036103
No. 9 – Checks and Balances
The best known expression that illustrates to me the interconnectedness of life is "the circle of life." We learn as children that vegetarians develop through eating plants, carnivores develop through eating vegetarians, and decomposers return all the above back into nutrients that can be fed back into developing plants with the aid of of the energy of the sun. We can identify our place in this scheme as humans as general consumers, and our cats as obligate carnivores, and watch worms degrade the compost that we can sprinkle over our corn fields so we can continue this interconnected circle of life. Yet, time and expanding research into the world of biology and microbiology have added greatly to this concept.
For example, how many of us have ever thought about how mold (a common class of fungi) and bacteria fit into this scheme? To many of us, they just elicit ideas of yet another loaf of week-old bread to be tossed in the trash, or of long nights running to the bathroom after eating seedy chicken, maybe even ending up in the hospital. But recently, a new tree of life was published that reveals that Earth’s biodiversity is greatly centered on the bacterial domain of life and that the number and variation of bacteria are incredibly greater than that of eukaryotes like us1. With the discovery of so many different types of bacteria, if these were all bad how could we avoid being constantly sick?
Well, it turns out that bacteria (and fungi) are so much more than just pests and pathogens. In fact, the plants, vegetarians, carnivores, and decomposers all rely on bacteria and fungi. In the end, the bad bugs that leave us sick are a very small minority. For example, we rely on Staphylococcus to protect our skin from pathogenic microbes, and on most of the bacteria that live in our gut to break down insoluble fibers and produce vitamins such as biotin2, all to our health benefit. Plants rely on bacteria, such as Rhizobium, to convert nitrogen in the air into workable nitrogen that goes into proteins3 that can then be passed on to nourish vegetarians and omnivores. The majority of our modern-day medicines are derived from plants, fungi, or marine bacteria. Without them, we would die far earlier from the bad bugs that have all made our lives miserable at some point.
These interconnected relationships are not all positive. In a fight for space very similar to that of conquerors who wanted to expand the land they ruled, bacteria and fungi expend much of their energy in fighting off and killing other bacteria or fungi, so that they can consume the wealth of nutrients available in a given site. Many of these, in true predator fashion, will consume at least parts of microbes they’ve killed, to thus gain nutrients and continue expanding. It is these very methods of killing each other that we have adopted for use as medicines.
This sort of a balance allows all living things to be connected to other living things without blowing our earth out of proportion. You could almost compare it to the three branches of the U.S. government that each check and balance each other so that everything is connected while functioning at the proper level. It is this way that life on Earth has managed to develop into what we know today.
- Hug, L. A. et al. A new view of the tree of life. Nat. Microbiol. 1, 16048 (2016).
- Said, H. M. Cell and molecular aspects of human intestinal biotin absorption. J. Nutr. 139, 158–62 (2009).
- Zahran, H. H. Rhizobium-legume symbiosis and nitrogen fixation under severe conditions and in an arid climate. Microbiol. Mol. Biol. Rev. 63, 968–89, table of contents (1999).
No. 10 – No exceptions
That living things interact with other living things is obvious. We humans interact with each other, our pets, and the plants in our gardens. Animals in nature interact with other animals to play, eat, survive, grow, hide, etc. What is less obvious is that all of these living things are connected. This is true for to all living things because of how they evolve. Organisms do not evolve on their own, but they do because their environment and other living things around them change. A classic example of coevolution are the mitochondria. They are thought to arise by bacteria that survived being swallowed by another cell, both coevolving into a eukaryotic cell (like ours). The connection of living organisms goes back to the beginning of time and majorly ties into the microbial world.
Long-time established connections are also revealed by looking at the genomes of various species . Most distant species share about one third of their genes. Thus, 37% of human genes share homologs with Bacteria and Archaea . These genes are exchanged by transfer of genes between special ("horizontal gene transfer"). What is probably a relatively recent event can be seen in some Japanese people who have a bacterial gene that allows the degrading of polysaccharides in seaweed . In fact, human-associated bacteria have a 25-fold higher horizontal gene transfer rate than bacteria in other environments . Such interactions result in the connection of all living things.
Many living things are also connected through symbiosis. Whether beneficial or parasitic, symbioses are numerous. For example, strains of brewers yeast breeds in the guts of social wasps . Or, Plasmodium species transfer to humans from mosquitos, resulting in the transmission of malaria [1,2].
Though generalizations usually have exceptions, I know of none here. All living things are connected. Billions of years ago, coevolution began and the microbial world became intertwined and connected all living things to one another.
 Crompton et al. Advances and challenges in malaria vaccine development. J Clin Invest. 2010 Dec; 120(12): 4168-4178.
 Mcfall-Ngai et al. Animals in a bacterial world, a new imperative for the life sciences. PNAS 2013 Feb; 110(9): 3229-3236.
 Stefanini et al. Social wasps are a Saccharomyces mating nest. PNAS 2016 Feb; 113(8): 2247-2251.
No. 11 – The Global Ecosystem
"All living things are connected to other living things." This is an easy enough quote to say, but its literary simplicity belies the greater complexity beneath. Depending on a person’s background, they could take this statement in many different ways; I prefer to pursue this statement to its biological meaning. That is to say, all living things are connected to other living things through what can be described as the global ecosystem, with the foundation of this global ecosystem being microbes. Microbial organisms, such as bacteria and fungi, play a much greater role than their small size would suggest.
To start, let’s focus on marine microbes. While difficult to prove definitively, the ocean has been estimated to contain approximately 1029 prokaryotic cells. These cells are involved in important functions to maintain larger eukaryotic life, including the about half of all primary production of organic compounds from CO2. The growth of marine microbes provide energy to unicellular eukaryotes, like diatoms and dinoflagellates, who then in turn become a food source for small fish, and so on, until we reach the larger predators such as marine mammals and humans. Marine microbes also contribute to nutrient cycling, particularly nitrogen and phosphorus, as well as the production of oxygen through photosynthesis.
Microbes also contribute to precipitation like rain and snow. Marine bacteria convert sulfur containing products from phytoplankton into dimethylsulfide (DMS). DMS reacts with other compounds in the air to form cloud-condensation nuclei, facilitating rain production. Even in the air far above us, bacteria can be collected and cultured. These "cloud-borne bacteria" have evolved to thrive in the cold temperatures of the atmosphere. Researchers like Bauer et al. have shown that the bacteria act as ice nucleation particles, thus producing ice crystals that make up precipitation like snow and hail. Previous studies showed that the bacterial species Pseudomonas syringae is able to catalyze the formation of ice crystals at temperatures around -2oC, much higher than the -12 to -25oC range of inorganic particles.
It follows that plants benefit from the rain. But they also enjoy more direct symbiotic relationships with bacteria and fungi. An example is mycorrhizal fungi associated with the roots of a plant. The fungi contribute to the uptake of both water and nutrients like phosphorus. Nitrogen-fixing bacteria are also prevalent among the root systems of plants and contribute to the global nutrient cycle like their marine relatives, in this case making nitrogen usable by plants.
Across the globe, microbes are the link between all living things. Therefore, I will end with a quote by the author Stewart Brand, "If you don’t like bacteria, you’re on the wrong planet."
 Holbrook S.J., and Schmitt R. J. (2005), Growth, reproduction and survival of a tropical sea anemone (Actiniaria): benefits of hosting anemonefish. Coral reefs, 24(1): 67-73.