by James Morris
Photograph by Randy Phillis
Great fleas have little fleas,
Upon their backs to bite ‘em,
And little fleas have lesser fleas,
and so, ad infinitum.
~ Augustus De Morgan, A Budget of Paradoxes
You are not alone. Certainly, you have family and friends to help, support, and encourage you. But you have even more intimate partnerships.
Of the trillions of cells that make up your body, only about half are human. What then are the rest? They are the numerous microbes that live on and in you.
We tend to think of microbes as harmful, causing everything from the common cold and flu to tuberculosis and malaria. But this is just a small subset of the microbes that are out there. Many are harmless, neither hurting nor helping you. And some are beneficial, even vital, not just to us, but to life on Earth.
Take the microbes that you carry around with you daily. Some are just along for the ride. But others play essential roles, ones you couldn’t live without. The microbes in your gut, for example, help you digest your food. Others help to train and instruct your immune system. Some of the residents of the skin keep away other, more harmful pathogens.
The collection of microbes in and on you is collectively called the human microbiome. You are a working, walking ecosystem, and your health depends importantly on these co-travelers with you. The microbiome is not static, but dynamic – it changes as you grow and is influenced by what you eat, whether you are taking antibiotics, where you live, and so on. There are different collections of microbes in different parts of your body, like your skin, nose, or gut. Even the two sides of your body have different groups of microbes.
A partnership between two organisms is a symbiosis, and a partnership where each benefits is a mutualism. Mutualisms are common throughout the natural world. Bees visit flowers, each one helping the other – the bee gets nectar and the flower is pollinated.
Cows eat grass made of cellulose, which they can’t digest. Within a specialized chamber in their gut called the rumen, they house bacteria that are able to break down cellulose into other molecules that the cow can digest. So, the bacteria and the cow both get a ready supply of food, and they couldn’t live without each another.
Some partnerships are even more intimate, where one lives in the tissues of the other. For example, certain bacteria live in the roots of some plants, where the bacteria get nutrients from plants. The bacteria in turn take nitrogen gas from the atmosphere, which is plentiful but unavailable in its form to plants and animals, and convert it to a form that is readily available.
A coral is a tight partnership between an animal and a photosynthetic alga. A lichen is a fungus and an alga that have taken a “lichen” to each other.
One of the more surprising discoveries of a tight partnership is a photosynthetic sea slug. A sea slug is a type of mollusk, which is an animal. How does an animal carry out photosynthesis, which we commonly associate with plants? The answer is that it has incorporated photosynthetic algae in its tissues.
In some cases, one organism lives not just within the tissues, but actually inside the cells of another. For example, aphids rely on certain bacteria that live in their cells to obtain the nutrients they need. There is even a species of mealybug that takes this one step further – within the cells of the insect is a type of bacteria, and within these bacteria is another type of bacteria.
When we think about evolution, images of struggle and competition might come to mind. But what these examples teach us is that cooperation has accounted for some of the key transitions in the history of life.
Your cells, in fact, are the result of an ancient partnership, as inseparable as the aphid and their bacteria. Your cells contain mitochondria, involved in energy metabolism, which bear unmistakable signs that they were once free-living bacteria.
How did these bacteria end up in your cells? A long time ago (in this galaxy), an ancient cell engulfed a bacterium capable of carrying out a form of metabolism similar to modern-day cellular respiration. Instead of digesting this bacterium, the early cell incorporated it, allowing it to live within the cell.
Plants cells contain mitochondria as well, but they also contain chloroplasts, where photosynthesis is carried out. Chloroplasts too were once free-living bacteria that were incorporated into an ancient cell.
Taking a broader view, you depend not just on the microbes in and on you, but on many organisms that share this planet.
The oxygen you breathe comes from bacteria, algae, and plants. The food you eat comes from animals, plants, and fungi. The cycling of many essential elements, like nitrogen and sulfur, depends on microbes. Decomposition is the realm of all kinds of organisms, like bacteria and fungi. Microbes would do just fine without us, but we would not do fine without microbes. Life would grind to a halt.
The fields of medicine and biology owe a tremendous debt to other organisms too. A common technique in research is the polymerase chain reaction (PCR), which uses an enzyme called Taq polymerase isolated from a type of bacteria that lives in hot springs. The blue blood of horseshoe crabs is used to test for deadly toxins from bacteria in medical equipment, saving millions of lives. Our very understanding of biology – from cell division to physiology to cancer – depends on the use of model organisms, like bacteria, worms, fruit flies, and mice, to name just a few.
We study diverse organisms to understand long-standing questions in biology. Planaria, a type of worm, can be split in half and regrow the missing halves; your spinal cord can’t. What is it about planaria that allow them to regenerate? Naked mole rats don’t get cancer, but we do – why is that? Bears hibernate, shutting down their kidneys over long periods of time – how come they don’t get renal failure, as would happen to us?
Even most of the medicines we take ultimately come from nature.
You are here not just with others, but because of others. They are intertwined in the very fabric of your cells, in and on you, and all around you.
© James Morris and Science Whys, 2018