Table of Contents
The Ubiquity of Living Together
No organism lives completely alone. From bacteria to humans, life is shaped by countless relationships with other organisms. These relationships can be brief or lifelong, loose or tightly integrated, beneficial, harmful, or seemingly neutral. Taken together, they show that “being alive” almost always means “living together.”
In the context of symbiogenesis, this chapter highlights how widespread and fundamental such interactions are, preparing the ground for more specific discussions of competition, symbiosis, and endosymbiosis in later sections.
Types of Biological Relationships: A First Overview
Whenever two organisms interact over time, each can be affected positively, negatively, or not at all. Without going into detailed definitions, several broad patterns are seen again and again:
- One partner clearly benefits, the other is harmed.
- Both partners benefit.
- One partner seems to benefit, while the other appears unaffected.
- One partner uses another as a habitat or food source.
- Several partners form intricate networks, where simple “+ / –” categories are not enough.
These interactions are not rare exceptions but are typical of nearly all habitats on Earth.
Living Together at Different Scales
On and Within the Body: Microbiomes
Every multicellular organism is inhabited by vast communities of microorganisms (bacteria, archaea, fungi, protozoa, and viruses). Collectively, these communities are called microbiomes.
Examples:
- The human body hosts trillions of microbes in the gut, on the skin, in the mouth, and at other body surfaces.
- Plants host dense microbial communities on their roots (rhizosphere) and leaves (phyllosphere).
- Insects, such as termites or aphids, depend on internal bacterial partners to digest food or provide missing nutrients.
Common features:
- Microbiomes form relatively stable “mini-ecosystems.”
- The host provides space and resources.
- Microbes process substances, produce vitamins, or influence development and immunity.
- Disturbances in these communities can affect the host’s health and performance.
Thus, practically every “individual” organism is itself a collection of many species living together.
On the Surface: Epibionts and Epiphytes
Many organisms use the surface of others as a living space:
- Barnacles on whales or crabs.
- Algae and small invertebrates living on shells or rocks covered by other organisms.
- Epiphytic plants such as orchids, bromeliads, and many mosses living on tree branches.
Key points:
- The “host” provides structure and position (e.g., access to light or flowing water), not food in the usual sense.
- The surface-dwelling organisms may have little direct impact, or they may alter water flow, light exposure, or vulnerability to predators for the host.
These examples show that living organisms serve as mobile or stationary “landscapes” for others.
Inside the Tissues: Endophytic Life
Some organisms live inside the tissues of plants and animals without immediately destroying them:
- Endophytic fungi and bacteria inhabit leaves, stems, and roots.
- Certain insects or mites live in plant tissues or animal skin layers.
- Microorganisms can occupy specialized cells or compartments in animal hosts.
Such internal residency can:
- Change how the host responds to stress.
- Influence growth, reproduction, or resistance to other organisms.
- Represent early stages of tighter, more specialized partnerships.
Built Habitats: Nests, Reefs, and Biofilms
Organisms often create physical structures that then host other species:
- Coral reefs, built by coral animals and algae, provide habitat for thousands of species.
- Beavers build dams and ponds that change entire landscapes and create new living spaces.
- Termites and ants build mounds and nests that harbor symbiotic microbes and other small animals.
- Microbes produce sticky substances and form biofilms (e.g., on rocks, teeth, or pipes), which become layered communities of multiple species.
In these cases, “living together” includes both the organisms themselves and the structures they create, which in turn support more life.
Extended Individuality: Organisms as Collectives
Because so many organisms host and depend on others, the line between “individual” and “community” becomes blurred.
Examples:
- A coral “individual” is a colony of many genetically similar polyps, each hosting photosynthetic algae.
- Lichens are stable associations of fungi with algae or cyanobacteria that function as a single, recognizable entity.
- Many insects (e.g., aphids, tsetse flies) cannot synthesize all necessary nutrients and rely on fixed populations of bacteria to do so.
- Humans and other animals are deeply dependent on their microbiota for digestion, immune function, and potentially even behavior.
In such cases:
- The “individual” is better understood as a composite of different species.
- Evolution can shape host and partner traits together, because they influence each other’s survival and reproduction.
- This composite view is crucial for understanding how entirely new forms of life could arise from close living-together, as discussed later in endosymbiotic theory.
Living Together Can Be Stable or Shifting
Relationships between organisms are not fixed categories. Over evolutionary time, the same pair of partners can move along a spectrum of interactions.
Possible shifts:
- A relationship that starts as harmful to one partner can become less damaging, then neutral, and eventually mutually beneficial if both sides gain from cooperation.
- A helpful association can turn harmful if environmental conditions change (e.g., if resources become scarce or partners compete more directly).
- Former enemies can become essential partners, or partners can become mere temporary users of each other’s resources.
Examples of such dynamics include:
- Microbes that start as invaders but later become long-term residents without causing disease.
- Animals that initially feed on plants but evolve more specialized, less damaging feeding strategies, while plants adjust their defenses or even benefit indirectly (e.g., through attracting certain herbivores that reduce competitors).
These shifts illustrate that “living together” is not a fixed state but a process, shaped by ecological circumstances and evolutionary change.
Living Together in Networks
Many organisms interact with more than one partner at a time, forming complex networks:
- A flowering plant may interact with numerous pollinators, herbivores, protective insects, microbes, and fungi simultaneously.
- In the ocean, tiny drifting organisms (plankton) engage in countless feeding, attachment, and chemical relationships.
- Soil communities consist of intertwined networks of roots, fungi, bacteria, and soil animals, exchanging nutrients, signals, and sometimes genetic material.
Features of such networks:
- The effect of any single pairwise interaction depends on the surrounding web of other interactions.
- Disturbances in one part of the network (e.g., loss of a particular partner) can ripple through many other relationships.
- Cooperative and conflict-driven interactions often coexist and may be inseparable.
Understanding life as a network of “living together” relationships prepares us to see how some of these networks can become so tight that new, integrated organisms emerge from them.
Living Together as a Source of Innovation
Persistent, close living together can generate entirely new biological possibilities:
- Continuous association allows partners to specialize: one may provide certain nutrients or metabolic pathways, the other offers protection, stability, or access to resources.
- Specialization can make the partners increasingly dependent on each other, reducing their ability to live separately.
- Over long periods, such dependence can become so strong that the partners effectively form a new, higher-level unit.
These processes do not just influence small details of biology; they can reshape entire branches of the tree of life. The idea that major evolutionary transitions may result from long-term living together is central to symbiogenesis and the endosymbiotic theory, which are addressed in subsequent chapters.
In summary, “life means living together” is not just a metaphor. It reflects the reality that:
- Organisms are built from, live with, and evolve through their interactions with other organisms.
- The boundaries between individuals and communities are often porous.
- Out of countless interactions, new forms of life and new levels of biological organization can arise.