The Inhabitants of Earth



Long version

Our planet Earth is inhabited by viruses, archaea, bacteria, and eukaryotes. These inhabitants contribute a tiny proportion of Earth’s total mass, but they pervade the land, the sea, and the atmosphere. There are trillions of some kinds of them almost everywhere. We study them scientifically and classify the ones we know about.

We, ourselves, constitute a small proportion of the eukaryotes.

All of these entities come in a multitude of sizes, shapes, and compositions, but all rely on DNA and/or RNA for their existence and operation. And they are all continually evolving.

Viruses, archaea, and bacteria are extremely small. They have no life-span as we envisage it, but they can exist for an appreciable time in environments with suitable temperature, stability, and absence of damaging conditions.

The beginning of all this is a mystery. We don’t know how life forms came into being, or whether viruses came before or after organisms. Some scientists think viruses were fairly late to the evolutionary process, forming as remnants from cells that had somehow lost the ability to replicate. That would mean that organisms came first, and viruses later. This seems reasonable, because viruses require the presence of organisms for their existence, as I will describe later. Other scientists think viruses could predate living organisms.

We don’t know whether archaea and bacteria had a common ancestor, or arose separately, or whether one arose from the other. Because of their similarities and differences, it is more likely that they had a common ancestor.

In this essay I will describe the four entities and their interactions.


Viruses are very different from archaea, bacteria, and eukaryotes. Viruses do not have the characteristics of metabolism and reproduction that are usually regarded to be necessary aspects of life. Archaea, bacteria, and eukaryotes have both.

Metabolism is the process of taking matter and energy into the body for the purposes of growth, reproduction and providing power for action. Reproduction is the process of producing potentially living entities within the body and releasing them to become separate organisms. Viruses do not reproduce, grow, or make any actions.

So, while the other three are recognised as living organisms, viruses are generally regarded to be inanimate, even though they have intimate associations with the other three.

Only a small proportion of viruses have been studied, so there are probably many things about them that we know nothing about. As at 2018, their classification system contained about 900 genera and 9000 species. The known species often have several strains, many with significantly different capabilities, and more strains are continually being created.

Viruses are sub-microscopic structures, ranging from about 20 to 300 nanometres in size. To give some idea of these sizes, COVID-19 has a diameter of about one ten-thousandth of a millimetre.

Viruses consist of a shell of protein that contains genetic material, that is, DNA and/or RNA. This genetic material provides instructions for replicating the virus. Some viruses, including coronaviruses, are covered by a phospholipid membrane (see later) in addition to the shell. These viruses are less stable and more vulnerable than other viruses.

Viruses can’t generate their own energy. They can’t move, but they get moved around indiscriminately by the various forces surrounding them.

Most viruses are inside the bodies of archaea, bacteria or eukaryotes, and some are in the bodies of other viruses. Such refuges are known as hosts.

Most viruses are useful or necessary to some kind of life. A small proportion are parasites. These infect bacteria, archaea, and eukaryotes, and are damaging or lethal to their hosts.

The “good” viruses perform a wide range of functions for a wide range of organisms. A tiny proportion of them are used in medicine such as in attacking cancer cells and infectious bacteria. Many work with other organisms, sometimes in symbiosis, sometimes acting as predators to keep the balance of ecosystems. They also act in conjunction with the immune systems of their hosts.

Viruses do not reproduce. They are replicated inside the cells of their hosts. All cells are many times larger than viruses.

Cells have processes for using DNA and RNA to assemble new cells and other tissues.

To get into a cell, the coatings of the virus and the cell must match. For example, the coating of SARS-CoV-2, the virus that causes Covid-19, is covered with a range of rough spikes. It is thought that, when one kind of its spikes comes in contact a particular protein called ACE2, which is part of the coating of the host’s cells, the interaction opens the cell’s coating and lets the virus or its RNA in. No other combination with that virus will open the cell. So most kinds of virus can infect only one kind of host. But a mutation that produces a new strain, might allow a virus to infect another species with similar characteristics to that host. Multicellular organisms can host several kinds of virus.

The new viruses get out of the cell when the cell bursts or when they get pushed out through a breach in the cell wall. They then get moved around by the processes of the host’s body, and are likely to infect other cells. After a few days, there can be trillions of them.

The replication is stopped when the population density of new viruses reaches a certain level. This occurs by the viruses reacting to particles that are released with each new virus. Restricting the numbers of replications stops the host from being further damaged or killed. This gives the virus a continuing habitat for survival, reducing the probability of the virus dying out.

A virus may lie dormant in the host’s tissues for decades. Then some incident may cause it to resume replicating. This often produces lifelong or chronic infections in hosts. Common human examples are herpes and hepatitis B and C.

The genome of a retrovirus, which is just RNA, can often get into its host’s DNA. This is usually advantageous for the host, such as increasing the host’s immunity. But it could be bad for the host when genomes of infective viruses such as HIV, are in the host’s genome and later get out and create more viruses. The genomes of most organisms, including ours, contains virus DNA.

During replication, the process will sometimes produce a mutation in the new virus. This may have no significant consequence, or it may produce an ineffective virus or a new strain of the virus. The high rate of replication creates new viruses with new abilities.

Some viruses, such as phages, which infect bacteria, have evolved to react to signals emitted by bacteria in a way that increases their ability to infest the bacterium, or resist the bacterium’s defences.

Viruses get out of animal host’s bodies by the host coughing, spitting, exhaling or excreting. Then the air or the water or whatever they land upon decides where they go to.

Viruses get into the host by being inhaled, or put in the host’s mouth, eyes or nose after having been picked up from some object that they have landed on, and transferred to the fingers, (or other body parts for other species). People touch parts of their face many times a day. Thorough handwashing with soap or detergent, etc., will remove viruses from hands. Viruses can also get in by being injected into body tissues along with infected blood or other medium.

People and other animals can be given some immunity from a specific kind of virus by being   vaccinated against that virus. Vaccination is a process of injecting some part of the specific virus into the body of the person or animal. This alerts the body’s immune system to some of the characteristics of that virus. A few weeks after a vaccination, the immune system will be able to recognise and attack that virus that virus if it gets in. But even then, the immune system must be able to come in contact with the virus.

Vaccinating an infected host will not be of any use. The immune system will already know about it. After infection, antiviral drugs will prevent the replication of RNA viruses, but different groups of these viruses need their specific drug.

Viruses that are outside can be made ineffective by high temperature, strong ultraviolet light, diluted ethanol, diluted bleach, and some detergents. But some viruses have evolved to tolerate bleach and detergents. The treatment has to damage the coating of the virus and disrupt the virus’s genome.

Viruses that are not inside a host will last longer in wet environments. The water provides a protective barrier from nearby solid mater, which otherwise could damage them if it is pushed against them. They also last longer on smooth surfaces than on rough.

No known viruses that infect archaea and bacteria also infect eukaryotes.

Most viruses that infect humans and plants are named according to the diseases they cause or sometimes according to their structure, e.g., the varieties of corona viruses.

Most viruses that affect other species are named according to their genomes. Bacteriophage is one exception.


Archaea are probably the most abundant life forms on Earth.

Archaea live in almost every location that supports life, including extreme environments, such as hot springs and very salty lakes where no other kinds of organisms can live. They are in the bodies of all eukaryotes, but no known archaea are pathogens or parasites.

We have archaea in many of our internal organs, and on our skin. They are essential for most of the eukaryotes on Earth. As a part of the microbiota of eukaryotes, they help to perform basic bodily functions, such as digestion of food.

Trillions of them are everywhere, in the atmosphere, in all waterways and in the soil.

They average about three micrometres is size, which is about 100 times the average size of viruses.

Archaea are single-cell organisms with no nucleus. Their genome and organs are all in a gelatinous liquid covered by a membrane that consists of phospholipids. These are strands of a kind of sugar, with one end having a phosphate head that associates chemically with water, and the other having a fatty tail that tends to avoid water. To make a membrane, the strands line up, side by side with the heads and tails attracting each other. Outside the membrane, there are one or more coatings that provide protection and enable various substances to get in and out. These comprise the cell wall.

This type of structure is used in all kinds of organisms, but the chemistry of archaeon membranes and walls is slightly different from that of the cells of all other organisms.

It is usually thought that eukaryotes arose through the amalgamation of an archaeon and a bacterium. Some of the DNA of archaea is so similar to that of eukaryotes that is now being suggested that eukaryotes are really versions of archaea. But there are very great differences as well as similarities.

Archaeon genes and metabolisms are more closely related to those of eukaryotes than they are to bacteria. But some parts of their biochemistry don’t appear in viruses, bacteria, or eukaryotes.

Unlike viruses, which have no active ability, archaea perform actions that require energy. So they use food, and energy, to reproduce, move, feed, grow, and keep safe.

Reproduction methods differ with different species. The most common method is binary fission. In this process the archaeon develops a second genome and other components, and then splits. Other methods involve taking in and using DNA from the environment or from other archaea to build new archaea.

Archaea detect environmental conditions and act accordingly. Different species have different needs, different environments, and different methods of moving. In liquids they swim using a rotating tail, called an archaellum, which is made of protein. Other species have bristles, called pilli, on their surface that allow them to slide or shuffle along. Alternatively, the pilli hook onto something and pull the archaeon towards it by retracting into the archaeon’s body.

The range of food that the various kinds of archaea eat includes, in addition to the remains of dead organisms, derivatives of iron, sulphur, uranium, carbon dioxide, methane, ammonia, and many other compounds. Their eating and excreting helps to sustain the condition of the environment, including reducing the amount carbon dioxide in the oceans, atmosphere and soil. It also helps eukaryotes to digest their food.

Archaea are our unseen, little-known, but essential, friends.


Bacteria are single-cell microscopic organisms about the size of archaea. Their cells are similar to those of archaea. But bacterial cell walls are different from the cell walls of archaea, and also from the cells of plants and fungi, which are eukaryotes.

Bacteria come in a wide variety of shapes and sizes, but most bacteria are more or less spherical or rod-shaped or helical. A spherical bacterium is called a coccus, a rod-shaped bacterium is a bacillus.

Most bacteria are benign or helpful to other organisms, but some infect other organisms or are parasitic. As with viruses, we generally regard them all to be the enemy to be eliminated, but a most of them are essential, or helpful for eukaryotes.

Some bacteria are unable to move by themselves, and get moved around by water.

Most move in similar ways to archaeaand can move around in a wide range of places, including in other organisms, looking for food.

Bacteria have pores in their outside wall for taking material in and ejecting it. Overall, they eat a vast range of organic material, each kind of bacterium having its particular kind in accordance with its species and its habitat.

The effects of bacteria come from what they eat and what they excrete. What they eat may be indigestible to other organisms. Also, their eating can degrade food or damage the bodies of their hosts.

What they excrete may contain toxins, or it may contain nutrients that other organisms can’t otherwise obtain. It may also be used in defence.

All of this can occur in the soil, in water, or inside their hosts, as with the gut microbiomes of animals.

All organisms employ electrons in all of their processes. Usually they obtain them from chemical reactions with their food. Some bacteria that live in water have processes for directly accepting electrons. In their environment, if an object with a negative electric charge enters the water, any nearby bacteria will extend strands of conductive tissue to meet the object. They will then extend more strands to other bacteria, feeding them with electrons. Some bacteria get all of their energy this way.

Reproduction of bacteria can occur by splitting, or by forming buds that protrude from their surface. The buds grow until they are ready to break free.

Other bacteria pick up and incorporate strands of DNA from the environment, or from a virus attached to DNA from another bacterium that it invaded, or receiving DNA from another bacterium. These processes give plenty of opportunities for mutations.

Bacteria reproduce at different rates, depending on the species and the environment, including food supply and the temperature. It can occasionally be as fast as ten minutes per generation. Under some conditions, some bacteria may change into a spore-like state until the conditions improve.

Most kinds of bacteria use products of their metabolism, that is, their shit, to disable viruses, fungi and other bacteria that are infecting them. Some bacteria use specialised proteins. The outcome depends on whether the defending bacteria have enough ammunition, and that will depend on how many invaders there are. Other kinds of bacteria use bits of virus DNA to block virus’s processes. And other bacteria have other kinds of defensive tricks.

Bacteria communicate with each other. One method of communicating is by secreting and sensing various kinds of chemicals. They also use electrons and light. They coordinate their actions, such as in defence and in building biofilms.

Biofilms are like tiny fortresses, giving security to the bacteria. They are flat circular mats, consisting of bacteria and their by-products. They are covered with sticky organic material, which protects from above and glues them to whatever they are attached to.

The members have special roles, depending on where they are within the circle, and this adapts the different members accordingly. Some find or produce food; some protect the surface and edges from enemies.

Biofilms grow by attracting bacteria from outside. There can be billions of bacteria in a single biofilm. More than 99% of bacteria live in some kind of community

Different species and strains of bacteria build biofilms with different of patterns and arrangements of management. The kind of biofilm also depends on the environment, such as on a tooth or in water or inside the body of a host. Unseen biofilms are almost everywhere where there is food.

While there is a large amount of coordination and cooperation within these communities, these is also intense competition between communities, including different kinds of bacteria and other microorganisms. There is even fierce competition between members of biofilms.

However, biofilms will negotiate with neighbouring biofilms and other communities for greater protection.

All of these kinds of action seem remarkable for such tiny organisms. But everything is controlled by population, population density, food supply, environmental conditions, and the structure of the organisms.

The same applies to archaea, but, because archaea do not threaten us, we haven’t studied them as intensively as with bacteria.

Bacteria inside the host’s body can be killed by antibiotics, which damage the structure of the bacterial coating or the bacterial internal organs. This can stop the function of the bacterium, including its ability to reproduce. (Antibiotics have no impact on viruses.)

Bacteria can also be killed by viruses, as already mentioned, and by the immune system.

Outside the body, bacteria can be killed by high temperatures, alcohol such as methylated spirit, acetic acid such as vinegar, and a few other liquids. Some bacteria can tolerate some of these.

Ongoing research has recently found that there are more than twice the number of archaea and bacteria living deep under the earth’s surface than on and above it. And there are at least as many kinds of these organisms down deep as those above. Some of them are fairly closely related to surface archaea and bacteria. Viruses and fungi also have been discovered there.

The volume of territory that they occupy is thought to be about twice the volume of Earth’s oceans, which occupy 70% of the earth’s surface and  on average are about 4 km deep.

Their underground conditions are very different from those above, with higher temperatures and very high pressure and difficulty of moving around. So these organisms meet much fewer other organisms than those above do. There are fewer dangers, the main one being depletion of their food supply. Everything happens very slowly, so they live a lot longer and reproduce at a very much slower rate.

Even more surprisingly, geomicrobiologists have recently revived bacteria that had been lying dormant for more than 100 million years in clay about 74.5 metres under the seafloor in the South Pacific Ocean, which at that point is under 5.7 km of water.
They may be the oldest-known organisms on Earth.
The scientists incubated them for up to 557 days in a laboratory until they were acclimatised to life on the surface.

The bacteria are aerobic, i.e., they need oxygen to live. Oxygen was present in the sediment  in which they were found. They must have had no nutrients during most of the 100 million years, but they were still alive.

They are of varied lineage, spanning 10 major groups of bacteria, and have a diverse range of food and of processing it.

This discovery is from a minute proportion of the sea floor, which has a huge range of depths, which have been subject for billions of years to the ups and downs of plate tectonics (which is sometimes referred to as continental drift). We can only wonder how much else is under the ocean floor, and what is it like.


Eukaryotes are much larger than, and have great differences from, viruses, archaea, and bacteria.

All of Earth’s multicellular organisms are eukaryotes. There are, however, many kinds of single-cell eukaryotes. With the exception of these, the cells of all eukaryotes have features that enable them to join with and work with other cells to build an almost unlimited range of complex shapes, sizes, and types. This gives eukaryotes a huge range of differences and abilities.

To the best of our knowledge, the first eukaryote came into being about 2.7 billion years ago when a bacterium and an archaeon fused into a single organism, with one of them becoming  the nucleus of the new organism. The descendants of that organism evolved in different ways to gradually produce groups of organisms more different from each other in appearance and function than the differences between and within archaea and bacteria. And yet the basic apparatus within all eukaryote cells is virtually the same. Almost all of the differences between the individuals and between the groups are the result of different details in the DNA in the nuclei of the cells.

Eukaryotes are the only organisms whose cells have an enclosed nucleus encasing their DNA. The nucleus has a lot of similarities with the bodies of archaea and bacteria. It is contained in a phospholipid membrane, and has outer coatings with pores through which material gets in and out. Its DNA is more similar to that of archaea than of bacteria.

The nucleus contains a gelatinous liquid similar to that of archaea and bacteria.

The nucleus also has differences from archaea and bacteria. The DNA or RNA of an archaeon or a bacterium is just one chromosome and is circular. The DNA of eukaryotes is in long twisted strings, and has several chromosomes. And the nucleus has no flagella or pilli, or any other means of propulsion. It doesn’t go anywhere.

In addition to the nucleus, the cells contain small organelles, i.e., tiny organs, that perform a range of functions.

The Golgi apparatus prepares proteins for secretion out of the cell, and helps in the distribution of lipids around the cell, and creates lysosomes.

Lysosomes break down excess or worn-out parts of the cell and any infesting viruses and bacteria.

Mitochondria produce and store energy for the functioning of the cell. They use a chemical process involved with the metabolism of the cell. They have their own DNA, which is different from the nuclear DNA.

And there are a lot more components that are part of the operations within the cell.

The cell also has an internal supporting structure made of microtubes that gives it its shape.

All of this is washed in a gelatinous liquid.

Eukaryote cells reproduce by cell division. The cells reproduce by taking in more nutrients, creating a new nucleus and organelles, etc., inside the cell, and then separating the new material which becomes new cell. This is a process controlled by the cell’s DNA, using RNA to produce amino acids and create and assemble proteins.

In multicellular eukaryotes, the organism and its components grow by reproduction of the cells. Although all the cells have the same genome, different types of cells are produced to build the different parts of the organism. This is controlled by different cells having different genes being activated or silenced.

Multicellular eukaryotes need distribution systems to transfer information and nutrients and other material throughout their bodies.

In our naming and classifying of Earth’s inhabitants into groups in accordance with their similarities and differences, we have found four distinct categories, called Domains: Viruses, Archaea, Bacteria and Eukaryotes. These has been subdivided and subdivided according to their characteristics until we reach the basic levels of genus and then species.

The eukaryotes have been classified into four distinct Kingdoms: Animalia, Plantae, Fungi and Protista. The first three are each thought to have their own common ancestor. The protists have miscellaneous ancestries, and none of them could be regarded to belong to any of the other kingdoms.


Animals come in a huge range of sizes, from microscopic rotifers, whose bodies have only about 100 cells, to the largest whales.

Animals have features that distinguish them from members of the other three kingdoms. They are built of soft flexible tissue that consists mainly of water. Their bodies are mostly supported by stiff material, either inside or coating the body. Almost all of them can move parts of their body. They can perform a range of functions, and most of them can move around their environment. Many kinds of animals can fly, walk, run and/or swim. Animals that live fixed to one spot, such as sponges and oysters, move parts of their bodies to obtain food and for defence.

Animals have mouths that takes in food and a hollow internal system where food is digested and from which nutrients are distributed and the depleted residue is ejected. All animals, with the exception of loricifera, a small group of microscopic inhabitants at the bottom of deep seas, must continually take in oxygen, which is distributed throughout their bodies as an essential source of energy. Carbon dioxide is produced in the body’s processes and then emitted, by larger species through the mouth and nose.

All kinds of eukaryotes are inhabited by viruses, archaea, and fungi. Some may benefit from this and others suffer. In animals, the benefit ranges from being trivial to being essential. In one group of animals, the mammals, their microbiome of trillions of viruses, archaea, bacteria and fungi in their digestive systems is vital for the physical and mental processes of the animal.

Animals have systems of nerves that send signals throughout the body to move various body parts and to sense their exterior and interior environment. In all but the smaller animals, this is controlled by a brain.

Animals have a wide range of senses and combinations of senses. Some of the main senses are sight, hearing, touch, taste, smell, and balance.

The most intelligent organisms are animals, particularly vertebrates, and the most intelligent animals are the species is Homo sapiens, that is, humans. But many kinds of animals have skills, abilities, and kinds of intelligence that most humans lack.

Most animals reproduce sexually, that is, with the fusion of a male cell and a female cell, which becomes an egg. An egg is a single cell that develops into a new-born animal that grows to be an adult. In most cases, the sexual process involves deliberate action.

Some small invertebrate animals have a multi-stage reproductive process, starting with an egg that is put outside the animal’s body. In one kind of sequence, after a time the egg produces a larva, which eats and grows and then builds a cocoon into which it enters and becomes a chrysalis. The chrysalis begins as a liquid and slowly develops into an adult animal that then breaks out of its container.

Most animals are unable to grow a replacement when a body part is lost. But there are a few oceanic animals that can be put through a shredder and all of the bits will grow to normal adults when thrown back into the water.



Most plants are rooted in the ground. They do not move around and the movements they make of their body parts are, with a few exceptions, opening and closing the petals of their flowers and turning the flowers towards the sun. Many kinds of plants will grow towards sunlight and/or to occupy empty spaces.

Most plants have stems and branches with green leaves attached in specific patterns. The stems and branches may be stiff or flexible, and tough or fragile. Plants have a range of outer coatings , from being very smooth to very rough. The stems and branches of trees are mostly composed of wood, which is the coating of the plants’ cells and is composed of cellulose and lignin. Plants also contain water.

Plants make their own food and internal material, using a process of photosynthesis of  atmospheric carbon dioxide, water and nutrients. Oxygen is emitted from this process. The process is powered by sunlight that falls on the plants’ leaves. The plant’s cells have an organelle called a chloroplast, that puts a green substance into the leaves and other body parts to increase their absorption of sunlight.

With plants taking in carbon dioxide and emitting oxygen, and animals taking in oxygen and emitting carbon dioxide, it looks a bit like symbiosis. But most animal food is derived, directly or indirectly from eating plants, so the symbiosis is a bit one-sided.

Most plants obtain nutriments, mainly nitrogen and minerals, through their roots in the ground. The roots need water to enable them to take in the nutriment. The roots, which stabilise most plants in the ground, are also the plants’ control organs. The roots send water and nutrients up to the rest of the plant and into the leaves.

Plants have rudimentary nervous systems. They also have rudimentary memories of  events that affected them. Some plants whose leaves are being eaten by insects will send toxins into their leaves. Others emit chemical signals to warn neighing plants to release toxins. (Some species of animals, fungi and protists also have toxins in their tissues to repel predators)

Most plants reproduce sexually, often with the incidental help of animals, mainly insects, that come to their flowers to feed on or gather the nectar and pollen produced by the flowers. This happens to bring male and female reproductive cells together to form seeds. The seeds ripen and become detached and get distributed to the surrounding area, where some of them will take root and grow into new plants.

Some plants are self-fertilising.

Many plants can grow from stems or leaves that are planted in the ground. For some plants, this is the only method of reproduction.

Most plants can grow new parts after parts have been removed.



Fungi, like the other kingdoms, are multitudinous and everywhere.

The larger ones consist of fleshy flexible filaments tangled together. They grow by extending one end of each filament and by building new filaments.

Fungi range in size from single cells to great accumulations of filaments. The largest organism on Earth is thought to be a fungus in Oregon USA. It occupies an area greater than 400 hectares under the ground. It is thought to be about 2,400 years old. Most of the mass of the earth’s fungi is underground, but some live in the tissue of other organs, including in rivers, lakes, and oceans.

Unlike plants, but like animals, they cannot make their own food. They get their food and energy from taking in material from living and dead organisms, and from decaying organic matter. So they do not need sunlight.

The food is taken into the filaments. Their inability to produce their food internally and the softness of their bodies is said to make fungi more similar to animals than to plants. But like plants, they don’t move around or move their body parts.

Many species of fungi live in a symbiotic relationship with plants. About 90% of plants depend on fungi for some of their nutrients, and the fungi receive nutrients from the plants. Lichens are symbiotic relations of fungi and algae.

Many kinds of single-cell fungi infect eukaryotes, but many more are in or on the bodies of eukaryotes as part of the usually helpful microbiota.

Fungi need water and oxygen and prefer moist and slightly acidic environments.

Fungi multiply sexually and/or asexually, mostly by producing spores, which get distributed into the environment. Spores are encapsulated cells that can become new organisms, somewhat similarly to seeds. Sexual reproduction involves the union of two spores. Mushrooms and toadstools are spore-bearing fruiting bodies of fungi. As with plants, the sexual process does not involve deliberate action.

Almost every species of organism on Earth, except fungi, can  be infected by some kind of fungus.

Fungi can repair damaged parts of their filaments, but not damaged mushrooms.



As with other eukaryotes, protists are very diverse.

Most protists are single cells, but a few can be larger. One exception is the giant kelp that can grow to about 50 metres. Kelp is one kind of alga, all of which contain chloroplasts similar to those in plants and cyanobacteria.

Algae reproduce in a wide range of ways, from cell division to sex, but most of the other protists use cell division.

Most protists live in water or damp soil. To move through water, most of the smaller ones use similar methods to archaea and bacteria.

Some protists produce their own food within their own bodies, others have to find it.

Many protists are parasites. One genus,Plasmodium, contains species that infect animals and cause malaria in humans. And many protists are symbiotic. One odd example of symbiosis is that sea otters shelter from predators in the huge thick layers of kelp. The otters eat sea urchins, which feed on kelp.

One group of protists is the slime moulds. There are about 900 species with similar characteristics but with a range of ancestries.

They can reproduce sexually, and end their lives as clusters of spores, which are released  into the wind or the water. When two matching spores come together, they become a new juvenile cell, which feeds and grows into being a single-cell adult.

Single-cell slime moulds can live individually, feeding on bacteria on dead vegetable matter. When food is scarce, some species unite to be something like larger multicellular organisms. These “quasi-organisms” have a range and sizes, the largest being a few centimetres. They can move, usually in a way resembling that of a slug or snail. They can detect the presence of food in their environment and move towards it. Some cells then form into fruiting bodies that release spores, a bit like tiny mushrooms.

Amoebas are protists that can change shape to produce the equivalents of legs. Their enclosing membrane is covered with hairs, which are used  in conjunction with shape-changing, to assist in moving around, feeding, and in other functions. They are mostly unicellular. A large proportion of them infect other organisms.

Despite their similarity, different groups of amoebas belong to different orders of protists.


With the possible exception of protists, my descriptions and classifications may make all this look like a tidy consistent system. But at every stage there are ambiguities and exceptions.

There are more than 30 different definitions of the concept of species. These refer to genomes, methods and possibilities of reproduction, ancestry, and kinds of body structure, which are invoked to suit the different kinds of organisms.

Plants are regarded to synthesise their own food, but there are several plants, with a range of ancestries, that are carnivorous. Most of them have features that attract and trap small animals. They use varieties of enzymes to break down the bodies of their victims to produce the nutrients that they rely on.

There is the insertion of virus DNA influencing the genomes of a large proportion of organisms. There is the continual exchange of DNA within and between archaea and bacteria, and characteristics of one kingdom of eukaryotes getting into another, such as snails that have chloroplasts. They are green in summer and go red in autumn like the autumn leaves of plants.

There are continual dynamic changes of evolution and extinction. There are continual changes in the environment throughout and beyond the earth. We are part of both the biological and the physical systems, as subjects and as actors.

All of this has occurred in accordance with the laws of physics. The characteristics of all of Earth’s inhabitants have been determined by the conditions on Earth and on its history. Their emergence and development seem to have happened through a succession of very unlikely coincidences. But all of the coincidences were statistically possible.

Any inhabitants of other parts of the universe would have similarly arisen and developed in accordance with the possibilities allowed by the conditions and histories of the object they occupied. I think it would be very unlikely that the first life on Earth arrived from somewhere else in the universe, and the same would apply to any extraterrestrial life.

We, Homo sapiens, are just one species among the eukaryotes, peeping from our little planet into the outer universe.

We know quite a lot about Earth’s inhabitants and their histories.

But we have no knowledge of how it all began, or whatinhabitants there are beyond our planet, or where we are going.