How Many Kinds of Living Things Are There?
For anyone who loves nature, the sheer diversity of life on Earth is breathtaking. There are so many kinds of birds and wildflowers, so many kinds of insects and trees. And yet there are distinct patterns that are repeated over and over. A bird has a head, two wings, a body, and two feet. It has feathers, a beak, and a bony skeleton. It lays eggs. This is certainly a different pattern than that of a fish, an insect, or a tree. For centuries, and even millennia, this combination of diversity and repeating patterns has fascinated humanity. People have tried to make sense of it all by categorizing the living things we see around us. We create categories such as birds, fish, and insects — and then we try to find, name, and count how many different kinds of living things there are in each category.
This effort soon leads to another question: How many distinct categories are there, and what are these categories? This question is tricky, because smaller categories can be grouped into larger categories. For example, birds, fish, reptiles, and mammals all have internal skeletons that include a skull and a backbone. This is quite different from worms, insects, mollusks, and crustaceans, which don’t have bony skeletons inside their bodies. Thus it is possible to divide all animals into two super-groups — the vertebrates (with backbones) and the invertebrates (without backbones). The upshot is that any serious effort to categorize all living creatures will result in a hierarchy of categories. We call this a classification system or a taxonomy.
From a practical standpoint, we can think about a taxonomy of living things by starting at the bottom (individual species), and then working our way up to bigger and bigger groupings — or else we can start at the top with the largest groupings and systematically divide them into smaller and smaller groups. Traditionally, dating back to the ancient Greeks, we have divided the world of nature into three large “kingdoms” — plants, animals, and minerals. In 1735, a scientist named Carl Linnaeus published a book that greatly expanded on this idea. He divided each of these three kingdoms into classes, which he divided into orders, then families, genera, and species. He further refined this classification system in later editions of his books. The editions he published in the 1750s are considered to be the starting point for modern biological classification. We still use his system for dividing groups into subgroups, and we still use some of the Latin species names that he invented. However, most of the details have changed since the 1750s, and every year scientists continue to make updates to the scheme.
These updates occur at every level of the hierarchy. We discover new species, or we decide that a particular species is actually two distinct species, or vice versa. We discover that a species assigned to a particular genus really ought to be assigned to a different genus, or that a particular genus ought to be split into two genera, or that two closely related genera ought to be combined into one. Likewise, we may discover that a genus assigned to a particular family really ought to be moved into a different family — and so on, up through the higher levels of orders, classes, and phyla. These issues even affect the highest traditional level of classification — the kingdoms.
In our modern system of biological classification, we include only living things, and therefore the mineral kingdom was dropped from the scheme a very long time ago. This left us with two traditional kingdoms — plants and animals. Back in the 1960s, the two-kingdom system was still widely taught in schools. But during the 1970s it was replaced by a model consisting of five kingdoms, and since then other kingdoms have been proposed. So what was the problem with the old, traditional two-kingdom model — plants and animals? The principal issues were:
1) The more that scientists studied fungi, the more it became obvious that fungi are not plants. The differences between fungi and plants are just as great as the differences between animals and plants. Solution: Create a new kingdom for fungi, separate from plants and animals.
2) As we studied the world of one-celled organisms, the idea of dividing them into plants and animals made less and less sense. Although some of these creatures seem animal-like, and some seem plant-like, other one-celled creatures did not fit well into either group. Even worse, some of them — such as Euglena — appeared to be plants and animals at the same time. Solution: Create a new kingdom for single-celled organisms. (This group is often called “protists”.)
3) Among single-celled organisms, some of them have large, complex cells — similar to the cells of multicellular organisms (plants, animals, and fungi). But other one-celled creatures (bacteria and blue-green algae) have tiny cells that are very simple, without a nucleus, mitochondria, or other internal structures. There is a sharp, easily defined distinction between these two groups. Solution: Create a new kingdom for bacteria, including the blue-green algae.
So starting in the 1970s, we began to teach our kids that the world of living things consists of five kingdoms: plants, animals, fungi, protists, and bacteria. This model appeared to be a much better depiction of the real world than the old two-kingdom model. However, we have since discovered an ancient group of single-celled organisms, called archaeans, that don’t fit very well into this model. At the very least, it appears that the archaeans deserve their own kingdom. On the other hand, because the archaeans are themselves quite diverse, it may make more sense to create multiple kingdoms to categorize these creatures. But a proliferation of kingdoms has a serious downside, because we lose the simple clarity that we had in the 5-kingdom model.
As we create more kingdoms, one way to simplify the highest level in our classification system is to create a new level, called a domain, which sits above the kingdom level. All living creatures can be divided into just three domains, based on details of their cell structure:
1) The eukaryotes consist of four kingdoms: plants, animals, fungi, and protists. All of these creatures have complex cells containing organelles such as a nucleus and mitochondria.
2) The bacteria form a single kingdom. All bacteria have very simple cells.
3) The archaeans consist of at least three kingdoms, and perhaps more. The cells are simple, as in the bacteria — but certain other attributes are more similar to eukaryotes. Still other attributes are distinct from both bacteria and eukaryotes.
So what should we teach our kids? Some educators continue to use the 5-kingdom model, while others have migrated to a 6-kingdom model (by including the archaeans as a single kingdom), and others have adopted the 3-domain model. None of these models is a perfect match to reality, because reality is more complex. However, the 3-domain model is generally considered to be the most realistic.
This dilemma comes up over and over in education — that the world is seldom as simple and neat as the models we teach our kids. We create these streamlined models as a way to organize our knowledge. Good models are quite useful for scientists and educators alike, because they provide a solid mental framework to which additional knowledge can be added bit by bit. However, we should let our students know that our universe contains additional complexities that our models don’t fully reflect.
It is also helpful to remember that a model that is useful to a professional biologist might not be useful to an 8-year-old child. The younger the child, the greater the justification for further streamlining the models and terminology we use. Such a reduced model should meet three essential criteria:
1. The model should provide an easy-to-grasp mental framework that enables the child to make sense of the world and to organize additional information that the child later encounters.
2. To the extent possible, the model should use age-appropriate language, focusing on terminology that the child is likely to encounter outside of school.
3. The model should be consistent with the more complex models that the child will be taught in later years.
For example, an excellent system of biological classification for very young children is the “four kinds” model. In this model, we teach young children that there are four kinds of living things:
All of these terms are easily understood, and can be related to experiences outside of school. This simple model can serve as a solid framework for organizing additional knowledge. As the kids grow older, we eventually teach them that microorganisms can be divided into several distinct groups, and that it is helpful to organize the various kinds of living things into domains and kingdoms.
By the way, at some point we also need to introduce the idea that even though bacteria and viruses can both be called “germs”, viruses are not living things — and therefore viruses are not included in our system of biological classification. (A virus is basically a rogue piece of DNA, biologically inert until it becomes inserted into a compatible living cell.) Still, there are many kinds of viruses, and therefore scientists have created a separate system for classifying the various types of viruses.
When we openly acknowledge that there are alternative models to describe a certain aspect of our world, then it raises an important philosophical question. If there are several models to choose from, and if none of them is a perfect reflection of reality, then what is really “true”? It can be uncomfortable to admit that the models we teach our kids are not the same thing as universal truth — and that our models must be updated from time to time. So should we present our system of biological classification as something that is real and true, or merely a human invention? The answer, as it turns out, is rather complicated.
When Linnaeus set up his classification system, there was no concept of biological evolution. Two species could be considered “related” simply because they looked very similar, or had similar characteristics. Few people considered the idea that two “related” species might have evolved from a common ancestor. Therefore the thinking was that biological classification is a system invented by humans, to help us organize our knowledge of the world. The patterns that we see in nature are usually real, but our classification system is determined by which patterns we choose as the organizing principles for our system. The most “natural” level in the system — existing without the need for human invention — was considered to be species. This was based on the idea that members of a species can interbreed with one another, but that they cannot breed with members of other species.
After Darwin, as biologists came to accept evolution as the driving force behind the diversity of life, they saw that our system of biological classification provides a rough approximation of actual evolutionary relationships. Biologists came to feel that our taxonomic system ought to be treated as a reflection of reality rather than an invention of man — and therefore any updates to the system ought to reflect the latest evidence regarding evolutionary relationships. And so began an ongoing process of revising the system as more and more data became available. This data included morphology (the shapes of living structures, such as the parts of flowers), developmental biology (the manner in which a multicellular organism grows from a one-celled zygote into a large, complex structure), and historical geology (especially the analysis of fossils and the surrounding strata). By the 1970s another important tool was biochemical analysis — looking for similarities in the natural compounds found in related species, along with similarities in the biochemical pathways employed by the organisms.
In recent years we have entered an era where we can directly compare the genomes of two organisms — providing us with the best tools yet to discern the genetic relationships between species, although the older tools remain important. The upshot is that scientists have been revising our biological classification system at an ever accelerating pace. These changes are especially noticeable at the family and genus level. Traditional botanical families such as Liliaceae (the lily family) have been broken up into several smaller families. Genera have been swapped back and forth between families, and species have been swapped back and forth between genera. For anyone who learned all of the major families and genera several decades ago, all of these changes can be rather unsettling.
At the same time, it has become clear that our concept of species is not quite as natural as we once assumed. We now have countless examples of closely related species interbreeding — even though it is quite clear that two distinct species were involved. In fact, a favorite technique of horticulturalists is to cross-breed two related species of plants. This approach is widely used for developing new varieties of crops and ornamental plants.
The driving principle for biological classification has now become genetic distance — the difference in the genomes of two distinct organisms. As it turns out, all creatures share a lot of genetic similarities with all other creatures. To take an extreme example, humans and earthworms are both eukaryotes, and we actually share a lot of genes. But humans share even more genes with fish, even more with lizards, even more with wolves, and even more with chimpanzees. So this gives us a good idea of how related we are to each of these other creatures. Also, it’s not just a matter of how many genes are shared. The shared genes tend to have slight differences between species, and the details of these small differences provide crucial clues as to how long it has been since we shared a common ancestor. Even within a species (such as humans), some of the genes will vary from one individual to another, and therefore genetic distance is a useful approach for determining the degree of relatedness.
As we continue to do genetic testing of individual organisms from many different species, our understanding of genetic relationships continues to improve rapidly. We are able to build constantly improving diagrams to illustrate the evolutionary relationships among all living things on earth. These illustrations usually take the form of tree diagrams. Every fork in the tree represents a point in the past where a single species diverged into two or more species (by first dividing into distinct populations, and then eventually into distinct species). To put it another way, every fork represents the last common ancestor for a particular clade of living creatures. (A “clade” is defined as a group of organisms consisting of a common ancestor and all its descendants, living or extinct.) If you were to build a diagram that traced the complete ancestry of one species — such as the African elephant — all the way back to a single-celled organism, then you would likely encounter hundreds of forks along the way, if not thousands.
This is the biggest issue with trying to match up our system of biological classification with the reality of evolutionary history. While our standard taxonomy consists of eight principal levels — domain, kingdom, phylum, class, order, family, genus, and species — there are no distinct levels in the real world. Instead, there is a continuous gradient ranging from two organisms being nearly identical to two organisms being tremendously different. This has led biologists to introduce all sorts of intermediate levels into the classification system — such as subfamilies, tribes, subtribes, subgenera, sections, and subsections. But no matter how many intermediate levels we invent, the system will never fully reflect reality — in other words, our system of biological classification will never reflect all of the forks in the evolutionary history of a particular organism.
In addition to all the intermediate levels between the eight principal levels, we can also divide the lowest level (species) into still lower levels, including subspecies, varieties, and forms. If we want to count how many kinds of living things there are in the world, we have to define what we really mean. Are we counting species? Or should we instead count subspecies, or varieties? In fact, the fundamental units of independent life are not really species — or even subspecies or varieties — but genetically unique individual organisms. Every species consists of individuals that are genetically different from certain other individuals in the same species. In the case of humans, every person in the world is genetically unique, unless that person has an identical twin.
So what does “genetically unique” actually mean? In one sense, every human has exactly the same set of genes as every other human. This was the concept behind “mapping the human genome” — a single genome common to all people. But for some of those genes, some people have slightly different versions than some other people. In fact, because your DNA contains two copies of every single gene, you yourself have two different versions of many genes. Everyone in the world, other than an identical twin, has a unique combination of these gene variants.
Therefore the word “gene” has two slightly different meanings in common speech. On the one hand, we could say that all humans have the same genes, just different versions of those genes. On the other hand, we commonly refer to each variant as a different gene. We might refer to a “blue-eyed gene” and a “brown-eyed gene”, even though these are really just two variants of the same gene. When someone undergoes a genetic test, the real question is not what genes that person has — we already know the answer — but which variants of each gene the person has. This ambiguity of meaning may be perfectly acceptable in everyday speech, but for scientists it is important to be clear. Therefore biologists refer to each variant of a gene as an allele, rather than a different gene.
So where does that leave us with regard to our initial question — How many different kinds of living things are? At one extreme, we could say that every genetically unique individual is a different kind of living thing — and therefore, considering only humans, we already know that there are more than 3 billion kinds of living things. At the other extreme, we could say that all living things can be divided into 5 or 6 kingdoms, or into 3 domains — depending upon our preferred model — and therefore there are no more than six different kinds of living things. But let’s instead assume that our objective is to count the number of species (ignoring subspecies and varieties). Therefore we can reframe the question as “How many different species of living things exist on earth at the present time?” This eliminates the problem of counting up extinct species from the past 4 billion years, and it eliminates the issue that life might exist elsewhere in the universe.
We still face two big problems in trying to generate an answer. The first problem is that because the concept of species has proven to be rather slippery, it can be difficult to perform a count even when a particular genus has been well studied. For example, in the genus Curcurbita (the squashes), some botanists say that there are only 13 species — most of which contain several distinct subspecies. Other botanists say that most of those subspecies are actually distinct species, and therefore there are actually 30 species in the genus Cucurbita.
The other problem is that there are many categories of organisms that have not yet been studied thoroughly. We know that there are many species of insects and spiders yet to be discovered. We know that there are many species of bacteria and other microorganisms yet to be discovered. But we don’t know how many. For example, among the nematodes (tiny wormlike organisms), there may be slightly more than 25,000 species in the world, or there may be a million species. We simply don’t know. In contrast, a few categories have been studied rather well. Thus we know that there are approximately 5400 species of mammals, and around 10,000 species of birds.
The most widely quoted estimate of the number of living species comes from a study published in 2011. This study estimated the number of species at 8.7 million, give or take a few million. However, only 1.2 million of those species have been identified so far. Furthermore, the figure of 8.7 million only includes the eukaryotes (plants, animals, fungi, and protists). It does not include the prokaryotes (bacteria and archaeans), whose species might outnumber the eukaryotes — we just don’t know.
So what is the best answer to our question? All we can truly say is this: There are millions of species of living things on earth, and it will be quite a while before we know exactly how many there are!