Two million
and counting
How do we classify the abundance of life?
Earth is the only planet that we know supports life.
Nearly everywhere we look is inhabited by organisms: high in the atmosphere, deep underground, in hot springs and encased in ice. Grouping living beings helps us to understand the sheer variety of life.
Scientists classify organisms into species based on anatomy and how they are related to each other. The question of how many species there are is one of the most fundamental in biology, yet we still do not know the exact answer. Scientists have named around two million species so far, but that is just the tip of the iceberg. Great uncertainties lie in estimating the real number of existing species for many groups, often varying by tens or hundreds of times, or even more.
What is a species?
One of the difficulties of counting species is deciding what the term 'species' means in the first place. Scientists use a variety of ways to define what a species is.
The most common is the biological concept of species. This defines species as a group of organisms which share most of their features and can successfully produce fertile offspring. However, this often isn't the most useful way of thinking about species across the diversity of organisms that are alive today, nor organisms that existed in the past.
Advancements in molecular studies have led to the genetic species concept where DNA and other genetic material are used as evidence to define and group different organisms. This is particularly relevant in the world of microorganisms, where reproduction often does not involve the exchange of genetic material. This species concept can also help distinguish between species that look alike but are genetically different.
As if the vast scale of Earth's biodiversity were not challenging enough, all these factors in defining the concept of species complicate our understanding of the true diversity of life on our planet, and make it a fascinating area of ongoing research.
Cataloguing life
Classifying related species discovered so far into groups can reveal what we know – and remind us of what we don’t know – about the sheer variety of life. The science dealing with the classification of organisms is known as taxonomy. Species represent the lowest of eight taxonomic levels. Individual species can be further grouped into categories of genus, family, order, class, phylum, kingdom, and domain.
The interactive graphic below shows a series of circles, each representing a group of related species. Most of the circles (with green backgrounds and on the left) show species grouped into phylum within the kingdom of Animalia. Other circles (on the right) represent entire kingdoms. The circles are scaled according to the number of species in the group that are currently known to science. Click on the circles below to find out more about how many species are known per group.
In addition to what is shown in the graphic, there are more than 20 smaller groups of animals with fewer than 10,000 known species, which are not represented here, but which are of utmost importance to understand the whole diversity of life.
The largest circle shown represents arthropods. One of the smallest circles shows the known number of viruses. Different factors have contributed to whether species have been studied and described. Larger and more visible organisms may have been easier to classify in the past. Other species have attracted specific attention from their particular characteristics or the personal interests of individual scientists. Scientific knowledge is expanding all the time as scientists develop new studies and techniques for researching species. Therefore, in a future graphic the size of these circles may look different, and reveal even more of the abundance of life, as we know it.
Data compiled in 2021.
Visit the Life, as we know it display
Opened between 2022 and 2024, our new displays on biodiversity showcase the variety of life on Earth and consider important questions about preserving this diversity for future generations.
The Life, as we know it display explores the classifications of life using specimens from the Museum's collections. Like the circles in the interactive graphic above, each disc in this display is scaled according to the number of species in the group that are currently known to science. In each disc one specimen stands for around 10,000 known species.
Most of the organisms on display in this case would fit in the palm of your hand. In reality, across the spectrum of living things the majority of species are very small, and organisms larger than your hand are rare.
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