-Helen Macdonald-
The human need to classify and sort things into categories is a natural behaviour that develops in early childhood. The capacity to recognise similarity or differences between objects is indeed a necessary step of our development. As soon as the toddler is capable of recognising likenesses in the observed objects, she will start to sort them into groups of colour, size, shape, etc..
Why do we classify things
Sorting things out is a natural behaviour that helps children understand how the world around them works and eventually builds the person’s capacity of seeing patterns, analyse objects and solve problems. Some studies have shown that the tendency to classify is innate, meaning that it develops independently, without the need of being taught. For example the part of the brain that is responsible for picturing images in our mind is able to differentiate between a living thing and an inanimate one without learning. Indeed it has been found that the exact same difference is present in people that are born blind have therefore never seen the visual difference between a living organism and an object. It means that the brain is pre-programmed to sort objects into two categories, the living ones, which are potentially dangerous, and the inanimate, which are usually not.
Classification
From the most ancients civilisations onwards, people have always separated things into categories. The first classification of the livings is believed to be the one created by Aristotle, the greek philosopher from the 4th century BCE. He classified living organisms in different categories depending on certain traits putting them in a hierarchy from the most complex (e.g. humans, or at least men, as he believed women to be inferior) to the most simple organisms. And he already used a binomial system, although not in the same way as we do currently.
Modern classification
More than 2’000 years later, in 1753, the Swedish botanist Carl Linnaeus created the modern classification system. Not only that, he also spent long time in the field collecting samples, describing and classifying more than 12’000 species of animals and plants. Linnaeus’ system was based on seven hierarchical levels named kingdom, phylum, class, order, family, genus, and species. The more two species are similar, the more levels they have in common. So two almost identical species will share the same kingdom, phylum, class, order, family, and genus and differ only by the species name. All species are named using the binomial system consisting in the genus name first followed by the species name. Both genus and species are written in italics, with the genus starting with a capital letter, like in the pictures below that show three different species of testate amoebae belonging to the same genus (pictures downloaded with permission from Microworld, world of amoeboid organisms.
Is modern classification obsolete?
Linnaeus in his classification separated all organisms into two kingdoms: animals and plants.
| Animals | Plants |
About a century later, in 1866 the German biologist Ernst Haeckel proposed the addition of a third kingdom called protista for unicellular organisms (in other words microorganisms).
| Animals | Plants | Protista |
Later on he recognised that structural difference between microorganisms procaryotes (bacteria) and eukaryotes (all other organisms) and suggested to put bacteria in a fourth kingdom called Monera.
| Animals | Plants | Bacteria | Monera |
Another century later, in 1969, Robert Whittaker proposed another addition to give fungi their own kingdom.
| Animals | Plants | Bacteria | Monera | Fungi |
Lastly, a subcategory of bacteria that used to be called archaeobacteria, was promoted to kingdom with the name of Archaea. Archaea are prokaryotes like bacteria, but also have proteins and functions in their cells that are similar to eukaryotes.
| Animals | Plants | Eubacteria | Archaea | Protista | Fungi |
The more you know, the less you know?
At that point we though we had it sorted. We had the 6 kingdoms of animals, plants, fungi, protists, eubacteria and archaea, in which we could classify all the organisms known, based on phylogenetic trees, which are analyses that calculate the distance among species based on traits similarities.
But then came high through-put DNA sequencing. DNA sequencing technology allowed more and more efficiently to read and compare the genetic code of different species. With each new study, it became evident that things are so much more complicated than we believed in the past. Studies of DNA sequences showed for example that many species thought different were actually just different expression of the same species, or that many populations thought to belong to the same species were actually different species. This kind of evidence started to introduce doubt in the accuracy of the classical classification system.
What are species
The definition of species used to be that two organisms are members of the same species if they can reproduce and produce a viable offspring. But molecular biology has proved that the classical definition of a species is not enough to classify the different types of organisms. It became commonly accepted that two individuals belong to the same species if they share more than 99% similarity in DNA sequences. As a comparison, humans share more than 98% of DNA with chimpanzee and bonobos.
But again these rules do not apply as smoothly as we would like to microorganisms. Everyday new species are discovered and each new species has the potential of shifting other species in a different category by introducing new genetic diversity in the group. Even the classification of macroscopic organisms such as plants is becoming less neat than we thought. For example, recent studies have shown that some species of plants can transfer DNA from one individual to another without the classical sexual reproduction, by lateral gene transfer, meaning that the DNA is transmitted by one adult individual to another, rather than by one or two parents to the next generation. There are some theories on how that might happen, but none has been proved yet.
At the moment there are more than 20 different concepts of species, which can bring some confusion when trying to estimate biodiversity or set conservation goals. And the tree of life is believed to look something like so:
With the former kingdom of Animals and Fungi now a subcategory of the Opisthokonta and plants a subcategory of the Archaeplastida (both at the bottom right of the tree, in green, in the image). Quite the downgrading from Aristotle’s classification that put humans at the top of them all.
Although it might seem overwhelming, I find it quite exciting the amount of life forms we still know nothing about and I cannot stress enough the need of protecting all of them at all cost.