Why do we need to classify things? I guess the human mind needs it to put things in perspective.
Imagine if there was no concept of centuries and half-centuries in cricket. Strictly speaking, there isn’t much difference whether one scores 100 runs or 99 or even 90. A batsman does not require any special ability to score those few extra runs. Worse, the 30-40 runs he scores after the century are not perceived as valuable enough as the previous 10 runs. Strange, but that’s what classification does. Still it’s a necessity. It’s hard for people to remember every individual score of a player, but it’s easy to remember the number of centuries in his career. So in a way classification is a tool to help us humans overcome our limitation of not being able to comprehend and assess randomness in a continuum.
Biology is no different. We have a fancy jargon here – taxonomy – which further creates fancier jargons. There is classification everywhere – vertebrates vs invertebrates, eukaryotes vs prokaryotes, aerobic vs anaerobic, and so on.
Classroom Biology has taught us a classification system which is represented as the Tree of Life. It follows from the Darwinian model of evolution. So we have species grouped as genera grouped as families grouped as orders grouped as classes grouped as phyla grouped as kingdoms grouped as domains. Phew! And then there are subgroups as and when required.
For instance, the domestic cat has this taxonomic signature: F. silvestris -> Felis -> Felidae -> Feliformia -> Carnivora -> Mammalia -> Chordata -> Animalia -> Eukaryota. So starting with Eukaryota, you keep on adding a distinct property to move up a level in the Tree of Life to finally arrive at your species. Thus a cat can be thought of as a living object containing nuclear cells (Eukaryota) lacking a cell wall (Animalia), which has a notochord (Chordata), with female secreting milk (Mammalia), and which feeds on other animals (Carnivora), has double-chambered bones covering middle and inner ear (Feliformia) and hunts alone (Felidae) without roaring (Felis) and is a wildcat. Of course taxonomy is not so simple and not free from debates and disputes either. For instance, not all Carnivores eat meat all the time.
Notwithstanding the opinion of an Indian junior minister, Darwin was a genius because his model of evolution withstood the onslaught of extensive genetic studies carried out more than a century later.
Darwin’s Tree of Life, however, does not cover the entire gamut of life as we now know. It leaves out microbes – bacteria and archaea. Archaea is a newly discovered domain of life consisting of microbes with appearance similar to bacteria but which differ in genetic processes. Bacteria and archaea are together referred to as prokaryotes to distinguish them from eukaryotes. Unlike eukaryotes, they do not possess nucleus in the cell.
The concept of ‘tree’ reaches its limits with eukaryotes and cannot be extended to include bacteria and archaea types. These types – defined mostly using their genetic characteristics – are related to one another more like a network rather than hierarchical branches. Even the notion of ‘species’ is nearly impossible to apply to microbes let alone the “Origin of the Species”.
One may want to dismiss these ‘invisible’ forms of life for their tiny size, but their quantity is mind-boggling (estimated microbes in the order of 1030). There are as many, if not more, bacterial cells in our body as human cells.
If that does not give an indication of the power of microbes, consider this: They are in a state of continuous evolution at a rate much higher than us eukaryotes because they can transfer genes among themselves laterally without having to wait for the arrival of offspring. The process is called horizontal gene transfer which dwarfs the vertical gene transfer we are capable of.
But wait, we haven’t finished with life yet. You haven’t seen anything until you consider viruses – the tiniest of all. Unfortunately, the dogma that viruses do not constitute life persisted in academic Biology for far too long. They were relegated to ‘particles’. That ostracizing was mainly on account of the fact that they cannot reproduce without entering a host cell. What was conveniently overlooked was that they carry the most important ingredient of life – genetic material – packaged inside a protein coating. And they outnumber all other life forms by a huge margin. There is a fresh drive now to include them within the scope of life. Some scientists call this “viral life” an empire, distinct from the other, our own, empire – “cellular life”.