Comment on three kingdom system of classification of organisms.

For a very long time, human beings classified all the living things as either plants (Planatae) or animals (or Animalia). However, with advances in science and microbiology, we figured that another classification is much better suited. Now we have a three-domain or three kingdom system of classification of organisms. According to this system, all the life is classified into three major categories: bacteria, archaea and eukaryota. The first two domains (bacteria and archaea) consist of prokaryotes or single celled organisms, while the eukaryota consists of multi-cellular organisms (such as human beings, plants, animals, etc.). All the three groups are genetically different from each other and do not share a common ancestor. Interestingly, archaea were, until very recently, not considered a different domain and were thought to be similar to other major prokaryote domain (bacteria). It was only in 1970s that they were designated as a different domain of life.

Hope this helps.