This is true that biological classification is a dynamic and ever evolving phenomenon which keeps changing our understanding of life forms.
At the beginning, living being were broadly classified into two groups, viz. animals and plants. This classification was based on apparent morphological features which were quite easy to understand. But when the scientists discovered certain flaws in this classification, they began to think about a better system of classification. The most accepted system of classification today is the Five-Kingdom Classification; proposed by
Robert Whittaker.
The Five-Kingdom Classification takes into account structural organization and phylogenetic relationship; while classifying the living beings. As per this system; all prokaryotes have been kept under one kingdom, i.e. Monera. Furthermore, eukaryotes have been divided into four kingdoms. All the photosynthetic autotrophs have been kept under plank kindom and all the eukaryotic heterotrophs (without cell wall) are kept under animal kingdom.
However, even this system of classification has certain limitations.
For example; the chemosynthetic autotrophs have been kept under the kingdom Protista.
Recently, some scientists have also suggested three domains of life and some others have suggested a Six-Kingdom classification. Such attempts of classification are in tune with our changing understanding of life forms.