Final answer:
The initial classification of life forms into just plants and animals was due to the limited understanding and observational capabilities of early scientists. Advances in cell biology and genetics led to an expanded classification system that includes a wider diversity of life forms.
Step-by-step explanation:
Scientists originally only included plants and animals in the initial classification levels because the understanding of life and its complexity was limited. Early taxonomic systems, such as those developed by Carl Linnaeus, were based on observable physical characteristics and thus were most inclusive of multicellular organisms that fit neatly into the plant or animal kingdoms. Over time, with the advancement in understanding of microbiology, cell biology, and genetics, the classification system has evolved to include a broader spectrum of life. The work of Carl Woese using comparative rRNA sequencing has expanded our knowledge leading to a three-domain system that includes Bacteria, Archaea, and Eukarya, acknowledging the existence of microorganisms and the more intricate relationships among different forms of life.
Likewise, Ernst Haeckel's introduction of additional kingdoms like Protista and Monera indicated a move towards a more nuanced and detailed taxonomy. Changes such as these illustrate that classification systems develop in response to increasing scientific knowledge, and the initial inclusion of only plants and animals reflects the historical understanding and observational capabilities of the sciences at the time.