At least 3 alternative classification systems regarding kingdoms have been proposed:
1. A six-kingdom system - This alternative divides the prokaryotes into 2 kingdoms
This modification is based on molecular evidence for an early evolutionary divergence between eubacteria (most bacteria) and archaebacteria (an ancient lineage of prokaryotes with many unique characteristics
2. A three-domain system - This scheme assigns even more significance to the ancient evolutionary split between eubacteria and archaebacteria by using a superkingdom taxon called the domain
The first 2 domains are bacteria and archaea; the third domain is the eukarya
The eukarya consists of 4 kingdoms of eukaryotic organisms
3. An eight-kingdom system - In addition to 2 separate prokaryotic kingdoms, this system also splits the protists into 3 kingdoms
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