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Figure 69.
In the evolution of life on Earth, the three domains of life—Archaea, Bacteria, and Eukarya—
branch from a single point. (credit: modification of work by Eric Gaba) 

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The phylogenetic tree in Figure 69. illustrates the pathway of evolutionary history. By tracing 
the evolutionary branches that connect the two points, one can determine the route from the beginning 
of life to any species. A single species can be used as a starting point to trace back to any branch 
point, the organisms related to it by various degrees of closeness can be identified. 
Phylogeny 
is the evolutionary history and the relationships among a species or group of 
species. The study of organisms with the purpose of deriving their relationships is called 
systematics
.The study of biology encompasses numerous fields, all of which help to build, maintain, 
and update the "tree of life" by shedding light on how past and present life evolved over time. The 
information gathered may come from studying morphology, body part structure, or molecular 
structure, such as the arrangement of amino acids in proteins or DNA nucleotides. It may also come 
from studying fossils. By considering the trees generated by different sets of data scientists can put 
together the phylogeny of a species. 
 
The Levels of Classification 
Taxonomy
(which literally means “arrangement law”) is the science of naming and grouping 
species to construct an internationally shared classification system. The taxonomic classification 
system employs a hierarchical model and is also known as the Linnaean system after its creator, 
Swedish naturalist Carl Linnaeus. A hierarchical system has levels and each group at one of the levels 
includes groups at the next lowest level, so that at the lowest level each member belongs to a series 
of nested groups. An analogy is the nested series of directories on the main disk drive of a computer. 
For example, Scientists categorize all living things into three domains: Bacteria, Archaea, and 
Eukarya. A kingdom is the second level of organization within each domain. There are multiple 
kingdoms within each domain. Within kingdoms, the subsequent categories of increasing specificity 
are: