The Systematic Classification of Life

Taxonomy is a system for classifying living things into daughter groups, within parent groups, within larger, ancestral groups, and so on. These groups-within-groups are called ‘clades’, and this site is an introduction to the concept of cladistics.  A clade indicates an ancestral phylogeny, a branch on the tree of life. Every twig on that branch is its own off-shoot, but each are still part of the main branch too, just as all the adjoining branches are still part of the same limb.

Evolution never suggests that one thing ever turned into another fundamentally different thing.  Every new species or genus, (etc.) that ever evolved was just a modified version of whatever its ancestors were.  To understand evolutionary Theory, one must first understand that the transformation of fish-to-amphibians, dinosaurs-to-birds, or apes-to-men are each are just a matter of incremental, superficial changes slowly compiled atop various tiers of fundamental similarities. Those successive levels of similarity represent taxonomic clades which encompass all the descendants of that clade. For example, amphibians are still stegocephalian [fish], birds are still dinosaurs, and humans are still apes -the same way we are still mammals, and for the same reasons; according to all the characters which define each of those groups. 

Sure, there are differences between different groups of apes, just like there are differences between people or cultures.  If we concentrate on differences, we can isolate any one man apart from all other men.  Citing such differences leads to prejudice.  So we observe similarities instead, with quite the opposite result.

In order to define any group, the definition must describe every member of that group without making exceptions for certain ones.  For example, if we were to define an automobile, we couldn’t say that they have four wheels because some European cars have only three.  We also couldn’t say they run on gasoline, or even internal combustion, because some cars run on electricity.  It is also logically impossible to define members of any category by what they are not. So we classify forms of life by defining the characters common to every member of that clade.  The only apparent exceptions are when some groups lose some feature defining one of the parent clades.  For example, whales and snakes are still defined as tetrapods, (four-legged vertebrates) because their structure is based on that of other tetrapods.  Thus their characters are all those of tetrapods + the loss of their legs, in addition to any other distinction particular to that group.  However human traits include no such exceptions, and we still belong to every taxonomic clade listed below according to all the criteria of each one. 
Each of the links above is an explanation of that clade, and its evolutionary evidence & significance. 

At right is an illustration of how the cladistic system of groups-within-groups works.

Introduction and history

Another way to understand the concept of clades is to remember that a bride may become the basis for a new family with a new name. But she is still part of her mother’s family too, and her grandmother’s, and so on, and so will her children and grandchildren always be. That is their heritage, and it can’t ever change no matter how much the bride’s new family does. In biology it is the same way. You can never grow out of your ancestry.  So you still always belong to whatever group you came from, and so will your descendants, even if they begin a new sub-group you don’t belong to.
Siblings and cousins can grow very far apart, and can become quite distinct over time. But there will always be key features to identify them as kin. So to assign an organism to a particular clade, there must first be a detailed character analysis, an in-depth study of every facet or aspect of that organism, its morphology (physical shapes) its physiology, (chemical composition) and of course its genetics. Using the first two factors, we can associate the subject with other species which may have some superficial surface differences, but which share fundamental, evidently-inherited structural similarities.  From there we can define a more precise (daughter) clade within that one by associating that organism with other species which share all the fundamental similarities as well as some of the superficial ones, so that they’re obviously the same ‘kind’ of organism, even if they’re genetically isolated, (can’t be bred together). All this is on the assumption that the physical attributes belie a biological relationship, a successive series of common ancestors. Because a comprehensive study of taxonomy reveals that to be an inescapable conclusion; Especially now that we can use the third factor, the genome, to confirm those relationships.

Long before genomic sequencing could confirm anyone’s ancestry, various historic figures recognized these similarities as indicative of a common group relationship. Lord Krsna mentioned proponants of this idea in the Bhagavad-Gita, an ancient sacred scripture in the Hindu religion. Aristotle spoke of man’s position on a scale of successive types of life. Another Greek philosopher, Anaximander even suggested that man had evolved from what he considered to be "lesser" life forms.  All of these comments were made thousands of years ago.

Then, an 18th century European naturalist named Carl Linn, (AKA Carolus Linnaeus) did a detailed comparison of all manner of animals and plants, and realized that all life on Earth seemed to conform to not just one, but several stages of fundamental commonality, where two sets of animals would share very specific structures in common, and the more basic structures were shared by an even larger set of other animals. This indicated that all life should be related in tiers of family groups flowering out of more primitive (simpler) apparent ancestors. But Linn was at a loss to explain how that could be, as later scientists (Cuvier & Lamarck) were also. It would be another hundred years before Charles Darwin and Afred Russell Wallace finally proposed ‘descent with modification by means of natural selection’ as the reason behind Linn’s enigmatic observation.
Darwin knew that for all this to happen the way he suggested, there had to be a mixture of some still-unidentified "units of information" that would allow a developing embryo to inherit the essence of both of its parents. He predicted this had to be so, or his theory would be falsified. Then one of his contemporaries, a Christian monk named Gregor Mendel, discovered genetics, the critical element Darwin needed to vindicate his ideas.

Alfred Russell Wallace and Charles Darwin

Since the discovery of DNA, the field of genetics has exploded, and is now used to prove paternity, confirm the modern heirs of historic estates, and even positively identify criminals -without any witnesses, and without need of any other evidence. Molecular evidence is the most conclusive ever used in a court of law, and these same techniques now confirm our relationship to all other life. The addition of genomic sequencing as a means of confirming relationships once determined only by traditional character analyses has made cladistic taxonomy a twin-nested hierarchy, and one of the most profoundly compelling arguments for biological evolution.

Please address questions or comments to
L. Aron Nelson

The purpose of this site is two-fold.  Its 1st agenda is to explain relatively new and somewhat revolutionary perspectives in taxonomic classification.  Its 2nd goal is to explain some of the myriad intermediate teirs of human phylogeny and evolutionary concepts to readers who may be unfamiliar with them or misinformed about them.