Primitive (biology)

In phylogenetics, a primitive (or ancestral) character, trait, or feature of a lineage or taxon is one that is inherited from the common ancestor of a clade (or clade group) and has undergone little change since. Conversely, a trait that appears within the clade group (that is, is present in any subgroup within the clade but not all) is called advanced or derived. A clade is a group in which all members have the same common ancestor.

A primitive trait is the original condition of that trait in the common ancestor; advanced indicates a notable change from the original condition. These terms in biology contain no judgement about the sophistication, superiority, value or adaptiveness of the named trait. "Primitive" in biology means only that the character appeared first in the common ancestor of a clade group and has been passed on largely intact to more recent members of the clade. "Advanced" means the character has evolved within a later subgroup of the clade.

Other, more technical, terms for these two conditions—for example, "plesiomorphic" and "synapomorphic"—are frequently encountered; see the table below.

At least three other sets of terms are synonymous with the terms "primitive" and "advanced". The technical terms are considered preferable because they are less likely to convey the sense that the trait mentioned is inferior, simpler, or less adaptive (e.g., as in lower and higher plants). The terms "plesiomorphy" and "apomorphy" are typically used in the technical literature: for example, when a plesiomorphic trait is shared by more than one member of a clade, the trait is called a symplesiomorphy, that is, a shared primitive trait; a shared derived trait is a synapomorphy.

"Primitive" and "advanced" are relative terms. When a trait is called primitive, the determination is based on the perspective from which the trait is viewed. Any trait can be both primitive (ancestral) and advanced (derived) depending on the context.

In the clade of vertebrates, legs are an advanced trait since it is a feature that appears in the clade. However, in the clade of tetrapods, legs are primitive since they were inherited from a common ancestor.

The terms "primitive" and "advanced", etc., are not properly used in referring to a species or an organism as any species or organism is a mosaic of primitive and derived traits. Homo sapiens, for example have large brains (a derived trait) and five fingers (a primitive trait) in their lineage. Species are constantly evolving, so a frog is not biologically more primitive than a human as each has been evolving continuously since each lineage split from their common ancestor.

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