PERSPECTIVE METAZOAN COMPLEXITY AND EVOLUTION: IS THERE A TREND?

Area of Science:

• Evolutionary biology
• Paleontology
• Developmental biology

Background:

• The concept of increasing complexity in evolution is widely accepted but lacks objective measurement.
• Existing evidence for evolutionary complexity trends is largely impressionistic.

Purpose of the Study:

• To propose a conceptual scheme for objectively measuring evolutionary complexity.
• To identify and categorize different types of complexity.
• To review evidence for trends in evolutionary complexity.

Main Methods:

• Development of a conceptual scheme for understanding and measuring complexity.
• Categorization of complexity into four independent types.
• Review of existing measures and evidence for trends in maximum and mean complexity.

Main Results:

• The proposed scheme provides a basis for objective measurement of complexity.
• Complexity encompasses four distinct, independent types.
• Evidence for evolutionary complexity trends is limited, with a clear trend only observed in early Phanerozoic metazoans for one type of complexity.
• Trends in other complexity types are documented but only within specific metazoan subgroups.

Conclusions:

• Objective measurement of evolutionary complexity is possible through a structured scheme.
• Evolutionary complexity is multifaceted, comprising four independent types.
• While some complexity trends exist, robust evidence across metazoans is scarce, particularly outside the early Phanerozoic and for specific complexity types.