Evolution of biological complexity

The evolution of biological complexity is one important outcome of the process of evolution.^[1] Evolution has produced some remarkably complex organisms – although the actual level of complexity is very hard to define or measure accurately in biology, with properties such as gene content, the number of cell types or morphology all proposed as possible metrics.^[2][3][4]

Many biologists used to believe that evolution was progressive (orthogenesis) and had a direction that led towards so-called "higher organisms", despite a lack of evidence for this viewpoint.^[5] This idea of "progression" introduced the terms "high animals" and "low animals" in evolution. Many now regard this as misleading, with natural selection having no intrinsic direction and that organisms selected for either increased or decreased complexity in response to local environmental conditions.^[6] Although there has been an increase in the maximum level of complexity over the history of life, there has always been a large majority of small and simple organisms and the most common level of complexity appears to have remained relatively constant.