Evolution of biological complexity

The evolution of biological complexity is one important outcome of the process of evolution. 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 being used to assess an organism's complexity.

Some used to believe that evolution was progressive and had a direction that led towards so-called "higher organisms," despite a lack of evidence for this viewpoint. This idea of "progression" and "higher organisms" in evolution is now regarded as misleading, with natural selection having no intrinsic direction and organisms selected for either increased or decreased complexity in response to local environmental conditions. 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 (the mode) appears to have remained relatively constant.

Nonetheless, the Last Universal Common Ancestor (LUCA) is the most recent common ancestor of all current life on Earth. LUCA should not be assumed to be the first living organism on Earth. The LUCA is estimated to have lived some 3.5 to 3.8 billion years ago (sometime in the Paleoarchean era). The composition of the LUCA is not directly accessible as a fossil, but can be studied by comparing the genomes of its descendents, organisms living today. By this means, a 2016 study identified a set of 355 genes inferred to have been present in the LUCA.

Usually organisms that reproduce more quickly and plentifully than their competitors have an evolutionary advantage. Consequently, organisms can evolve to become simpler and thus multiply faster and produce more offspring, as they require fewer resources to reproduce. A good example are parasites such as Plasmodium - the parasite responsible for malaria - and mycoplasma; these organisms often dispense with traits that are made unnecessary through parasitism on a host.

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