Aquatic locomotion

Aquatic locomotion or swimming is biologically propelled motion through a liquid medium. The simplest propulsive systems are composed of cilia and flagella. Swimming has evolved a number of times in a range of organisms including arthropods, fish, molluscs, amphibians, reptiles, birds, and mammals. Swimming evolved a number of times in unrelated lineages. Supposed jellyfish fossils occur in the Ediacaran, but the first free-swimming animals appear in the Early to Middle Cambrian. These are mostly related to the arthropods, and include the Anomalocaridids, which swam by means of lateral lobes in a fashion reminiscent of today's cuttlefish. Cephalopods joined the ranks of the nekton in the late Cambrian, and chordates were probably swimming from the Early Cambrian. Many terrestrial animals retain some capacity to swim, however some have returned to the water and developed the capacities for aquatic locomotion. Most apes (including humans), however, lost the swimming instinct. In 2013 Pedro Renato Bender, a research fellow at the University of the Witwatersrand's Institute for Human Evolution, proposed a theory to explain the loss of that instinct. Termed the Saci last common ancestor hypothesis (after Saci, a Brazilian folklore character who cannot cross water barriers), it holds that the loss of instinctive swimming ability in apes is best explained as a consequence of constraints related to the adaptation to an arboreal life in the last common ancestor of apes. Bender hypothesized that the ancestral ape increasingly avoided deep-water bodies when the risks of being exposed to water were clearly higher than the advantages of crossing them. A decreasing contact with water bodies then could have led to the disappearance of the doggy paddle instinct. Microbial swimmers, sometimes called microswimmers, are microscopic entities that have the ability to move in fluid or aquatic environment. Natural microswimmers are found everywhere in the natural world as biological microorganisms, such as bacteria, archaea, protists, sperm and microanimals.