<Table> <Tr> <Td> </Td> <Td> It has been suggested that Human swimming be merged into this article . (Discuss) Proposed since October 2017 . </Td> </Tr> </Table> <Tr> <Td> </Td> <Td> It has been suggested that Human swimming be merged into this article . (Discuss) Proposed since October 2017 . </Td> </Tr> <P> Aquatic locomotion 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, reptiles, birds, and mammals . </P> <P> 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 . </P>

Which are the two forces that are primarily responsible for propulsion in swimming