Scientists believe they have got to the bottom of anal evolution . A team of Norwegian researchers has identified two sets of genes that cause some creatures to have a fully-functioning anus , while other only have 'transient ' ones . The study also reveals how some creatures , such as the Penis worm , have very similar digestive systems to humans , even though they are very different species . Scroll down for video . The latest study builds on earlier research which looked at the genes expressed during the development of intestines , stomach and anus in the Penis worm , pictured . Biologist Dr Andreas Hejnol and Dr Chema Martín-Durán , of the University of Bergen , spent several years studying how genes change the development of creatures . 'While several organ systems have been investigated , such as the nervous system , the anal opening has been largely neglected , ' Dr Hejnol told BBC Earth . The study , published in the journal Zoologischer Anzeiger , identified genes known as brachyury and ParaHox , that are crucial to the development of an anus . According to a report by Matt Walker at the BBC , their study reveals how the anus is perhaps one of the most important organs in our body , helping our digestive system work . He notes that , interestingly , not all animals have one . Some animals have simpler versions , in others the organ appears to be transient , while another has multifunctional anuses . Animals that do have an anus , however , often grow to be larger , more sophisticated creatures . A team of Norwegian researchers has identified two sets of genes that cause some creatures to have a fully-functioning anus , while other only have 'transient ' ones . In 2013 , researchers discovered that one of our earliest ancestors was a worm with no anus . The worm , named Xenoturbella bocki , has no brain , no sex organs and no vital organs . The 1m worm is regarded by some scientists as a key biological link between many species . Researchers believe the worm is a distant progenitor of humans and a host of other animals living today . Swedish researchers say the worm might belong to a branch of the animal family tree called deuterostomes , of which humans are also members . Trying to understand when and why the anus evolved is also complicated because it also appears and vanishes again within various groups of animals , such as flatworms . The latest study builds on earlier research which looked at the genes expressed during the development of intestines , stomach and anus in the Penis worm . What surprised the researchers was not just that the development was the same in the worms as in humans , fish and frogs , but that exactly the same genes were involved in the process . Penis worms live in shallow parts of the oceans , including off the Swedish west coast where they propagate during the winter . 'This means that the origin of the human stomach is further back in time than previously thought , probably about 500 million years ' , says Ralf Janssen , postdoc in professor Graham Budd 's research group on paleobiology at Uppsala University . The team are now conducting further research into whether the anus has evolved independently a number of times among animals . 'The pattern of presence and absence of the anus in animals is quite fascinating , ' Dr Hejnol told the BBC .
Brachyury and ParaHox genes could shed light on anus evolution . They may indicate why some creatures only have a 'transient ' anus . Comment: