<P> Fish do not have necks, so the head is directly connected to the shoulders . In contrast, land animals use necks to move their heads so they can look down to see the food on the ground . The greater the mobility of the neck, the more visibility the land animal has . As lineages moved from completely aquatic environments to shallower waters and land, they gradually evolved vertebral columns that increased neck mobility . The first neck vertebra that evolved permitted the animals to have flexion and extension of the head so that they can see up and down . The second neck vertebra evolved to allow rotation of the neck for moving the head left and right . As tetrapod species continued to evolve on land, adaptations included seven or more vertebrae, allowing increasing neck mobility . </P> <P> The sacrum connects the pelvis and hindlimbs and is useful for motion on land . The aquatic ancestors of tetrapods did not have a sacrum, so it was speculated to have evolved for locomotive function exclusive to terrestrial environments . However, the Acanthostega species is one of the earliest lineages to have a sacrum, even though it is a fully aquatic species . Once species moved onto land, the trait was adapted for terrestrial locomotion support, which is evidenced by additional vertebrae fusing similarly to permit additional support . This is an example of exaptation, where a trait performs a function that did not arise through natural selection for its current use . </P> <P> As the lineages evolved to adapt to terrestrial environments, many lost traits that were better suited for the aquatic environment . Many lost their gills, which were only useful for obtaining oxygen in water . Their tail fins became smaller . They lost the lateral line system, a network of canals along the skull and jaw that are sensitive to vibration, which does not work outside of an aquatic environment . </P> <P> For successful land invasion, the species had several pre-adaptations like air - breathing and limb - based locomotion . Aspects such as reproduction and swallowing, however, have bound these species to the aquatic environment . These pre-adaptations have allowed vertebrates to venture onto land hundreds of times, but were not able to accomplish the same degree of prolific radiation into diverse terrestrial species . To understand the potential of future invasions, studies must evaluate the models of evolutionary steps taken in past invasions . The commonalities to current and future invasions may then be elucidated to predict the effects of environmental changes . </P>

When did invertebrates first invade the terrestrial realm