The Original Feet
The year was somewhere around 400million BCE when evidence of the first aquatic animal - a relative of the fish - began to walk using its pectoral and pelvic fins; which were well on their way to becoming adapted as feet (or podia)... the first tetrapod was born...
Until only recently (around 2010) it was believed that fish haddeveloped the ability to walk in fresh water conditions, such as swamps where debris perhaps precluded efficient swimming or, indeed, provided an escape opportunity from preditors.
However, it seems that perhaps 18 million years prior to this swamp scenario became likely, there is evidence that some fish became "tetrapods" - evolving the ability to walk in the shallows, as shown above.
The tracks found at Zachelmie quarry, in Poland's Holy Cross Mountains, were made by a tetrapod of about two meters in length, and show the distinct impressions of individual digits with each step.
The tetrapods were probably submerged in water as they walked, the team said. If the creatures were partially floating, that would explain the absence of a central mark left by the body as it dragged along the sediment.
More information can be found here.