Most likely, a change in environment (which includes a change in food availability).
If you are looking for the 'cause' of life in general you'll have to wait for the study of abiogenesis to bear fruit or look to your religion.
"Using the Schroedinger's cat illustration, these critters either lived in the water or they didn't, as being equally efficient on both is illogical.
This is a false dichotomy. There are animals which spend time both in the water and on land and are adapted to both - pinipeds, otters, beavers, penguins and others. In the Cetartiodactyl sequence there is a gradual change in the hip joint that shows loss of function on land that is accompanied by changes in the spine that show better adaptation for water.
"Anyone who came across these, who had no preconceived ideas about them would say they were all different creatures an none gave rise to the other.
That is doubtful. Don't forget that recognition of sequences in the fossil record predates Darwin and all Darwin did was recognize a few of the main mechanisms that created those sequences. Scientists prior to Darwin spent many years trying to find Biblical explanations for the obvious geological stratification that imposed a relative time sequence on lines of similar fossils. It was not some bias against creationism that caused them to recognize the sequence, the sequence is there and obvious.
There were, in fact, many attempts to work out a series of creations in which God got better at making things. The historical sequence was accepted long before Darwin's explanation.
Like I said in another reply; "There's been a whole LOT of environmental changin' goin' on!"