What first caused people to consider that the continents were once one large landmass?

Continental drift theory of plate tectonics given Alfred Wagner.

Explanation:

The theory was developed in the year 1912 and was published in the year 1960 when the due significance was given to the drifting and the spitting nature if the landmass across the entire world. All started with the discovery of the divergent plate boundaries in the mid-ocean ridge in the oceans and thereby along with the study of the seafloor spreading and the evidence of the paleomagnetism.

None   they are   equally likely to occur on any plate boundary