For some invertebrates that live in water, the sense of taste and the sense of smell are identical. why do you think separate organs for taste and smell might have evolved in animals that live on land but not in some animals that live exclusively in water?

The answer relates to the medium in which the sensed molecules travel. On land, animals detect a scent through organic molecules transported in the air. Land animals will experience a taste sensation by grasping an object using their mouth. Here, the organic molecules providing the sensation are transported by saliva or organic fluids of the object. So on land, there is a clear distinction between molecules transported in the wind and those that can be tasted. Water is a much more dense medium than air, and can carry small and large organic molecules, unlike air. Therefore, aquatic animals have little need for olfactory organs as all molecules can be 'tasted' from the water. 

Ithink it’s around 500 million or something. kinda scary honestly but that’s just my opinion.