Forams live today in all the world's oceans, but they first appeared in the rock record approximately 525 million years ago. Between that long ago time and today, the size, shape and construction of foram tests changed significantly. Tests might be single-chambered or multi-chambered, right coiling or left coiling, larger or smaller, or they may be constructed from scavenged grains or self-produced CaCO3, depending on the conditions and time period in which the foram lived.
Traditionally, forams have been grouped into species on the basis of their characteristic test shape; and their distribution has been studied. Scientists discovered that some species existed only during narrow geologic time bands, while other species survived much longer. Some were restricted in their geographic distribution, while others were global in extent; and some species were found exclusively in shallow water environments, while others preferred deeper water.
Over time, scientists have painstakingly pieced together the geologic time period, geographic distribution and preferred environment for many individual foram species, and from this work some major events in foram evolution are evident.