Modern gaviiformes are found in many parts of North America and northern Eurasia (Europe, Asia and debatably Africa), though prehistoric species were more widespread.

[2][3] More recently, it has become clear that the Anseriformes (waterfowl) and the Galliformes are the most ancient groups of modern birds, and these being distinct by the end of the Albian 100 million years ago (Ma), while just possible, is not at all well-supported.

They were once believed to be related to grebes, which are also foot-propelled diving birds, and both groups were once classified together under the order Colymbiformes or in even older classifications as the Urinatores.

The similarity is so strong that even the most modern cladistic analyses of general anatomical features are easily misled into grouping loons and grebes.

Namely, other recent authors have considered loons to share a rather close relationship with seabirds such as penguins (Sphenisciformes), tubenoses (Procellariiformes), waders (Charadriiformes) – and perhaps the newly discovered clade Mirandornithes which unites grebes (Podicipediformes) and their closest living relatives, the flamingos (Phoenicopteriformes).

The conflicting molecular data regarding their relationships is not much resolved by the fossil record; though they seem to have originated at the end of the Late Cretaceous like their presumed relatives, modern loons are only known with certainty since the Eocene.

A piece of a carpometacarpus supposedly from Oligocene rocks near Lusk, Wyoming was described as Gaviella pusilla, but this handbone also shows some similarities to the plotopterids which were flightless wing-propelled divers and if these are apomorphic would make an unconvincing member of the Gaviidae (though it still could be a small-winged gaviiform in a yet-undescribed family "Gaviellidae"[13]): while the carpometacarpus in Gavia is somewhat convergent to that of wing-propelled divers, enabling the wings to be used as rudders for quick underwater turns, Colymboides still had an unspecialized plesiomorphic hand.