Cormorants seem to be a very ancient group, with similar ancestors reaching back to the time of the dinosaurs.
In fact, the earliest known modern bird, Gansus yumenensis, had essentially the same structure.
The details of the evolution of the cormorant are mostly unknown.
Even the technique of using the distribution and relationships of a species to figure out where it came from, biogeography, usually very informative, does not give very specific data for this probably rather ancient and widespread group.
However, the closest living relatives of the cormorants and shags are the other families of the suborder Sulae—darters and gannets and boobies—which have a primarily Gondwanan distribution.
Hence, at least the modern diversity of Sulae probably originated in the southern hemisphere.

While the Leucocarbonines are almost certainly of southern Pacific origin—possibly even the Antarctic which, at the time when cormorants evolved, was not yet ice-covered—all that can be said about the Phalacrocoracines is that they are most diverse in the regions bordering the Indian Ocean, but generally occur over a large area.

Similarly, the origin of the family is shrouded in uncertainties.
Some Late Cretaceous fossils have been proposed to belong with the Phalacrocoracidae:

A scapula from the Campanian-Maastrichtian boundary, about 70 mya (million years ago), was found in the Nemegt Formation in Mongolia; it is now in the PIN collection.
It is from a bird roughly the size of a spectacled cormorant, and quite similar to the corresponding bone in Phalacrocorax.
A Maastrichtian (Late Cretaceous, c. 66 mya) right femur, AMNH FR 25272 from the Lance Formation near Lance Creek, Wyoming, is sometimes suggested to be the second-oldest record of the Phalacrocoracidae; this was from a rather smaller bird, about the size of a long-tailed cormorant.

As the Early Oligocene Sula" ronzoni cannot be assigned to any of the suloid families—cormorants and shags, darters, and gannets and boobies—with certainty, the best interpretation is that the Phalacrocoracidae diverged from their closest ancestors in the Early Oligocene, perhaps some 30 million years ago, and that the Cretaceous fossils represent ancestral suloids, "pelecaniforms" or "higher waterbirds"; at least the last lineage is generally believed to have been already distinct and undergoing evolutionary radiation at the end of the Cretaceous.
What can be said with near certainty is that AMNH FR 25272 is from a diving bird that used its feet for underwater locomotion; as this is liable to result in some degree of convergent evolution and the bone is missing indisputable neornithine features, it is not entirely certain that the bone is correctly referred to this group.

During the late Paleogene, when the family presumably originated, much of Eurasia was covered by shallow seas, as the Indian Plate finally attached to the mainland.
Lacking a detailed study, it may well be that the first "modern" cormorants were small species from eastern, south-eastern or southern Asia, possibly living in freshwater habitat, that dispersed due to tectonic events.
Such a scenario would account for the present-day distribution of cormorants and shags and is not contradicted by the fossil record; as remarked above, a thorough review of the problem is not yet available.

Two distinct genera of prehistoric cormorants are widely accepted today, if Phalacrocorax is used for all living species:
Limicorallus (Indricotherium middle Oligocene of Chelkar-Teniz, Kazakhstan)
Nectornis (Late Oligocene/Early Miocene of Central Europe – Middle Miocene of Bes-Konak, Turkey) – includes Oligocorax miocaenus
The proposed genus Oligocorax appears to be paraphyletic – the European species have been separated in Nectornis, and the North American ones are placed in the expanded Phalacrocorax.
A Late Oligocene fossil cormorant foot from Enspel, Germany, sometimes placed herein, would then be referable to Nectornis if it proves not to be too distinct.
All these early European species might belong to the basal group of "microcormorants", as they
Cormorants seem to be a very ancient group, with similar ancestors reaching back to the time of the dinosaurs. In fact, the earliest known modern bird, Gansus yumenensis, had essentially the same structure. The details of the evolution of the cormorant are mostly unknown. Even the technique of using the distribution and relationships of a species to figure out where it came from, biogeography, usually very informative, does not give very specific data for this probably rather ancient and widespread group. However, the closest living relatives of the cormorants and shags are the other families of the suborder Sulae—darters and gannets and boobies—which have a primarily Gondwanan distribution. Hence, at least the modern diversity of Sulae probably originated in the southern hemisphere.
While the Leucocarbonines are almost certainly of southern Pacific origin—possibly even the Antarctic which, at the time when cormorants evolved, was not yet ice-covered—all that can be said about the Phalacrocoracines is that they are most diverse in the regions bordering the Indian Ocean, but generally occur over a large area.
Similarly, the origin of the family is shrouded in uncertainties. Some Late Cretaceous fossils have been proposed to belong with the Phalacrocoracidae:

A scapula from the Campanian-Maastrichtian boundary, about 70 mya (million years ago), was found in the Nemegt Formation in Mongolia; it is now in the PIN collection. It is from a bird roughly the size of a spectacled cormorant, and quite similar to the corresponding bone in Phalacrocorax. A Maastrichtian (Late Cretaceous, c. 66 mya) right femur, AMNH FR 25272 from the Lance Formation near Lance Creek, Wyoming, is sometimes suggested to be the second-oldest record of the Phalacrocoracidae; this was from a rather smaller bird, about the size of a long-tailed cormorant.
As the Early Oligocene Sula" ronzoni cannot be assigned to any of the suloid families—cormorants and shags, darters, and gannets and boobies—with certainty, the best interpretation is that the Phalacrocoracidae diverged from their closest ancestors in the Early Oligocene, perhaps some 30 million years ago, and that the Cretaceous fossils represent ancestral suloids, "pelecaniforms" or "higher waterbirds"; at least the last lineage is generally believed to have been already distinct and undergoing evolutionary radiation at the end of the Cretaceous. What can be said with near certainty is that AMNH FR 25272 is from a diving bird that used its feet for underwater locomotion; as this is liable to result in some degree of convergent evolution and the bone is missing indisputable neornithine features, it is not entirely certain that the bone is correctly referred to this group.
During the late Paleogene, when the family presumably originated, much of Eurasia was covered by shallow seas, as the Indian Plate finally attached to the mainland. Lacking a detailed study, it may well be that the first "modern" cormorants were small species from eastern, south-eastern or southern Asia, possibly living in freshwater habitat, that dispersed due to tectonic events. Such a scenario would account for the present-day distribution of cormorants and shags and is not contradicted by the fossil record; as remarked above, a thorough review of the problem is not yet available.
Two distinct genera of prehistoric cormorants are widely accepted today, if Phalacrocorax is used for all living species:
Limicorallus (Indricotherium middle Oligocene of Chelkar-Teniz, Kazakhstan)
Nectornis (Late Oligocene/Early Miocene of Central Europe – Middle Miocene of Bes-Konak, Turkey) – includes Oligocorax miocaenus
The proposed genus Oligocorax appears to be paraphyletic – the European species have been separated in Nectornis, and the North American ones are placed in the expanded Phalacrocorax. A Late Oligocene fossil cormorant foot from Enspel, Germany, sometimes placed herein, would then be referable to Nectornis if it proves not to be too distinct. All these early European species might belong to the basal group of "microcormorants", as they
Cormorant