As movie theaters across the U.S. prepare to welcome throngs of bipedal primates to screenings of Rise of the Planet of the Apes this weekend, it seems appropriate to reflect on a time in Earth's history when nonhuman apes actually did reign supreme. It's hard to imagine, because so few ape species exist today and all are imperiled. But during the Miocene epoch, between roughly 23 million and 5 million years ago, the planet harbored as many as 100 ape species. There were tiny apes (think housecat-size) and giant apes, leaf-eating apes and nut-eating apes, apes that walked on all fours and apes that swung from branch to branch. And they roamed throughout the Old World from France to China, Kenya to Namibia.

In 2003, paleoanthropologist David Begun of the University of Toronto wrote an article for Scientific American on the apes of the Miocene. It's a fascinating story. And we commissioned portraits of some of the apes from paleoartist extraordinaire John Gurche to illustrate it (above). Begun makes the case that the taxonomic group that encompasses great apes and humans, the hominids, originated in Eurasia, because most of the known fossils of great apes have come from Eurasia. Specifically, he posits that either an ape called Dryopithecus or one dubbed Ouranopithecus (center and far right, respectively) was the ancestor of African apes and humans.

Begun's theory is controversial. Other researchers favor an African origin for the group, a model that has gained support from discoveries in recent years of fossils of large-bodied Miocene apes in Africa, such as the 10 million-year-old Chororapithecus from Ethiopia and its Kenyan contemporary Nakalipithecus. But there's still not much to go on.

And the fossil record of African great apes doesn't get better after the Miocene. But the human fossil record sure does. In fact, the human fossil record is pretty darn good. It's strange to think that we know so much about human evolution yet so little about that of our closest living cousins. Chororapithecus might be a gorilla forebear, as might a 9.5 million-year-old creature from Kenya called Samburupithecus. But aside from those tentative links and three fossil chimp teeth from Kenya that are around half a million years old, scientists have no other fossils to document the rise of the modern African apes. And clues may continue to prove elusive: the forested environments that apes tend to inhabit are generally not conducive to fossil preservation. Until more fossils come to light, the story of exactly how the planet of the apes became the planet of the humans remains very much incomplete.