Researchers have discovered an ancient species of Old World monkey in the badlands of northwest Kenya, according to a study published in the Proceedings of the National Academy of Sciences.
A team from the University of Texas at Austin, the National Museums of Kenya and other institutions found 22-million-year-old teeth in three different sites, enabling them to identify a new species—now named Alophia metios.
The new find could help shine a light on the origin and evolution of Old World monkeys (Cercopithecoidea)—the largest family of primates, comprising 138 species native to Africa and Asia, such as baboons and macaques.
Important aspects of this group’s early evolution remain shrouded in mystery as there are only two fossils representing it from before 8 million years ago—a 19-million-year-old tooth from Uganda and a 25-million-year-old tooth from Tanzania. Thus, the latest discovery helps to fill in a 6-million-year gap in the Old World monkey fossil record.
"For a group as highly successful as the monkeys of Africa and Asia, it would seem that scientists would have already figured out their evolutionary history," John Kappelman, an author of the study from UT Austin, said in a statement.
"Although the isolated tooth from Tanzania is important for documenting the earliest occurrence of monkeys, the next 6 million years of the group's existence are one big blank,” he said. “This new monkey importantly reveals what happened during the group's later evolution."
The region in which the researchers found the monkey teeth is very arid today. But millions of years ago, it was a verdant woodland. During their excavations there, the team also found the remains of numerous other animals, including reptiles and early elephants.
The monkey teeth described in the new study are more primitive than those found in later Cercopithecoidea specimens. Specifically, they lack a key dental trait of the molar teeth present in all modern-day Old World monkeys, known as bilophodonty.
"These teeth are so primitive that when we first showed them to other scientists, they told us, ‘Oh no, that isn't a monkey. It's a pig," Ellen Miller, another author of the study from Wake Forest University, said in a statement. "But because of other dental features, we are able to convince them that yes, it is in fact a monkey."
Analysis of the fossilized teeth indicated that A. metios probably ate hard foods, such as fruits, seeds and nuts. But the researchers suggest that later Old World monkeys began to eat more soft foods, such as leaves, which could have driven the evolution of bilophodonty. This key trait—which evolved long after the monkeys first appeared—enabled them to process a wider range of foods, thus contributing to their success.
"You can think of the modern-day monkey molar as the uber food processor, able to slice, dice, mince and crush all sorts of foods," Mercedes Gutierrez, an author the study from the University of Minnesota, said in the statement.
A fossil jaw of the newly discovered ancient Old World monkey named Alophia metios. John Kappelman.