Monkeys are some of the most varied and diverse mammals alive today. Known for their oftentimes arboreal lifestyle and playful nature, these creatures have a long and fascinating history in the fossil record. In order to best understand the evolution of monkeys, however, we must first understand the evolution of their greater order, the primates.
Early Primates
During the Late Cretaceous, the Earth was ruled by some of the largest and most powerful animals to ever walk the planet: the dinosaurs. But in the background of the reign of the terrible lizards, a new development was taking place among a much smaller group of animals—the early primates. Mammals far below the eyeline of the dinosaurs scurried about as small shrew-like animals. Despite appearances, these were not shrews but some of the first members of a brand-new lineage of mammals: the primates. Molecular studies put the date of birth of this creature anywhere from 70 to 66 million years ago. As iconic creatures such as Tyrannosaurus rex and Triceratops roamed the Earth, so too did the earliest primates.
Following the end-Cretaceous extinction, primates continued their evolution. At this time, they still looked close to their shrew-like ancestors. Purgatorius is the oldest known of these animals, evolving 66 million years ago right after the death of the dinosaurs. This animal is part of a group known as the plesiadapiforms and contains many other early primate relatives such as Plesiadapis. There’s controversy over the placing of plesiadapiforms: some view them as true primates, others see them as ancestral, and others still see them as sister to the line containing primates. Regardless, these animals strongly resembled what were at the time the earliest primates.
Anatomical Advancements
Primates during this time made advancements such as abandoning the claws of their earlier ancestors in favor of nails with finger pads, something very much unlike their arboreal contemporaries. These changes can still be seen today in primates like monkeys. Adaptations such as highly mobile ankles, shoulders, and hip bones helped these animals easily traverse through the trees. Another point of interest was their diet. While not carnivorous or even really that omnivorous, these animals set themselves apart by consuming calorie-rich fruits found in trees that they were comparatively far more equipped to explore. This diet led to several new developments, most notably an increase in brain size and further adaptations to their hands for better gripping ability when trying to grab onto fruit.
By this point, primates were beginning to look more and more like the species we are familiar with today. Their eyes grew larger and forward-facing, they became even smarter, and their faces began to change to be much more rounded compared to the shrew or rodent-like skulls of their ancestors.
Divergence and Specialization
Shortly after, primates branched off into two major lines: the Strepsirrhini and the Haplorhini. The Strepsirrhini include animals such as lorises and lemurs. The Haplorhini, on the other hand, is the group most relevant to us today. There are several differences in the evolution of Haplorhini that set them apart from the Strepsirrhini, including possessing shorter faces as opposed to the elongated ones of the Strepsirrhini like lemurs. The most notable difference is their lack of a rhinarium, or wet nose; instead, their noses are dry and oftentimes covered in hair. One of the animals inside the Haplorhini are the strange-looking tarsiers of Southeast Asia, but today we are going to focus on another group: the simiiforms, which contain all monkeys.
Evolution of Monkeys
Monkeys evolved towards the end of the Eocene, just as the Earth began to cool. They built upon the many changes that primates up to this point had developed, and they developed in some more unique ways. For example, they became more active during the daytime as opposed to the nocturnal lifestyle of their ancestors. One of the earliest monkey groups is believed to be Eosimiidae, containing genera such as Eosimias found in China and Myanmar. These monkeys possessed many characteristics of later monkeys but also retained more traits found in their ancestors, such as more primitive dentition.
Prior to the discovery of primates such as Eosimias, it was thought that monkeys had evolved in or around Africa. This discovery changed the viewpoints of many researchers on that fact. While primates were present on continents such as North America, Europe, and Asia for millions of years, this is the closest we have to the true origin of monkeys on one of the northern continents during the Eocene. Alongside the Eosimiidae, there were some older lines of monkeys as well, including the Parapithecidae, containing Apidium found in the Egyptian famed fossil sites. These early groups, while certainly monkeys, were still not the lines of monkeys present with us today.
New World and Old World Monkeys
Around 40 million years ago, monkeys saw perhaps their most significant branch in their evolution: the simiiform split off into two groups—the Catarrhini (Old World monkeys) and the Platyrrhini (New World monkeys). Platyrrhines set themselves apart from their Old World counterparts in several ways, but the most notable difference is in the structure of their noses. New World monkeys have flatter noses with nostrils that point out towards the sides. They also have an additional premolar tooth and prehensile tails that can grab onto branches, aiding in movement. The most distinguishing feature, however, is their location; these monkeys are found exclusively in South and Central America.
Despite the earliest primates having a presence in North America, the continent only connected with South America during the Pliocene. However, fossil evidence of Platyrrhines in South America dates back as far as the Oligocene. It is speculated that the ancestors of the first New World monkeys came from Africa and managed to reach South America via rafting on masses of vegetation that floated in the Atlantic Ocean. While this type of travel seems near impossible at a glance, the span of millions of years would have provided numerous opportunities for successful crossings.
Today, Platyrrhini consists of five families: Callitrichidae (marmosets and tamarins), Cebidae (squirrel monkeys and capuchins), Aotidae (owl monkeys), Pitheciidae (titi monkeys, sakis, and uakaris), and Atelidae (howler, spider, and woolly monkeys). The isolation of these New World species has led to some of the strangest-looking monkeys on Earth, including bald-headed monkeys, bearded monkeys, and monkeys with giant mustaches.
The Old World monkeys, grouped under Catarrhini, are part of a separate sister lineage to groups such as Parapithecidae. Unlike the New World monkeys, the Old World monkeys have only one family, Cercopithecidae, which includes a wide variety of monkeys such as mandrills, baboons, macaques, snub-nosed monkeys, langurs, colobus monkeys, and others. These monkeys can be found in Africa and Asia, with one species, the Barbary macaque, being found as far north as the tip of Spain. The Catarrhini is also notable in that it contains the apes, meaning all apes evolved from Old World monkeys, with the oldest ape in the fossil record, Proconsul, having evolved around 28 million years ago.
Conclusion
The great variety of monkeys we see both in the fossil record and in our world today is a true testament to their adaptability and versatility. They have set themselves apart from other mammals and branched out into niches that they truly made their own. Monkeys are a staple of Earth’s mammalian fauna, and it is hard to imagine any rainforest or jungle setting today without these fascinating creatures. Their continued survival and the protection of their diverse habitats are essential for maintaining the rich biodiversity of our planet.
