The similarities between orangutans and humans, and the closeness of our evolutionary history, is often cited as a primary reason for the importance of orangutan conservation initiatives, but what this actually means, and just how closely related we are to Asia’s only great ape, is often confusing.
Genetic studies in to the DNA structure of orangutans and humans has shown that our genome, our hereditary information, differs by a remarkably small amount. As such, orangutans and humans both belong to the scientific order known as the primates, the group of mammals that contains all the monkeys, prosimians (the pre-monkeys) and apes living today. One of the oldest surviving mammal groups, the primate lineage is thought to go back at least 65 million years ago, when small, arboreal, insect eating mammals, referred to as the Euarochonta, were beginning the process of speciation that eventually led to the primates we see today, one of the most diverse and varied groups of animals (Dawkins, 2004). Primates inhabit almost every part of the world, with the non-human species’ found predominantly in tropical and sub-tropical regions of Africa, Asia and Central and South America.
While there isn’t one unique characteristic that defines a primate, there are a number of characteristics that are found throughout species of this order, including an opposable thumb, which aids strong, grasping hands; forward facing eyes, which allow three-dimensional viewing; eye sockets, which protect the eyeball; fingerprints unique to each individual, and much larger brains, in comparison to body size (Redmond, 2008).
Primates are divided in to two scientific sub-orders. The primates considered the most primitive, or those that have retained the most number of features of the ancestral primates, are classified as being Strepsirrhines, and include the lemurs of Madagascar, the galago’s potto’s and angwantibo’s of Africa, and the lorises of Asia. The tarsiers, monkeys and apes are classified as Haplorhines, on account of their larger brains, specially adapted hands and feet, and wider range of facial features, a result of the lack of a rhinarium, or a wet snout.
Around 30 million years ago, the primates of the Haplorhini divided in to two further groups, the Platyrrhini, which now consists of the modern day New World monkeys of South and Central America, and the Catharinni, made up of the Old World monkeys of Africa and Asia, and the apes. This latter group split in to two superfamilies at approximately 20-25 million years ago, as evolutionary pressures gave rise to drastic differences in the primate species. Modern day Old World monkeys make up the superfamily Cercopithecoidea, The apes- the gibbons, chimpanzees, bonobo’s, gorillas, orangutans and humans- are the only surviving members of the superfamily Hominoidea.
Around 20-19 million years ago, a primate had evolved in central Africa that had characteristics of both Old World monkeys and apes. This primate, named Proconsul, included four known species and had a posture that was similar to that of a monkey. However, its lack of a tail, facial structure and strong grasping capabilities marks it out as an ape, and studies in to the growth and form of fossilized Proconsul teeth have shown that this primates life history was similar to that of a modern day gibbon, with additional features of other apes and monkeys (Palmer, 2010). It is likely that the first apes evolved from a monkey-like creature that had descended to the ground, and, through the process of evolution, lost many of their ancestral features, acquiring new ones as they adapted to their new environment. For example, a tail would be useful for a life in the canopy, aiding as it does movement and balance. On the ground, a long tail would be a hindrance to terrestrial movement, and would be one of the first adaptations to disappear (Morris, 2008). Whether or not Proconsul is an intermediate between monkeys and apes, and a common ancestor of present day apes in the evolutionary tree, is still debated, but, regardless, Proconsul is one of earth’s first known apes.
Between 9 and 17 million years ago, apes from the genus Dryopithecus were living not just in Africa, but were the first known species’ of ape to have migrated in to Europe and Asia, giving clues to the migratory patterns of our ancestors (Palmer, 2010; Begun, 2004)). Although there are variations in the five species of this genus, like Proconsul, it resembled a monkey in many ways, but the bones in the forearm and elbow suggested it moved about in the tree tops like an orangutan or a gibbon. However, its skull formation is similar to the chimpanzee. Like all fossils, its evolutionary position is debated, but some scientists have claimed that Dryopithecus and its close relatives were the ancestors of all the extant great apes (Palmer, 2010).
During this period, between 8.5-12.5 million years ago, three species of the genus Sivapithecus were living in the rainforests of Asia. Sivapithecus was around 1.5 meters in height, and many of its physical attributes resembled those of a chimpanzee. However, like an orangutan, it had a concave face with projecting incisors and large canines (Palmer, 1999). Fossils of Sivapithecus species have been found in Turkey, China and Pakistan, and analysis of bone structure indicates they were adept at movement on both the ground and in the trees, and fed on a diet of savannah grasses and seeds (Palmer, 1999). The genus Sivapithecus is now acknowledged as being the direct ancestor of modern day orangutans (Fleagle, 1999), and scientists believe that this line, the lineage that descended to modern day orangutans, branched off from the line that descended to modern day gorillas, chimpanzees, bonobo’s and humans at around 12 million years ago (Palmer, 2010; Fleagle, 1999).
The orangutan is the only non-human great ape found in Asia, but this development is relatively recent, as another group of primates may have evolved from Sivapithecus (Fleagle, 1999), and lived at the same time as orangutans, in what are now China, India and Vietnam (Morris, 2008). Gigantopithecus lived between 300,000 to a million years ago, and was, as its name suggests, a giant, the largest apes ever known, with males of the largest known species weighing as much as 540kg, twice as much as a male gorilla (Morris, 2008). Gigantopithecus was thought to feed chiefly on plants, and live terrestrially, walking on all four limbs, like a gorilla (Morris, 2008; Galdikas, 1999). Why Gigantophithecus died out is unknown, but it is possible it was hunted to extinction by early man (Galdikas, 1999; Morris, 2008)
While the orangutan lineage was evolving in Asia, the primates that led to the modern day African great apes and humans were still in Africa. Fossil evidence of ancestral gorillas, chimpanzees and bonobo’s is sparse, due to the acidity of rainforest soil, which tends to dissolve, rather than fossilize, bones, but fossils of early humans found in open savannahs, and genetic mapping of the human and non-human ape genome, have given a clearer picture of how the other great apes evolved. The line that descended to modern day gorillas is believed to have diverted from the line that chimpanzees and humans followed at around 8 million years ago, with the chimpanzee lineage diverting from the human lineage at around 4 million years ago. The line that led to modern day bonobo’s is thought to have split from the chimpanzee line at around 2.5 million years ago (Palmer, 2010). It should be noted, however, that some scientists date the split between the human and chimpanzee lineages at around 7 million years, on account of fossils found in Kenya, named Orrorin, which have distinctly human characteristics and have been dated at 6 to 5.8 million years old (Senut et al, 2001).
The human, or hominin, fossil collection is reasonably large, and although there is confusion and debate about the exact route the lineage from ancestral apes to human has taken, the evolution from a quadruped covered in fur to the hairless bipedal ape we are today is well documented, and current estimates put the emergence of Homo Sapiens in Africa at around 200,000 years ago, with migration to the rest of the world dated at between 80,000-60,000 years ago (Palmer, 2010; Lockwood, 2007).