Ii. the First Apes

Miocene Epoch (c.23-5.3 million BP)

Tectonic forces continued to play an enormous role in the formation of the African continent during the Miocene. In the northeast there were a number of plates operating—the Arabian, the Nubian, the Somalian and the Eurasian. Their movements led to the formation of a land-bridge between Eurasia and Africa and the transfer of species from one to the other. It also led to rifting along the whole length of east Africa, from the Red Sea to Malawi. The uplifting of highlands gradually changed the prevailing weather patterns so that this part of Africa became gradually drier and the vegetation had to adapt accordingly.

Elsewhere in Africa, the picture is somewhat murky, mainly due to the difficulties presented by the environmental conditions, in the Congo basin, for example, and the lack of datable material—particularly  in South Africa.

Miocene African Fauna. © Mauricio Anton

In addition to increased aridity during the Miocene, there was also a sharp decrease in atmospheric carbon dioxide to levels more or less the same as today’s (c. 400 ppm) which contributed to the slow but steady expansion of grassland. The grasslands contained areas of scrub bush as well as stands of trees and, wherever there was permanent water, there were areas of closed forest. This type of landscape is known as savannah (or savanna) and is characterized by seasonal fluctuations in rainfall, with alternating wet and dry seasons. The latter became more prolonged and extreme as time passed, putting a severe strain on the plants and animals that depended on water to survive. Initially these savannahs were characterized by C3 plants including grasses but, as drying process continued over the next several million years, they slowly gave way to C4 grasses which were better equipped to deal with the changing circumstances.

The new animals on the scene included a number that were able to flourish on the savannah, both herbivores and the carnivores that preyed on them. The Early and Middle Miocene were characterized by a number of very large animals, among them the Rhinocerotidae and Hippopotamidae, both of which were newly arrived from Eurasia. Mammals that have since become extinct include a relative of modern tapirs known as the chalicotheres, a huge beast which used its large claws to tear branches down from the trees; and the sivatheres a short-necked member of the giraffid family with bony antlers antlers (Sivatherium giganteum). Wear analysis of their teeth indicates that they were all browsers who ate mainly leaves and fruit, rather than grazers who cropped grass.

The dominant predators of the Oligocene and Early Miocene belong to an extinct group known as creodonts and are classed separately from the Order Carnivora which superseded them. The first to appear were the Hyaenidae of which there were at least 5 families. At first, they were all dog-like animals who chased down their prey but by the onset of the Pliocene these had been replaced by animals with bone-crushing jaws, more akin to modern species.  The first big cats to arrive were sabretooths, which appear in the Late Miocene. Meganterion had the typical muscular forelimbs and dagger-like canines seemingly designed to penetrate the thick skins of  some of the larger prey such as young rhinos or elephants. Homotherium, who first shows up in the Pliocene, seems to have been an ambush predator operating in woodlands where flight was more difficult. Lokotunjailurus emageritus was more gracile and perhaps better suited to chasing down its prey.

The main problems with depending on grass or leaves for nutrition is that huge quantities of it are required and the fact that they are very difficult to digest due to their high cellulose content. Breaking it all down into fatty acids that are usable requires microbial fermentation in the gut. In some species such as the equids and elephants, this is done by ‘hindgut fermentation’ which takes place in an enlarged colon and a chamber known as the cecum. In the case of ruminants, such as the bovids and giraffids, it takes place in a large chamber, the rumen, whereupon the forage is regurgitated and broken down some more by further chewing before being swallowed again. It then passes through two more chambers, the reticulum and the omasum before it reaches the stomach proper, the abomasum. There are advantages and disadvantages with both methods and, although all of these herbivores can both browse and graze, hindgut fermenters digest grass poorly and ruminants have trouble with leaves. The amounts of forage that had to be consumed and its seasonal availability tended to favour larger species, which require less nutrition for their body mass and are better able to survive prolonged dry seasons.

The earliest ruminants are thought to belong to the genus Eotragus appeared around 18 million years ago in Asia. They gave rise to the Bovidae, the ancestors of all horned, cloven footed grazers—wild cattle and buffalo (Bovini) and antelopes (Tragelaphini) as well as sheep and goats (Caprini)— which spread widely throughout Europe and North America as well as to Africa. Originally they were mainly browsers but, by about 10 million years ago, grazers began to appear in significant numbers and not long afterwards shift their diet to C4 grasses. This is shown from the ratio of carbon isotopes (13C vs. 12C) in the enamel and, in some cases, by the size and shape of their teeth. Hipparions (an early equid) were among the first to do this but were followed by White Rhinos (but not Black Rhinos), many of the bovids, some of the elephants and short-necked giraffes.

The First Apes (Family: Hominidae)

Apes begin to appear in the fossil record about 15 million years ago but, because they mainly inhabited the tropical rainforest, where preservation of fossils is virtually nil, little is known of their immediate antecedents. Some fossilized teeth have been found in East Africa,which may belong to ancestral gorillas and chimpanzees, but little else.

The most obvious characteristic that distinguishes them from monkeys is their lack of tails, which most quadrupeds use for balance.  Instead, they had a number of shoulder adaptations needed for swinging through the treetops, a technique known as brachiation. They also have larger brains for their size than most other mammals, which means that they are better at communication and organization, but there is a price to pay for this. Big brains need time to fully develop, which means a longer gestation in the womb, an extended period of parental dependency, and delayed sexual maturity. For these reasons, births tend to be fewer and more widely spaced. Group survival depended on a longer reproductive lifespan among mothers and a reduced mortality rate for them and their offspring.

Most modern apes eat leaves and fruit and their dentition reflects that. Their canines are ‘designed’ to rip through the skin of ripening fruit and their molars were used slice and chop it into mulch. Some of them may include animal protein in their diet, normally in the form of insects, eggs and hatchlings. As rainfall levels decreased and became more seasonal during the late Miocene their preferred foods became unavailable for extended periods of time and adjustments had to be made in their food procurement strategies. Changing seasons meant that unripe fruit and the more succulent leaves were not to be found. It then became necessary to forage on the ground, gathering fallen fruit and nuts, digging for bulbs and tubers, or scrounging amid the leave litter for bugs and larvae. Cracking nuts, opening tubers and digging into termite mounds would have been greatly facilitated by the use of tools, an activity observed among modern chimpanzee communities.

As the savannah grew, the forests shrunk and they often needed to move from one stand of fruit-bearing trees to another, often separated by hundreds of metres which meant spending mo retime simply moving around and less time collecting food. Increasingly, this meant crossing stretches of open country with ever-increasing distances. The method they devised is known as ‘knuckle walking’, which involved turning the forefoot over and sparing wear and tear on the ‘palm of the hand’. Chimps and gorillas are capable of a few steps in an upright posture, particularly when they are being aggressive, but that is not their habitual means of getting around.

Although some apes, such as orangutans, are solitary creatures most African varieties tend to congregate in groups. For gorillas this might be as small as a dozen or so individuals lead by a dominant male; for chimpanzees, on the other hand, group size may consist of a hundred or more individuals, including a number of males competing for sexual partners. Group cohesion is maintained through the practice of mutual grooming to create social bonding. It is estimated the chimpanzees spend as much as 20% of the daylight hours engaged in this activity. Vocalization also plays a role, as it does for many other animals, and is useful for conveying basic information—danger signals, mating calls, etc.

Towards the end of the Miocene about 7 million years ago, a subfamily of the hominidae (great apes) known as the Australopithecina or Homina (scholarly opinion is divided), split off from the rest and ventured out of the forests and into the open. They were much better adapted for fully upright, bipedal walking and, in addition to being a more efficient means of covering ground, would have given them a better chance of spotting potential danger and freed the hands for other uses. They now seem to have developed the technique up in the trees, moving along the tops of branches, grasping them with their prehensile feet and using their ‘arms’ for balance and support. The latter were long, apelike and well-adapted for climbing too.

The earliest members of this group were Sahelanthropus tchadensis, Orrorin tugensis, Ardipithecus kadabba, and Ardipithecus ramidus, all of which emerged in the more forested areas of East and Central Africa. The location of the foramen magnum, the opening at the base of the skull through which the spinal cord passes in Sahelanthropus meant that its skull sat directly on top of the spine rather than projecting forward. The femur of Orrorin was longer and shared many features with later humans rather than apes. Changes in the articulation of the femora with the pelvis together with the upward tilting of the ilia, enabled a more or less permanent upright posture. With their opposable big toes, they would have had a rather slow and awkward gait but their usefulness when it came to tree-climbing was very useful in escaping predators or finding a place to nest for the night was a necessity for survival.

Further Reading