Primates branched off from the mammalian insect eaters in the upper Cretaceous period (see Fig. 7.22), and today there are six natural groups of nonhuman primates, located in the following regions:
1. Lemurs: Madagascar
2. Lorises: Africa, South and Southeast Asia
3. Tarsiers: certain islands in Southeast Asia
4. New world monkeys: South and Central America
5. Old world monkeys: Africa, South and Southeast Asia
6. Lesser and great apes: Africa, South and Southeast Asia
These primate groups are classified into two suborders. The first three groups belong to the prosimians. They are more distantly related to the last
three ones, called simians or anthropoidea. Lemurs and lorises split from the main primate line in the upper Cretaceous period (see Fig. 7.22), and the tarsiers branched off in the Paleocene. New world monkeys departed from the path leading to the great apes and man in the Eocene, old world monkeys in the early Oligocene, and the lesser apes (gibbons) in the middle Oligocene periods.
The geographical distribution of today's primates (see Fig. 7.23) roughly coincides with the band of the Tropics, from 23.5° northern to 23.5° southern latitude (dotted), reflecting their preference for a warm and moist climate, especially the rain forest. This is essentially the environment in which they evolved from their insectivore ancestors.
Throughout most of the Tertiary period, temperatures were much higher than today and the range of the tropical forests extended to much greater latitudes. This can be deduced from the isotope ratios of oxygen found in skeletal deposits of deep-sea dwelling organisms (benthic foraminifera), recovered from tropical ocean drilling cores (see Fig. 7.24). Because ocean water containing the lighter 16O evaporates more easily than with the heavier 18O isotope, the water with the lighter isotope gets deposited in glaciers and ice sheets in the polar regions, while the remaining ocean water (which the organisms take up to build their skeletons) gets enriched with the heavier oxygen isotope. A persistent increase in the 18O/16O ratio from the early Eocene up to the Pliocene (see Fig. 7.24) therefore indicates increasing glaciation and steadily declining temperatures during primate evolution. The present extent of the tropical forest is therefore only a fraction of what it was in Eocene and Oligocene times, when the extensive ice sheets of the Arctic and Antarctic regions had yet to develop. This explains why most fossils of early primates have been found in central Europe, and at similar latitudes in North America.
While insectivores typically led a life on the ground, the ancestors of primates began to follow insects up the trees, where also a great abundance of plant materials - leaves, fruit, and nuts - could be found. From the early Cretaceous period onward, as discussed above, newly evolving angiosperm (flowering) plants increasingly utilized insects, birds, and mammals for the fertilization and the dispersal of their seeds. In warmer regions, these flowering plants soon replaced the gymnosperm (conifer-like) plants, which let the wind carry their seeds. A new mutualistic relationship became established between these animals and the angiosperm plants, which in return for fertilization and seed dispersal were provided with nourishment.
Having climbed one tree, the problem for an early small primate was how to get to another. This was essential because plants, in order to maximize
the service from animals, typically provide their nectar and fruit sparingly. By letting the fruit ripen at different times, they also ensure that the limited number of collecting animals will not be overwhelmed. With the forest floor infested with superbly adapted and camouflaged predators, getting all the way down from one tree to transit on the ground and climb up the next one was not just tedious but also dangerous. By jumping from branch to branch, primates were able to completely avoid the hostile forest floor, but to do this their mental equipment first needed to be improved. It was this arboreal environment in a tropical forest that stimulated the growth of the primate brain.
Two primate species represent important examples in this evolution. Both are now extinct simians, which lived near branching points of the evolutionary tree. Figure 7.25 shows the flora and fauna in the tropical forest in early Oligocene times in Egypt, some 35 million years ago. Here Aegyptopithecus, the common ancestor near the point (marked A in Fig. 7.22) at which old world monkeys departed from the lesser and great apes, is shown in the lower left-hand corner of the figure. Weighing about 500 g, it led the life of a quadrupedal tree-dweller.
In this environment, stereoscopic vision and a capability for fine-tuned hand and foot movement were essential, as false estimates of distance or branch thickness, or an imprecise grip could be life-threatening when jumping from tree to tree. To perfect stereoscopic vision, the eyes moved into a more frontal position, allowing a greater overlap of the visual fields. But
this forward vision also increased vulnerability to predators. This, however, was compensated by the fact that the primate's arboreal environment was far less dangerous than the forest floor.
In addition, these animals required a better memory, as well as an improved pattern and color recognition capability, since the trees carrying ripe fruit in the rain forest at any given time are often widely separated. Moreover, unlike their ground-dwelling ancestors roaming about on a two-dimensional surface, monkeys had to remember pathways and locations in a much larger three-dimensional space. The greater mental capabilities required for this arboreal lifestyle and a more intense social interaction meant that primates developed a much larger brain, of roughly twice the size compared to that of the other mammals of the same body weight.
The other important stage (marked P in Fig. 7.22), past the branching point at which the lesser apes (gibbons) left the line leading to the great apes,(orang-utan, gorilla, and chimpanzee) and humans in the middle Oligocene period, is represented by Proconsul (see Fig. 7.26). This early Miocene ape, which lived about 20 million years ago, serves as a useful model for the common ancestor of the great apes and the hominids, especially since it had already acquired a weight of about 5 kg. This tenfold increase in weight necessitated a very different lifestyle compared to that of the much lighter Aegyptopithecus, and like today's orang-utan, it had to climb and move on sturdier tree branches and avoid jumping in the tree canopy, where the thinner branches might no longer bear its weight. This more sturdy mode of arboreal movement had led to a loss of the tail, which in the smaller monkeys serves as a steering rudder for jumping and as an additional gripping instrument. It also became necessary for Proconsul to visit the ground more frequently.
Ground-dwelling thus became increasingly important along the evolutionary path to man, as the apes grew and acquired the weight that characterizes today's chimpanzees and gorillas. About 14 million years ago, orang-utans branched away from the evolutionary line leading to man. The recently discovered 12.5-13 million-year-old middle Miocene ape Pierolapithecus catalau-nicus (Moya-Sola et al. 2004) is a likely model of the last common ancestor of the great apes and humans, which means that the departure of orang-utan
Fig. 7.26. The ape Proconsul lived in the Miocene, 20 million years ago (after Kelley 1992)
Fig. 7.26. The ape Proconsul lived in the Miocene, 20 million years ago (after Kelley 1992)
could be more recent (Fig. 7.27, dashed). Roughly 9 million years ago gorillas separated from the line of the great apes and some 6-7 million years ago man's closest living relatives in the animal world, chimpanzees, became separated as well (see Fig. 7.27). Another reason why ground dwelling became more important was because the tropical forest receded due to the climatic evolution toward the ice ages (Fig. 7.24).
The creatures leading to us from this last branching point are called ho-minids. For an up-to-date account of our knowledge on hominids see Foley (2005). Their line of development began with the australopithecines, comprising Ardipithecus ramidus, which flourished some 4.4 million years ago, Australopithecus anamensis, which existed at around 4.2 million years ago, Australopithecus afarensis, at 3.9 million years ago, and finally Australopithecus africanus, which lived approximately 3 million years ago. Note that Fig. 7.27 includes the recently discovered earliest 5.7 to 7 million year old hominid fossils Ardipithecus ramidus kadabba, as well as Sahelanthropus tchadensis which are very close to the branching point to the chimpanzees (Haile-Selassie 2001), (Brunet et al. 2002). The australopithecines, weighing roughly 10% more than
Chimpanzee Paranthropus pithecus
10 11 12
Chimpanzee Paranthropus pithecus
Fig. 7.27. The phylogenetic tree of the great apes and humans (modified after Kelley 1992, Wood 1994). The recently discovered Miocene ape Pierolapithecus could change the branching time of orang-utan (dashed). Heavy lines indicate the time spans over which fossils were found
Fig. 7.27. The phylogenetic tree of the great apes and humans (modified after Kelley 1992, Wood 1994). The recently discovered Miocene ape Pierolapithecus could change the branching time of orang-utan (dashed). Heavy lines indicate the time spans over which fossils were found chimpanzees, were all upright walkers, but despite their striking similarity in appearance to later humans and modern man (see Fig. 7.28), they did not possess a significantly larger brain (and probably mental capability) than chimpanzees or gorillas (Fig. 7.33). Accustomed to ground-dwelling, they nevertheless remained very accomplished climbers and retired to the trees to avoid danger and to sleep at night.
Their upright walk (Figs. 7.28, 7.29), a development for ground-dwelling, was not just an energy-efficient way of covering large distances. More importantly, it freed the arms and hands for the transport of food and infants, and the use of tools. With an upright gait, enemies hidden in high grass and bushy environments are more easily spotted, and plant food on high branches better obtained. All these factors support the view that our ancestors left the dense rain forest and entered more open places, particularly the wide savannahs, while gorillas and chimpanzees, which normally use all four extremities for walking (knuckle walk), remained behind in the receding rain forest, where they survive to this day. As clear proof of the upright walk, Fig. 7.29 shows tracks preserved in moist ash from a volcanic outbreak which afterwards dried and became cement-like. These were made by A. afarensis about 3.7 million years ago, in the Laetoli area of northern Tanzania.
Perfection of the upright walk, which is sometimes also clumsily performed by gorillas and chimpanzees, necessitated a number of important anatomical changes (Fig. 7.28). The gripping toe, for example, was modified into an acceleration tool for striding with longer legs, which were straightened and positioned more directly below the pelvis. This supported the body weight better and avoided spending energy to hold it upright. The fingers and toes changed from long curved phalanges, specialized for climbing, to the short straight forms of modern man. The pelvis was modified for the different attachment of the muscles for upright walking, the spine acquired an S-form to act as a shock absorber for the walking motion, and the head became balanced straight on top.
The first humans developed out of australopithecines and flourished some 2.5 million years ago as Homo rudolfensis and Homo habilis (see Fig. 7.27). Then, about 2 million years ago, a particularly efficient upright walker, Homo erectus, appeared (Figs. 7.28, 7.29). Recent fossil discoveries on the island of
Java in Indonesia suggest that this ancestor, whose chest had become barrellike to improve breathing, was still alive possibly as late as 27 000 years ago (Swisher et al. 1996). About 1-1.5 million years ago, Homo erectus began to use fire as a tool (Brain and Sillen 1988).
Finally, about 500 000 years ago, the archaic Homo sapiens, and roughly 200 000 years ago modern Homo sapiens appeared (McDougall et al. 2005) who, in the past 30 000 years, developed from a Stone Age collector and hunter to a farmer, builder, and producer of machinery. The Neanderthal man, or more properly called Homo sapiens neanderthalensis, was a side line of human evolution. He lived throughout Europe and the Middle East from roughly 230 000 to about 30 000 years ago and became extinct due to the intensive competition with modern H. sapiens (Mellars 2004). It has been shown using mitochondrial DNA sequencing that there was no interbreeding with modern H. sapiens (Serre et al. 2004).
As an example of an archaic Homo sapiens, also called Homo heidelber-gensis (Balter 2001), Fig. 7.30 shows the skull of the Tautavel man, found in 1971 in the Arago cave at the village of Tautavel, near Perpignan, France. It has a brain volume of 1150 cm3 and an estimated age of 450 000 years.
As the demands of fine-tuned hand manipulation and the social interaction between our ancestors became more complex, culminating in the development of speech, the volume of the brain increased rapidly, from an average of 400 cm3 in A. afarensis to 900 cm3 in H. erectus and finally to 1350 cm3 in H. sapiens. This dramatic threefold growth of the brain is all the more staggering when compared with the body weight, which went up by only about 30% during the same period (see Fig. 7.33). Figure 7.28 compares
Australopithecus afarensis, Homo erectus, and Homo sapiens. Given a time separation of roughly 2 million years between each of these hominids, both the similarities and the differences are striking.
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