BIOLOGY Volume 3 - A Guide to General Biology - 2004

26. EVOLUTION, OR THE HISTORY OF LIFE ON EARTH

26.8. Human Evolution

The Study of Human Evolution relies primarily on the investigation of the fossil record, which is notoriously incomplete. Nevertheless, these fragmentary data enable paleoanthropologists to begin piecing together the phylogeny (evolutionary history) of primates.

Cytology/cytology/16.html">Early stages of human evolution are studied by drawing on comparative anatomy of fossils, as well as by comparing numerous characteristics of modern humans—ranging from biochemical to behavioral ones—with those of other mammal species.

To investigate later stages of human evolution, archaeological evidence is also employed. Discoveries of artifacts (objects made by humans), such as stone tools, pottery, and hearths, allow us to reconstruct the pathways along which both the biological and cultural development of modern humans took place.

Undoubtedly, the greatest challenge encountered in studying human phylogeny is obtaining adequate fossil remains. Exceptional human fossils were discovered, for example, in the deposits of the Olduvai Gorge in northern Tanzania by Louis, Mary, and Richard Leakey; however, usually only skulls and Teeth are recovered. These structures are preserved thanks to their extraordinary thickness and hardness (Fig. 26.19).

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Fig. 26.19. Changes in Skull Structure during the course of evolution from Dryopithecus to Homo sapiens.

The study of fossils begins with dating, i.e., determining their age based on the age of the rock strata (layers) in which they were found, as well as the strata lying above and below them. This is known as relative dating. Absolute dating is performed using radiometric Methods described in Appendix 4. Combining these two methods yields much more reliable age estimates than either method used independently.

Any Conclusions regarding humankind and its putative ancestors should be considered tentative and subject to revision in light of new discoveries. Nevertheless, there are several widely accepted views on human phylogeny, which are outlined below.

26.8.1. Human Phylogeny

Humans belong to the mammalian order known as primates, which also includes tarsiers, lorises, lemurs, and Old and New World monkeys (Table 26.10). Many traits typical of this order are adaptations to arboreal life, and it was precisely these characteristics, essential for an arboreal lifestyle, that played a crucial role in the subsequent evolutionary development of our ancestors. These traits enabled human ancestors to exploit new ecological niches that opened up as the lush forests of the Miocene epoch gave way to the drier grassy savannas of the Pliocene.

The Order Primates comprises three groups of animals referred to as anthropoids: New World monkeys (marmosets and spider monkeys), Old World monkeys (baboons and proboscis monkeys), and hominoids (apes and humans). Humans and their ancestors are more closely related to apes than to any other anthropoids, while apes, in turn, are phylogenetically closer to Old World monkeys than to New World monkeys (see Table 26.8).

Table 26.10. Typical Features of the order Primates

It is generally accepted that the ape and human lineages diverged from the Old World monkey stock approximately 25–30 million years ago during the Oligocene, with the subsequent split between apes and human ancestors occurring 5–10 million years ago in the mid-Miocene. Since then and continuing to the present, the family Pongidae (fossil forms and modern gibbons, orangutans, gorillas, and chimpanzees) and the family Hominidae (fossil forms and modern humans) have evolved along divergent paths. Judging by comparative biochemical data (Section 26.7.8), it can be inferred that gorillas and chimpanzees diverged from the human evolutionary line relatively recently, a mere 5 million years ago. However, no paleontological evidence has yet been found to confirm this.

Various fossils pertaining to human phylogeny are presented in Table 26.11. These remains belong to representatives of four genera and six species. They illustrate changes in such biological characteristics as skull shape, tooth structure, brain size, the transition to bipedalism, and dietary habits.

Of particular significance in human evolution were The Development of bipedalism and the increase in brain size.

Table 26.11. Key features underlying human phylogeny

The transition from quadrupedalism to bipedalism (bipedalism) had consequences extending far beyond its impact on the Skeleton and musculature. It is currently believed that an upright posture and the associated changes in The Nervous system made the subsequent enlargement of the cerebral hemispheres possible. The common ancestors of humans and apes most likely moved on all fours, much like chimpanzees, but starting with Ramapithecus, our ancestors began spending most of their time upright. Approximately 4 million years ago, they made a definitive transition to walking on two legs with a fully straightened spine.

Once the forelimbs were freed from locomotion, it became possible to use them for carrying various objects and manipulating the environment; all such activities prepared hominids for subsequent dexterous and agile behaviors associated with their cultural evolution. Furthermore, an upright posture increased the stature of hominids and broadened their field of vision, which must have provided a distinct advantage in their habitat—the open expanses of the savanna.

The advantages conferred by bipedalism were accompanied by brain enlargement, as evidenced by the expansion of the cranial vault. As shown in Table 26.11, hominid cranial capacity increased from about 450 to roughly 1400 cm³. However, sheer volume alone does not fully capture the functional potential of the brain as it evolved during human history. The complex folding of the Cerebral Cortex increased its surface area, significantly enhancing its functional capabilities. This increased efficiency allowed for the control and coordination of complex behavioral activities, such as toolmaking, hunting, and speech.

A remarkable feature of human evolution is that the gradual changes in physical traits (skeleton, locomotion, diet) were accompanied by an accelerated development of social behavior. This process of becoming human is termed hominization, and it is believed to have been driven by the following factors:

1) the development of labor skills and speech;

2) changes in Sexual Behavior that made pair-bonding possible and strengthened parental care of children;

3) community integration and The Emergence of social responsibility rooted in the law of food sharing.

These biological and social changes were accompanied by developments that were transmitted from person to person through communication rather than inherited genetically. They indicated the growth of culture, defined as a store of information and a set of "behavioral stereotypes transmitted not genetically, but through learning via imitation or example." (Stephen Tomkins (1984), The Origins of Mankind, CUP.) Culture encompasses many different aspects of human life: customs, rituals, the transmission of knowledge, language, beliefs, laws, religion, food, and labor activities. Our knowledge of early human cultural evolution is limited to artifacts discovered by archaeologists. These are mostly stone tools, yet their study provides insight into early human activities.