increase the organism’s adaptability, or chance for survival, are passed on to offspring in greater numbers than those traits which hinder survival and result in early death of the organism.
An example of this would be certain color markings on animals, such as the spots on newborn deer, which camouflage the animal and hide it from predators. This is a positive trait that increases the chance of survival. A negative trait would be albinoism, which makes the animal more visible to predators and would likely result in an early death before the animal reaches the age of bearing offspring. The elimination, often through early death, of those possessing negative traits and the passing on of favorable traits is known as natural selection, which is the result of selective environmental pressures. The process of natural selection therefore addresses the issue of “survival of the fittest” in terms of which organisms are best adapted to survive in their environment.
Charles Darwin is remembered as the father of evolutionary theory, although his proposal was actually presented simultaneously with that of Alfred Russell Wallace, who independently discovered natural selection during the same time. Today, the modern or synthetic theory of evolution has expanded upon their ideas and combined them with data drawn from genetic studies. The synthetic theory states that variability among individuals within a population is produced through random mutation, an inheritable change in the chemical composition of DNA, which produces new or modified enzymes and alters physical characteristics. The differing characteristics, or genetic variability, is then acted upon by natural selection in the environment. All species of life currently existing on earth are a result of this developmental process. Therefore, in light of evolutionary theory, let’s examine the evolutionary development of our species.
The Emergence of the Human Species
Modern-day human beings are classified as “hominids,” a term which refers to members of the human family Hominidae. The Hominidae family belongs to the Primate Order, which also includes apes, New and Old World monkeys, and the prosimians. Humans are placed in the Primate Order based on anatomical and physiological characteristics, as well as biochemical makeup. Numerous studies have revealed that modern-day humans and apes are very similar in terms of skeletal structure, muscular anatomy, physiological processes, serological reactions, and chromosome patterns.
Over the years, science has defined our species in a variety of ways—an intelligent, tool-making, social, self-aware animal. Today, however, paleoanthropology bases its definition of the human species primarily on its physical characteristics. Yet if we look at humankind’s physical make-up, there is not one single characteristic that makes us unique. Rather, a combination of four specific traits, referred to as a “complex,” distinguishes us as human beings. The human or hominid complex includes: (1) bipedal walking, (2) manipulative hands capable of a precision grip, (3) binocular (three-dimensional) color vision, and (4) a large brain-to-body ratio.
The traits of the hominid complex are interrelated; as one trait developed, it often enhanced the effectiveness of another, thereby creating a positive feedback loop within the trait complex. For example, the development of efficient bipedal walking left the forearms completely free from the task of support and locomotion, thus enabling the hands to be used for other purposes. Our forward-facing eyes, sensitive to color, depth, and shape, allowed us to manipulate objects with our now-freed hands. The ability to precisely manipulate objects led to habitual tool use, which in turn increased our chances of survival. Then, as brain size increased relative to body size, convolution of the brain surface gave rise to increased neural interconnections, thus enabling even further refinement of hand-eye coordination and the development of more efficient tools.
A scarcity of fossil evidence makes it difficult to date precisely the division of the hominid lineage from the ape or pongid lineage, although it seems likely that the human lineage split from its closest relatives approximately 6 to 12 million years ago. An early genus of apes, Dryopithecus, which dates from 22-15 million years B.P. (before present), is believed to include the evolutionary ancestor of both humans and later apes such as the chimpanzee and gorilla. It’s likely that another genus, called Ramapithecus, dating back 12 million years B.P., was the first direct ancestor of the hominid line. Until recently, it was believed that the first true hominids, called Australopithecines, appeared in the fossil record as early as 4.2 million years ago. The recent discovery of a new genus, Ardipithecus, dating to 4.4 million years ago, however, may displace the Australopithecines as the first hominid species.
A discussion of hominid forms found in the fossil record is a complex issue due to several factors. First, there are a large number of taxonomic classifications within the human lineage. Further, there is often disagreement within the scientific community as to how a specific fossil find should be classified. The “lumpers” tend to minimize differences, saying that they are no more significant than the genetic variation we see in modern-day humans. The “splitters,” however, see even small variations as significant and label similar specimens as different species. (As you can tell, I consider myself a “lumper.”) Add to this the fact that a new find can seriously alter our view of the human evolutionary landscape. In the last decade alone, the discoveries of four new species have completely altered previously long-held beliefs.
Rather than give a lengthy discourse on the various hominids found in the fossil record, I will present a general overview of the human evolutionary landscape. Those desiring more detailed information on hominid forms are referred to references listed. Table 2, Evolution of the Hominid Lineage, outlines the distinguishing characteristics of each species. I suggest that readers refer to that table during the following discussion.
The Early Hominids
A likely candidate for the first true hominid is Ardipithecus ramidus, found in Ethiopia. This species dates to 4.4 million B.P., was forest dwelling, and likely bipedal. Originally placed within the Australopithecine genus, it is now proposed that the differences are great enough to warrant assigning it a new genus. However, the fossil evidence is sparse, and an absolute determination as to its place in the hominid family remains to be seen.
There is no doubt that the genus Australopithecus belongs in the hominid family. There are at least several (depending upon your classification) species: anamensis, afarensis, africanus, and garhi, dating from 2.3-4.2 million years ago, all found in Africa. This genus clearly shows the transformation from ape-like features to modern human features. Afarensis contains the famous “Lucy” skeleton, whose discovery thrilled the scientific world in the early 1970s. Lucy dates to 3.2 million years ago, and her species is the likely link between the Australopithecines and the Homo genus. Australopithecines walked completely upright, had a brain size averaging 420-480 ml (milliliters), ranged in height from three to four feet, and lacked projecting canine teeth (an ape characteristic). They lived in family groups, appeared to be semi-nomadic, and made crude shelters. Some finds have been associated with tool use, once thought to be a strictly Homo trait.
Another group of hominids existed at the same time as the Australopithecines. The Paranthropus species include aethiopicus, robustus, and boisei. Robustus and boisei were once classified as an early form of Australopithecine. Fossil evidence now shows that they had a separate evolution and eventually died out, playing no part in the evolution of the Homo genus. Although bipedal with large brains, they also had primitive features, including a saggital crest, huge teeth and molars, and large jaws. They appear in the fossil record as early as 2.8 million years ago and disappear about one million years B.P.
The Homo Genus
The Homo genus, to which modern humans belong, began about 2.3 million years ago, and has two species: the earlier habilis and the later erectus. Both species show significant brain development and mastered tool use, language, and demonstrate the beginnings of developing a human culture. They also exist outside of Africa, with finds discovered in China, Asia, Indonesia, and Europe. One of the earliest specimens