twenty million years ago the Indian plate collided with Asia; this generated the Himalayan mountain range, which drastically altered the earth's climate forever after. Basically, "The climate became drier and the forests of what is now Africa and Asia contracted. The result was an increased area of savanna habitat, with fewer trees." (Campbell, 711). This brought about environmental and ecological changes that animals already adapted to one way of life needed to alter in order to survive. It is widely believed that humans and apes diverged from a "common humanoid ancestor only about 5-7 million years ago." (Campbell, 711). It is not difficult to imagine that one of the leading forces that caused the early hominoids to split lineages was the widening of the savanna. Essentially, many groups of animals that had become suited to tree-dwelling needed to adjust to life on the open plains; out of these pressures came the hominids.
Of the differences that can be observed in the fossil record between all hominoids, there are several key variances that biologists and anthropologists utilize to tell us the most about our evolutionary history. These features include: brain size and skull shape, jaw shape, bipedal posture, size difference between the sexes, social structure, and tool usages (Campbell, 711). Each of these characteristics possesses the capacity to tell us volumes about what sort of life an individual hominoid lived, as well as who they might be related to. From such observations, major portions of human evolution have been pieced together into the broad picture that most scientists accept today.
Over the course of evolutionary history there have been approximately a dozen different species of hominids; however, we -- the Homo sapiens -- are the only one who has survived. Still, "As many as six earlier species of Homo have become extinct, and another six hominid species have usually been placed in the genus Australopithecus." (Cowen, 352). Between 4.3 and 4.4 Ma, Ardipithecus ramidus appeared in what is now Ethiopia; and Australopithecis anamesis fossils have been found in Kenya, dating from a few hundred thousand years later.
Homo appeared, however, as another significant global climate change was underway. The first great glaciations made the Africa of about 2.4 Ma cooler and drier than it had been, and it is within this habitat that Homo gained an ecological toehold. There were significant changes to the body patterns that Homo exhibited: "Early Homo was small by modern standards, perhaps just over a meter tall, but was at least as heavy as contemporary robust autralopithecines at about 30-50 kg. The difference in brain size is striking, however. The brain size was about 650 cc, considerably larger than the brain of an australopithecine." (Cowen, 357). Further ecological changes took place in Africa about 1.5 Ma and, "It is tempting to associate them with the appearance of a new species of human, Homo erectus." (Cowen, 360). Brain size continued to be a contributing factor to the fundamental evolutionary changes as Homo erectus first appeared around this time period
The fossil of "Turkana Boy," found in 1984 by Lake Turkana in Kenya, was a significant find because it seemed to be an early link between species of Homo habilis and Homo erectus. Since the boy's brain was large enough to suggest that an adult of his species would have had a brain of about 900 cc, "It is a reasonable hypothesis that Turkana Boy represents hominid forms liking H. habilis to H. erectus in our phylogeny." (Campbell, 713). The very classification of Homo erectus indicates the evolutionary changes that the fossils of this species represent. Essentially, the name itself means "upright human"; so, its skeleton is such that it is far closer to that of modern humans in posture than fossils from just a million years earlier. Additionally, "It had fairly modern human features, with a larger cranial capacity than that of Homo habilis. The forehead is less sloping and the teeth are smaller." (Wikipedia 2005). Homo erectus also stood approximately as tall as modern humans -- about five feet ten inches -- but although its brain mass was significantly greater than that of Homo habilis, it still was only about 74% of typical brain masses in modern humans (Wikipedia 2005). So, Turkana Boy essentially linked fossils of earlier hominids to finds that had already been made of more advanced Homo erectus.
The anatomy of Homo erectus can be differentiated from other species of hominoids in a number of ways. Clearly, brain volume has always been one indicator; the range of brain volume within Homo erectus remains somewhat wide: between 750 and 1225 cc (Class Notes 2005). Also, the teeth are reduced in size, relative to H. habilis, and his jaw is parabolic and massive. Homo erectus also has a sloping forehead with a sharply pointed skull in the back he has a very robust brow ridge (Class Notes 2005). The leg to arm ratio has also increased in Homo erectus relative to his ancestors. He may have stood about five feet tall, and has been characterized as being almost a modern human below the neck.
Homo erectus' anatomical characteristics are not the only thing that make him important, however. One of the initial features of Homo erectus that made him fascinating was where he was found: "Since the 1950s, discoveries of Homo erectus fossils have been made sporadically, principally in Africa, but also in Asia." (Geocites 2005). This meant that Homo erectus was the first hominid species to migrate out of Africa: "These humans colonized Asia, including the Indonesian archipelago, leaving fossils known by such names as 'Beijing Man' and 'Java man.'" (Campbell, 712). It has been estimated that Homo erectus lived between 1.8 Ma and 500,000 years ago, but it had already managed to populate regions of Asia as early as 1.5 Ma. Obviously, this suggests that modern humans must be a direct descendent of Homo erectus, because of the regions in which both ancient Neanderthal and human fossils have been found.
Broadly, Homo erectus is perhaps the most significant stepping stone in human evolution because he is the clear predecessor of all of the current races of humanity; however, there remains debate as to whether Homo erectus is the first common ancestor of all the human races, or if some later, modern human descendent of Homo erectus supplied this role. Regardless of this debate, what is certain is that the expansion of Homo erectus onto the other continents of the globe was indelibly linked to his particular characteristics -- both biological and behavioral -- and that these characteristics laid the foundation for the later global dominance of the Homo sapien.
Given the terrestrial proliferation coupled with the emergence of Homo erectus, it should not be wholly surprising that, "Quite suddenly, at about 1.4-1.5 Ma, all over East Africa, Homo erectus is found associated with a completely new set of stone tools." (Cowen, 361). For some reason, Homo erectus began making use of a type of tools that, perhaps, was more difficult to make, but was also more suited to the butchering of large prey. It is called the Acheulian tool kit; it has been shown to be more effective than the more ancient Oldowan tool kit in most situations. So, it is possible that such advances in technology helped in the spread of Homo erectus across the continents; yet, there is much stronger evidence to suggest that in Eastern Africa at least the Acheulian tool kit allowed Homo erectus to better compete for food. This may have been a leading factor that led to the extinction of certain other major predators: robust australopithecines, A. africanus, and the saber-toothed cats.
Some experts believe that this drastic alteration of the African ecological order is a direct result of Homo erectus. First, there is the emergence of more workable tools; and second, Homo erectus much larger than the other hominids against which he competed for food. Largely, according to some, this indicates that Homo erectus should be thought of as the first truly effective large game hunter among the hominids (Cowen, 361). Still, many others believe that the invention of the Acheulian tool kit should not be attributed to Homo erectus in general, but only to the specific population that lived in Eastern Africa. This, according to them, is because of how old the Java fossils of Homo erectus might be; they could indeed date back to 1.8 Ma, "If this date is correct, it implies that H. erectus left Africa almost as soon as it evolved there, before its African branch invented the Acheulian tool kit." (Cowen, 361). Overall, it may be difficult to conclude that hunting prowess alone allowed for the exponential expansion of Homo erectus out of Africa.
Another indication that Homo erectus had become somewhat more like modern humans was the fact that males were only about 1.2 times the size of adult females of the same population. This is a size ratio very close to that of modern humans; "Some…