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 anthropologists interpret this to mean that monogamy on pair-bonding had evolved in H. erectus societies, replacing a more polygamous system where the largest, strongest males generally out-competed smaller males and left the most offspring.” (Campbell, 712-13). This implies that by the time Homo erectus emerged, females were choosing mates based upon characteristics other than mere physical prowess; this too, could help to explain the success of the species.
In addition to the more advanced tools that Homo erectus employed, many scientists have suggested that the spread of such a species across Eurasia and into temperate climates suggests that he may have made use of another major advancement: fire. Still, this assertion is almost pure speculation — growing only out of attempts to explain hew Homo erectus could have been so widely successful — and may remain so because the tools that could have been used to make fire are not likely to leave traces: “It is possible that they used an instrument like a bow drill which involves constant rubbing of wood together. This type of evidence would not preserve well.” (Class Notes 2005). Altogether, it is plausible that Homo erectus used fire for multiple purposes; there is some evidence that it was used to drive herd animals over cliffs some 700,000 years ago in China; yet there is little definitive evidence one way or the other (Class Notes 2005). Even so, the use of fire would supply a much needed explanation for how a species native to the warm climes of Africa could so rapidly have come to dominate much of the terrestrial world.
Nonetheless, there remain many difficulties with the hypothesis that Homo erectus should be thought of as a very near relative of the species that led to the modern Homo sapien. These difficulties are not primarily associated with the evolutionary path that the geologic record suggests; instead, there are many competing theories concerning the sequence of events that eventually led to modern humans. In other words, among paleontologists, it is fairly widely accepted that the fossils that fall under the header Homo erectus are very near the direct lineage that leads towards humans, but it is unclear as to whether Homo erectus can be considered a single species. This is very significant, because if it is not a single species, then it is more likely that modern Homo sapiens colonized Eurasia independently of the migration of Homo erectus. One trouble is thus summarized:
“Some paleontologists insist that the variation among them is no more than it is among Homo sapiens today, and therefore that all the specimens should be placed into one species, Homo erectus. Here is the problem. If hominids were spread widely but thinly over the Old World, the widely separated populations would tend to diverge in some characters, both anatomical and cultural.” (Cowen, 362).
Consequently, it has been difficult for many experts to accept the notion that a single species of Homo erectus can be thought to have stretched from South Africa to Southeast Asia; accordingly, some prefer to classify African varieties of erectus as Homo ergaster. Understandably, this would be a somewhat easy assertion to uphold if the discussion was of anything other than possible ancestors of human beings. After all, it is counterintuitive to think that populations so geographically separated, and exhibiting such variation could ever be the same species. Yet, if they were indeed separate species, then how is it that Homo sapiens subsequently came to populate six of the seven continents and all be members of the same species?
In order to grasp the consequences of this question we must first define what a homo sapien is. To do this, the biologic definition of a species must be used: “a set of individuals who are potentially or actually interbreeding to produce fertile offspring.” (Cowen, 43). Yet, this definition leaves the door open for many gray areas in nature: instances where it is impossible to determine whether animals are members of the same species or not. Lions and tigers, for example; when they do interbreed they produce fertile offspring, but it is unclear whether they would interbreed in nature freely. They no-longer coexist in the same habitats — because of human actions — so they are not clearly different species. Similarly, you could, based upon the biologic definition of species, claim that an individual born with a genetic defect who is unable to have children is not a homo sapien — another gray area.
Ultimately, if we are to believe that the migration of Homo erectus comprised the temporal and geographical split that eventually produced differing races of Homo sapiens, then we must believe two things: first, Homo erectus is our direct ancestor; and second, that Homo erectus and modern humans must be the same species. After all, if a modern human and an ancient Homo erectus — from this point-of-view — could not have produced fertile offspring, then it would also be impossible for a modern American Indian and an African to produce fertile offspring. So clearly, whenever the geographical split occurred, it must have occurred within a population very genetically similar to our own.
Sechrest and Brooks note that with reference to plant and animal life in general, “Species diversity is unevenly distributed across the globe, with terrestrial diversity concentrated in a few restricted biodiversity hotspots.” (Sechrest and Brooks 2002). In other words, of the millions of plant and animal species presently inhabiting the planet earth, a significant portion of them reside in geographically small expanses of land. The consequence of this approach to the issue of global diversity is, “If this history is disproportionately extensive, we may face losses of phylogenic diversity and/or evolutionary ancient lineages even more devastating than reflected by species losses alone.” (Sechrest and Brooks 2002). Yet, the mere existence of Homo sapiens across the entire globe sheds some level of doubt upon such understandings of how biologic diversity must occur.
Our knowledge of human evolution after Homo erectus, however, is even messier than what paleontologists know of the pre-modern hominids. It has been postulated by some scientists that Homo erectus must have coexisted with modern humans as they migrated on their own across Eurasia and into Australia. It is thought, by these paleontologists, that a form of modern humans may have evolved only one million years ago in Africa; supposedly, this species is represented by fossils found in Spain which have been called Homo antecessor, and must have lived about 500,000 years ago (Cowen, 363). Soon after, “Around 400,000 BP, H. heidelbergensis was making beautifully crafted hunting spears in Germany.” (Cowen, 363). It would seem that these species developed into Neanderthals in Europe, while Homo erectus still thrived in Asia. Next, as many theorize, modern Homo sapiens swept in, around 45,000 years ago, and out-competed all other groups into extinction.
This scenario seems to be at least somewhat backed-up by genetic evidence. Today, there is fundamentally very little variation across the human population; yet, there are compelling divisions between “African” and “non-African” types along many genetic lines (Cowen, 364). This observation suggests that the human beings who colonized Eurasia and Australia both broke away from the African population recently — only a few tens of thousands of years ago — and that they were a small, isolated population of their own. To geneticists, the evidence implies very strongly that modern humans are descended from a population of Homo that was not only very small 10,000 years ago, but had been very small for a long time beforehand (Campbell, 715).
Overall, the geneticists’ line of reasoning explains a lot. First, it has been noted that modern human DNA varies very little relative to modern Chimpanzee DNA. This also indicates a very explosive adaptive radiation among humans as we spread across the globe. Such would not be the case if Homo sapiens, in the form of Homo erectus, had migrated out of Africa as early as 1.8 Ma. Additional credence has been afforded this theory with the recent analysis of Neanderthal DNA; it has been found to be relatively dissimilar to our own; thus, indicating that Neanderthals had colonized Eurasia well before human arrival.
However, this is not the only interpretation of the DNA evidence. For instance, if we look at Chimp DNA as being an analog to Human DNA, we should expect what could be called a species of humans as being far more diverse than we currently are. So, although Neanderthal — and we can infer Homo erectus — DNA is dissimilar, it is close enough that they could be considered the same species as modern Homo sapiens. In other words, the level of variation among Chimps, were it applied to humans, could easily account for the differences between Homo sapiens, Neanderthals, and Homo erectus. Accordingly, it is possible to imagine a single species of humans being genetically and geographically distributed along such lines 1 Ma. If this was the case, then approximately 10,000 years ago there must have been some event that caused humans of our type to survive while the other breeds of human beings — Neanderthals and Homo erectus — were wiped out. Next, it must be inferred that some new behavior or adaptation — like speech — originated in Africa and facilitated the spread of modern humans over their near relatives. Broadly, I believe that it is very likely that Homo erectus could have been considered the same species as Homo sapiens; yet, it also seems likely that the spread of Homo erectus was independent of the spread of modern humans because of the lack of variability in modern humans suggests a far more recent adaptive radiation.
1. Campbell, Neil A. Jane B. Reece. Biology: Sixth Edition. New York: Benjamin Cummings, 2002.
2. Class Notes. “HPN: Homo Erectus.” 2005.
3. Cowen, Richard. History of Life: Third Edition. Malden: Blackwell Science, 2003.
4. Geocites. “Homo Erectus.” Geocites.com, 2005. Available: http://www.geocities.com/palaeoanthropology/Herectus.html.
5. Sechrest W, Brooks T. “Hotspots and the Conservation of Evolutionary History.” The National Academy of Sciences, 19; 99(4): 2067-2071, 2002.
6. Wikipedia. “Homo Erectus.” Wikipedia the Free Encyclopedia, 2005. Available: http://en.wikipedia.org/wiki/Homo_erectus.
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