This is taken from my book The Emergence and Nature of Human History, Volume One. The section of the book from which this is taken is a chronology of the emergence of consciousness. The chronology employs Carl Sagan's device of condensing the Universe's age down to one year, with the Big Bang occurring on 1 January. The section also measures time through the use of an imaginary timeline one million meters in length. The chapter is lengthy, so I'll publish part two in the middle of tomorrow night. This diary will go over big with your creationist/ID acquaintances.
EVOLUTION OF THE GENUS HOMO
31 DECEMBER, ABOUT 10:30 PM; ABOUT 999,818 METERS UP THE LINE
Only now, in the last hour and a half of our one-year Universe, can we begin to examine the branch of life that led directly to the modern humans. Only now, after having considered (if only very briefly) the whole story of space-time and energy-matter’s origins and evolution, can we look for our specific place in the tumultuous story of reality. We have traced the (at least) 60 million-year evolution of the primates. We have noted in particular how the ardipithecines and the australopithecines evolved in eastern, southern, and perhaps central Africa, and how their upright posture lent a crucial survival advantage to them. Are we their descendants, or is our true lineage yet to be uncovered? We will weigh the various ideas of those who are attempting to find that nexus between non-human and human, the point—perhaps impossible to define—at which distinct members of our genus stood in the African sun for the first time. What traits, if human only in rudimentary form at first, did they possess? From what lineage did the first animals that everyone acknowledges were genuine humans, Homo erectus, emerge and how did they bring forth our particular species and subspecies? In examining all of this and how it unfolded, we must bear in mind an inescapable fact: over all these developments the processes of natural selection and genetic drift reigned, indifferent and without the capacity for remorse or intent.
Attempting to Define the Boundary Between Non-Human and Human
In general, in our examination of the evolution of the primate line, we have seen the following trends or tendencies in particular:
1. A significant increase, over millions of years, in the average size of primates. In particular, with the evolution of the family Hominidae, there was now a lineage of primates that produced animals of more substantial height and weight than the average monkey, even when modern mandrills and baboons are taken into consideration.
2. The increasing ability of certain of these larger animals to function both in trees and on the ground, an ability facilitated by the evolution of bipedalism. This bipedal capability probably began to emerge when the ability to stand upright while in an arboreal setting proved to be adaptively advantageous. What was useful in the trees proved to be even more so on the ground. Hominids tended to retain significant climbing abilities and the ability to move through branches along with this ability to live on the ground.
3. An increase in the ability to reach, manipulate, and transport objects, a direct consequence of the ability to stand upright for increasing periods of time and the freeing of the forelimbs from use in locomotion. This tendency favored the evolution of hands good at examining, manipulating, and using objects to facilitate survival.
4. A general increase, in Hominidae, in the average brain size, and just as significantly, a change, in certain lineages, in the average brain shape. These changes were indicative of animals with an increasing ability to use fluid intelligence—intelligence that is brought to bear in processing information and the solving of novel problems—to meet the challenges of existence. Such intelligence allows certain animals to notice objects in the environment and conceive of novel uses for them. Those animals that evolved brains not just of large absolute size and large size relative to the rest of the body, but configured and organized in particular ways, had a tremendous advantage over animals that did not possess such brains. In connection with this, we must note something that we have not touched on before: intelligence gives an animal that is not particularly strong, fast, or agile in comparison to its predators behavioral options. It does not have to rely on a single strategy to cope with deadly threats. It can run or it can hide or it can fight or, most crucially, it can live in social groups capable of organized defense, a defense which can include the use of objects as weapons. Primates evolved a tendency toward social living, perhaps very long ago. Now, intelligence and social living in combination proved to be tremendously useful from a survival standpoint.
5. The retention, in many members of Hominidae, of primate tendencies toward omnivorousness, and the evolution of dentition that reinforced and facilitated these tendencies. Omnivores have obvious survival advantages, and their ability to ingest other animals and obtain nourishment from the protein of those animals was a significant factor in their ability to flourish.
So one would think that the task of identifying a set of traits particular to humans would be a fairly straightforward one. But such is not the case. Few problems have been more difficult in the area of paleoanthropology than defining the boundary line between human and non-human. In 1964 the great paleoanthropologist Louis Leakey and his colleagues Philip Tobias and John Napier said that an animal that possessed all of the following traits was a human: a physical structure that permitted habitual upright posture and bipedalism, arms shorter than legs, a hand with a fully opposable thumb capable of precision grip, a facial structure distinct from that of the typical australopithecine, a rounded dental arch and smaller premolars and molars than the australopithecines, and the possession of a brain at least 600cc in capacity.1 Certain brain sizes have often been thought of as the “Rubicon” beyond which an animal was definitely one of our genus, and Leakey’s definition of the minimal human brain size is probably one of the smallest ones in the literature. The 600cc figure seems to have been chosen specifically so that Homo habilis, which Leakey’s team had both discovered and designated, would be included in our genus. Other researchers considering the location of the boundary line have included the fashioning of tools into the mix along with the other criteria. For several decades, a view much like Leakey’s tended to prevail in anthropology and paleontology.
In 1994 Richard Leakey, Louis’s son, and a prominent paleoanthropologist in his own right, offered an unusually broad view of the term human. Leakey said that since they were capable of upright locomotion—the key development, in his opinion, in our evolution—all hominids should be considered human. Leakey stressed that this did not mean that ancient hominids saw the world in the same mental way in which we do. Leakey’s definition would stretch the human line back 7 million years, by his calculation (the point at which, at that time, the split between hominids and ancestral chimpanzees was thought to have taken place).2 It must be said that while there are some researchers who have defined the word human in a similarly expansive way, most seem to apply more restrictive criteria.
Recent discoveries have raised new issues in regard to the boundary between human and non-human. If Australopithecus garhi or A. sediba were able to use objects as primitive tools, if they were capable of true bipedalism, if they had, in many respects, human-style dentition and dental arches, if they had flexible hands capable of precision grip, and if the largest of their brains (not the average ones) crossed Louis Leakey’s 600cc “Rubicon”, how finely do we have to draw the line to distinguish ourselves from the more human-like australopithecines? Is the idea of a cerebral “Rubicon” a valid one? Does the crucial criterion boil down to the humerofemoral index? Is relative finger length where we draw the line?
Two specialists in human evolution, Mark Collard and Bernard Wood, believe that any species we wish to place in the genus Homo must meet the following criteria:
… for an extinct hominid species to be allocated to the genus Homo, two conditions must be met. First, cladistic analyses should indicate that the species is more closely related to the type species of Homo, H. sapiens, than it is to the type species of Ardipithecus, Australopithecus, Kenyanthropus, and Paranthropus. [This is to say that the animal must clearly show evidence that it is not a member of one of the possible ancestral or sister clades to Homo.] Second, the body mass and shape, the inferred locomotion, the rate and pattern of hard tissue development, and the relative size of the masticatory apparatus of the candidate species should be more similar to the strategies used by the type species of Homo than they are to the type species of the other early hominid genera listed above.3By the term strategies in the passage above Collard and Wood mean adaptive strategies. An adaptive strategy is that set of traits that allows a species to survive and reproduce. For example, the kind of animal that engages in obligate bipedalism is using a different strategy than one adapted for both walking and arboreality. Adaptive strategies exist within an adaptive zone, a particular kind of environmental setting that influences the adaptations of the life forms within it. An adaptive zone is not simply a place. It is a way of living within a place, the methods by which an animal exploits, and survives within, a particular environment. It is within an adaptive zone that the interplay of genetic variability and environmental influence takes place. Several different kinds of animal taxa in a given adaptive zone (which may be thought of as ecological niche) may have similar adaptive strategies. An adaptive zone provides opportunities for evolutionary changes and often tends to produce phenotypes (body forms and structures) that are similar across many taxa. Animal lineages that occupy a range of diverse settings and become adapted to them by selection are undergoing adaptive radiation. (In a sense, the entire biosphere of this planet is a shifting collection of adaptive zones, emerging and disappearing in an irregular way over time.) Did the first purported humans live within adaptive zones characteristic of those occupied by primates we know to be part of Homo? This is part of what paleontologists are looking for when they compare different varieties of human-like primates. The question they want answered goes beyond, “How was this animal built”? They also want to know, “how did this animal survive and make a living?”
Criteria for Inclusion in the Genus Homo
So after considering all of the arguments above, what defines a human? A primate can be considered a member of Homo only if all of the following criteria are met:
--The estimated volume of the cranium is greater than that of the average australopithecine or modern great ape. Naturally, scientists use cranial capacity to estimate brain size. The definition of what constitutes the minimum human brain size is still a matter of great debate. Many paleoanthropologists are reluctant to consider animals with an estimated 600-650cc cranial volume humans. They feel more comfortable when the cranial volume is at least in the 750-800cc range. With fossil remains, measuring cranial capacity can be problematic, inasmuch as many of the specimens examined have missing, partial, or damaged cranial bones. Further, the cranium contains more than the brain. It also contains the meninges, three layers of membrane that protect the brain, and this factor has to be taken into account when doing estimates. The relationship of brain size to body mass is significant as well. If researchers do not possess adequate post-cranial specimens, ascertaining this ratio in a particular primate can be a matter of great difficulty, or even an impossibility. Moreover, the sheer size of a brain cannot always tell us much about its organizational structure. We cannot, for example, infer the possession of language skills by examining the endocranium, the skull’s inner surface, despite the assertions of some researchers in the past. (In regard to the brain’s internal organization, the shape of the cranial vault can give us clues, but no more than that.) Human cranial capacity indicates, broadly, an animal of sufficient intelligence to survive in a variety of settings.
--The animal must be a true obligate biped, one that moves most efficiently by bipedal walking on the ground to the exclusion of knuckle-walking, quadrupedal movement, or methods of moving through branches, such as brachiation. This bipedalism must be demonstrated by the position of the foramen magnum, distinct pelvic and vertebral structures, the angle of the femur, the structure of the knee, the arch of the feet, the anatomy of the tarsal bones, and the positioning of the big toes. The gait of the walk revealed by these traits must be characteristically human as well. In short, the locomotory apparatus must indicate an animal that made or makes its living in a terrestrial setting.
--The animal must possess a forelimb structure distinct from that of the australopithecines or great apes. The humerofemoral index is around 100 in the modern great apes, meaning simply that their arms tend to be as long as their legs. In modern humans, the index is around 70. The animal must have a hand structure that exhibits recognizably human morphology, i.e. fingers suited for prehension, but not of the kind of length in relation to the thumb characteristic of chimpanzees or gorillas. The thumb must be large and fully opposable, suitable for both a precision grip and a power grip (the ability to wrap the fingers and thumb around an object). In short, the arms and hands of the animal must allow it to exploit the environment by examining and manipulating objects within the recognized human range of ability.
--The animal must possess molars and premolars smaller than the huge vegetation-crushing teeth of australopithecines and certain great apes. It also has to have canines less prominent than those of the other hominids as well. The dental arch must be (approximately) in the form of a parabola. The dentition must indicate an animal fully able to exploit a variety of potential food sources in the environment.
--The structure of the face and cranium must be distinct. Specifically, the jaw must not be as prognathic (jutting out from the rest of the face) as that of the apes. The supraorbital torus (or brow ridge) must be less prominent than in the apes or australopithecines. The zygomatic arch, running from the maxilla to the temporal bone and including as its main section the cheek bone, must be relatively less prominent as well. The calvaria (skullcap) must be distinct from the other hominids, meaning there must be no pronounced sagittal crest at the apex of the skull, the forehead which forms its anterior section must be distinct (in some respect) from that of an australopithecine, and the endocranial volume must fall within recognized human limits. (See above.)
As you see, I am using many of the criteria proposed by Louis Leakey and his colleagues while agreeing with the critics of those criteria that the minimum human brain size is probably significantly greater than that postulated by Leakey. I think we would be on safe ground in regarding any primate that exhibited all of the features listed above as a human. Naturally, specific traits can exhibit a great range of variation from individual to individual.
What are the criteria we cannot use, and what knowledge will we never possess?
--We cannot define humans by the phrase, “those who made the first tools”. There is strong evidence, as we have seen, of tool use or tool fashioning, albeit very crude, in some of the later species of australopithecines. As we will see, it was the incredible elaboration and development of tool use that set humans apart from other animals, not tool use itself.
--We cannot say that social living defines humans either, as those who have observed troops of chimpanzees or baboons could tell us. It is the great complexity of human social life that sets us apart, not the basic fact of group interaction.
--Finally, we do not know the point at which genuine human consciousness emerged. It is likely that consciousness existed on a continuum in the evolving primate order. The point on that continuum at which the characteristic human way of seeing the world first manifested itself can never be known. Our attempts to infer this by assessing brain size and shape will always be educated guesses, and our definition of the minimum human brain size will always be to some extent arbitrary.
Competing Views of Human Phylogeny
Paleoanthropologists have produced numerous cladograms that offer hypotheses about the line of descent leading from the late Miocene apes to Homo sapiens. For all their diversity, these charts do tend to have certain views or assumptions in common:
1. They agree that, in order, the apes that split off from the lineage leading to humans were the gibbons, orangutans, gorillas, and chimpanzees.
2. Most agree that the genus Homo is monophyletic.
3. They agree that the genus Paranthropus was a side branch and an evolutionary dead end.
4. They agree that the first undoubted member of Homo was Homo erectus.
Additionally, as we saw in the previous chapter, a rough chronology of primate evolution is beginning to emerge, thanks to back-breaking field work and pains-taking, exhaustive analysis in the laboratory. To recapitulate, here is the chronological sequence again:
Sahelanthropus tschadensis, 6 million to 7 million ybp.As I also stated in the last chapter, simply because we have established a chronology, it does not, by necessity, mean that every animal in it has a phylogenetic relationship to the others, and paleoanthropologists are very careful to point that out in their research. There are, as we saw, highly interesting similarities among the various species, but it is, as yet, not possible to establish an unassailable, definitive lineage that leads from the Miocene (much less the late Oligocene) primates to us.
Orrorin tugenensis, 6 million ybp.
Ardipithecus kadabba, 5.5 to 5.8 million ybp.
Ardipithecus ramidus, 4.4 million ybp.
Australopithecus anamensis, 4.2 to 3.9 million ybp.
Australopithecus afarensis, 3.7 million (perhaps) to 3.0 million ybp.
Kenyanthropus platyops (disputed taxon), 3.5 million ybp.
Australopithecus bahrelghazali (disputed taxon), 3.5 million to 3.0 million ybp.
Australopithecus africanus, 3.0 million (perhaps) to 2.3 million ybp.
Australopithecus garhi, 2.5 million ybp.
Australopithecus sediba, 1.95 million to 1.78 million
Genus Paranthropus (three identified species), 2.7 million to 1 million ybp.
Having said this, therefore, what are the major points at issue in tracing our phylogeny?
WHAT WAS THE RELATIONSHIP OF THE AUSTRALOPITHECINES TO HOMO?
As we saw in the previous chapter, there are paleoanthropologists who reject the notion that Homo emerged out of the australopithecines, hypothesizing instead that the australopithecines and humans stemmed from a common ancestor and went their separate ways. Kenyanthropus platyops has been proposed as a direct ancestor to humans, for example. Some scientists contend that the ability of even the latter australopiths to live in an arboreal environment casts doubt on their role in the rise of the committed terrestrial bipedal genus Homo. But it must be said that the majority of researchers are convinced that humans did indeed evolve out of one line of these upright animals. There seem to be no other credible candidates—yet.
A great many cladograms have been constructed over the years offering hypotheses about the phylogeny of our genus in general and the relationship between australopithecines and humans in particular. To cite only a few examples, in 2005, before the discovery of A. sediba, paleoanthropologist Roger Lewin presented, in his standard text on human evolution, four different hypotheses offered by different researchers concerning the phylogenetic relationship of various finds to us. In all four of the hypotheses, Ar. Ramidus lies at the base of the phylogenetic tree (although one of the groups of paleoanthropologists constructing these trees puts a question mark next to ramidus). All of them postulate that A. anamensis is descended from ramidus. All of them postulate that afarensis (or in one case, what are called simply the specimens found at Hadar and Laetoli) is descended from anamensis.
But after that, there are sharp differences of opinion. In one scenario, the animals the remains of which were found at Hadar and Laetoli lead to P. robustus and P. boisei, while some of the Hadar animals might have a relationship to africanus. In another scenario, afarensis splits, one group evolving into boisei, another into africanus. Another hypothesis is that afarensis splits and on one hand produces all of the genus Paranthropus and on the other evolves into africanus. In the first three hypotheses, it is africanus that gives rise to Homo. The fourth scenario is more intriguing. The scientists constructing it have left room for three different unknown species. One of these unknowns is descended from afarensis and gives rise to P. aethiopicus in one direction and africanus in another. Africanus gives rise to a second unknown species, that splits in two directions. One product of the second unknown is a third unknown. The third unknown produces robustus and boisei. The other product of the second unknown is Homo.4 Many other phylogenetic schemes have been offered over the years, and new finds invariably alter our ideas. (In the notes for this chapter, I have incorporated links to a number of different cladograms to allow the reader to see the variety of hypotheses that have been presented.)5
As of this writing (2012), the two strongest contenders for the line directly ancestral to Homo are A. garhi and A. sediba, whose traits we have already examined. If A. garhi ultimately turns out to be our ancestor, this will mean our genus probably had an east African beginning. If it was A. sediba, then the human race was probably born in southern Africa. But there is another possibility: that our genus was not monophyletic. Indeed, multiple lineages of humans may have evolved separately, and the lineages we have found that were contemporaneous with each other (if not necessarily living in proximity to one another) may have been the product of this polyphyletic origin. Certain evidence seems to point in that direction. It also may be that we have yet to uncover our direct ancestor. There is yet another factor we must admit is possible: that the animals once considered to be the first humans, Homo habilis and Homo rudolfensis were, in fact, particularly distinct forms of australopithecine. (See below)
In my view, the major issues are those of brain size/shape/organization/relation to body size. Complete obligate bipedalism is crucial in defining a human, yes, but it is the ability to comprehend the world (to some degree, at least) that to my mind separates a very bright non-human from a not-so-bright human. Where is the line between the two? No one can say. Was the very bright non-human standing on one side of that line the product of an australopithecine lineage? There is no consensus either way. But what can be said is that there was a significant increase in brain size in certain hominin lineages, and this becomes detectable in specimens dating back to about 2.5 million to 2.0 million ybp. The only bipedal primates we know of from that time are the australopithecines A. africanus and A. garhi and the purported members of Homo, Homo habilis and Homo rudolfensis. Further, the first stone tools, as we will see, date from somewhere around 2.6 million to perhaps 2.5 million ybp. No genus of upright animal other than Australopithecus or Homo has ever been associated with them. The ability to conceive of using objects found in the environment to extend the body’s ability to do work, or to modify objects found in the environment to do work, is indicative of a brain that is physiologically complex. What change in the brain’s neurochemical organization might be associated with this increased mental sophistication? A team of researchers may have found a possible answer.
In humans, sialic acid, specifically N-acetylneuraminic acid (abbreviated as Neu5Ac), is found in the greatest concentrations in the brain, where it plays a significant role in building synapses and facilitating “signaling” between neurons.6 Humans are unable to synthesize a different kind of sialic acid, common to all other mammals, known as N-glycolylneuraminic acid (abbreviated as Neu5Gc). A mutation that affected our genome rendered the gene responsible for Neu5Gc’s production inactive. It is thought that this mutation occurred after the split between the ancestral chimpanzees and the hominins and well before the evolution of modern humans. Studies of Neanderthal specimens have detected Neu5Ac but not Neu5Gc. This means the mutation that blocks the synthesis of the latter acid must have happened before the emergence of the common ancestor of Neanderthals and us. After taking the Neanderthal data into consideration and comparing our genome to that of the great apes, the research team investigating this issue estimated that the key mutation occurred between 2.8 million and 2.7 million ybp—just before the great increase in encephalization (the ratio of brain size to body size) among certain hominins. The researchers caution that it would be premature to ascribe the increase in hominin encephalization to the absence of Neu5Gc in humans, since the brains of other mammals retain relatively low levels of it (while possessing greater amounts in other organs). Still, it is a suggestive phenomenon.7 Current techniques make sialic acids difficult to detect in fossil specimens in the 2 million year-old range or greater. But if it can ever be demonstrated that Neu5Gc was absent in the later australopithecines while Neu5Ac was present, that might be a powerful argument in favor of a phylogenetic relationship between Australopithecus and Homo.
WHAT WAS THE ROLE OF THE EARLIEST PURPORTED MEMBERS OF HOMO?
Although it was not designated as H. habilis at the time, the first fossil example of this species was discovered in 1959 by the Leakey research team. The specimens came from the Olduvai Gorge of Tanzania, and that area rapidly became the epicenter of research on human origins. It was in Leakey’s research paper announcing habilis that he and his colleagues set forth the criteria by which Homo is defined. Homo habilis (the name literally means “Handy Man”) has generally been accepted as the first true human, but there are increasing doubts about its place in our phylogeny (see below).
Specimens attributed to habilis have been discovered not only at Olduvai Gorge (from which several examples have been extracted) but also at Koobi Fora (also known as East Turkana) in Kenya, Uraha, in Malawi, Omo and Hadar in Ethiopia, and Sterkfontein and Swartkrans in South Africa.8 (The hominids/hominins that existed after the ardipithecines are designated in the following way: AL meaning Afar Locality, Omo meaning Omo, Ethiopia, OH meaning Olduvai Hominid, KNM meaning Kenya National Museums, SK meaning Swartkrans, South Africa, or Sts or Stw meaning Sterkfontein, South Africa). Chronologically, the earliest appearance of habilis (and the possible related species, Homo rudolfensis) is difficult to ascertain. Three distinguished experts in paleoanthropology and primatology have concluded that, on the basis of very fragmentary remains, the earliest examples of habilis/ rudolfensis may go back as far as 2.4-2.3 million ybp. They also point out, however, that the fossil record from 3.0-2.0 million ybp is very sketchy, and that little is known of the environmental conditions in which the first members of Homo evolved. Further, they note that around 1.8-1.7 million ybp there were perhaps three different species of Homo—habilis, rudolfensis, and erectus—living at the same time, and that it is not yet possible to establish firm phylogenetic relationships among them. 9
Several problems arise when we try to describe habilis in a satisfyingly consistent way. First, there is the fragmentary nature of the actual specimens. There have been more than 30 separate discoveries of habilis remains, but only KNM-ER 1805 (dated to 1.7 million ybp), KNM-ER 1813 (dated to 1.9 million ybp), OH-13, OH-16, OH-24, SK-847 and Stw-53 are significant parts of the cranium, sometimes including sections of the maxilla and the dentition. Post-cranial fragments from other finds consist of some femurs, a couple of humeri, a hip, a partial foot, and a number of small skeletal fragments, including some parts of the hand. Chronologically, all specimens date from about 2.3 million ybp to about 1.5 million ybp (although ranges of estimates for certain specimens vary widely).10 So although we possess a relatively large number of specimens, they are not comprehensive in nature by any means.
Second, habilis-like remains display a considerable range of diversity, which was why the species rudolfensis was originally designated. In cranial capacity, habilis remains have ranged from about 500cc to 700cc [and some researchers put the upper range higher].11 This diversity is further illustrated by the work of one scientist studying the brain-body ratio (and dietary habits) of early members of Homo. She has pointed out that there appears to be what she describes as “substantial overlap between larger specimens of H. habilis…and smaller specimens of H. erectus”.12 Still, the idea that habilis was a single species that showed a normal range of variation has its defenders. A group of researchers carefully analyzed variations in habilis-like cranial specimens KNM-ER 1470 [often classified as rudolfensis], KNM-ER 1813, OH-24, and Stw-53. They then analyzed the range of variation in gorilla skulls. Their conclusion: “Craniofacial variation in early Homo…is not excessive as previously claimed when compared to gorillas.” They stress that the degree of sexual dimorphism in habilis has not yet been firmly established, which might affect the interpretation of the finds.13 It must be said, however, that the bulk of scientific opinion seems to be moving toward the opinion that habilis-like specimens are best thought of as being part of at least two species.14
Third, habilis specimens sometimes display an unusual mosaic of traits, some of them quite australopith-like in nature. The cranium designated KNM-ER 1813, cited above, is an example of this. The specimen was discovered at Koobi Fora and dates from 1.9 million ybp. The cranial capacity is only about 510cc, which seemingly puts it in the australopithecine range. (The modest cranial capacity is not because the animal was a juvenile. Analysis of the teeth indicates it was an adult.) Yet the facial features and dentition are unlike those of the australopithecines.15 Further, although habilis specimens give strong evidence of bipedal capability, they also indicate that this primate retained a strong climbing ability, meaning that it was probably a facultative biped (an animal capable of walking on two feet but not necessarily doing so as its normal mode of locomotion).
Detailed analysis of OH-62 (a large set of fragments that includes post-cranial material) has uncovered further evidence of the somewhat primitive nature of habilis. The specimens, dated at 1.8 million ybp, were probably from an adult female, only about 36 inches in height, with hands that reached her knees. The morphology of the hand bones appears to be similar to that of an australopithecine. The anatomical gap between this specimen and erectus appears to be significant. Some researchers looking at this evidence have concluded that habilis was simply not advanced enough to be considered a part of Homo. As one put it, “Homo habilis remains more of an evolutionary idea than an example of anatomical fact linking one species to another.”16
Homo rudolfensis is similarly problematic. The first specimen that is now usually considered to be part of the species was discovered by the Leakey research team in 1972, but the designation rudolfensis [from Lake Turkana’s previous name, Lake Rudolf] was not suggested until 1986. The chief differences between habilis and rudolfensis are all craniodental, inasmuch as there are no post-cranial remains associated with any rudolfensis cranium. The most significant specimen is KNM-ER 1470, a very complete cranium and maxilla. The brain size is estimated at 775cc. It has been dated at 1.8 million ybp. It is distinct from habilis in its supraorbital torus, its facial dimensions, the shape of the maxilla, and indications that it must have possessed large post-canine teeth.17 The only other significant craniums or cranial fragments of rudolfensis are KNM-ER 3732, which consists of the calvaria and part of the zygomatic arch, and KNM-ER 3891 which contains, among other pieces, fragments of the cranial vault, parietal bone, and palate.18
Habilis has been associated with very simple tools, those of the Oldowan tradition or the Oldowan mode. The first Oldowan tools to be discovered, at Olduvai Gorge, were hammer-stones and cutting instruments called flake tools. Bifacial tools (rocks modified on both sides) came somewhat later. It is thought that rudolfensis must have been a tool user, but as yet no tools can be definitely associated with it.19 Most Oldowan tools appear to fall into the 2.0-1.5 million ybp timeframe, although older examples have been identified. It appears, however, that the oldest stone tools, discovered at Gona, in the Afar region of Ethiopia, predate habilis, with estimated dates of 2.5 million to 2.6 million ybp ascribed to them. These tools are, according to some researchers, attributable to Australopithecus garhi. Since the tools found at Gona stretch chronologically from about 2.6 million to 1.5 million ybp, they may be part of the broader Oldowan tradition. Their significance is potentially very great. From a principal researcher of the area:
Late Pliocene hominids began manufacturing and utilizing ﬂaked stones c. 2•6 Ma, and the Gona localities provide the earliest evidence of a high density of stone artefacts… The beginning of the use of modiﬁed stones was a major technological breakthrough which opened windows of opportunities for eﬀective exploitation of available food resources including high nutrient meat and bone marrow from animals. The cut-mark and bone fracture evidence from Bouri provides strong evidence for the incorporation of meat in the diet of Late Pliocene hominids as early as 2•5 Ma. The sudden appearance of thousands of well-ﬂaked artefacts documented from several localities in this time interval is intriguing. It may mean that the beginning of the manufacture and use of ﬂaked-stones was a novel adaptive strategy which appeared abruptly c. 2•6 Ma and spread through populations quickly.20Naturally, this scientist points out, chronologically older finds may point to a more gradual technological evolution. But the implication is clear: habilis and rudolfensis may have been the inheritors of the Oldowan tradition, not its main creators. Other researchers note that non-human primates, especially chimpanzees, use objects from the environment as tools, and that A. africanus and A. afarensis may have had brains of a size comparable to chimpanzees, and hence may have had similar tool-using capabilities. Further, the features of the hand in africanus, afarensis, (as well as habilis) may all have been suited for the use and modification of objects in the environment, and not only ones made of stone. Bone tools from both southern and eastern Africa, dating from 2.0 to 1.5 million ybp, have been found, and there is also evidence that wood may have been used for tools at that time as well. 21
There are scientists who argue that habilis and rudolfensis belong together in a separate genus, although this would complicate our picture of hominid phylogeny. Alternatively, it has been suggested that what we think of as the first two species of human are actually better thought of as Australopithecus habilis and Australopithecus rudolfensis. There is as yet no consensus in the scientific community about what are sometimes called the habilines (habilis-like primates), and it will take the discovery and careful analysis of other specimens to put the various remains in their proper context. Certainly, the discovery of Australopithecus sediba has given those who doubt the human status of the habilines new arguments. The possible roles of the habilines can therefore be summarized in this way:
1. The habilines were the first members of the genus Homo, and they gave rise to erectus and hence every modern human.The discussion of the role of the habilines therefore is directly related to our next questions…
2. The habilines were side branches of the genus Homo that died out, leaving no legacy.
3. The habilines were actually australopithecines and had no relationship to Homo.
4. The habilines were australopithecines that gave rise to Homo ergaster, and thus
they were the nexus between the genus Australopithecus and the genus Homo.
5. The coexistence of the habilines with erectus is due to the fact that erectus is the
product of a subpopulation of habilines that has yet to be firmly identified.
WHAT IS THE ORIGIN OF HOMO ERECTUS? WHAT PHYSICAL CHARACTERISTICS MADE HOMO ERECTUS DISTINCT?
The first species within Homo to be universally recognized by all researchers as an undisputed human was, as we noted at the outset of this chapter, Homo erectus, simply meaning, as one might surmise, “Upright Man”. The oldest specimen attributed to erectus is thought by some paleoanthropologists to be approximately 1.9 million years old, although its true affinity might be habilis or rudolfensis. There is more substantial evidence for erectus at 1.8 million ybp,22 and at 1.7 million ybp the specimen labeled KNM-ER 3733, a fairly complete cranium discovered at Koobi Fora, gives us very firm evidence. Its cranial capacity was 850cc.23
It is perhaps necessary here to discuss how paleoanthropologists tend to describe hominid remains. When they apply very specific and restrictive criteria to a species, they are defining it sensu stricto—that is to say, in the strictest sense of the term. Sometimes a paleoanthropologist might describe a species by the term sensu lato—in the broadest sense of the term. The application of broadly-defined criteria is often necessary, given the somewhat indistinct boundaries between species and the frequently ambiguous features of fossil specimens. A broadly defined species can also be thought of as a species that shows a mixture of “primitive” and derived traits while a species defined in the strict sense is more fully derived. So in the earliest specimens of erectus we have, we are often limited to trying to identify the species sensu lato, and new finds sometimes cause researchers to redefine what is meant by erectus sensu stricto.
The most complete example of Homo erectus, and the most complete ancient human skeleton of any kind ever found, was discovered in Kenya in 1984. Nicknamed Turkana Boy (for Lake Turkana), it is also known as Nariokotome Boy, for the specific location in which it was unearthed. Its museum designation is KNM-WT 15000. This specimen was once thought to have been a twelve year-old boy, based on the anatomy of his skull and the characteristics of his teeth. However, a recent study, incorporating new data and new analytical methods suggests that he was probably about eight years old, about 154 centimeters in height, and had probably reached over 90% of his adult height at the time of his death. This would indicate that erectus was not as tall in stature, perhaps, as we had once thought, and not as drastic a break with what had come before. Such a human would have had an adult brain in the area of 850cc. The specimen is dated at approximately 1.5 million ybp.24
Because of such specimens as KNM-ER 3733 and Turkana Boy, most scientists have reached the conclusion that erectus was of African descent. But others are now asking: Was there an Asian origin of Homo erectus? There are scientists prepared to say there was, based on the evidence gathered from several important finds at Dmanisi, in the Republic of Georgia. One researcher has argued that the evidence for an east African origin of erectus is ambiguous at best, and the Dmanisi specimens, although originally categorized as erectus, are much more similar to habilines.25 As he has put it:
… there is now a rival hypothesis to the canonical one that H. erectus originated in Africa and was the first hominin to disperse into Asia. The alternative proposition is that H. erectus originated in Asia from a population of Homo that dispersed out of Africa before 1.8 Ma; some of this Asian population of early H. erectus then dispersed back into East Africa and others dispersed eastwards to Java.26The picture is further complicated by the contention of some researchers that the very earliest specimens of what have usually been called erectus, are in fact examples of a more primitive species known as Homo ergaster or “workman”. Ergaster was first proposed in the 1970s. It status as a separate species is argued on the basis of its distinct molars, premolars, cranium, mandible (bigger than erectus specimens), and generally more rounded head. Some scientists consider ergaster to be strictly African in heritage, while viewing the animals classified as erectus as strictly Eurasian in nature. These same scientists contend therefore that sapiens was the product of ergaster, not erectus. There are also researchers who argue that erectus evolved from ergaster, and that habilis may have given rise to ergaster.27 Moreover, despite the contention that the Dmanisi finds are habilines, other researchers have argued that most Dmanisi specimens clearly fall into the ergaster/erectus range of cranial size. Three of the crania found at Dmanisi in fact appear to contain a mixture of African ergaster and Asian erectus traits.28
Those scientists who agree that ergaster is a valid taxon generally view Turkana Boy as an example of the species. They also see KNM-ER 3733 as ergaster as well. Other significant finds that are said to fall into ergaster are KNM-ER 992, a mandible found in northern Kenya and dated at 1.5 million ybp, KNM-ER 3883, a probable adult male specimen that contains the cranial vault and some of the facial structure (including a very large zygomatic arch), discovered at Koobi Fora and also dated at 1.5 million ybp, and OH-9, from Olduvai Gorge, the upper part of a skull with a large, semi-rounded cranial vault and a prominent brow ridge.29 It should be noted, however, that there is substantial disagreement among paleoanthropologists concerning the status of OH-9. Some scientists contend that OH-9, even though recovered in Africa, resembles the Asian forms of erectus, thus making its phylogenetic status uncertain.30
The erectus/ ergaster controversy is far from resolution. But whether erectus-like animals were part of one species or two, we can summarize their physical traits in the following way:
--Erectus’s cranial capacity increased significantly over time. In the earliest specimens ascribed to erectus, cranial capacities ranging from 600 to 900cc were most common, with some specimens crossing the 1000cc threshold. By the time the latest members of erectus appeared, cranial sizes of 900cc were the lower end of the scale, and most specimens exhibited cranial capacities of 1000cc or greater, with some ranging up to 1200cc. This means that the estimated brain size of late erectus overlapped with the lower ranges of brain size attributed to modern humans.
--The cranial vault itself tended to be low compared to modern humans, and the typical erectus skull was wider than it was high, in contrast to humans like us. The shape of the braincase itself was similar in certain respects to that of modern humans. The sagittal ridge (at the top of the skull) was prominent, and the nuchal torus, a thick ridge of bone in the rear of the skull, was pronounced.
--The brow ridge was prominent, the forehead sloped, the chin receding. The face was large, the mouth was somewhat prognathic, and the neck appears to have been strong. The jaw and teeth were strong as well, but noticeably more derived than those of habilis.
--The dimensions of the postcranial skeleton resembled ours in many ways. Erectus was taller than the primates that had preceded it. It was also probably stockier and more muscular than modern humans. Significantly, it seems to have exhibited a less marked degree of sexual dimorphism than earlier primates, putting it in the more modern range in this respect.31
Probable erectus/ ergaster footprints have been discovered at Ileret, Kenya. They have been dated at 1.51 to 1.53 million ybp. In 2009 it was announced that these footprints strongly resemble those of modern humans, and are markedly different from the tracks at Laetoli which were apparently made by afarensis. These footprints are clearly from animals that were fully bipedal. As the researchers publishing the results put it,
The Ileret footprints show the earliest evidence of a relatively modern human–like foot with an adducted [toward the midline of the body] hallux, a medial longitudinal arch, and medial weight transfer before push-off…these modern human characteristics, in combination with the large size of the prints, are most consistent with the large size and tall stature evident in some Homo ergaster/erectus individuals. These prints add to the anatomical and archaeological evidence pointing to a major transition in human evolution with the appearance of hominins with long lower limbs, conferring advantages at a lower energetic cost, and archaeological indications of activities in a variety of ecological settings and the transport of resources over long distances.32We do not yet know the deep origin of ergaster/erectus. Since habiline specimens dating from as recently as 1.44 million ybp have been found, and erectus-like humans and habilines apparently coexisted for hundreds of thousands of years, many scientists now argue that habilis did not give rise to erectus. The fossil record from 2 million to 3 million ybp is, as we have noted, not very complete. The ancestral group that gave rise to erectus-like animals is still not clear.33 Was it A. sediba, or an unknown habiline-like subpopulation, or A. garhi, or a line of animals that has yet to be unearthed? It is one of paleoanthropology’s major objectives to find answers to these questions.
1. L. S. B. Leakey, P. V. Tobias, and J. R. Napier, “A New Species of the Genus Homo from Olduvai Gorge” in Nature, April 4, 1964
2. Leakey, Richard, The Origin of Humankind, pp. x-xv
3. Bernard Wood and Mark Collard, “The Meaning of Homo” in Ludus Vitalis, Vol. IX, Number 15, 2001
4. Lewin, Roger, Human Evolution: an Illustrated Introduction, p. 149
5. Cladograms of various kinds tracing the phylogeny of the hominids may be found at the following locations:
Access Science: http://accessscience.com/... [Subscription may be required]
The Smithsonian National Museum of Natural History:
Darwiniana and Evolution:
An unusual site that nonetheless displays about two dozen different cladograms:
6. B Wang and J Brand-Miller, “The role and potential of sialic acid in human nutrition” in European Journal of Clinical Nutrition (2003) 57, 1351–1369
7. Hsun-Hua Chou, Toshiyuki Hayakawa, Sandra Diaz, Matthias Krings, Etty Indriati, Meave Leakey, Svante Paabo, Yoko Satta, Naoyuki Takahata, and Ajit Varki, “Inactivation of CMP-N-acetylneuraminic acid hydroxylase occurred prior to brain expansion during human evolution” in PNAS, August 21, 2002
8. The Cambridge Encyclopedia of Human Evolution, edited by Steve Jones, Robert Martin, and David Pilbeam, pp. 242-243; Encyclopedia of Human Evolution and Prehistory, Second Edition, edited by Eric Delson, Ian Tattersall, John A. Van Couvering, and Alison S. Brooks, p. 329-330
9. Grine, Frederick E., John G. Fleagle, Richard E. Leakey, The First Humans: Origin and Early Evolution of the Genus Homo, pp. 34-36
10. From a compilation by Steven Heslip of Michigan State University, located at: https://www.msu.edu/...
11. Lieberman, Daniel, The Evolution of the Human Head, p. 480
12. S. C. Anton, “Framing the Question: Diet and Evolution in Early Homo” in Primate Craniofacial Function and Biology, edited by Chris Vinyard , Matthew J. Ravosa, and Christine Wall, pp. 455-456
13. Joseph M. A. Miller, Gene H. Albrecht, and Bruce R. Gelvin, “Craniometric variation in early Homo compared to modern gorillas: a population-thinking approach” in Shaping Primate Evolution: Form, Function, and Behavior, edited by Fred Anapol, Rebecca Z. German, and Nina G. Jablonski, pp. 66-93
14. “Homo habilis” from the Institute on Human Origins, located at: http://www.becominghuman.org/...
15. Smithsonian, eFossils
16. Reader, John, Missing Links: In Search of Human Origins, pp. 329-332
17. Archaeology Info.com, a website maintained by two researchers who have worked at Lake Turkana in Kenya, located here: http://archaeologyinfo.com/...
18. Heslip, located at https://www.msu.edu/...
19. Sawyer and Deak, The Last Human, pp. 127-128, 119
20. Sileshi Sema, “The World’s Oldest Stone Artefacts from Gona, Ethiopia: Their Implications for Understanding Stone Technology and Patterns of Human Evolution Between 2•6–1•5 Million Years Ago” in Journal of Archaeological Science, (2000) 27, 1197–1214
21. Melissa A. Panger, Alison S. Brooks, Brian G. Richmond, and Bernard Wood, “Older Than the Oldowan? Rethinking the Emergence of Hominin Tool Use” from Evolutionary Anthropology, 11:235–245 (2002)
22. Handbook of Paleoanthropology, p. 1656
24. Ronda R. Graves, Amy C. Lupo, Robert C. McCarthy, Daniel J. Wescott, Deborah L. Cunningham, “Just how strapping was KNM-WT 15000?” in Journal of Human Evolution, 2010, 06.007
25. Robin Dennell, “’Out of Africa I’: Current Problems and Future Prospects” in Out of Africa I: The First Hominin Colonization of Eurasia, edited by John G. Fleagle, pp. 247-249
26. Dennell, p. 250
27. Willoughby, Pamela R., The Evolution of Modern Humans in Africa: a Comprehensive Guide, pp. 165-167
28. Willoughby, pp. 172-173
29. eFossils, http://www.efossils.org/...
30. Gilbert, W. Henry, and Berhane Asfaw, Homo erectus: Pleistocene Evidence from the Middle Awash, Ethiopia, pp. 355-356
31. Prins, Harald E. L., William A. Haviland and Bunny McBride, Evolution and Prehistory: The Human Challenge, pp. 182-183
32. Matthew R. Bennett, John W.K. Harris, Brian G. Richmond, David R. Braun, Emma Mbua, Purity Kiura, Daniel Olago, Mzalendo Kibunjia, Christine Omuombo, Anna K. Behrensmeyer, David Huddart, and Silvia Gonzalez, “Early Hominin Foot Morphology Based on 1.5-Million-Year-Old Footprints from Ileret, Kenya”, in Science 27 February 2009: Vol. 323 no. 5918 pp. 1197-1201