This is a chapter from my book The Emergence and Nature of Human History, Volume One. The main section of the book is a chronology of the events that led to the emergence of human 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.
Because of the tremendous importance of primate evolution in this chronology, I examine this subject particularly closely. The chapter is very long, so I have divided it into three parts.
I am not a scientist; I am a (very minor) historian and former history teacher. But I am writing a "deep history" of the world, and so I was forced to venture into subjects which are typically the province of physicists, chemists, biologists, and geologists. If I have made egregious errors of any kind, or even minor ones, please correct me.
If you are interested in reading other parts of the chronology, just look at my diary list starting with "The Beginning of the Physical Universe".
The Geography and Climate of the Miocene Epoch
By the middle part of the Miocene Epoch, about 14 million ybp, the world map had largely assumed its present form, although Florida, sections of Europe and northeast Africa, and parts of southeast Asia were submerged [indicative of a warm climate].70 A team of scientists studying the Miocene Epoch has ascertained that there were major fluctuations in atmospheric CO2 concentrations over its course, with high levels corresponding to warmer periods and low levels corresponding to greater ice cover, especially in Antarctica. Moreover, these climatic shifts had discernible effects on the animal species of the time. Most intriguingly, the shifts in CO2 seem to have affected the respective growth and recession of forest areas and grasslands, with cooler periods reducing forested areas and stimulating the spread of grasses, although the paleobotanical evidence is incomplete on this point. Herbivorous animals seem to have coevolved with the spread of grasslands, especially the ungulates (hoofed animals). The kind of grasses that dominated in the cooler, drier, lower CO2 environments are known to botanists as C4 grasses. C4 grasses evolved a more efficient way of acquiring and using CO2. In areas that were now saturated with sunlight (because of the reduction of forest canopies), they flourished. The data seem to point to a major spread of C4 grasses in the late Miocene, about 8 mya.71 If there was indeed such a reduction in the forested areas of the world, including Africa, could this have influenced the evolution of animals capable of using their bipedal abilities in both arboreal and terrestrial settings? Would a premium now be put on the ability of a primate to move with ease through both kinds of landscapes, albeit not necessarily with equal fluidity? The research suggests that the increasing encroachment of grasslands on the forested areas of the world may have had just such an impact.
At the Threshold of Humankind
Sometime in the late Miocene Epoch the first hominins evolved, and over the course of several million years an animal that can be called human arose out of them. It is impossible for us to draw a sharp distinction between our most advanced non-human ancestor and our earliest human ancestor. As we will see in the next chapter, our definitions of the boundary between them will always be, to some degree, arbitrary.
We should also keep in mind, as we examine the various discoveries of possible hominids and hominins that have been made, that we cannot infer that they all have a phylogenetic relationship with the others. In other words, we cannot assume that just because we can present these finds in chronological order that we have established a genuine lineage. That job is still very much a work in progress.
Discovered in Chad, six different specimens have been found. Based on a comparison with the remains of other animals in the area, these specimens are believed to be at least 6 million, and possibly 7 million years old. Those fragments recovered include a cranium, with a relatively small brain case, a narrow basicranium, and a prominent supraorbital torus (a bony ridge that forms a prominent brow above the eyes), among other features. The animal appears to have had a small chin, as well. Several of the features of Sahelanthropus appear to differ from those of modern apes, particularly the smaller canines, and various structures of the face (with different features standing in contrast to those of both particular modern apes and extinct hominoids). Sahelanthropus appears to have marked similarities to other primates thought to be hominid, and its discoverers claim hominid status for it. If the analysis of these remains is correct, it is the earliest hominid yet discovered, a mixture of both primitive and derived features. The discovery is noteworthy for another reason as well: it was made more than 2,500 kilometers from the Rift Valley of East Africa [see below], the region believed by many to have given rise to the hominid line. A hominid that long ago and that far west may cause us to reevaluate our conceptions of the path that led to human evolution.72 This animal is also known as Toumaï'.
Announced in 2001, prior to the publication of the Sahelanthropus find, Orrorin was declared the earliest hominid yet discovered. Found in Kenya and dated at 6 million ybp, 13 fossils from what were believed to be five individuals were recovered. The specimens include parts of the animals’ jaw, dentition, femora, and humerus. Analysis indicates that the structure of the femur was that of an animal capable of bipedal walking on the ground while the structure of the humerus indicates an animal that retained the ability to climb. Orrorin is thought to have been the size of a modern chimpanzee. Its discoverers claim that its mixture of primitive and derived features implies a split in the ape-hominin line prior to 6 mya.73
The first specimens of the genus Ardipithecus were discovered in 1992 and announced in 1994. The new genus and species, discovered at Aramis in the Middle Awash region of Ethiopia, was designated Ardipithecus ramidus. After its announcement, an extraordinary fifteen year-long project of excavation and analysis followed, necessitated by the extremely poor physical condition of the specimens. (See below.)
From 1997 to 1999 a set of primate remains discovered in the same region were identified as those of an ardipithecine. In 2001 the animals from which these specimens came were designated Ardipithecus kadabba. The fossil fragments of kadabba were dated at approximately 5.5 to 5.8 million years of age. The remains included a great many teeth and samples of mandible, ulna, toes, fingers, humerus, and clavicle. Kadabba was an animal generally more primitive in many respects than the australopithecines (see below).74 Whether kadabba was bipedal or not, and whether it was part of the common ancestry of humans and chimpanzees or a post-split hominid, has not yet been definitively ascertained.
However interesting researchers have found kadabba, by far the greatest excitement and interest have surrounded ramidus. After an epic feat of removal, reconstruction, and analysis by means of the most sophisticated technology available, Ardipithecus ramidus was formally described in 2009. The remains are a spectacular find, and an entire issue of the journal Science was devoted to their description. Ramidus was an animal of the Pliocene Epoch [around 5.3 to 2.6 million ybp]. The fossils were found in a layer of volcanic ash dated to 4.4 million ybp. The specimens, more than 110 of them, were from numerous individuals, with extensive post-cranial remains of two. There were enough remains, in fact, to reconstruct much of an entire individual. What emerged from the findings is a possible ancestor of the genus Homo.75
The skull of Ar. ramidus resembles that of Sahelanthropus, although somewhat smaller. The difference in size may be due to the fact that the specimen is from a female. The face was less prognathic [characterized by a jaw that jutted out past the facial plane] than that of modern African apes. The teeth of ramidus suggest that it was an omnivore, and its dentition is markedly different from that of modern apes. In particular, the upper canine teeth are less prominent.76
The most complete set of remains is a partial skeleton designated ARA-VP-6/500. It was probably a female. Standing on two feet, its height would have been about 120 centimeters, or about 47 inches. Its body mass is estimated to have been about 50 kilograms, or around 110 pounds. Although this skeleton does not include the humerus, comparisons with other Ar. ramidus humeri indicate an animal that was not very different in size from the males of its kind, in contrast to the sexual dimorphism (differences in size) that developed in australopithecines. The individual’s hind limb was primitive, with an opposable big toe. However, other structures in the feet provided stability. Hence the foot was flexible and capable of grasping while being able to help propel the body. It was a foot adapted for bipedal movement, when required. The individual’s leg structure was typical of quadrupedal primates, but its arms were proportionally shorter and its legs proportionally longer than modern apes. ARA-VP-6/500 seems to have been an animal capable of either walking or clambering around in the branches. Tellingly, its hands lacked the adaptations characteristic of knuckle-walking. The authors of the study describing Ar. ramidus make these conclusions about it:
The adoption of bipedality and its temporal association with progressive canine reduction and loss of functional honing now constitute the principal defining characters of Hominidae. The orthograde positional behaviors of hominids and apes were thus acquired in parallel, generated by early bipedal progression in the former and suspension and vertical climbing in the latter. Overall, Ar. ramidus demonstrates that the last common ancestors of humans and African apes were morphologically far more primitive than anticipated, exhibiting numerous characters reminiscent of Middle and Early Miocene hominoids.77All the ardipithecines recovered so far are from Ethiopia, not very far from where significant discoveries of other hominids have been made. Ar. ramidus appears to have lived very close, chronologically, to the split between ancestral chimpanzees and hominins (by some calculations). Are the ardipithecines ancestral to the next major group we will examine, the australopithecines? And if the australopithecines were the line that gave rise to the humans—a proposition about which there is disagreement—could we say that we can now trace our heritage back at least 4.4 million years, and possibly 5.8 million?
The name Australopithecus was first suggested by anthropologist Raymond Dart. It was the name he gave to a small hominid skull that had been discovered embedded in dolomite in the Taung quarry in South Africa in 1924. The specimen was shipped to Dart, who extracted it from the rock, analyzed it, and formally described it in a paper of 1925. Dart called it Australopithecus africanus—the “southern ape of Africa”.78 This discovery, widely hailed at the time as the “missing link” (an unfortunate term that paleontologists try to avoid using) helped reestablish the view that Africa was the ancestral home of the hominids (as opposed to those who believed Asia to have been our place of origin). Since the 1920s, a series of major finds has convinced many researchers, but not all, that one of the many lineages of australopithecines is probably ancestral to our genus.
There is disagreement about the proper taxonomic classification of the australopithecines and the terminology associated with it. Many paleontologists and primatologists (such as John Fleagle) place the ardipithecines, australopithecines, and a genus known as Paranthropus (see below) into a broad subfamily of Hominoidea called Australopithecinae.79 Other researchers prefer a taxonomic scheme in which the subfamily Homininae is divided between the Tribe Panini, which gave rise to the genus Pan (the chimpanzees), and the Tribe Hominini, which is first divided into a subtribe known as Australopithecina. Australopithecina, in this view is divided into the following genera: Ardipithecus, Australopithecus, Orrorin, Paranthropus. Most interestingly, in this taxonomy, the second subtribe that evolves out of Hominini is simply called Hominina—the genus Homo. The implication is that humans share a common ancestor with the australopithecines but are not evolved directly from one of the australopithecine lineages.80 As we will see, this latter view is not universally shared.
Part of the difficulty in reconciling the various ideas that have been offered is semantic. For example, the term Australopithecus afarensis (see below) might be rejected, seemingly removing a key australopithecine from our ancestry. Yet, in a given taxonomic scheme, what is known as afarensis might be placed into a different genus. For example, the name Praeanthropus has been proposed by some, and afarensis renamed Praeanthropus afarensis—a possible ancestor to both humans and a more restricted group of australopithecines.81 Different views can lead to the use of different terms.
The australopithecines are spread out over three million years in time, and many species have been identified. But there are some general traits and tendencies that stand out. First, australopithecines were bipedal. They may not always have been fully upright, and their gait may not have been fully human-like. But the evidence is clear: when they had to move on the ground, they walked on two feet to do so. Additionally, there is evidence that over the centuries australopithecine bipedalism evolved in the direction of distinctly hominin bipedalism, an evolution indicated by changes in the hip and femur. Many of the earlier australopithecines retained the ability to move about in arboreal settings as well. Second, australopithecines had average brain capacities ranging from about 420cc to about 530cc, and encephalization quotients ranging from 2.5 to around 2.7. Moreover, the shape of the brain in certain australopithecines hints at changes in brain organization. Third, there was a general reduction in the size of the premolars and molars over time.82 There seems to be, in short, a set of traits in certain australopithecines that we can recognize as being similar, albeit not identical, to those of our own genus.
So let us examine the major australopithecine-like finds that have been made, and weigh the evidence that paleoanthropologists have gathered about their possible role in our evolution.
Australopithecus anamensis (4.2 to 3.9 million ybp)
Discovered in Kenya at Kanapoi and Allia Bay, the specimens include pieces of both maxilla and mandible with dentition, some isolated teeth, some fragments of skull, pieces of tibia, and part of the knee joint. The tibia and knee confirm anamensis’s bipedalism. Could anamensis have been descended from Ardipithecus? Given the time frame in which anamensis lived, and the significant anatomical differences between it and Ardipithecus, it does not appear likely.83 There is, however, evidence that anamensis is related to another australopithecine species, afarensis (see below). The oldest specimen of afarensis yet discovered is 3.6 million years old, and until recently the 300,000 year gap between it and the most recent anamensis remains had not been bridged. The discovery of australopithecine remains in the Woranso-Mille area of the Afar triangle in northern Ethiopia, specimens dated between 3.8 and 3.6 million ybp, appears to have bridged this gap. [Not all agree.] These remains have a mixture of anamensis and afarensis dental and jaw features. Since, in the view of some researchers, there are no significant anatomical differences between anamensis and afarensis, they do not see the two as separate species at all, but just the earlier and later types of the same lineage.84
Australopithecus afarensis (3.6, possibly 3.7 to 3.0 million ybp)
The first specimens of this (then unnamed) species were unearthed by a team led by paleoanthropologist Donald Johanson at Hadar, in the Afar region of northern Ethiopia, in 1973. However, the most famous example of this species was unearthed by Johanson’s team in 1974, working in the same locale. A significant part of the skeleton, about 40%, was discovered at Hadar, an extraordinarily rare find. The specimen was nicknamed Lucy, and showed unmistakable evidence of upright posture. (Johanson’s team recovered more specimens the next year, representing at least 13 individuals.) The Lucy specimen, analyzed by a group led by paleoanthropologist Tim White, contained significant samples of cranium, jaw, teeth, humerus, ulna, radius, fingers, vertebrae, ribs, pelvis, femur, and tibia. Initial analysis indicated that it was in the range of 3.3-3.4 million years of age. [It has since been determined to be about 3.2 million years old.] Lucy was a complete surprise: a little upright-walking primate with a small brain, and an apparently ape-like head. She stood 3½ feet tall, and probably weighed 60 pounds. (Specimens of afarensis from Hadar and Laetoli yielded individuals up to five feet in height and probably much heavier.) Johanson became convinced that afarensis was ancestral to the human lineage.85 In 1992 the skull of a large male afarensis was discovered at Hadar by researcher Yoel Rak. The skull was designated AL 444-2, and is approximately 3 million years of age. Discovered in about 50 fragments and reconstituted into 8 major sections, the skull is 75% to 80% complete, with a brain size of about 500cc.86 It seemed that afarensis’ place in the phylogeny leading to humans was secure.
However, the discovery of another largely complete skull, AL 822-1, unearthed in 2002, has cast some doubt on the proposition that afarensis is an ancestor of Homo. The part of the lower jaw in primates that ascends upward toward the temporomandibular joint is known as the ramus. The ramus of each species of “higher” primate is distinct, or species-specific, but broadly the configuration of the ramus falls into one of two groups: on one side, chimpanzees, orangutans, and humans, and on the other, gorillas. The ramus exhibited in AL 822-1 is clearly gorilla-like, and is most similar to that of a species known as Paranthropus robustus [often referred to as Australopithecus robustus]—an evolutionary dead-end. There are many other morphological similarities between afarensis and robustus, so the authors of the study announcing these findings concluded (in 2007):
Additional support for the phylogenetic hypothesis proposed here comes from another early hominin, Ardipithecus ramidus,[see above] whose ramus was recently unearthed at an Ethiopian site dated at 4.51–4.32 million years ago. In our analysis, the specimen's posterior [referring to the posterior section of the ramus] probability is highest with chimpanzees, at 98%. In other words, the Ar. ramidus ramal morphology is almost identical to that of a chimpanzee and thus constitutes further evidence that this morphology is primitive for the chimpanzee and human clade.87But as is always the case in paleontology, new discoveries can change our picture rapidly. In 2011, evidence of a very important afarensis find was presented: a complete fourth metatarsal, a bone of the foot. Its structure strongly suggests that afarensis’s foot had an arch, and its features in general indicate that afarensis had a foot that was strikingly similar to that of humans. If this was the case, it indicates that the species was largely terrestrial as well.88
In further response to the argument that afarensis has many similarities to robustus, several observers have pointed out that it has long been hypothesized that afarensis is ancestral to both Homo and P. robustus. In regard to the issue of a gorilla-like ramus, we have no idea what the upper part of that structure looked like in the very earliest examples of Homo. Further, afarensis shows a mosaic of human-like and ape-like features in its face and cranium. (Its dentition, however, is distinctly ape-like.) The humerofemoral index, which measures the length of the arms in relation to the length of the legs, has been determined for only one specimen. In the smallest humans, the index is about 74; in the smallest great ape it is 98; in the afarensis specimen, it was 85, almost exactly intermediate (although the femur in afarensis is ape-like in nature). The afarensis humerus is Homo-like. The hand has many human-like features (although it is not identical to ours, of course). In short, we do not yet have definitive evidence about afarensis’s phylogenetic status.89
Afarensis has been unearthed at three locations in Ethiopia, one in Kenya (Koobi Fora), and at Laetoli in Tanzania. Some 400 specimens have been discovered. If we can answer key questions about its locomotor abilities and how they evolved (whether by the piecemeal acquisition of anatomical traits or not), its paleoenvironment, its diet, and the extent of sexual dimorphism in the species over time, we may get closer to the day when we can say whether these east African bipeds were in the line that led to us.90
Kenyanthropus platyops (3.5 million ybp)
Discovered in 1998 by paleoanthropologist Meave Leakey’s team and announced in 2001, the name, translated literally, means “flat-faced man of Kenya”. The physical evidence consists entirely of a partial skull. It was unearthed on the shore of Lake Turkana, in northern Kenya. It was given the status of a new genus by Leakey. Its features bear a striking similarity to a skull designated KNM-ER 1470, which is now believed to have been an early variety of human, Homo rudolfensis.91 Kenyanthropus remains a controversial find. The skull which is the only specimen may have been seriously distorted by first an expansion and then a collapse of one of the sides. In particular, Tim White has been highly critical of the analysis of the find, and he sees it as another example of afarensis.92 Is Kenyanthropus a direct ancestor of the human line or is it a misidentified specimen? At this writing it can only be said that the question of Kenyanthropus’s phylogenetic relationship to Homo is unsettled.
Australopithecus bahrelghazali (3.5 million to 3.0 million ybp)
We have only the most limited physical evidence for this australopithecine, a single mandible and some dentition. Many researchers question whether it deserves designation as a separate species, seeing it as a variety of afarensis. The most striking fact about it is its place of discovery: Chad, well outside the east African locale of other australopithecines.93 (See Sahelanthropus tschadensis above.)
Australopithecus africanus (Perhaps as early as 3.0 million, to 2.3 million ybp)
First named by Dart (see above), all A. africanus remains have been located in South Africa. The most important sites at which examples of africanus have been located are at Sterkfontein, Taung, and Makapansgat. Anthropologist Robert Broom is notable for his extensive work on this species, especially in the 1930s and 1940s. In the literature, africanus is often described as a gracile australopithecine, as opposed to the robust variety. Gracile was originally intended to mean slender or with smaller, less pronounced teeth and facial features. The terms gracile and robust are misleading; there are some very stocky australopithecines that have been labeled “gracile” and some rather small ones labeled “robust”. In fact, the terms are outmoded, and some researchers advocate their outright discontinuance.
Africanus’s chief physical features were as follows:
-- Anatomical structures consistent with bipedalism
-- Cranial capacity: average male, 485cc, average female 428cc
-- Average height (when sex can be determined): Males, 138 cm, females 115 cm
-- Body mass (when sex can be determined): Males, 41 kg, females 30 kg94
It should be noted that the estimated brain size of africanus, while small by our standards, is of considerable size in relation to its body mass. The face of africanus was ape-like but its dentition was unlike that of modern apes, with smaller, more human-like canines, and premolars and molars similar in many ways to ours.95 In the post-cranial anatomy, africanus gives indications that its arms and legs may have been of more similar length to each other than those of afarensis, a possible indication of greater arboreality. This question, however, has not been settled. Nor have questions about whether its vertebrae, forelimb bones, or hip were more or less human-like, although the ilium appears more human-like than that of afarensis.96
The significance of africanus in the lineage leading to humans is a matter of great debate. The problem, as is the case with all of these species, is the often ambiguous nature of the physical evidence, which can be interpreted plausibly in a number of different ways. The differences between species can be very subtle as well. Africanus had an unusual mixture of traits, but such features as its human-like dentition and the human-like curve of the jaw may be of great significance. Indeed, there is a group of paleoanthropologists who have reached an intriguing conclusion. In their view, the population ancestral to humans underwent a split into two or possibly more lineages about 2.5 million years ago. Many are ready to say that the lineage that led to us was derived from one of the later variants of africanus, and that africanus is the common ancestor to three kinds of primate: P. robustus, P. boisei, (see below) and Homo habilis, the latter being the (probable) first true human. One particular paleoanthropologist has catalogued sixty-four features shared by habilis with either robustus or boisei, or with both. This is a very strong indicator of common ancestry. All of these features represent changes from africanus. Given the time frame and the narrow geographical area in which africanus lived, this researcher believes that a population of africanus that existed outside of South Africa is very probably the ancestor of our genus.97
Australopithecus garhi (2.5 million ybp)
A. garhi is a yet another hominid from the Awash region of Ethiopia. In the 1990s, researchers led by Tim White recovered specimens from several locations that include mandibles, dentition, partial ulnas, partial femurs, a bone from the foot, and most significantly, part of a cranium. There are indications that it might be a descendant of afarensis, but its cranial anatomy is not similar to africanus. In other words, A. garhi is not the africanus-like animal the existence of which was postulated above. Additionally, A. garhi may have used simple stone tools to get at the marrow of animal bones. There are animal remains with cut marks on them that may indicate this. As the researchers announcing this find put it, in assessing garhi’s place:
It is in the right place, at the right time, to be the ancestor of early Homo, however defined. Nothing about its morphology would preclude it from occupying this position. The close spatial and temporal association between A. garhi and behaviors thought to characterize later Homo provide additional circumstantial support.98Australopithecus sediba (1.95 to 1.78 million ybp)
The debate about human origins has revolved around several key issues: Are the australopithecines ancestral to the human line, or are they a lineage that diverged from an ancestor common to both australopithecines and humans? What is the place of Homo habilis in our lineage? Was it truly the first identifiable human genus and species or was it another variety of australopith? (See the next chapter for a more extensive examination.) Did humankind evolve in east Africa, perhaps somewhere along the enormous geological feature known as the Great Rift Valley (a 3000 mile-long complex rift and tectonic plate boundary line that runs from southwestern Asia to southeastern Africa)? Or did humans first emerge deep in southern Africa? In 2010 a discovery of potentially enormous significance was announced, one that bears on all these questions. A new australopithecine species, A. sediba, was first described. Its remains were discovered in a cave at the Malapa site in South Africa. The first fossils recovered were parts of two individuals, one apparently an adolescent male and the other an adult female. From the initial stage of the investigation of these two individuals, sections of the cranium, mandible, dentition, vertebrae, humerus, ulna, radius, hips, pelvis, knee, femur, tibia, shoulder, scapula, rib cage, fingers, and toes were recovered, meaning that we have a pretty good idea of how this primate was put together.99
The cranium, from the adolescent, is fragmented, and distorted to a small degree. It has a cranial vault capacity of at least 420cc. The cranial vault itself is ovoid in shape. The extension of the chin is just about in line with the forehead. The dental arch is in the shape of a parabola, much like ours. The teeth are small in size, even in relation to the earliest purported humans. In the post-cranial remains of these specimens, the hip, knee, and ankle are strongly indicative of an animal that was habitually bipedal. Many features of sediba set it apart from other australopithecines, most notably the face and teeth, and it resembles early examples of Homo in certain respects more than it does most other australopiths. The closest resemblance to another australopithecine is to africanus, and even here sediba is distinctive. The authors of the first major paper describing sediba did not consider it a member of Homo because of its limited brain size, a somewhat prominent brow, certain features of the cheekbones, and certain features of its dentition. But it shares more evolutionarily novel, or derived features, with Homo than any other kind of australopithecine.100
Investigation of the Malapa site has continued, and it is yielding even more spectacular results. Almost all the bones of the right hand of the first female discovered there have been located and reassembled, revealing a hand that had a unique combination of australopithecine and Homo qualities. The hand of sediba is the hand of an animal able to manipulate objects, and yet it is also a hand suitable for arboreal activities. The scientists who rearticulated it believe that sediba had a good precision grip. The anatomy of the thumb (although the thumb is longer than ours) would certainly seem to suggest so. Perhaps most significantly of all, sediba’s hand may be the hand of a tool-maker. Even if sediba did not make tools, certain features of its hands are derived enough for us to say that it could have. Certainly it seems to have been just as capable in this respect as Homo habilis (albeit in a different manner), an animal associated with basic stone tool technology. Sediba may in fact change our ideas of how a tool-making hand looked, offering us a broader range of possibilities.101
Since 220 specimens have already been recovered, and at least five individuals identified, there will no doubt be many more revelations about sediba to come. Sediba is not a member of Homo. But the leader of the team investigating and analyzing it, Dr. Lee Berger, had this to say about its possible role in hominin phylogeny:
The fossils have an overall body plan that is like that of other Australopiths – they have small brains, relatively small bodies and long and seemingly powerful arms. They do, however, have some features in the skull, hand and pelvis that are found in later definitive members of the genus Homo but not in other Australopiths. However, given the small brains and Australopith-like upper limbs, and features of the foot and ankle, the team has felt that keeping this species in the genus Australopithecus was the conservative thing to do. Nevertheless, sediba is turning out to be one of the most intriguing hominins yet discovered, and it certainly shows a mosaic of features shared by both earlier and later hominins… Our study indicates that Australopithecus sediba may be a better ancestor [than habilis] of Homo erectus …102Finally, there is the electrifying possibility that some of the skin and hair of these primates has been preserved, meaning they may contain accessible proteins and perhaps even DNA. If this turns out to be the case, it will be the first skin of an ancient hominin ever recovered, and it may give us a deeper understanding of the evolution of our own genus.103
The Side Branches: Paranthropus aethiopicus, P. boisei, and P. robustus
Three primates that were once placed inside the genus Australopithecus have been recategorized. They are still seen as part of the broader clade of australopithecine types, but they have been placed in their own genus: Paranthropus. As far as we know, they left no modern descendants. They appear to have been of monophyletic origin. P. aethiopicus is the most poorly documented in the fossil record, while both skulls and fragmentary examples of the post-cranial anatomy have been recovered for boisei and robustus. Some researchers believe that although they were bipedal, they retained a significant capacity for arboreal living. Aethiopicus (as one might gather from the name) and boisei have been found in east Africa, while robustus was an inhabitant of southern Africa.104 There are researchers who question whether Paranthropus robustus was in fact an obligate biped, based on an examination of the (limited) samples of hip and vertebrae recovered. They surmise that robustus, living between about 2 million and 1 million ybp in a climate very much like the one found in South Africa today, may have engaged in four-legged locomotion at certain times. Intriguingly, there is some evidence that robustus may have used objects as simple tools, and may even have used fire at times.105 Boisei had a large skull and a prominent jaw, with large molars and premolars. With a braincase that averaged around 500cc, its brain was apparently large relative to many australopithecine types. Boisei’s skull had a very prominent sagittal crest, a ridge of bone at the top of the skull running from front to back. Boisei remains have been found in Kenya, Tanzania, Ethiopia, and Malawi, and it appears to have flourished in those regions between 2.3 million and 1.4 million ybp.106
Other Developments in Mammalian Evolution in the Cenozoic Era
Many mammalian lineages underwent major diversification and adaptive radiations in the ages following the death of the non-avian dinosaurs. Mammals, which originally had been small in size and rodent-like, ultimately evolved lineages such as that which produced Paraceratherium, an enormous, hornless rhinoceros that lived in south Asia from around 20 to around 30 million ybp, the largest land mammal that has ever lived. It is gone now, but other mammalian lines would ultimately play a major part in the story of the genus of upright primates that was evolving in eastern and southern Africa.
More than 50 million years ago, the first members of an order called Proboscidea evolved. The elephants of Africa and India are the largest carriers of their heritage today.
Some 55 million years ago, a small mammal, dog-like in size, known to scientists as Hyracotherium first evolved. Many millions of years later, its descendants lived in the open plains, and selection pressures on them began to favor size and speed. By 4 million years ago, about the time Ardipithecus ramidus existed, the most famous genus that evolved from this line had come into being—Equus. The equines were then about the size of ponies. Their path and that of the upright primates were to intersect much later—and change the world.107
About 50 million years ago, along the shores of what is now Pakistan, there lived an animal about 650 pounds in weight, with huge feet, an animal capable of walking or swimming, as required. Known as Ambulocetus, its descendants now make up the whale population of the planet Earth.108
Some 60 million years ago, an order of mammals evolved that gained its nourishment exclusively from the flesh of other animals. The order Carnivora, with its deadly teeth, had arrived. It spawned many distinct families. Two of them, Canidae and Felidae, were ultimately the producers of the hundreds of millions of dogs and cats that roam the world’s wild lands or inhabit the homes of its consciousness-bearing primates.109 Those primates’ ancestors feared the cats (with good reason), and may have first banded together to defend themselves from these fearsome predators. The humans tamed the wolf-dogs only over a very long time, and bent them to their purposes. The humans, through breeding, were to transform the felines and canines into a multitude of diverse types.
All of the other mammals that humans ultimately domesticated, the cattle, the sheep, the camels, the pigs, and the rest, all have their roots in the tens of millions of years prior to the evolution of the hominins. It was the emergence of consciousness that was to give the humans their mastery over them. The other mammals would serve as beasts of burden, sources of milk and meat, sources of leather and wool, and sources of fuel and fertilizer for the last hominins left standing—Homo sapiens sapiens.
In sum, what we can finally say is that eventually, from one of the families of terrestrial primates, there emerged a uniquely specialized group. The primates of this group moved about the landscape by walking in it, as did other types. They possessed hands capable of grasping and examining objects, as did their relatives. In all likelihood these primates lived in groups and acted to protect each other, just as the other small packs of primates did. They were unimpressive at first, but they possessed a crucial advantage: they had the most advanced brain that had ever existed in the animal kingdom. When they looked about the world, they did not merely perceive it as just a mass of physical sensations. It was a world of causes and effects, a world of objects, a place, a place existing in time. There had emerged an animal capable of not just being shaped by the world, but of shaping it in turn. Many animals had, of course, shaped the world unintentionally. But this animal was different:
Sometimes it would shape the world because it chose to.
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