When viewed up close, the Australopithecus-Homo transition has always been murky. Daniel Lieberman, Nature 449:291 (2007)
In a world where more than half the population live in cities, and reality is defined by roads, concrete buildings, supermarkets, electricity, other human beings – by what we have created ourselves – it is easy to forget that man is an animal distinct from all other animals. We have subdued the world to such an extent that wild nature is excluded from experience, and re-enters only through the portals of a cultural system that processes it into entertainment. Nonetheless, our otherness requires explanation. One explanation is that we have always been different. Another is that we evolved from the animals that most resemble us, the great apes.
- finger nails, an opposable thumb and a big toe
- stereoscopic, colour vision
- two mammary glands located on the breast
- usually single offspring
- a large brain compared to body size
The list could be longer, but however long it is, the striking thing is that the features are not unique. Opposable thumbs also distinguish the opossums; birds of prey have stereoscopic colour vision; elephants have fingernails, two pectoral mammary glands and single offspring; and mice have a large brain-to-body-mass ratio. Although also not unique, the main specialisation of the primates is arboreality – the ability to live in trees.
So is man just an ape who left the trees, or does he possess features sufficiently distinct, as a package, for him to be differentiated from the primates? Similarities include the lack of a tail, the ability to move the arms above the head, a broad chest, a short lower back, more flexible hips, ankles and thumbs, and a more erect posture. But similarities may or may not be evidence of an evolutionary relationship. Differences are also important, especially if the interval in which they are supposed to have arisen is short. The principal differences are:
|1||Hemispheres of brain symmetrical and equal in size||Hemispheres of brain asymmetric, left hemisphere longer and shorter than right|
|2||No neck, spine joins skull from back; extensive muscular connections between head and shoulder; muscles attached to nuchal crest stabilise forward-oriented head||Distinct neck, spine joins skull from below; no nuchal crest; head stabilised during running by nuchal ligament (not present in apes)|
|3||C-shaped spine for walking on all fours; sacrum narrow, straight and parallel to spine; typically four lumbar vertebrae||S-shaped spine for upright stance and walking; sacrum broad, curved and tilted; typically five lumbar vertebrae, larger than in apes|
|4||Blades of iliac crest (upper part of hipbone) flat and parallel to each other||Blades turned backward and projecting to increase leverage of spine muscles|
|5||Can stand only with legs bent; hip joints do not extend fully upright and pelvis is broad||Hip joints extend upright; large buttock muscle with different origin and insertion aids running|
|6||Small semi-circular canals in inner ear||Large semi-circular canals in inner ear, for balance in the vertical plane|
|7||Arms longer than the legs||Legs longer than the arms|
|8||Chest funnel-shaped||Chest barrel-shaped|
|9||A weak relatively short thumb, long curved finger bones and long palm||Strong mobile thumb (3 additional muscles) connected to wrist by saddle joint; finger bones straight; apical tufts|
|10||Feet like the hands, with opposable big toes for grasping; apes usually walk on all fours||Feet unlike the hands and used for walking rather than climbing; big toe not opposable; large heel|
|11||Knee joints do not lock upright||Knee joints are bigger and lock upright|
|12||Short Achilles tendon; medial arch of the foot is low||Long Achilles tendon and high medial arch for running|
|13||Hairy body for heat insulation, no female pubic hair or male top lip; nails grow quickly||Constantly growing hair on top of head; hairy male face, pubic hair, elsewhere hair is fine and sparse; nails grow slowly|
|14||90% fewer sweat glands than humans||5-10 million sweat glands|
|15||30 facial muscles for expression and recognition||54 facial muscles for complex social relationships|
|16||Sclera of the eye (surrounding the iris) is brown||Sclera is white and larger; eyes horizontally more elongate|
|17||Larynx and vocal cords capable of only simple sounds||Larynx and vocal cords capable of complex sounds|
|18||Ears sensitive to frequencies of 1-8 kilohertz||Ears sensitive to frequencies of 2-4 kilohertz|
|19||Ovulation (fertility) apparent||Ovulation concealed|
|20||No hymen in female; males mate from behind||Hymen; vulva at 45° permits frontal mating|
|22||Male has a penis bone (baculum) for erection; female has a clitoral bone.||Penis boneless, erection achieved by blood; no clitoral bone|
|23||Birth of young easy and rapid; broad pelvic canal||Birth of young painful and slow (may have been easier when crania were smaller)|
|24||Mammary glands purely for milk production; female breasts small||Female breasts sexually sensitive; enhanced by fat|
In total, these differences represent a high mountain for natural selection working on DNA copying errors to climb, to say nothing about mental and spiritual differences. It is often claimed that in their genetic make-up chimpanzees and humans are very similar. Were that so (recent findings indicate otherwise), the similarity in genes would be paradoxical, for anatomically there is a gulf. The anatomical facts do not support the supposition that apes and humans as known today belong to the same family tree.
Differences capable of showing up in the record of fossil bones may, of course, have been smaller in the past and have diminished back in time towards a common stem. Or the two groups might not be related at all, whether or not the differences were smaller. That remains to be seen. As often, the reader will need to put a little effort into mastering some new terms (mostly species names). However, do that and by the end of this chapter you will be equipped to form your own judgement as to whether apes and man are related. Whatever your starting position, be prepared to change your mind, for the story is not as generally presented.
Perhaps the best introduction to the fossil record is by way of a diagram such as the one below (Lieberman 2009, adapted), showing the species considered relevant to the story of man’s evolutionary prehistory and their suggested relationships.
As can be seen, humans (blue in the diagram) are not thought to descend from modern-day apes but from the australopithecines (light and dark orange). While the differences between extant humans and apes are certainly relevant, we are not retracing the history of both lineages when we go back through the fossil record. We are comparing human beings with apes that are long extinct – possibly as a result of human beings hunting them to extinction. That is a handicap, in addition to the poverty of the fossil record. Gorillas and chimpanzees are irrelevant to the question of man’s immediate ancestors, and in any case have no fossil record at all across the period 7-0 Ma, with the solitary exception of one mid-Pleistocene chimpanzee.
This fossil consists of a jawless skull reconstructed from tiny fragments. Niklas and Kutschera ranked it among their top ten transitional fossils, characterising it as a mosaic of chimpanzee and human features, and it epitomises the whole debate – enormous significance read into evidence that is predominantly missing, great media fanfare at the time of its discovery, academic disagreement thereafter. A mosaic of features from distinct lineages does not make for an ideal ancestor, and in this case the specimen had more features that were not chimp-like than that were, such as thick continuous brow ridge (thicker even than in gorillas), no muzzle, relatively thick tooth enamel and low rounded teeth cusps. The animal may have been able to support itself on two legs, though this has been disputed (Wolpoff et al 2004). The scientific report of the fossil neglected to mention that it was found out of context, on desert sand [photographed here]. It is dated to 7.2-6.8 Ma.
This taxon, from the Afar region of Ethiopia, is much better known. It lived in woodlands, moved along the branches with its palms and was to some degree bipedal. It had very long arms and, like other apes, had an opposable (divergent) big toe, suited to climbing. However, according to the report (Lovejoy et al. 2009), its body-plan bore ‘little or no functional relationship to the highly derived suspension, vertical climbing, knuckle-walking, and facultative bipedality of extant African apes’. Its cranial capacity was estimated to be 280-350 cc. To some specialists, its remoteness from the chimpanzee body-plan implied it was close to the common ancestor of what later became the distinctive chimpanzee and australopithecine forms. Others argued that it was on another line altogether and such features as it shared with the australopithecines evolved in parallel. Either way, Ardipithecus is unlikely to have been the australopithecines’ ancestor, despite living at the right time (4.4 Ma). No one knows where the australopithecines came from.
In fact, what constitutes an australopithecine is itself unclear. Australopithecus is not a genus in the conventional sense, comprising all the species of a certain grade that are most closely related to each other (MacLatchy et al. 2010). The most important inferred attribute is bipedality, the ability to walk on two legs. This can be facultative, habitual or obligate. ‘Facultative bipedality’ means an ability to walk on two legs in short bursts, and seems to have been the characteristic mode of the australopithecines. But the degree of bipedality is difficult to assess, since the relevant pelvic and foot bones are often either missing or not found with the rest of the skeleton. The overall body plan was ape-like, not human. The relatively high arm-to-leg-length ratio and forearm-to-humerus length ratio suggest they spent most of their time in the trees, where, like orangutans, they moved along branches with an erect posture as their hands obtained additional support from the branches above. Habitual hand-assisted bipedality in trees – a credible scenario for at least some of the australopithecines – should not be confused with habitual free-standing bipedality on the ground, for which the australopithecines show no clear evidence.
Take, for example, the semi-circular canals of the inner ear, important for balance. While Lucy herself is virtually headless, the semi-circular canals of another partial skeleton attributed to afarensis resemble those of extant great apes more than modern humans (Alemseged et al. 2006). Lucy’s skeleton includes hardly any foot bones, but toe bones (AL 333-115) from relatively nearby show the curvature characteristic of arboreal apes. An arboreal habitat is also indicated by curved finger bones and an upwardly oriented shoulder socket, but again these are not part of AL 288-1, so we do not know that these belong to the same species. Conversely, the fourth metatarsal AL 333-160 is indistinguishable from that of a human foot, and the only reason it is attributed to an australopithecine called afarensis is because ‘A. afarensis is the only hominin known from Hadar’ – not the best of logic, considering that the region has thrown up quite a few human-like bones, to say nothing of the Lucy skeleton itself. Heel bones from the site also have a human morphology. Only when AL 288-1 is supplemented with the head of an ape from elsewhere in the region (on the basis that ‘afarensis is the only hominin known from Hadar’) does ‘A. afarensis’ become much less human-like. New finds, nonetheless, are beginning to challenge the belief that all bones referred to afarensis represent a single species, albeit not that Homo was around at the time (Haile-Selassie et al. 2016).
There is no consensus as to which extinct ape species might have given rise to Homo. The ancestor proposed in the diagram mainly because it seems chronologically intermediate, is Australopithecus garhi, from the Awash Valley, dated to c. 2.5 Ma ago. The remains consist of teeth and a partial cranium, enough to infer a cranial capacity of about 450 cc but not enough to make any confident link with Homo, or even Australopithecus. What we have is distinctly non-human. It had a sagittal crest at the top of the skull for anchoring large chewing muscles, its upper jaw (unlike that of Sahelanthropus) jutted out prominently, and although the dental arcade was U-shaped, the molars and canines were huge (three times the size of modern human teeth). A few hundred metres distant lay an antelope bone with cuts and percussion marks made by tools. The discarded femur of a horse also bore cut marks. Large mammals at the lakeside site were being butchered for meat and their long bones broken to extract marrow. Although the tools themselves were not recovered, stone cores and flakes were found 96 km to the north at Gona, where the raw materials for tools were plentiful. At the lake there were no such materials, presumably because the butchers were keeping their tools for future use. The skull itself appears smashed from the top, possibly so that they could the brains, a practice that has been documented even in modern times (Blanc 1962). Clearly textbooks need to be rewritten if Homo was the predator rather than descendant of the australopithecines.
The type species of Australopithecus is A. africanus, but it is very poorly known. The species name was coined in 1924 to describe the skull of a young human child (the ‘Taung Child’), from a quarry in South Africa. Some other material, including a hip joint showing the functionality of a habitual biped, is also human; other elements attributed to A. africanus are not and may be genuinely australopithecine. Initially the so-called ‘Little Foot’ skeleton, Stw 573, was understood to be another representative of the species, but it took 15 years to extract it from the enclosing rock and after further analysis it was assigned it to a previously almost unknown taxon, A. Prometheus. Researchers had been lumping two different species into one – probably three, A. prometheus, A. africanus and Homo. Like afarensis, africanus is a composite, a modern-day ‘Piltdown Man’, except that no hoaxer put his disjointed bones together and the palaeoanthropological community have deceived themselves. Some of the bones are no older than the earliest acknowledged human fossils.
Also contemporary with early Homo, the later australopithecines boisei and robustus developed thicker cranial walls, pronounced sagittal crests, huge teeth and huge jaws. In these respects they were on a trajectory at odds with the evolutionary trends shown by man, and some researchers side-line them to another genus called ‘Paranthropus’. Boisei changed little over the presumed more than 1 million years of its existence and appears to have eaten grass. Boisei lived in Tanzania, robustus in South Africa.
Australopithecus sediba was also an inhabitant of South Africa, and was found just 15 km from Stw 573. It represents an even greater evolutionary puzzle. Its discoverer assigned it to the genus Australopithecus chiefly because it had a small skull (albeit the preserved specimen was that of a juvenile). However, it also had a small body, at a time when human body size was both smaller and more variable. Most of its features were like those of early Homo, including those seen in the more human specimens of A. africanus; other features suggested that it was better able than modern humans to climb trees, an important source of food. It was capable of fully extending the knee during the swing phase of walking.
Homo floriensis – the diminutive ‘hobbit’ inhabitant of the island of Flores, dating to comparatively recent times – had a brain of only 426 cc, about the same size as sediba. In the light of such finds we all need to rethink what it means to be human. In any case, as with Lucy, the non-human name given to Australopithecus sediba is confusing and, as argued by Been et al. (2014), the bones may be a mixture of australopithecine and human – a disturbingly common possibility.
At the point where unambiguously human fossils appear on the scene, the record gets even more interesting – and difficult to decipher. There was no smooth transition from Australopithecus to Homo. The dotted lines in the diagram indicate a gap, due either to lack of relationship or lack of decisive fossils. The oldest acknowledged Homo is H. habilis. Fossils consist mainly of a crushed, incomplete cranium, a few teeth or a few limb bones. One of the controversies surrounding H. habilis concerns how much of the assigned material actually belongs to the taxon. If australopithecine limb bones and human skulls are mixed together, its to some extent intermediate character may be spurious. Like the cranial material, postcranial fossils are ‘highly variable and further confounded by a lack of associations with craniodental material. … Current evidence best supports the presence of more than one species’ (MacLatchy et al. 2010). The oldest sufficiently complete skulls to permit an estimation of brain volume (at 510–750 cc) date to 1.9 million years ago. The skull KNM-ER 1813 is clearly human.
Though not as changeable as some animals, Homo should not be imagined as having an ideal form, as in Greek statuary or as the species concept itself implies. At any one time, variability within species – including human beings – can be enormous, and species in their entirety evolve. They change because the environments to which they are adapted change. They must adapt. A shift in climate may result in a less wooded, grassier landscape and a consequent shift in the kind of food available, in which case the size and shape of the teeth, the jaw and the musculature around the jaw may need to be reconfigured. The purpose of the jaw apparatus is to enable efficient diet-appropriate mastication, not to facilitate differentiation between species. Man has not always been an agriculturalist, or a meat-eater, and may not always have used fire for cooking. Indeed, the size of our jaw and teeth has shrunk even into historical times, as food before entering the mouth has become more processed. Nor should we suppose that the trend is the result of those with larger jaws and teeth leaving fewer offspring.
Taxonomists (being themselves subject to classification) tend to be either ‘lumpers’ or ‘splitters’, according to whether they are disposed to interpret differences as evidence of separate species or as minor variations within the same species. Theoretically, a species is the generalised identity of all individuals that can interbreed, a possibility that cannot be determined from fossils. Until recently, whether, say, Homo erectus and Homo neanderthalensis could interbreed was a matter of judgement, with the different species names implying that they couldn’t. DNA evidence has since proved that they not only could, but did; Homo sapiens too (Callaway 2016). It may suit the palaeo-racism of anthropologists to set modern man apart by designating those ancestors that look different from themselves a different species; a succession of species may lend more credibility to the idea that man evolved from the apes than a single continually evolving species; and a new species may earns its discoverer more prestige than the variant of an existing one. But the genetic evidence indicates that even the lumpers do not lump as much as they should. The same applies to australopithecine evolution. Dissatisfied with the chaos resulting from the lack of an agreed methodology for identifying species, one researcher went so far as to call for the assigning of new names to be suspended (Quintyn 2009). The call was not, of course, heeded.
Bipedality is often regarded as the key to becoming human, but as with many other traits, bipedality is not unique to humans or hominids. Some of the earliest reptiles were bipeds. So were some dinosaurs, including the earliest. We know of at least one extinct non-hominid ape that was bipedal (Oreopithecus). Among extant facultative bipeds are several kinds of lizard, bears, kangaroos, several kinds of rodent (including kangaroo rats and mice), various kinds of flightless bird (if not all birds), cockroaches and some insects. The zebra-tailed lizard can run at up to 5.3 metres per second – 50 body-lengths per second. While all humans are bipeds, not all bipeds are humans.
On the other hand, no animal walks or runs in quite the same manner as humans. Substantially larger articulating surface areas relative to body mass in the joints of the lower body, including the femoral head and knee, the sacroiliac joint and the lumbar vertebrae, lower the impact stresses generated during walking, and help to dissipate the much higher impact loads during running (Bramble & Lieberman 2004). ‘We keep the trunk erect; in walking, our knees are almost straight at mid-stance; the forces our feet exert on the ground are very markedly two-peaked when we walk fast; and in walking and usually in running we strike the ground initially with the heel alone’ (Alexander 2004).
Apart from man, no primates have ever been obligate bipeds, none appear to have been habitually bipedal except, possibly, in the trees, and none have ever been capable of long-distance running. The living great apes are habitual quadrupeds. Whether, then, one interprets the optimal design of the human body for walking and running as showing that man has always been distinct from apes, or that he represents the final stage in a trend of increasing bipedality, depends on philosophical disposition. The fossil evidence shows no such trend. The australopithecines had a greater degree of bipedality than other apes, but they came and went; the living apes are not bipedal.
Homo‘s foot is unique and an engineering marvel, with 26 bones, 30 joints, 19 muscles, 115 ligaments and 250,000 sweat glands. But ligaments and sweat glands do not leave fossils. As regards the skeletal elements, one of the key ape-to-man transformations relates to the big toe (hallux). In apes this toe is a divergent digit designed for grasping, in man a convergent digit designed for weight-bearing, balance and propulsion. Evidence for such a transformation is lacking. ‘There is no evidence of any ape living or extinct that gave up hallucial divergence.’ (McHenry & Jones 2006) Even apes that normally live on the ground, such as gorillas, have divergent big toes.
The oldest bone fossil (distinct from tools and tracks) acknowledged to belong to Homo is a temporal bone dated to 2.4 million years ago in the received chronology. A relatively well-preserved foot from Olduvai, labelled OH 8, dates to around 1.8 million years ago. Although not preserved, the big toe seems to have been convergent, and as in humans, the foot was longitudinally arched (Berillon 2003). Arching improves the efficiency of locomotion in two ways: by acting as a spring during running and by providing rigidity to the foot as it toes off.
The oldest members of Homo to preserve both a complete hind limb and partial foot are those from Dmanisi, Georgia. Dated to 1.77 Ma and among the earliest humans known outside Africa, the Dmanisi individuals were short-statured (1.45 to 1.66 m) and had small brains (600 to 775 cc). Relative to body mass, the hind limb (leg) was longer than that of African apes and A. afarensis and within the lower range of values for modern humans (Pontzer et al. 2010). The foot was also functionally similar to that of modern humans, with the characteristic ankle-joint, mid-foot arch and convergent toe. Cultural indications – stone tools and evidence of hunting and/or scavenging – were consistent with this evidence of walking and running ability.
The oldest virtually complete skeleton, KNM-WT 15000, from Nariokotome, Kenya, dates to 1.53 Ma – a photograph appears on the website’s rotating banner. In morphology it is obviously and entirely human, and belongs to an adolescent boy about 5′ 4″ (163 cm) tall (Graves et al. 2010, who revised the age and height downwards). As with the Dmanisi individuals, he was capable of travelling long distances, and in all probability lived by foraging and hunting.
The evidence so far reviewed indicates that (1) there has always been a wide anatomical gap between apes and humans, notwithstanding that each have been continually evolving, and (2) the discontinuity corresponds with the appearance of Homo fossils around ‘2 million years ago’. In other words, the distinction between Homo and Australopithecus coincides with a difference significantly greater than that between the perceived species within those categories. In the framework developed here, the true date would be much lower. Dating systems based on radioactive decay exaggerate true time, but to a progressively lesser extent as time passes. Consequently the final 2 million years as measured by isotope dating represent much more than 2/4567ths of Earth’s total history – around one third.
The lateness of man’s appearance was proportional to the time environments took, following the end-Hadean cataclysm, to stabilise towards the conditions experienced as normal today, conditions that permitted long-distance exploration, hunting, settlement, and eventually agriculture. The extraordinary acceleration in the pace of change over the last five thousand years, recording the rise of civilisation from ziggurats to skyscrapers and the growth in population from a few million to almost seven billion, is what followed as man responded to the new opportunities. The Earth had returned to something close to the stability that existed before the cataclysm. In comparison with the rest of history, today’s slow pace of geological change and rapid pace of cultural change are abnormal.
Fossils of human beings are extremely rare when they first appear because their population was then neither numerous nor widespread. The entrance of our species is unspectacular, while extant great apes have virtually no fossil record. In the received chronology five hundred thousand years separate the earliest human skull and the earliest complete skeleton of a human – longer in real terms, probably, than the planet’s entire history. Five hundred thousand years also separate the oldest known stone tools from the first human skull. Hominids of some description, we have seen, were making tools at Gona and butchering animals with them as early as 2.5 Ma ago. The very oldest known tools, from the same region, go back to 2.6 Ma. The identity of the tool-maker has to be determined on circumstantial evidence, involving an assessment of which animal might have been capable of the technology. Palaeoanthropologists are reluctant to suppose that it could have been Homo, despite the ‘sophisticated understanding of stone fracture mechanics’ revealed by the tools, the clear association with Homo later in the record and the inability of any living ape to replicate them. The tools were used for preparing his dinner, which, at 1.95 Ma, included turtle, crocodile, fish, bovids, hippopotamus, rhinoceros (Braun et al. 2010) and probably also australopithecines. The discovery of similar tools and butchered animal bones in northern Algeria, on the other side of Africa, only adds to the mystery. Dated to 2.4 Ma, they put back man’s presence in the region by six hundred thousand years.
|Received date||Evidence of Homo|
|2.58 Ma||Oldest stone tools (Ethiopia)|
|2.42 Ma||Oldest bone fossils|
|2.1 Ma||Oldest stone tools outside Africa (China)|
|1.53 Ma||Oldest human skeleton|
In addition to bones and tools, there is also the evidence of footprints. Until recently, the oldest known human-like footprints were those at Laetoli, impressed 3.66 Ma ago on a surface of wet volcanic ash from the Ngorongoro volcano (East African Rift Valley). They precede the oldest acknowledged human body fossils by 1 million years. The pattern is clearly bipedal, and the stride length within the range of modern humans (though small). The feet were arched and had big toes parallel to the other toes.
Because of their age the prints have been much debated. Were they really as human-like as they appeared? Could perhaps an australopithecine have made the prints – the contemporaneous ‘Lucy’, for example? While it was difficult to imagine feet with curved phalanges of afarensis fitting neatly into them, some argued that they were formed in the course of a ‘bent knee, bent hip’ gait. However, subsequent experiments showed that the latter type of gait would have resulted in toe impressions significantly deeper than heel impressions, whereas at Laetoli the impressions are about equally deep. As such they are characteristic of fully erect, human-type walking (Raichlen et al. 2010) – the kind of walking the fourth metatarsal labelled AL 333-160 was capable of.
The Laetoli footprints were discovered in the 1970s. In 2016, a new set of footprints was reported from the same horizon, even more recognisably human. They were also somewhat larger. The stature of one of the individuals was put at 1.65 metres (5 feet 6 inches), far taller than the diminutive ‘Lucy’ (Masao et al. 2016). Until 2017 the oldest known human footprints outside Africa were those at Happisburgh, England, where a storm exposed mudflats dated to 800,000 years ago.
What do you do if you are convinced that Homo could not have been in existence 3.5 million years ago and you find, in the same region as that inhabited by his presumed ancestor afarensis, a human-looking skeleton dating to 3.6 million years ago? This was the problem posed by the partial skeleton KSD-VP-1/1, whose discovery in the Afar region, not far from Gona, was reported in June 2010. The solution chosen was to announce the fossil as that of an australopithecine and then characterise the Homo-like features as evolutionarily advanced. If the long-touted Lucy had to be repudiated, so be it:
The new A. afarensis specimen stood between 5 and 5 1/2 feet tall, towering over Lucy’s 3-foot height. Other fossil fragments suggested that Lucy was an unreliable measuring stick for A. afarensis, but the new fossil is the most conclusive evidence yet. Dubbed ‘Kadanuumuu’ or Big Man, it is described June 21 in the Proceedings of the National Academy of Sciences.
Big Man’s limbs also appear well-suited for running, in contrast to the shortened gait implied by Lucy’s skeleton. The proportions compare to those found two million years later in Homo erectus, and would not be out of place in a modern human, said study co-author Owen Lovejoy, a Kent State University paleoanthropologist.
“The difference between Australopithecus and humans is much less than everyone expected” said Lovejoy. “Upright walking and running were pretty advanced at 3.6 million years ago, and they didn’t change much over the next two million years. Most of the changes in that period of time took place elsewhere.”
The report itself did almost nothing to justify the attribution, comparing the fossil chiefly with two known species, A. afarensis as represented by Lucy (who, although diminutive, was a human being) and Homo sapiens. As often as not, the comparisons showed that Kadanuumuu was more similar to Homo than to Lucy, a later Homo. Over 5 feet tall, erect and capable of running, Kadanuumuu was of similar stature to the individual with large stride at Laetoli. He appears to have been just a big man, as his curious nickname signifies.
Two months later, in August 2010, came news that animal bones with ‘unambiguous stone-tool cut marks for flesh removal and percussion marks for marrow access’ had been found a few miles from Hadar and Gona. Yet more evidence of Homo in the region? Not so far as the discoverers were concerned. As they wrote in Nature, ‘The only hominin species present in the Lower Awash Valley at 3.39 Myr ago to which we can associate this tool use is A. afarensis.’ They were no doubt speaking for the whole palaeoanthropological community.