Earth-logs

A biface, Acheulean hand axe is more than object of beauty produced by exquisite skill, this industrial genre was invented by African Homo ergaster around 1.6 Ma ago, became a central feature of Palaeolithic archaeology, and lasted until the last few hundred thousand years. Nobody doubts that production of these objects implies a brain that fashioned able to visualise a complex shape within a shapeless lump of rock and to devise a way of achieving it. Moreover, its longevity spanning several species of Homo to our own shows that skills were efficiently passed down through hundreds of thousand generations: possible evidence for linguistic skills in the makers and teachers. But what was it for? Experts have been at a loss to agree on a function: too heavy for hafting to a spear; more awkward for cutting than earlier Oldowan pebble fragments; produced with careful three-dimensional symmetry when a hand tool needs none; time consuming to make yet often found in great abundance and apparently hardly used. One idea is that they were in fact for throwing, in the manner of a discus, yet broken biface axes are rare. A more appealing hypothesis is that they were made for ‘show’ as an element in human sexual selection (Kohn, M. & Mithen, S. 1999. Hand axes: products of sexual selection? Antiquity, v. 73, p. 518-526). Kohn and Mithen argued that the primary function of hand axes was to advertise a maker’s “good genes”: an indicator of the knap­per’s geographic knowledge of suitable resources; his ability to execute a plan; his dexterity and patience; and his so­cial awareness. Those are all attractive qualities in a potential mate. They also suggested that the axes’ often near-pristine quality and occurrence in great numbers at some sites indicate that once their purpose was served, they were thrown away: ‘That man is so cool, he must be good at surviving’. Ten years after Kohn and Mithen first mooted the hypothesis it has come under criticism by April Nowell and Melanie Lee Chang, of the universities of Victoria, Canada and Oregon USA, respectively  (Nowell, A. & Chang M.L. 2009.The case against sexual selection as an explanation of handaxe morphology. Paleoanthropology, v. 2009, p. 77-88).

The critique begins by examining Kohn and Mithen’s interest in symmetry as an element in attractiveness, that Nowell and Chang concede, but consider to have arisen not in a sexual context but in development of vision, despite vision being an evolutionary ‘given’ vastly older than hominins. After a discussion of how fully modern human females base their sexual choices on non-physical attributes of potential mates, such as “niceness,” intelligence, sense of humour, compatibility, willingness to work hard and evidence that the partner in question is attracted to them, Nowell and Chang examine available archaeological evidence. Much of this concerns the ‘absence of evidence’. For instance, there is no evidence to suggest that females did not make hand axes and living females in gatherer-hunter societies do make tools. Other criticisms include: the absence of hand axes from Asia until migration there by H. sapiens [but the biface axe had not been invented when H. ergaster migrated there from Africa around 1.8 Ma]; not all biface axes are symmetrical [but they are nonetheless impressive]; and axes in large numbers generally occur where prey has been butchered, as at Boxgrove, and may have accumulated by hundreds of years of use and loss at such sites by seasonal hunting. The most serious criticism is that some hand axes do show minute patterns that indicate that they were used; although most axes have never been examined for wear patterns. My own conclusion is that the critique is based on absence of evidence for biface axes as ritual objects in sexual selection, but that is not evidence of absence, and I wonder if the 10 years taken to bring together contrary evidence has a bit to do with casting doubt on a not quite ‘PC’ idea. There are many intriguing facets of the fossil and archaeological records of hominins, none more so than those which may have a cultural connotation, like ochre caches (see Deeper roots of culture in EPN of March 2009) and the tear-shaped Acheulean axe. For most we may never know their true context, but can be sure that any curiosity and imagination we apply are reflections of imaginative and curious forebears.

Homo erectus in a cold climate

The famous Zhoukoudian Cave where Peking Man, now known to have been Homo erectus, was first found in 1929 is a lugubrious place. It seems the hominin fossil remains of at least 40 individuals were dragged there and eaten, hopefully by predators. They are by no means the oldest Asian hominins at less than 1 Ma, and their ancestors, probably African H. ergaster, migrated that far around 1.6 to 1.8 Ma ago. Until this year, decent ages from Zhoukoudian were a problem: the errors on estimates of around 500 ka were too large (the likely time lies in a ‘datability gap’ between the capabilities of Ar-Ar and ¹⁴C dating methods) to see if the hominins were living at such a high latitude (40ºN) in warm or cold conditions. The latter would be of great interest as it suggests both the use of fire and clothing, and probably adaptation to cooked tubers. In fact, even in the current interglacial episode Beijing gets mighty cold in winter. However, cosmic-ray bombardment can produce unstable isotopes that are suited to dating in that gap, provided materials have been exposed to them. The fossil-containing sediments in Zhoukoudian Cave contain quartz that was exposed at the surface and washed in at the same time as H. erectus individuals were dragged in. Decay of cosmogenic ²⁶Al to ¹⁰Be and measurement of parent and daughter isotopes in quartz grains have yielded ages of 770±80 ka, somewhat older than earlier estimates (Shen, G. et al. 2009. Age of Zhoukoudian Homo erectus determined with ²⁶Al/¹⁰Be dating. Nature, v. 458, p. 198-200). This age roughly correlates with layers in the western Chinese windblown loess deposits that were deposited during the dry conditions of a minor glacial episode.

See also: Ciochon, R.L. & Bettis, E.A. 2009. Asian Homo erectus converges in time. Nature, v. 458, p.153-154. Gibbo0ns, A. 2009. Ice age no barrier to ‘Peking Man’. Science, v.  323, p. 1419.

 

Walking with the ancestors

From time to time the most evocative hominin trace fossils come to light, such as the Australopithecus afarensis footprints fount by Mary Leakey at Laetoli in Tanzania. A recent one is of footprints of a probable H. ergaster dating back to 1.5 Ma near Lake Turkana in Kenya, not far from the site of the famous ‘Turkana Boy’ skeleton of the same species (Bennett, M.R. and 11 others 2009. Early hominin foot morphology based on 1.5-million-year old footprints from Ileret, Kenya. Science, v. 323, p. 1197-1201). Not only does the trackway reveal details of flesh, skin and bones of the feet, but careful analysis of 3-D scans of the prints, in the context of the mechanical properties of the material walked upon, allows the authors to show that the person who left them moved in essentially the same way as do we when walking through soft mud. They are distinctly different from the Laetoli prints, showing arches and very distinct big toes that are so necessary for ‘springiness’ and bipedal balance respectively.

See also: Crompton, R.W. & Pataky, T.C. 2009. Stepping out. Science, v. 323, p. 1174-1175.

There has long been a pervasive aroma of eurocentrism in cultural palaeoanthropology, encouraged by the spectacular cave paintings in southern France and northern Spain that are no more than 40 ka in age and the first to be discovered. This undoubted flowering of art as we appreciate it today has been linked to much more than figurative expression. Some have argued that Homo sapiens only became fully human after Europe was colonised. Thankfully, the archaeological record is rapidly being set straight by more and more discoveries of symbolic representation from elsewhere (Balter, M. 2009. On the origin of art and symbolism. Science, v. 323, p. 709-711). Blomberg Cave In South Africa is a repository for 100 ka old inscribed ochre artefacts (Balter, M. 2009. Early start for human art? Ochre may revise timeline. Science, v. 323, p. 569), which represent symbolism of some kind and the imagined uses to which the ochre was put – ritual or cosmetic body painting? But there are tantalising objects that push art back even further. In 1999 a cache of stone tools at Tan-Tan in Morocco was found to include a 6 cm quartzite chunk that looks like a rough version of the ‘Aurignacian Venuses’ of later times, yet the find dates back to 300 to 500 ka. Something similar turned up in the 250 ka site of Berekhat Ram in the Israeli-occupied Golan Heights of Syria. Both predate the evolution of fully modern humans. And what of the tear-drop shaped biface ‘axes’ associated with H. erectus and H. ergaster as far back as 1.6 Ma? These are extremely odd objects, for several reasons: it is hard to visualise their use; many finds are in pristine condition, as if never used; to make one demands a mental model of what potentially lies within a rock; they are more difficult to make than later blade tools that are more utilitarian. Arguably, the ‘Acheulean hand axe’ may be more of a symbol than a tool.

The reason for renewed discussion in print of these matters is, of course, the bicentenary of Charles Darwin’s birth and the 150th anniversary of publication of his Origin of Species. Darwin drew a link between tool making and language in his Descent of Man. He would have been delightedly surprised to learn details of the emergence of new tool-making skills in Africa, from where he insisted we all came (Morgan, L.E. & Renne, P.R. 2009. Diachronous dawn of Africa’s Middle Stone Age: New 40Ar/39Ar ages from the Ethiopian Rift. Geology, v. 36, p. 967-970). Morgan and Renne, of the University of California at Berkeley, discovered that the oldest sites in the Main Ethiopian Rift that contain the novel tools that mark the onset of the Middle Stone Age (MSA) span a much greater interval than assumed hitherto. In one site such tools date to 276 ka, whereas at another such objects appear only at 183 ka. The more delicate work to make MSA points and blades, and a much diversified ‘tool kit’ has been called the Levallois technique, thought to have been associated with a cognitive leap from the Lower Palaeolithic Oldowan and Acheulean techniques. For some it came to signify more: the appearance of fully modern humans. But the new ages do not tally with the fossil record of H. sapiens or with estimates from mitochondrial DNA molecular clocks. All in all, culture, whether art or technology, seems to be characteristic of the genus Homo. Given a push bike, could H. ergaster have ridden it and, more important, had fun? What would a Neanderthal, male or female, have done with a tube of lipstick?

The Neanderthal genome is coming!
Some computer owners take part in the search for extraterrestrial intelligence, allowing SETI to combine their processing power with that of hundreds of others, on the off chance that the meaning of  (pi) pops up in a systematic burst of non-static microwaves. Personally I would far rather wait for a message from a relative than from some seriously weird being whose motives we might never guess. A Neanderthal lady – more precisely her leg bone –from Croatia is very close to speaking volumes about our own history. Two teams of DNA sequencers are putting the finishing touches to her genome. That it would ever happen was a fevered dream not so long ago. That it will opens up a revolution in understanding our origins. To keep in touch, read Elizabeth Pennisi’s account of the pending revelations (Pennisi, E. 2009. Tales of a prehistoric human genome. Science, v. 323, p. 866-871). Svante Paabo gave a glimpse of his team’s rough draft of the genome at the AAAS annual meeting in February 2009. When analyses are finished palaeoanthropology will explode onto the news channels, blogs, and among the twittering classes. Should SETI get a result, I would first eat my trousers and then prepare to be eaten myself. As for Darwin, maybe you have noticed his prominent brow ridges…

The period between 300 and 30 ka was critical for the evolution of modern humans. Our mitochondrial DNA indicates that fully modern humans emerged around 200 ka. Projectile weapons that help define the epoch first appeared. Clear signs of self-adornment and symbolism also turn up during the Middle Palaeolithic. All of these developments took place in Africa, and the last two are reflections of the increased efforts by archaeologists in the continent from which we all originated. There is a long way to go to match the density of sites from which later periods in human history have been outlined in Europe, but progress is accelerating. One great hindrance has been dating sites, for the Middle Palaeolithic lies in a time zone where the Ar-Ar and ¹⁴C methods are ineffective. A developing chronological ‘workhorse’ for this difficult period depends on the way in which exposure of sand grains to sunlight ‘heals’ the defects in their molecular structure formed when radioactive isotopes in soils emit ionising radiation. Artificial illumination of sand grains containing these defects causes them to luminesce. The degree of luminescence is related to the time over which the defects have built up. Optical dating relies on grains having been exposed at the surface for a time to ‘reset’ the luminescence clock, and then being buried so that new defects can accumulate. Having lots of sunlight and a superabundance of bare sand, Australia has become a hotbed of research into optical dating of events associated with its peopling during the last ice age. Expertise developed there has been applied to many Middle Palaeolithic sites in Southern Africa (Jacobs, Z. et al. 2008. Ages for the Middle Stone Age of Southern Africa: Implications for human behaviour and dispersal. Science, v. 322, p. 733-735).

Archaeological work in South Africa and Namibia has revealed two distinct stone industries in the Middle Palaeolithic, both of which made hafted weapons that would have made hunting more efficient than the whatever weapons were used in earlier times – the most distinctive of the preceding Lower Palaeolithic tools was the bifacial hand axe, whose use is obscure. Both cultures involved the earliest recognisable ornamentation, such as shell beads and materials engraved with symbols, together with indirect evidence for the use of hematite and goethite pigments for body painting (see When and where ‘culture’ began in EPN of November 2007). Genetic evidence famously places modern human origins and their global migration out of Africa within this time frame. So, dating the archaeological sites as accurately as possible is a crucial importance, and a tremendous start has been made by the multinational team lead by Zenobia Jacobs of the University of Woolangong in Australia. Optical ages span 90 to 30 ka, with clusters between 71.9 to 71 ka and 64.8 to 59.5 ka, with a statistically significant gap of about 6.7 thousand years between them. When compared with climatic-change indicators from the Antarctic ice record the developmental episodes do not seem to correlate clearly with any specific warm of cool periods, though the earlier spans the time of the Toba super-eruption in Indonesia and the later one was a period of warming. So any environmental cause for the technological and cultural changes is unclear. However, both fall within the estimated time span of the genetic ‘bottleneck’ between 80 and 60 ka, and the most likely times for the initial ‘Out of Africa’ migrations, probably across the Straits of Bab el Mandab linking Eritrea and Arabia across the Red Sea shallowed by ice-cap linked falls in global sea level.

Childhood and families

Human females are unlikely to break 10 seconds for the 100 metres because of their sashaying gait. It can’t be helped, being due to the evolution of the pelvic girdle of bipedal females to deal with birthing of infants with increasingly large heads. Supposedly, the human female pelvis is now close to the limit that will permit walking on two legs. Such problems do not plague other living primates partly because their young have small heads relative to their bulk, and pelvic anatomy is not constrained by an habitually upright gait. It seems not to have been an ‘issue’ for australopithecines either: they did not possess ‘child-bearing hips’. The intermediate species, Homo erectus, despite having a 1 Ma fossil record (maybe as long as 1.8 Ma for the Asian form) only recently provided substantial pelvic remains (Simpson, S.W. et al. 2008. A female Homo erectus pelvis from Gona, Ethiopia. Science, v. 322, p. 1088-11092). In the words of the authors, this pelvis is ‘obstetrically capacious’ and demonstrates that female skeletal evolution responded to increasing foetal brain size: it would have permitted infants with heads 30 to 50% of the adult size to have been born. Homo erectus has been widely supposed to have had a tall willowy frame analogous to that of fully modern human inhabitants of tropical savannahs, yet the Gona woman was stocky. So, environmental influences seem to have had less of an evolutionary role than the advantages of greater brain development before birth. That places H. erectus even more firmly on the human line; indeed greater in utero brain development seems to have taken place than in modern humans.

The Gona pelvis demands re-evaluation of how foetal and childhood development has progressed over the last two million years (Gibbons, A. 2008. The birth of childhood. Science, v. 322, p. 1040-1043), the unique attributes having appeared during the evolution of our own genus. Among chimpanzees, infants can fend for themselves, with a little help from elders, after 3 years old. Street children from Asia and South America need to be 6 before they can survive without parental care. Growth lines on teeth that appear week by week reveal that previous age estimates for a number of immature australopithecines whose first adult molars had erupted were large overestimates: instead of 6 they point to 4 years old. Another signal feature of human development is the lengthy period to full development (marked by the eruption of the 3^rd molar as well as the end of significant growth in stature). The average age when human child bearing begins is around 19, while chimpanzees start at about 11. A fresh examination of the famous Turkana Boy’s skeleton, an H. erectus, that uses tooth microstructure reduces his age at death from 13 to 8, suggesting an earlier onset of independence than in modern children. He grew much more quickly too, and would have reached adulthood somewhat earlier: around 14.5 years old. The picture with Neanderthals is not completely clear, some tooth studies suggest that their children grew significantly more quickly than modern ones, other studies point to the same rates or even longer development if adult brain sizes of Neanderthals are taken into account (larger on average than those of modern humans). Using average life expectancy of gatherer-hunter humans and chimps who survive dependent childhood – 45 and 70 years respectively – along with evidence for child development, suggests that australopithecines could have reached 45 while H. erectus adults could have expected to reach 60 years old.

There are other differences that begin to slot into space with the new data. Both human and chimpanzee females have a similar child-bearing period of around 20-25 years. The difference is that, on average, the natural interval between births is about half as long for human mothers as for chimpanzees. The greater number of human offspring gives a greater chance of the survival of some to reproduce themselves. On the other hand, slower child development places a greater burden on mothers, even after weaning. So there is quite a contradiction between the evolutionary effects, if only child-mother relationships are taken into account. This contradiction was resolved, to some extent, by a seminal paper in the late 20^th century by a group of anthropologists from the Universities of Utah and California (see O’Connell, J.F., Hawkes, K. & Blurton Jones, N.G. 1999. Grandmothering and the evolution of Homo erectus. Journal of Human Evolution, v. 36, p. 461-485). They focussed on the potentialities of the early onset of infertility or the menopause among women relative to its appearance among female chimpanzees, which gives, on average, a 30 year non-child-bearing period to older women. This approximately coincides not only with child-rearing periods for their daughters, but for their granddaughters as well. The ‘grandmothering’ hypothesis for human development centres on the great evolutionary advantages of post menopausal women assisting with child rearing. O’Connell et al. suggested that this arose among H. erectus, as far back as 1.8 Ma, and the Gona pelvis together with other new views of H. erectus development add considerable weight to that concept. As well as freeing younger women for food gathering, the cultural significance of older women caring for children adds another dimension that may link to the advantages of delayed post-weaning development that we see today, albeit in many annoying contexts!

Which human species first controlled and used fire has been debated for as long as archaeologists began to realise we had a long and complex ancestry. Because sites can easily be contaminated by charcoal from natural fires it has been difficult to present convincing evidence. But there is a way to get believable data. Stone tools and fragments from their manufacture may have fallen in fires set by hominins, and show changes caused by intense heating. One such example comes from a long-occupied site in Israel (Alperson-Afil, N. 2008. Continual fire-making by Hominins at Gesher Benot Ya‘aqov, Israel. Quaternary Science Reviews, v. 27, p. 1733–1739). Nira Alperson-Afil of the Hebrew University of Jerusalem investigated small flint artefacts, probably flaked off during tool making, from eight levels excavated at the site. In all of them some flint shards showed signs of extreme heating, such as discoloration, crazing and tiny bowl-shaped pits (‘potlids’) resulting from exfoliation of hot flint surfaces. The features are reproduced by experimental heating of flint shards, and do not occur in those that have not been heated above 300ºC.

Gesher Benot Ya‘aqov was first occupied around 790 ka, by Homo antecessor, and the excavation levels may span around 100 ka. The site is the earliest to provide convincing evidence not only for the use of fire, but that it was a continuous part of the hominins’ culture: they could make it at will. Alperson-Afil suggests that fire making may have been an integral part of the Acheulean culture, well known for finely crafted biface axes, since its inception around 1.6 Ma ago. Ambiguous evidence for hominin fire use, such as burnt bones and reddened sediments, has been found at several sites in Africa dated between 1 and 1.5 Ma. Alperson-Afil’s meticulous micro-forensics should help African archaeologists and those working at very old sites left by migrating hominins in Georgia and Asia to check whether fire has such a long-lived place in our evolutionary history.