Chapter 23 Humans: the emergence of the latest model

Sections

The roots of modern people

Human superhighways

Humans’ impact on their world

Apart from their occupation of Dmanisi in Georgia around 1.9 to 1.8 Ma, Europe as a whole was not the prime destination for H. erectus. The earliest sign of any hominin living in Europe  dates to around 1.2 to 1.1 Ma in the Atapuerca Mountains in northern Spain, in the form of a well-worn molar, a partial mandible and some worked animal bones that are insufficient for a species designation. More revealing fossils from that area represent five or six individuals dated to 850 to 780 ka, which palaeoanthropologists have assigned to a species named Homo antecessor that was a possible intermediary between H. erectus and more abundant later humans. Despite the fragmentary nature of the skull bones, some provide evidence of high cheek bones, different from those of H. erectus. Yet about a quarter of the bones show signs of having been defleshed; a product of ritual practices or cannibalism? A clear link to the African hominin scene is a well-preserved skull from the Danakil Depression of Eritrea dated at 1 Ma, which shows similar transitional features. Whichever species was involved, during two interglacial periods at 950 and 800 ka, represented by estuarine sediments at Happisburgh (‘Haze-burra’) in Norfolk, England, human visitors mislaid biface stone axes and left footprints in a stratum of river sediments. At 52.5°N, this is arguably the highest latitude ever reached by early hominins.

The most spectacular evidence for early European culture, though sadly without human fossils, comes from sites up to ~400 ka old on the Castillian Plateau of Spain, the Mediterranean coast of south-west France and Germany. In the headwaters of the River Ebro, at Torralba, is an ancient butchery site with the remains of at least seventy straight-tusked elephants, plus those of rhinoceros, deer, horse and extinct oxen. Nearby are twenty circular heaps of elephant long bones, with sharpened tusks that may have supported skin tents. The site is swampy, and charred branches and twigs throughout the killing ground suggest that herds of animals were driven by burning brands to become bogged down. The excavators found a few pointed stone tools, but the unusual numbers of round, heavy stones at the site may have been used to bludgeon the panicked animals to death. The sheer size of the deposit, together with the large number of dwellings suggest repeated use of the unique terrain by more than a hundred early humans. Dating of different levels in the stratigraphy suggest a 200 ka period of use.

By 400 ka yet more sophistication had appeared. A coastal site at Terra Amata near Nice, France, comprises a group of post holes, hearths and aligned stones marking out oval huts up to 5 x 7 metres in size. Imported flat blocks of limestone may well have served as seats. Among the remains are a wooden bowl and pieces of red ochre trimmed into pencil-like points, undoubtedly for art of some kind. Equally exciting are precisely crafted, wooden spears found in a 400 ka soft-coal mine at Schöningen, Germany, which are equal to modern javelins in their aerodynamics. Some even have grooved tips that might have been used to attach stone points with sinew and resin. A 500 ka old rhinoceros shoulder blade from Boxgrove in Sussex, England, has a circular hole, as if it was brought down by such a heavy spear. Whoever they were and from wherever they came, the earliest Europeans were big-game hunters, not unlike more recent humans in their life style.

Between about 1.0 Ma and 500 ka a major cultural and evolutionary advance had reached Western Europe. Although African evidence is not so rich, representative fossils of those who left it are present, although they have been assigned a non-African species name, H. heidelbergensis, from the place where the first fossil skull was unearthed and described. Very similar remains emerged from southern, eastern and northern Africa, given the name H. rhodesiensis and then had it dropped in favour of the European type specimen. It has been suggested that finds of biface hand axes in West and South Asia of the same broad age indicate even wider dispersion. Like H. erectus, these were people with similar stature to modern humans but a comparable brain size (~1250 cm3), the main differences being a significantly more robust lower body and a cranium with massive, arched brow ridges in front of a sloping forehead.

Fossils of 32 individuals from the Sima de los Huesos (‘Cave of Bones’) in the Atapuerca Mountains of northern Spain provided an opportunity for an unprecedented statistical analysis of a single hominin population. The microwear on teeth show that these members of  H. heidelbergensis displayed ‘handedness’, in fact the proportion who were right-handed is much the same as among living humans – chimpanzees are ambidextrous. The findings are thought to point towards higher cognitive functions; interesting, but perhaps premature, because similar analyses have not been conducted on earlier hominins. Nevertheless, brain capacity of these robust people significantly exceeded that of H. erectus, even being within the range for anatomically modern humans (AMH). Almost certainly the species indicates a branching from H. erectus, probably by way of H. antecessor, and most authorities consider them to be our ancestors, although, as you will see, that is now regarded as a somewhat simplified view.

In terms of stone tools these early big-game hunters were not especially inventive and used much the same ‘tool kit’ as African H. erectus, including the biface hand axe that had first appeared more than a million years earlier. This long period of little technological advance is a puzzle, considering the changes in brain size that had taken place since 1.8 Ma. Many anthropologists consider it to reflect limited cognitive change, yet such a dim view sets aside the other revolutionising advances: fire, building, large-scale hunting, and of course repeated forays well beyond Africa. In the spirit of the adage, ‘Absence of evidence is not necessarily evidence of absence’, maybe a far broader tool kit of tools made of perishable wood, skin, bone and sinew had been developed through using the biface axe as a multipurpose tool. Perhaps the hand axe may be regarded as the first machine tool; i.e. a tool for making other tools.   Beginning around 250 ka ago in Africa tools occur that were made in an entirely new way. Instead of the massive biface axe fashioned from large pieces of rock, the focus shifted to flakes prised from these cores. Flake tools (Figure 23.1) are light, razor sharp and easily trimmed into many shapes, including blades that can be attached to wooden hafts as spears, knives and perhaps arrows. The first appearance of flake and blade industries marks the beginnings of the archaeologists’ Middle and Upper Palaeolithic. Following this cultural breakthrough, fossil humans in Africa show a steady reduction in the bone mass of the skull, a general increase in skeletal lightness, steadily smaller teeth and flatter faces with emphatically jutting chins. Fully modern humans had arrived on the African scene.

F23_1Figure 23.1 Examples of Upper Palaeolithic tools

The human picture in Europe, Central Asia as far east as Tadzhikistan and the Middle East after about 300 ka ago is different. The famous Neanderthals were the only occupants who have left substantial fossils. Recognition in the early 20th century that their remains were much older and very different from those of modern humans, Neanderthals suffered a bad press. This stemmed partly from the most complete individual turning out later to have been a crippled old man with badly deformed spine and limbs, and partly a touch of racism applied to the low brow, prognathous face and small chin common to all Neanderthals. Until very recently our immediate predecessors in Europe were regarded as shambling brutes, the epitome of the ‘caveman’. Neanderthals were indeed very different from us, yet they survived at least two full ice ages at high northern latitudes. Their skulls retain the robustness of H. heidelburgensis, but with enlarged nasal passages; Neanderthals had truly  enormous noses adapted to breathing frigid air. Crania are flat and long (Figure 22.2), yet brain capacity is often well in excess of that for modern humans. Much of the brain expansion occupied its rear parts, the occipital and parietal lobes. Respectively these involve visual processing, and information storage, language, learning and memory. The parietal lobe is also a key region for intelligence. There seems to be no reason whatever to regard Neanderthals as dim. As with the toothless H. erectus lady from Georgia, the crippled Neanderthal could only have survived if he was looked after; they were caring people, at least as regards members of the same group. Although bodies were within the same range of stature as our own, the robustness of Neanderthal limb bones and muscle attachments indicate enormous body strength. Muscles seem to have been so powerful that the overexertion sometimes produced several kinds of bone injury and distortion. Geneticist Steve Jones of University College, London once remarked that if a groomed modern human from 50000 years ago in a pin-stripe suit sat next to him on the London Underground he might change seats, but he would probably change trains if a Neanderthal in City clothes leapt aboard! As you will discover later, human attitudes towards strangers have changed dramatically in the last 100 thousand years or so …

Neanderthals were well equipped and undoubtedly wore clothing, made shelters, hunted, used fire and famously lived in caves, where available. Deliberate burial of their dead, in some cases arguably with remains of flowers, indicates some form of ritual and belief system. Those in Spain wore necklaces and pendants of bivalve shells, some of which retain evidence of having been painted. The excavators even found a paint container and painting tools made of small bones from a horse’s foot. The container and tools retain traces of the common iron colorants goethite, jarosite and hematite. One large, perforated scallop shell, perhaps used as a pectoral pendant, shows that its white interior was painted to match its reddish exterior. Given the evidence for adornment by earlier hominins, to find that Neanderthals created art should not be surprising. In May 2016 it emerged that about 177 thousand years ago and earlier, they had broken stalagmites off the cave roof to create curious semi-circular structures in Bruniquel Cave near Montauban in southern France. All contain incontrovertible evidence that fires were made within them. Rather than being near the well-lit cave entrance the structures are more than 300 m deep within the cave system surrounded by spectacular stalagmites and stalactites that are still in place. Were the structures younger than 42 ka they would probably have been attributed to the earliest anatomically modern Europeans and to some ritual function. Instead they were made during the climatic decline to the last but one glacial maximum, so some palaeoanthropologists are wary of drawing such a conclusion attributable to Neanderthals. The Neanderthals’ occupancy of Europe and central Asia went unchallenged for more than 250 ka, so they were extremely successful, considering the conditions that they survived. About 30 ka ago the Neanderthals vanished from the fossil record.

The ranges of AMH and Neanderthals overlapped in the Middle East around 130 to 90 ka ago. But there are so few well-dated fossils that the data permit back and forth movements of both, perhaps with no direct contact. From 40 ka ago, there was definitely co-occupation by modern humans and Neanderthals in France. Fully modern humans probably did not gain a foothold and advantages by virtue of a more advanced technology. Their earliest tools in Europe are functionally little more advanced than those developed by the late Neanderthals, although some scientists have suggested that the latter were copied or traded from fully modern sources. An overlap of at least 10 ka is certain, so the disappearance of the Neanderthals is unlikely to have been due to genocide. So why did these superbly adapted people become extinct?  Research conducted by a team from Harvard University provides one likely answer. They studied the remains of food animals, particularly the teeth of ruminants, found at modern-human and Neanderthal sites. The teeth of ruminants continually grow to replace wear and tear. In doing so they acquire an annually layered structure, dark layers being laid down when grazing is poor in winter and light layers in summer when the grass is more nutritious. The research focussed on the last layer formed before the animal was slain and eaten. At Neanderthal sites both light and dark layers are mixed. Sites occupied by AMH have a preponderance of either dark or light final layers, rather than a mixture. The simplest conclusion is that Neanderthals occupied sites permanently, whereas modern humans had a nomadic lifestyle with shifting summer and winter hunting areas.

Permanent occupants of a site must obtain and manage food resources from a fixed home range. Nomads move from place to place, as local resources wax and wane. When they enter the foraging grounds of a sedentary population, the same natural resources must support maybe twice the usual numbers. Food stocks drop rapidly and recover slowly. The nomads move on to pastures new, leaving the locals to face reduced resources. A lifestyle focused on a home range demands very hard physical work at the best of times, whereas roaming to find rich pickings is less demanding. Perhaps the former explains the extreme muscularity of Neanderthals and the evidence of work-related injury among their skeletal remains. Yet both may stem from not having weapons suited to killing at a distance, such as javelin-like spears and bows, and perhaps they relied on lunging at large and dangerous prey animals. Bones of the modern Europeans show far less evidence of potentially crippling labour. The only surprise is that the Neanderthals lasted so long in the face of such inevitable competition for sustenance. The last Neanderthals seem to have died out 30 ka ago in rocky fastnesses of southern Spain, where fully modern humans had previously been conspicuous only by their absence.

The roots of modern people

Biological theories of human evolution are as short on data as they are long on hotly disputed ideas. Accounts that I have covered so far rest on constructing a human evolutionary chart (Figure 22.7) since the time of the last ancestor shared with chimpanzees. It relies on using imprecise times of appearance of a variety of species that are defined on limited, purely anatomical features. The chart qualitatively links beings who were culturally human – making tools; operating socially with diminishing signs of social dominance, such as sexual dimorphism and threatening canine teeth; and signs of pair bonding to maintain a lengthy childhood – with the changes in their environment and their responses to that. Divergence of genetically separate populations that may or may not have been incapable of interfertility (i.e. distinct species) must have been involved. Human morphological richness, some trends within it, and more distinct ones to do with culture emerged over three and a half million years. This took place in populations with growing advantages over other animals by virtue of their development of material culture and using it socially, together with growing consciousness indissolubly linked to these ‘social-industrial’ practices. Increasingly cushioned from purely ‘natural’ selection, genes that might otherwise have been extinguished, such as the tendency for neotonous birth and long childhood, for lighter skulls without great crunching jaws and for lithe bodies able to run long distances, survived in the population. The advantages of increased brain size permitted by these tendencies, and more sophisticated use of information-processing power would have allowed those individuals so endowed to outcompete other, less brainy individuals. Such a process points towards a unique combination in human evolution of social forces together with those stemming from environmental change and natural selection. Until the end of the first decade of the 21st century, the only real grasp that we had on the genetics of human evolution was that expressed in the cells of living people.

Molecular biologists have revolutionised knowledge of the source of all living people through examining genetic differences between individuals from modern populations that separated geographically at different times in the past. Development of methods to extract fragments of DNA preserved in fossilised bone, to amplify them using the polymerase chain reaction, to check them for contamination by DNA from other sources and to reassemble them in increasingly complete sequences has recently added the opportunity to seek evidence for relatedness that penetrates further back in human evolutionary history. But all these analytical advances by no means imply that genetic studies have simplified our ideas on the evolution of anatomically modern humans and their extinct relatives; rather the opposite.

The first fruitful approach was to compare nucleotide sequences in the DNA of human-cell mitochondria (mtDNA), which are passed on exclusively in the female line. As a result, their DNA accumulates mutations but they are not ‘shuffled’ every generation. The male line passes on Y chromosome DNA similarly. The global variability of these matrilineal and patrilineal sequences, or haplogroups, between population groups with different ancestral roots is no more than one tenth of one percent of their entire genome;. But, numerically that is quite a lot (~3 million base pairs). Most of the variation is that among different African populations and between them and non-Africans. That in itself hints at the antiquity of African peoples compared with those who now populate the other habitable continents. It is possible to calibrate the rate of genetic change by comparing haplogroups whose time of physical separation is known from dated geological evidence. Good examples are those provided by the geographic separation of the first Australians from people in New Guinea about 50 ka ago, and over the last 10 ka by the successive spread of Polynesian people throughout the Pacific. Such a molecular ‘clock’ helps link living populations in a tree or relatedness.

Increases in the speed and affordability of documenting the entire nuclear genome, from more and more individuals willing to donate samples promises to amplify the genealogy of living populations and its degree of certainty. One of the outcomes of the simpler mtDNA and Y-chromosome methods is that the ‘root’ of the genealogical tree of all living people, the source from which their genetic divergence arose (Figure 23.2), is estimated to have been about 160 and 140 to 150 thousand years ago, respectively, starting with a single African female ancestor and a single male from the same continent. This does not mean that the ancestral female was a solitary African ‘mitochondrial Eve’, the very first modern human female, but that all other women living at the same time did not begin a line of descendants that has survived to the present. The same applies to the notional ‘Y-chromosome Adam’, and the ‘Eve’ and the ‘Adam’ never met. These estimated timings for the start of genetic divergence provide a minimum date for the appearance of AMH. The oldest fossil remains of AMH are from the Omo valley of southern Ethiopia, dated at 195 ka. But whether its appearance marks a rapid speciation from an earlier form of human in that area or a gradual transition will probably never be resolved without more fossil evidence.

F23_2Figure 23.2 (a) Relatedness of modern and archaic people based on DNA analyses (simplified). (b) Genetic flow between modern and archaic people. Names in the Neanderthal and Denisovan fields relate to sites where analysed samples were found.

Studies of mtDNA helped resolve a major dispute. Had modern humans in Asia, Europe and Africa evolved separately from the earlier inhabitants of those regions, such as the Neanderthals, H. erectus or variants of either, the genetic patterns in each area would show clearly separate groupings. The fact that there is a narrow continuum among the living individuals who were tested is powerful evidence against such a ‘multiregional evolution’ hypothesis suggested by Milford Wolpoff in 1984. His ideas were in response to an earlier model suggested by Steven Jay Gould and Niles Eldridge from which emerged the ‘Out of Africa’ model, which mtDNA studies seemed to have vindicated, but see later. Another implication is that whenever human migrants left Africa they carried language, for every living human group has that capacity. However, linguistic studies suggest that the wide modern variations of language cannot be linked in a similar way to the mtDNA evidence, despite evidence that the complexity of languages decreases with distance from Africa. They have probably evolved much more quickly than did genetics. Moreover, there are many documented instances of borrowing from and merging between existing languages on all continents, so obscuring the language group spoken by those who originally left Africa for Eurasia.

One of the greatest scientific achievements and surprises of the early 21st century has been the extraction from fossil humans and sequencing of ancient DNA by a team led by Svanti Paabo at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. So far, they have produced results from several fossils of Neanderthals, a possible H. heidelburgensis and a small finger bone that turned out to be of an unknown hominin. When compared with the genomes of different groups of living people these new data show beyond much doubt that Neanderthals successfully mated with modern humans; the greatest surprise of all. The progeny of this liaison entered the AMH population of Asia and Europe, to the extent that most Asians and Europeans host between 1 to 4% of Neanderthal DNA. Further analysis suggests that the interchange was with both male and female Neanderthals. Yet modern Africans show no sign of that interbreeding. However, the DNA of three surviving hunter-gatherer groups in West Africa contains traces of their having been sexually involved with an unknown, archaic hominin group.

The finger bone, at first thought to be from a Neanderthal, yielded DNA that is not, so the single tiny bone, for the moment, carries the burden of being from a hitherto unknown species. It was found along with more substantial Neanderthal remains in Denisova Cave, central Siberia. So for the moment the finger, more precisely the DNA it yielded, is termed Denisovan. Like the Neanderthals, the Denisovans turn out to have had intimate relations with humans, but just those migrating across the central Asian steppes. As a result, sections of their DNA comprise up 6% of that of modern Melanesian people living on islands of the West Pacific, together with some from Neanderthals perhaps acquired earlier (Figure 23.2). In 2012 a skull found in Guanxi Province, SW China, was reported to be very different from modern humans, Neanderthals or Asian H. erectus. It may prove to be Denisovan, once genetically analysed, but it is only 14 to 11 ka old.

Extraction of DNA from human fossils penetrated back to 430 ka with the publication in 2013 and 2016 of mitochondrial and partial nuclear DNA sequences from H. heidelbergensis from Sima de los Huevos in northern Spain. The nuclear DNA showed greater resemblance to that of Neanderthals than to the Denisovan, as anticipated from physiological comparisons. So when and where did the DNA sequences of the four species arise and the exchanges take place? Taking Neanderthals and modern humans alone, fossil evidence suggests a minimum age of 450 ka for their common ancestor. According to genetic comparison between a Siberian Neanderthal bone, the Sima de los Huevos H. heidelbergensis and modern humans, the population leading to Neanderthals diverged from an ancestral line to anatomically modern humans much earlier, between 550 and 765 ka. This may mark the time when the population that became Neanderthals left Africa for Europe and western Asia. It seems that they interbred with modern humans at least three times, first as long ago as 130 ka, during the previous glacial maximum, and then twice again in the climatic decline to the latest ice age. There is fossil and archaeological evidence from the Middle East that the two populations lived in close proximity around the first instance, but no such coincidence for the later liaisons. The Denisovan – modern East Asian/Melanesian connection seems to date to about 45 ka in eastern Asia. Comparing mtDNA from Neanderthals, H. heidelbergensis and the Denisovan points to their sharing an ancestor before about 500 ka, at which time Denisovan descent split from that to the first two (Figure 23.2).

Interbreeding among at least three groups over tens of thousand years and inheritance of their genes by modern human populations raises the possibility that all were, in a practical sense, members of the same species, albeit having been genetically separated for around 700 ka. There is even evidence for a fourth, ‘mystery’ human having contributed bits to Denisovan DNA, circumstantially ‘fingering’ the H. erectus population known to have inhabited east Asia until about 27 ka ago. Moreover, all these admixtures did not prove fatal to their offspring. Had it done so their genetic signatures would soon have disappeared from the modern human genome. Their outcomes were a mixed bag of pros and cons.

The human leucocyte antigen (HLA) group is a vital part of our immune system in the form of ‘killer cells’. Part of modern Eurasian DNA that codes for the group appears in the Neanderthal and Denisovan genomes. Indeed more than half the HLA alleles of modern Eurasians may have originated in this way, and could also have been introduced into Africans subsequently. This legacy would have bolstered the resistance of migrants from Africa to Eurasian pathogens. Neanderthals passed on another 15 genome regions to Eurasians with mixed blessings, which include those involved in energy metabolism, possibly associated with type 2 diabetes; cranial shape and cognitive abilities, perhaps linked to Down’s syndrome, autism and schizophrenia; hair and skin pigmentation; and barrel chests. Neanderthal ancestry may have helped enhanced blood coagulation, so assisting recovery from wounds and haemorrhage when giving birth, but also proclivities to heart attacks and strokes. The remarkable resistance of Tibetans to hypoxia or altitude sickness, brought on by exposure to low atmospheric oxygen, may be attributable to sections of Denisovan DNA in their genome.   On the distinctly ‘down’ side we can also count ‘weak bladders’; solar keratoses that confer skin cancer risk; a tendency to malnutrition from modern diets low on meat and nuts; depression; and even a tendency to nicotine- and other addictions. But even a ‘pure’ line of modern human descent, shared by most Africans, has its positive and negative heritable traits. There is surprising evidence that modern human males are missing stretches of Neanderthal DNA on their Y chromosomes that would be expected to remain, unless subsequent evolution had stripped them out. Apparently, this ‘editing’ may have resulted from modern humans and Neanderthals being at the edge of biological compatibility. Y-chromosome DNA from a 49 ka Neanderthal contains mutations in three genes, one of which may have led to a maternal immune response to hybrid male foetuses, resulting in miscarriage. Most of the Neanderthal DNA stretches present today seems to have been passed down through the female line

Human superhighways

It is not yet possible to tie the biological changes in human evolution after 2 Ma ago to increasingly stressful environmental changes – the fossil record is too scanty and dating it with the same precision as the climate records has yet to be achieved. However, there are climatic links to humans’ most characteristic behaviour; migration. Biological change among early humans probably stems partly from their habitual migration, which may have resulted in small populations isolated from the main African gene pool. The episodic spread of arid lands during glacial epochs would also have carved up the source population in Africa into separate regional groups. While Africa was well-watered during interglacials there would be little need for humans to move to satisfy their appetite for meat. Growing aridity in the tropics as glaciers advanced far to the north would have driven grazing animals to more restricted refugia, for instance at the centre of major drainage basins, or to follow the regionally shifting vegetation belts. In the first case human populations would have become isolated from others. In the second, opportunistic humans would have had little choice other than to follow, thereby opening new possibilities in entirely new lands. Their wanderings were no longer random. Figure 23.3 shows the ranges of H. erectus, H. heidelbergensis and Neanderthals. Their early diffusion out of Africa that began at around 1.8 Ma would have been determined by tectonics, regional geography and climatic conditions.

F23_3Figure 23.3 Migrations of early humans to Eurasia. Pink arrows – likely routes followed by Homo erectus; green dots – occurrences of H. heidelbergensis fossils; orange band from Europe to central Asia – range of Neanderthals.

The first requirement for land animals and plants is a reliable source of water. The bulk of rainfall in East Africa either sinks into the sands of fringing deserts on its way to the Indian Ocean, or fills lakes along the East African Rift or flows seawards down the narrow corridors of rivers such as the Nile. When conditions became more arid topography would largely dictate the likely diffusion of game and plant resources, and the hominins following them. Movement would be coralled northwards and southwards along the necklaces of rivers and lakes of the Rift. Moving north along the Rift leads to what is now the Red Sea. Until about 1.5 Ma its southern end was closed across the Strait of Bab el Mandab, and periodically connected to the Mediterranean. Early northward migration from the Rift valley system by H. erectus would probably have crossed the Bab el Mandab land bridge to skirt Arabia to the Persian Gulf. Proceeding west from there is barred, even today, by the deserts of Kuwait, Saudi Arabia, Iraq and Syria. They would have been even more inhospitable during cold-dry episodes. Moving further east was the only option, a northern route being blocked by the Caucasus-Himalaya mountain chain and increasingly severe winters beyond ~40°N. A route following the coast of the Indian Ocean would have offered marine resources, leading directly to Indonesia. During periods of low sea level, characteristic of glacial episodes, a huge area of tropical lowlands – Sundaland – replaced the complex scattering of large islands in the south-western Pacific. Australia and New Guinea (Sahul), while linked together by exposed areas of low elevation, remained cut off by deep, open sea, so deterring migrants lacking rafts and any idea of what lay beyond the horizon.  Yet continued diffusion would lead into well watered lands of what is now China. Rising sea level during interglacials might easily have stranded inhabitants of islands in the Indonesian archipelago and others in the westernmost Pacific Ocean. The tiny, ‘Hobbit’-like people (Homo floresiensis) recently discovered on the island of Flores on the east of the Indonesian archipelago may have evolved from stranded H. erectus. Faunas of small islands, limited in resources often show signs of insular dwarfism through natural selection among a very limited gene pool.

Around 1.5 Ma ago the Strait of Bab el Mandab opened. Thereafter, although migrants would have been guided there by the Rift, the loss of the former dry-land crossing would have forced diffusion towards Egypt, either along the coast or the course of the Nile. The loss of the Bab el Mandab route perhaps helps explain why H. erectus living in east Asia remained isolated until only 100 thousand years ago. Provided the Isthmus of Suez remained dry, onward passage along the west coast of the Red Sea led to Palestine. Even today, this route from Eritrea to Suez is arid, but possesses the advantage of seafood. Nevertheless, migrants did reach the Middle East and eventually Europe, in the cases of H. heidelburgensis and Neanderthals, but archaeologists have yet to find their remains along the Red Sea coast. There is another possible northwards route  – following the White Nile from its source in the western branch of the Rift. Winding through the largest desert on Earth, the Nile has formed a narrow, watered and vegetated route to the eastern Mediterranean lands for at least 5 Ma.

The deserts of the Middle East, especially during cold-dry periods, bar passage to the east. Equally daunting to the north are the Zagros, Kurdistan and Taurus mountains rimming the Tigris-Euphrates plain. The outflow from the Black Sea to the Mediterranean through the Bosporus is impassable today without boats. But when sea level falls as ice caps accumulate the Bosporus is shallow enough for it to have been passable at the height of most of the glacial periods since about 1.2 Ma ago. Homo antecessor, heidelbergensis, then Neanderthals and finally fully modern humans colonised Western Europe, most probably during glacial epochs. Mastery of fire may have been the key to reaching Europe and staying there, given the climatic conditions that made it possible. Finds of biface hand axes in Pakistan and the Indian sub-continent, dated approximately at between 500 to 125 ka, indicate pre-modern human ventures into South Asia, but skeletal remains are not associated with them.

The earliest signs of AMH who left Africa were unearthed from the Skhul and Qafzeh caves in what is now northern Israel. Their context was that of deliberate burial at a time when climate was cooling from the last interglacial, between 90 to 120 ka. Evidence of intermittent Neanderthal occupation of the same area in Tabun cave since 200 ka includes fossils of two individuals dated at ~122 and ~90 ka. This makes the Levant a likely place for genetic exchange between the two human groups at roughly 130 ka ago as suggested by genetic comparisons. In 2015 a new date for modern humans’ first arrival in Asia was a by-product of mining for bat guano from the floor of Fuyan Cave, in Hunan Province of southern China. The perfectly preserved teeth of several individuals turned up in waste material. Dating of the teeth using the 230Th method indicates that humans lived in the cave at least 120 thousand years ago; an astonishing 60 ka earlier than earlier discoveries had suggested. Rock shelters on the southern shore of the Persian Gulf have yielded 120 and 95 ka tools, very like those associated with modern humans then living in NE Africa. So this period marks a definite push out of Africa, possibly using a northern route via the Levant.

What may have been a separate dispersal began to populate Asia around 75 ka ago. The evidence for this is revealing, for it is associated with probably the greatest natural disaster ever to have affected humans of any age. Sulfate and rock dust from the Greenland ice cores show a major ‘spike’ at 74 ka, which coincides with the radiometric age of volcanic ash found as a layer that once covered most of South Asia and the floors of the northern Indian and West Pacific Oceans. The age coincides precisely with that measured in pyroclastic debris on the flanks of a 100 by 30 km elliptical caldera now occupied by Lake Toba on the Indonesian island of Sumatra. From the area and varying thickness of the Toba ash geoscientists reckon that the Toba eruption explosively ejected 2800 of magma, about 800  km3 falling as ash as far afield as the Greenland ice cap; the greatest volcanic event of the last two million years. The entire Earth would have been enveloped by sulfuric acid aerosols in the stratosphere to result in several years to decades of low temperatures – perhaps 10°C below ‘normal’ – and reduced vegetation growth (Chapter 15). Excavation of the Toba Ash in South India unearthed Middle Palaeolithic core- and flake tools, from below and above the ash layer. People were in the area before the eruption and some survived it. Note that some geneticists have suggested that populations may have fallen to a level between 1,000 and 10,000 breeding pairs around 74 ka. They cite genetic evidence suggesting that today’s humans are descended from a very small gene pool that passed through such a bottleneck.

Archaeology indicates that the survivors spread quickly throughout what is now the western Indonesian archipelago as had H. erectus before them. Falling sea-level as glaciers grew connected Malaysia, Sumatra and Java by dry land, termed Sundaland. Modern humans’ relationship with earlier migrants to the Far East is undocumented, yet the record is extremely clear as regards the way that they spread their influence. By 60 ka people may have arrived in Papua-New Guinea and by 48 ka in Australia. To reach Papua-New Guinea required travelling in boats or on rafts, because the intervening sea is far deeper than the lowest glacial sea level. Moreover, it would have required hopping from island to island in the eastern part of the Indonesian archipelago. From even the nearest island  neither Papua-New Guinea nor Australia is visible. With sea-level depressed by no more than 25 to 50 metres early Papuans could simply walk across the Torres Strait to Australia using a broad plain now termed Sahul. At the depth of the last Ice Age the Bering Straits were dry land (Beringia) connecting Asia to the Americas. By 20 ka, and maybe earlier, north-east Asian populations discovered the Americas a thousand generations before Europeans did. Within a hundred generations they had occupied both continents, previously virgin as regards humans and their culture. The much stormier conditions of glacial epochs delayed colonisation of the ocean hemisphere of the Pacific until the present interglacial, but in the 10 ka before Europeans entered the Pacific, virtually every island had been visited if not colonised by intrepid Melanesian and Polynesian voyagers.

Mapping out how people dispersed to fill all the habitable continents in more detail than permitted by dated archaeological sites stems from the blends of genetic  haplogroups carried by people who now occupy different areas. The timing of each genetic ‘branching’ and the location of living people who carry mixtures of haplogroups helps estimate where each haplogroups emerged, the routes taken by the ancestral populations which population carried it and where they split into different subpopulations – the ‘dots’ that, when joined-up, trace human dispersion patterns through time (Figure 23.4).

F23_4Figure 23.4 Probable migrations undertaken by anatomically modern humans, estimated from the distribution and age of emergence of different Y-DNA haplogroups in living people. Based on 2004 data from Family Tree DNA. A more recent version (2010) can be downloaded here, one showing modern language groups is available here, and one based on mtDNA haplogroups is here.

The first modern humans were speaking as they moved out from Africa. So there is a distinct possibility that language ability is extremely ancient. Two outer parts of the lateral brain, Broca’s and Wernicke’s areas, are definitely linked to speech and its understanding. They leave imprints on the inner skull wall. The skulls of H. habilis and erectus show them. However, their presence in fossil hominins is not absolute proof of language ability, for similar but unused features occur in some living apes. Moreover, lacking the means for speech in the structure of the larynx and the tongue could thwart language.  The search is on in earlier human fossils for the small thyoid bone, whose shape and position relative to the larynx is critical for speech, together with subtle structures related to breath control. A Neanderthal hyoid bone found in the Kebara cave system of Israel is so like that of modern humans that is is consistent with a capacity for speech in the Neanderthals.

Following the extinction of the Neanderthals, modern human culture famously exploded in Europe. The human stone tool kit expanded to include delicately crafted blades and points, some of which could be for arrows or harpoons, and a host of so-called microliths including sickle teeth, wood- and bone-working tools. Yet art was not an invention by European AMH. Stratigraphic levels dated at 100 ka beneath the floor of Blombos Cave in coastal South Africa have yielded finely worked, double pointed bifacial tools, surely intended for hafting in spears and knives. Accompanying them were signs of the use of fire-heating to encourage more intricate means of working rock flakes and cores. Polished bone awls, shells bored to be strung on sinews and engraved with abstract patterns suggest highly crafted clothing and adornment. Drawing materials in the form of grooved blocks of ochre together with shells that contain powdered pigments and grinding implements clearly point to painted art; perhaps body art, for the cave walls show no sign of paintings. The appearance 70 ka later in Europe of wonderful depictions of animals and stencils of human hands on cave walls was once thought to show that the focus of explosive artistic development was in Europe (Figure 23.5a). But they are definitely not the earliest figurative works of art. That appeared on the walls of a far-distant cave on the Indonesian island of Sulawesi beneath flowstone dated at almost 40 ka (Figure 23.5b). Taken together with similar although undated depictions of long-extinct birds and animals in north-western Australia, the European and Indonesian paintings more likely signify import of graphic technologies, and a cognitive drive to use them, came with the first migrants from Africa. As outlined earlier, art in the broadest sense has been documented for Neanderthals as far back as 177 ka, in association with H. erectus on Java and by subtle signs from even earlier times.

Although 12 ka has long been cited for the first actual portraits of humans (Figure 23.5c) there are examples of what seem to be 3-D human figurines from Berekhat Ram on the Golan Heights of Syria (233 ka) and Tan-Tan Morocco (500-300 ka), contested by some but accepted by others, that deepen the controversy about just how much smarter modern humans are than their predecessors. The half-million year old ochre ‘pencils’ from Nice and etched shell from Java (Chapter 22), both comparable with the oldest modern-human artwork from Blombos Cave, show that art is older still – perhaps as a means of adorning bodies rather than rock walls. Art is a good deal more important than most of us think, particularly that which is abstract that signifies depiction of the world of spirit as well as that of tangible matters. Did Homo erectus have rituals and beliefs?  They leave little if any sign outside of art. Next time you feel transported by music, tap your feet to a rhythm, or even rise to dance unselfconsciously, ask yourself, ‘Did I have to think about this, or is it an instinct?’

F23_5Figure 23.5  Human art: (a) lions from Chauvet cave, France (~32ka); (b) extinct bovid from Sulawesi, Indonesia (~40-35 ka); (c) human caricatures from La Marche cave in France (12 ka); (d) possible female figurine, Berekhat Ram, Syria (233 ka); (e) possible female figurine, Tan-Tan, Morocco (500-300 ka). Scale bars for (d) and (e) are 1 cm.

Humans’ impact on their world

The world average population of humans from 2.5 Ma until the Holocene has been estimated at about 200 thousand people. When times were good it may have risen to one or two million at most. These figures give population densities of about 1 to 10 persons per thousand square kilometres in occupied parts of the world, around the averages for modern gatherer-hunters in the Arctic and arid central Australia, and in tropical rain forest respectively. From a very low population of stone tool users at 3.5 Ma, the possible founders of the genus Homo, it built to about a million by the end of the Pleistocene, when human numbers reached the limit for sustainable foraging across all continents except Antarctica. The increase in carrying capacity presented by agriculture at the outset of the Holocene undoubtedly meant a leap in the total population. The third population explosion, for which real figures are available, followed the beginning of the Industrial Revolution. We now number eight to thirty thousand times the ‘natural’ carrying capacity of the environment.

A tiny number of foraging humans might suggest that its impact on the environment in previously uninhabited areas would be immeasurable small. Not so. The clearest cases come from the colonisation of the Americas and Australia, beginning at around 20 and 50 thousand years ago respectively. We come to them shortly. First we need some means of detecting human influences on the fossil record.

During the late stages of the Cenozoic Era, the number of new additions to the fossil record of animal species is roughly matched by corresponding disappearances, presumably by extinction. The record for three terrestrial mammal groups (rodents, grazers and carnivores) shows a steady rise through time, probably due to increasingly good preservation of faunas. Suddenly, at the end of the Pliocene the appearance of new genera jumps by ten times, and the number of extinctions increases by about four times. Since Pliocene strata are preserved just as well as those of the slightly younger Pleistocene, these sudden changes are unlikely to be an artifact of the fossil record. When continental ice sheets began to wax and wane in the northern hemisphere after 2.5 Ma ecosystems underwent a globally decisive change. Some of the resulting shifts and diversification of vegetation zones opened opportunities and others stressed animal life on land. The jump in new genera is probably adaptive radiation to fill new niches, mainly on tropical grassland and steppe, together with genetic drift in small populations stranded in habitable pockets of land. Increased extinction must be dominated by the demise of some of these new genera. So how can we judge any human influence?  By looking specifically at animals that may have been prey for human hunters or with which humans competed. Narrowing the focus to large mammals weighing more than 45 kg it is possible to get the best idea.

Africa has had the greatest diversity of large mammals anywhere since the start of the Pliocene. The earliest Pleistocene (2.5 – 0.7Ma) saw the loss of 21 genera there, at a time when early Homo had no hunting tools. In the mid- and late Pleistocene (0.7 – 0.1Ma and <0.1Ma), when hunting was more likely, the number of extinctions in Africa falls unexpectedly to 9 and 7 genera. The inference is that African Pleistocene extinctions were not entirely a result of slaughter by humans. The story is very different for those continents to which anatomically modern humans migrated. The largest extinctions in Europe are in the mid- and late Pleistocene. For Australia and the Americas the peaks of extinction occurred in the last 100 thousand years. There does seem to be a coarse pattern following human diffusion. Southern Europe was successively colonised by H. antecessor, heidelbergensis and then Neanderthals in the mid-Pleistocene and was continuously occupied thereafter, whereas modern humans entered Australia and the Americas only in the Late Pleistocene. Late Pleistocene Australia lost 86 percent of all its large mammals. In South America 80 percent disappeared, and the North American fauna suffered a 73 percent loss.

Clearly, there is a significant difference for the Middle and Late Pleistocene between Africa and previously unpopulated continents – a much smaller proportion of large mammals became extinct in Africa than elsewhere. Yet Africa had both more genera and a more widespread human population than either the Americas or Australia. The natural question to ask is whether the declines were due to human factors or some other combination of circumstances. There is no doubt that humans with modest hunting equipment have devastated large animal populations in recent times. The records from Madagascar and New Zealand provide a good yardstick. Madagascar was colonised about 1500 years ago. Within 600 years ten genera of large mammals, including 7 lemurs, together with the strange flightless elephant birds were eaten to extinction. Polynesians reached New Zealand only a thousand years ago. Within 700 years 34 bird species, including the giant flightless moas, had disappeared. Bones of all these groups in both areas show clear evidence that they were hunted and eaten. In both cases the new prey, although in some cases formidable, succumbed mainly because they had never encountered humans before; they were naive. Although assuming naivety may seem frivolous, there is a large body of evidence that animals pass on experience of predators from generation to generation – they adapt their behaviour. This is easily observed when we encounter familiar species in wilderness areas. They are often surprisingly ‘tame’ compared with those in our back gardens.

Although both the Americas and Australia had fearsome mammalian predators before humans entered the arena, there were none with two legs and spears. Although small in number at first, colonizing bands may literally have eaten their way through three continents in a matter of only a few thousand years. However, there are other possibilities. Humans followed game and, presumably, other predators across the dry Bering Straits – the Beringia land bridge – perhaps during and certainly just after the last glacial maximum. Perhaps the other incoming predators presented insuperable competition to the native fauna. That might be a justifiable conclusion for high latitudes in North America, where mammals would have been adapted to much the same conditions as were those crossing from Asia. However, other animals than humans would have crossed Beringia up to 7 times since around 800 ka – each time that sea level fell sufficiently during the most severe ice ages. Yet there is no evidence for pre-human decimation of comparable magnitude. For the Americas, human-induced mass extinction seems inescapable. Australia’s extinctions seem to have been delayed relative to human entry by at least 20000 years. Central Australia is dry, and perhaps the early colonists lived along the coast, only venturing into the Red Centre when population pressures enforced it.

In the areas in which human induced extinctions are well supported, large predators such as sabre-toothed cats of North America were decimated to the same extent as prey animals. They may have been out-competed, despite their ferocity. Examining the record of smaller animals reveals no such dramatic events. No doubt they were hunted and eaten by humans, but their larger populations and faster reproduction than large mammals would have cushioned them from extinction. The last mammoth or giant deer would have been a more valuable target for human predation than its equivalent weight in voles. Meat is not the only inducement for hunters; in treeless steppes early Europeans, Asians and Americans built shelters from elephant bones probably covered by their skins. Africa, in contrast, has vast numbers of large mammalian prey animals that have adapted to defend themselves against all indigenous predators. Neither a Cape Buffalo nor an African Elephant can by any stretch of the imagination be regarded as naive. Despite our justifiable fears that today’s economic system threatens mass extinction, the process may have begun 50 millennia ago. It is also possible that early humans inflicted similar changes on vegetation.

In East Africa and Australia tree communities are dominated by fire-resistant species and those whose seed germination actually depends on fire. To some extent this may stem from the effects of fires ignited by lightning in seasonally arid areas. But fire has been part of human culture for more than a million years. As well as for warmth and cooking, hunter-gatherers use it in two other important ways. Fire panics game into traps and ambushes, a particularly effective strategy for hunters lacking projectile weapons. It can also be used to clear impenetrable scrub, giving less cover to prey animals. Fire-resistant flora have been endemic in East Africa for more than a million years, possibly as a result of erects’ and later humans’ strategies. In Australia they suddenly became dominant more recently. The evidence for that is accurate, coming from offshore sediment cores in which pollens change from being dominated by those of fire-sensitive trees to fire-tolerant species after a 140 ka old layer rich in charcoal particles. The only evidence for drying in the Australian climate comes at 30 ka. If humans were responsible for the fires that swept the continent, then they colonised Australia much earlier than they left signs of their presence, except for controversial 170 ka art in the Northern Territories. Deforestation can be a major causative factor in climatic drying. Transpiration by trees increases humidity of regional airmasses, while resins and dusts that it emits act as important ‘seeds’ for the condensation of clouds. It may be that early humans created the barren centre of Australia. But additional evidence that bears on possible human devastation emerged from studies in the early 21st century that used a sort of ‘grab-bag’ genetic approach – environmental DNA (eDNA) that was first deployed successfully for assessing unseen diversity in samples of ocean water.

Such palaeoecological ‘bar-coding’ was first applied to permafrost in Siberia and Alaska. Dividing the samples into 3 time spans – 50-25, 25-15 (last glacial maximum) and younger than 15 ka – the researchers found that these major stages in the last glacial cycle mapped an ecological change from a dry tundra dominated by abundant herbaceous plants or forbs, to a markedly depleted Arctic steppe ecosystem then moist tundra with woody plants and grasses dominating. The eDNA of dung and gut contents from ice-age megafauna, such as mammoths, bison and woolly rhinos showed that forbs were the mainstay of their diet. Bones of large mammals also established the timing of extinctions in the last 56 ka, showing 31 regional extinction pulses linked to the rapid ups and downs of climate during Dansgaard-Oeschger cycles in the run-up to the last glacial maximum.  By the end of the last glacial maximum, large animals were highly stressed by purely climatic and ecological factors. Human predation probably finished them off. Another imaginative approach exploited a fungus (Sporormiella) that thrives on the dung of large herbivores and can only complete its life cycle after they have digested plant matter. Spores of this fungus extracted from a lake core in New York State, USA, fell from dominance to less that 2% of all spores and pollen about 13.7 ka ago, before the first appearance of the first Native Americans in the area. The same fungus breaks down dung in Australia. Lake-floor sediment from Queensland showed a similar collapse in the abundance of Sporormiella between 43-39 ka. But in this case the decline accompanied a marked increase in fine-grained charcoal – a sign of widespread bush fires – followed by a steady increase in pollen of scrub vegetation at the expense of that of tropical rain forest trees. The shifts do not correlate with any Southern Hemisphere climatic proxy for cooling and drying that might have caused ecosystem collapse. That still does not mark out newly arrived humans as the culprits, as the early archaeological record of Australia, as in North America, is sparse and only estimated to have started at around 45 ka. Yet this is quite strong circumstantial evidence for human interventions, such as deliberate fire clearing to improve visibility of game attracted to new green growth. As we have noted before, increasingly sophisticated research rarely simplifies attempts at explaining events.

Rapid global warming and increasing humidity since the end of the Younger Dryas 11.7 ka ago saw grasses of various types spread in the Middle East. None was a new species, and the seeds of each were edible for humans, but no means of gathering them efficiently had previously been possible. By making composite tools, such as sickles of microliths embedded in wood or bone, around 23 ka humans had begun to adopt eating habits previously confined to paranthropoids, despite having teeth unsuited to.chewing uncooked grain. Cooking them or grinding then baking makes grain an easily digested and abundant source of carbohydrate and protein. By 7 ka agriculture based on selective planting of the most productive grains and other staples and thus unconscious breeding had arisen on all continents except Australia: The Fertile Crescent of the Middle East (wheat, barley and oats – 10 ka); Africa (sorghum and millet – 7 ka); east Asia (millet and rice – 8 ka); central America (maize and squash – 7.5 ka) and New Guinea (taro, banana/plantain and yam – 11 ka). At roughly the same time some naturally herding ruminant animals became domesticated. The timing of the spread of agriculture, either through migration of early farmers or diffusion of the technology, seeds, livestock and ideas is often charted by declines in tree pollen and increased evidence for weeds in cores through Holocene lake-bed sediments and bogs; sure signs of deforestation during a period of stable climate.

The seemingly obvious fact that controlling a food supply is a great deal easier and more productive than perpetually looking for one meant that more people could survive than those needed to be involved in supplying food. This partial sidestep into paranthropoid habits has in 250 generations moved humans from the Stone Age to silicon-based culture, into various forms of class society, nation states, global warfare, racism, massive pollution, human-induced climate change, and dental decay. The human history of the Holocene, for that is when our modern social habits truly began, concludes the book in Chapter 24. The last ten thousand years also mark a mass extinction event comparable in pace with any previous one, yet neither an impacting asteroid nor a mantle superplume is in any way involved.

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