Holocene migrations of people into Britain

People assigned to a variety of human species: Homo sapiens H. neanderthalensis (Swanscombe, 400 ka and several later times ) H heidelbergensis (Boxgrove, ca 500 ka, )H. antecessor (Happisburgh, ca 950 ka) – have left signs of their presence in Britain. Human occupancy has largely depended on climate. Around 9 times since the first known human presence here, much of Britain was repeatedly buried by glacial ice to become a frigid desert for tens of thousands of years. Between 180 and 60 ka only a couple of flint artefacts found in road excavations in Kent hint at Neanderthal visitors. For most of the Late Pleistocene the archipelago seems to have been devoid of humans. Arguably, Europe’s first known anatomically modern humans occupied several caves in Devon, Derbyshire and South Wales as early as around 43 ka, while climate was cooling, only to abandon Britain during the Last Glacial Maximum (24 to 18 ka ago). As climate warmed again thereafter, sporadic occupation by Late Palaeolithic hunter-gatherers occurred up to the sudden onset of the frigid Younger Dryas (12.9 ka). Once warming returned quickly 11,700 years ago, sea level was low enough for game and hunter gatherers to migrate to Britain; this time for permanent occupancy. Bones of the earliest known of these Mesolithic people have yielded DNA and a surprise: they were dark skinned and so far as we can tell remained so until the beginning of Neolithic farming in Britain around 6100 years ago. The DNA of most living Britons with pale skins retains up to 10% of inheritance from these original hunter gatherers.  Much the same is known from elsewhere in NW Europe. In the early Holocene it was possible to walk across what is now the southern North Sea thanks to Doggerland. Following a tsunami at around 8.2 ka this rich area of wetland vanished, so that all later migration demanded sea journeys.  

Mesolithic people remained in occupation of the British Isles for another two millennia. A wealth of evidence, summarised nicely in Ray, K. & Thomas, J. 2018, Neolithic Britain, Oxford University Press, suggests that there was a lengthy period of overlap between Mesolithic and Neolithic occupation around 4100 BCE. The main difference between the two groups was that Neolithic communities subsisted on domesticated grains and animals, while those of the Mesolithic consumed wild resources. Cultural clues in archaeological finds, however, suggest a lot in common, such as the erection of various kinds of monuments. Posts of tree trunks, sometimes arranged in lines, were raised in the Mesolithic and lines of probably ritual pits were dug. Both ‘traditions’ continued into the Neolithic and evolved to stone monuments, to which were added burials of different kinds. It is worth noting that Stonehenge was developed on a site that held much earlier, large totem-pole like posts, with a nearby spring that had hosted regular gatherings of Mesolithic people. Signs of Mesolithic occupation in Britain extend just as widely as do those of Neolithic practices. A study of DNA from 7 Mesolithic skeletons and 67 of early Neolithic age (Brace, S. and 20 others 2019. Population Replacement in Early Neolithic Britain. Nature Ecology & Evolution, v. 3, p. 765-771; DOI: 10.1038/s41559-019-0871-9) revealed that early Neolithic people did not wipe out the genetic make-up (either by complete displacement or annihilation) of their predecessors. About 20 to 30% of Neolithic DNA was inherited from them; as would be expected from assimilation of a probably much smaller number of hunter-gatherers into a larger population  of  immigrants who brought farming and herding from Asian Turkey (Anatolia). Such ‘hybrid’ genetics was widespread in Europe and they are referred to as the Early European Farmers (EEF). As Ray and Thomas suggest, aspects of Mesolithic culture may have been adopted by the newcomers across the British Isles from Orkney to Wiltshire.

Around 2400 BCE the earliest Neolithic ceremonial site at Brodgar on Orkney was destroyed to the accompaniment of an enormous feast that consumed several hundred cattle. At about the same time several men, whose tooth geochemistry indicated an origin in the European Alps, were buried on Salisbury Plain together with the earliest metal artefacts known from Britain (copper knives), the accoutrements of archery and distinctive, bell-shaped pottery beakers. Stonehenge was ‘remodelled’ shortly afterwards, with the addition of its giant trilithons, four of which were later adorned with carvings of metal axes and daggers. The Early Bronze (or Chalcolithic) Age had arrived! A 2018 study of ancient DNA from Bronze Age burials in Europe suggested a far more drastic swamping of Neolithic genetic heritage by the ‘Beaker people’ (Olalde, I. and a great many others 2018. The Beaker phenomenon and the genomic transformation of northwest Europe. Nature, v. 555, p. 190-196; DOI: 10.1038/nature25738). The skeletons from Britain analysed by Olalde et al. apparently suggested that, within a few hundred years, up to 90% of the Neolithic gene pool had been removed from the British population. Who were these people who used metals and the distinctive Bell Beakers, where did they come from and what did they do?

The closest match to the British and western European Bronze Age DNA was that associated with the Yamnaya people from the steppes of SE Ukraine and Southern Russia who had developed a culture centred on herding. They had also adopted the wheel from people of the Mesopotamian plains and had domesticated the horse for riding and pulling carts: ideal for their semi-nomadic lifestyle and for moving en masse. After 3000 BCE they spread into Europe, as widely recorded by their distinctive beakers and the presence of their DNA in the genomes of later Europeans. Their burials – in ‘kurgans’ – resembled the round barrows that appeared on Salisbury Plain and elsewhere during the Bronze Age. The DNA replacement data from 2018 were limited and held few clues to how it happened. One possibility for such a dramatic change could be a violent takeover that drove down the population of British Neolithic people. To address the broader influence of migration in more detail and over a loner time span, a team led by the Universities of York and Vienna, and Harvard Medical School (Patterson, N. and a great many others 2021. Large-scale migration into Britain during the Middle to Late Bronze Age. Nature, early online release; DOI: 10.1038/s41586-021-04287-4) used ancient DNA from 793 individuals excavated in Britain (416 individuals) and continental Europe (377) from Bronze- to Iron Age sites (2300 to ~100 BCE).

The proportion of Early European Farmers DNA in British individuals from the Bronze Age (2400 BCE) to the Iron Age (750 BCE to 43 CE). Note the ‘fuzzy’ nature of the data, and that the decline in EEF in British individuals was not as great as earlier analyses had shown. Remarkably, the ‘Amesbury Archer’, who brought the first metals to Britain, had a higher proportion of EEF ancestry than the Early Bronze-Age average. (Credit: Patterson et al. Fig. 3)

The new data from Britain suggest that the migrants, who crossed the Channel later in the Bronze Age, were of mixed ethnicity, but most carried EEF genes. The influence of earlier migrants from the Yamnaya heartlands is present, but so too are relics of Mesolithic ancestry. Interestingly, the British data show a much larger increase in the genes associated with lactase persistence, which marks the ability of adults to digest milk, than was apparent in the wider European population (50% compared with about 7% in Eastern Europeans of the time). Whatever the impact of the first influx of metal-using people – it may have been culturally decisive in Britain – by the end of the Bronze Age the EEF ‘signature’ had increased in peoples’ genomes. Rather than some kind of invasion, the influx was more likely to have been a sustained movement of people to Britain over several hundred years By the Iron Age, almost half the ancestry of Britain, particularly in England and Wales, was once again predominantly of EEF origin (around 40% of the mixture), but culture had become completely different. There are even suggestions that the influx brought with it the beginnings of Celtic languages. Yet the data leave a great deal of further analysis to be undertaken.

See also: Drury, S.A. 2019. Genetics and the peopling of Britain: We are all hybrids, People and Nature; Ancient DNA Analysis Reveals Large Scale Migrations Into Bronze Age Britain, SciTechDaily, 28 December 2021.

Did ice-age climate changes across Europe happen at the same time?

Although the frigid conditions at the last glacial maximum, around 19 to 20 thousand years ago, gradually relinquished their grip through slow global warming, this amelioration came to sudden stop around 12 800 years before the present. Northern hemisphere ice-core and other climate records show that there was a return to glacial conditions over a period of a few decades at most, to launch what is known as the Younger Dryas stadial that lasted over a thousand years until about 11 500 years ago, with the onset of the warm, climatically more stable Holocene that launched the transformation of the human way of life. The start of the Younger Dryas had dramatic effects throughout the northern hemisphere, the cold conditions emerging suddenly from an immense oceanographic change; a weakening or the halt of the North Atlantic thermohaline circulation in which cold, very salty surface waters at the fringe of the Arctic Ocean sink to drag warmer water to high latitudes. In short, the Gulf Stream slowed or stopped its warming influence at high northern latitudes.  Current thoughts centre on a freshening of surface sea water following the collapse of the North American ice sheet to gush meltwater and icebergs into the North Atlantic to buoy-up surface waters.

Iceage time 18kyr
Major climate shifts in Europe since 18 ka (credit: Wikipedia)

Most of the data about this climatic shock can only be dated accurately to within a few centuries: it is clear that the initial cooling was very rapid, on the scale of a few years, as was the warming that closed the Younger Dryas and marked the start of the Holocene, but the ‘when’ is known only to within a few hundred years. To resolve the start and stop ages needs records that include several indicators: clear signs of the beginning and end of the episode, an accurate means of dating them and confirmation from other sites, which presupposes a cast-iron means of correlating the records over large distances. The most reliable markers for correlation are volcanic ashes that can be dated radiometrically and which drift on the wind to be deposited over very large areas. If sedimentary sequences that accumulated continuously preserve such ashes, contain clear signs of climatic change and clearly record the passage of time in great detail, there is a chance of resolving climatic events very accurately; but they are no common.  A British-German team have located and analysed two such promising sites (Land, C.S. et al. 2013. Volcanic ash reveals time transgressive abrupt climate change during the Younger Dryas. Geology, v. 41, p. 1251-1254). One of them is from the bed of a lake that formed by a single volcanic eruption (Meerfelder Maar) in the Eifel region of western Germany. Quiet sediment accumulation has occurred there continuously to form very narrow, alternating dark and light layers, the variegation being due to sedimentation under ice in winter and open water in summer respectively. Twelve thousand of these annual varves provide a means of dating potentially with a precision of ± 1 year, but calibration to absolute time is necessary. The maar sediments contain three ash layers, two of which are from small local eruptions; the older having an age of 12 900 years before 2000 AD, the other being 11 000 years old, showing that the entire Younger Dryas is spanned by the Meerfelder Maar sediments. The third was dated by varve counting, showing the eruption had taken place 12 140 years ago. That age coincides closely with that of major eruption in Iceland.

Panorama Weinfelder Maar oder Totenmaar, Eifel
A typical volcanic maar in Eifel Region of Germany (credit: Wikipedia)

One prominent climatic feature of the Younger Dryas of Europe is a shift around halfway through: it started with the fiercest cold and then ameliorated. This change shows up in the Meerfelder Maar record as a reduction in mean varve thickness and an increase in the titanium content of the clays, the latter taking place in about a year (12 250 years ago) some 100 years before the Icelandic ash was deposited. The same kind of change occurs in records from lakes as far north as the Arctic Circle. One of the core records from Kråkenes in Northern Norway also contains the tell-tale Icelandic ash (as do ice cores from Greenland), but in its case it occurs 20 years before the abrupt climate shift. This clearly shows that major climate changes at the end of the last ice age occur at different times from place to place. The authors ascribe the 120 year difference between the two records to the times when prevailing, warm westerly winds began to affect central and northern Europe, linked to a gradual northward migration of the polar front. The data from both lakes also suggest that the Younger Dryas ended about 20 years earlier in Norway than in Germany, although Lane et al. do not comment..

Hitherto, correlation between climate records has been based on an assumption that major climate changes were at the same time, so that climate proxies such those discussed here have been ‘wiggle-matched’. Quite probably a lot of subtleties have thereby been missed.

Enhanced by Zemanta

A sign of the times; the ‘Anthropocene’

Alternative version of image:Wooden hourglass ...
Image via Wikipedia

On 11 May 2011, the Geological Society of London hosted a conference, co-sponsored by the British Geological Survey, to discuss evidence for the dawn of a new geological Epoch: the Anthropocene, supposed to mark the impact on the Earth of our species. The Society, and no doubt others internationally, is interested in gathering thoughts, reflections and observations about the Anthropocene. There is indeed a a powerful and vocal, though not necessarily large, lobby directed at the International Commission on Stratigraphy (ICS) to enshrine this new division. That lobby  has been active since 2000 (see: No escape from global warming;  Changing the world; Epoch, Age, Zone or Nonsense in EPN issues of November 2000, April 2005 and March 2008 respectively)

We currently live in the Holocene (‘entirely recent’), an Epoch with ICS imprimatur. Yet the last 11.7 ka has been but one of very many interglacials since about 2.6 Ma ago; the start of the Pleistocene Epoch and the Quaternary Period – Arduino’s last surviving division of geological time, and lately resurrected from an untimely demise! The ‘golden spike’ for the Pleistocene/Holocene boundary is at the agreed combination of signals – ‘deuterium excess values, accompanied by more gradual changes in 18O, dust concentration, a range of chemical species, and annual layer thickness’ – of the end of the Younger Dryas/Greenland Stadial 1 in a single Greenland ice core (NGRIP) held in a cold store in Copenhagen.

The Holocene itself was based on anthropocentric grounds; i.e. it roughly coincides with the transition from human foraging to sedentary life, agriculture, the relentless development of exploitation of the majority of humans and the commodification of the physical and organic environments following the Younger Dryas stadial. I guess that paraphrases how the ‘Anthropocene’ is proposed to be defined – a signal of the beginning of irreversible global change due to human activities whose future we cannot predict.

Even if it was possible to agree on some definitive signal of the onset of human-induced global change in the geological record there remains the formal difficulty for the ICS of agreeing on the location as well as the age and likely durability of the GSSP that would mark the beginning of the ‘Anthropocene’. The originator of the idea, Nobel Prize winning chemist Paul Crutzen, argued vaguely for the ‘start of the Industrial Revolution’. Recently it has been proposed by some to be 6 August 1945 marked by long-lived radioactive fallout from the atomic massacres of civilians at Hiroshima and Nagasaki.  Quaternary researchers decided some time back that the ‘present’ (as in ‘before present’ or b.p.) should be the year 1950 when atmospheric testing of thermonuclear weapons created excess 14C that will make radiocarbon dating of the next 50 ka somewhat more uncertain than it otherwise would have been. The ICS may well have a lengthy debate on its hands if the proposal ever reaches its deliberations.

Furthermore, the advocates are concerned that we are living in the transition into their ‘Anthropocene’ and that it will be so rapid and biologically disastrous as to manifest itself in stratigraphic sections of the future as a mass-extinction event. No previous mass extinction event has been allocated epochal status, being so brief, though never so brief (~10 ka) as the Holocene or any other interglacial of the past 2.6 Ma.

All that I can conclude is that should there still be geologists in, say, a million years time, who will be living in conditions and possessing intellects about which we would be ill advised to guess, they will still be in awe of the vast tracts of geological time and their stratigraphic and tectonic records over the last 4.55 Ga. Consequently, it is possible that they may well regard the then ancient proposal for an ‘Anthropocene’ as premature, hubristic and not a little reminiscent of the fable of Chicken Little; a humorous legacy of their somewhat startled predecessors. By all means let us be concerned  about and take action to halt adverse human influences on the planet, but sloganeering to climb aboard a bandwagon does neither. At the Geological Society meeting, Paul Crutzen observed  “… it will probably take another 20 years before it is formally accepted.” Thank goodness for a sense of reality: we may all be extinct by then…

Added 12 August 2011: Between 11.5 and 3.5 ka the greatest event in the evolution of modern humans took place on all continents except Australia and Antarctica; a foraging lifestyle gave way to settlement and the domestication of both plants and animals – the Neolithic Agricultural Revolution. The production of surplus value, stored in the form of livestock herds and grain, marked by this transition set humanity on the road to its current social, ecological and economic crisis. Interestingly, William Ruddiman of the University of Virginia in 2005 noted a shift in the CO2 content of glacial ice around 8 ka, which he ascribed to intense farming and suggested that if there were to be an Anthropocene Epoch it should coincide with the start of agriculture. Combining geological and societal factors points unerringly to the start of the Holocene, so there is little need for a new Epoch. That sensible view receives support from a palaeo-demographic survey of 133 burial sites in the Northern Hemisphere: some before the local transition to agriculture, others following it (Boquet-Appel, J.-P. 2011. When the world’s population took off: the springboard of the Neolithic demographic transition. Science, v. 333, p. 560-561). The proportion of 5 to 19 year-old remains in the cemeteries shows a marked rise in the thousand years after the first local signs of agriculture thereafter to stabilise at a new higher level. This indicates a significant increase in female fertility, perhaps by as much as two births per woman. That would set in train the relentless, 1200-fold rise in world population from the estimated 6 million at the start of the Holocene to 7 billion at present.

Related articles