More time for modern humans to have mingled with Neanderthals

When anatomically modern humans (AMH) became established in Europe the days of the Neanderthals were numbered. Yet, genomic evidence is mounting for many instances of interbreeding between the two groups (see Human evolution links). The longer they were in contact the chances of meeting and having sex were likewise increased. So, for how long were the two groups able to make contact? Neanderthals declined and eventually disappeared between 41 and 39 ka, except for a possible refuge for a tiny number in southern Spain until 37 ka and maybe in the northern Urals where there are disputed Mousterian stone tools as young as 34 to 31 ka. Undoubtedly, the appearance of AMH somehow contributed to the demise of our close relatives, but there are many possible reasons why. Until recently, the earliest European entry of AMH had been placed at around 41 ka, based on dating of H. sapiens remains in Romania (but note: a single 210 ka possible AMH skull from Greece). This is now exceeded by data from a Bulgarian cave.

Bacho Kiro cave in Bulgaria (credit: Getty images)

The Bacho Kiro site was first excavated in the 1970s, and revealed stone tools that represent the earliest Upper Palaeolithic culture, known as the Bachokirian. Mitochondrial DNA from excavated bone fragments is clearly of AMH origin (Hublin, J.-J. and 31 others 2020. Initial Upper Palaeolithic Homo sapiens from Bacho Kiro Cave, Bulgaria. Nature, v. 581, online; DOI: 10.1038/s41586-020-2259-z). Dating the Bacho Kiro cave sediments has been difficult, but new analytical and statistical approaches using the radiocarbon (14C) method have yielded ages between 46 to 44 ka and perhaps as far back at 47ka (Fewlass, H. and 20 others 2020. A 14C chronology for the Middle to Upper Palaeolithic transition at Bacho Kiro Cave, Bulgaria. Nature Ecology and Evolution, v. 4, online; DOI: 10.1038/s41559-020-1136-3). This is the earliest unequivocal, direct evidence of our species in Europe and its association with the initial Upper Palaeolithic culture. Among the finds are perforated animal teeth and ivory beads that probably formed pendants, which resemble those found elsewhere in association with late Neanderthals: the Chatelperronian culture that seems to have been shared between AMH and Neanderthals.

The new data add up to 6 thousand years to the period of AMH-Neanderthal co-occupation of Europe, or about 400 generations. Plenty of time to ‘get to know one another’, and perhaps to assimilate genetically

See also: Rincon, P. 2020. Longer overlap for modern humans and Neanderthals. (BBC News 11 May 2020); Metcalfe, T. 2020. A tooth offers evidence modern humans reached Europe earlier than previously thought. (NBC News 11 May 2020)

Human evolution links

Time and energy permit me to summarise only one or two research developments each week. Yet there is a continual flow of other publications in fields which interest me, and hopefully most readers of Earth-logs. I come across them during my weekly search for suitable inspiration, so have decided occasionally to provide links to informative summaries in other blogs.

Last week, Science Daily reported on a paper in the journal Genetics that evaluates new genetic evidence that interbreeding between anatomically modern humans (AMH) occurred more often than previously suggested, when the two groups were in contact in Eurasia (New research adds to growing evidence that our ancestors interbred with Neanderthals at multiple times in history. Science Daily 1 April 2020). Other Neanderthals also left signs that around 40 ka ago they wove cordage from woody cellulose (in Scientific Reports): they were clearly as technologically adept as contemporary AMH (40,000 year old evidence that Neanderthal’s wove string. Science Daily 9 April 2020).

The early-April issue of Science also published dating of a key site in South Africa to show that around 2 Ma ago the earliest known Homo erectus co-inhabited the surrounding area with Australopithecus naledi and the earliest known Paranthropus. One of the highlights is that this rules out A. naledi as a direct human ancestor, as previously claimed by some. (When three species of human ancestor walked the Earth. Science Daily 2 April 2020).

In its last March issue Science carried a paper suggesting that Neanderthals in Portugal were avid consumers of seafood (Neanderthals ate mussels, fish, and seals too. Science Daily 26 March 2020).

Further back in the Eurasian human story

About 800 to 950 thousand years (ka) ago the earliest human colonisers of northern Europe, both adults and children, left footprints and stone tools in sedimentary strata laid down by a river system that then drained central England and Wales. The fossil flora and fauna at the Happisburgh (pronounced ‘Haze-burra’) site in Norfolk suggest a climate that was somewhat warmer in summers than at present, with winter temperatures about 3°C lower than now: similar to the climate in today’s southern Norway. At that time the European landmass extended unbroken to the western UK, so any hunter-gatherers could easily follow migrating herds and take advantage of seasonal vegetation resources. These people don’t have a name because they left no body fossils. A group known from their fossils as Homo antecessor had occupied Spain, southern France and Italy in slightly earlier times (back to 1200 ka). Since the discovery of their unique mix of modern and primitive traits, they have been regarded as possible intermediaries between H. erectus and H. heidelbergensis – once supposed to be the predecessor of Neanderthals and possibly anatomically modern humans (AMH). Since the emergence about 10 years ago of ancient genomics as the prime tool in examining human ancestry the picture has been shown to be considerably more complex. Not only had AMH interbred with Neanderthals and Denisovans, those two groups were demonstrably interfertile too, and a complex web of such relationships had been pieced together by 2016. But there has been a new development.

700 ka Homo erectus from Java: a possible Eurasian ‘super-archaic’ human (credit: Gibbons 2020)

Population geneticists at the University of Utah, USA, have devised sophisticated means of making more of the detailed ATCG nucleotide sequences in ancient human DNA, despite there being very few full genomes of Neanderthals and Denisovans (Rogers, A.R. et al. 2020. Neanderthal-Denisovan ancestors interbred with a distantly related hominin. Science Advances, v. 6, article eaay5483; DOI: 10.1126/sciadv.aay5483). In Earth-logs you may already have come across the idea of the ancestral ‘ghosts’ that are represented by unusual sections of genomes from living West African people. Those sections seem likely to have resulted from interbreeding with an unknown archaic population – i.e. neither Neanderthal nor Denisovan. It now seems that both Neanderthal and Denisovan genomes also show traces of such introgression with ‘ghost’ populations during much earlier times. The ancestors of both these groups separated from the lineage that led to AMH perhaps 750 ka ago. Rogers et al. refer to the earliest as ‘neandersovans’ and consider that they split into the two groups after they entered Eurasia, at some time before 600 ka – perhaps around 740 ka. This division may well have occurred as a result of a population of ‘neandersovans’ having spread over the vastness of Eurasia and growing genetic isolation. The reanalysis of both sets of genomes show evidence of a ‘neandersovan’ population crash before the split. Thereafter, the early Neanderthal population may have risen to around 16 thousand then slowly declined to ~3400 individuals.

A ‘state-of-play’ view of human interbreeding in Eurasia since 2 Ma ago (credit: Gibbons 2020)

However, the ‘neandersovans’ did not enter a new continent devoid of hominins, for as long ago as 1.9 Ma archaic H. erectus had arrived from Africa.  Both Neanderthal and Denisovan genomes record the presence of sections of ‘super-archaic’ DNA, which reflect early  interbreeding with earlier Eurasian populations. Indeed, Denisovans seem to have repeated their ancestors’ sexual exploits, once they became a genetically distinct group.  From the ‘ghost’ DNA fragments Rogers et al. conclude that the ‘super-archaics’ separated from other humans about two million years ago. They were descended from the first ‘Out-of-Africa’ wave of humans, represented by the fossils humans from Dmanisi in Georgia (see First out of Africa, November 2003 and An iconic early human skull,  October 2013 in Earth-logs Human evolution and migrations). A measure of the potential of novel means of analysing available ancient human DNA is the authors’ ability even to estimate the approximate population size of the interbreeding ‘super-archaic’ group at 20 to 50 thousand. Long thought to be impossible, it now seems possible to penetrate back to the very earliest human genetics, and the more DNA that can be teased out of other Neanderthal and Denisovan fossils the more we will know of our origins.

See also: Gibbons, A. 2020. Strange bedfellows for human ancestors. Science, v. 367, p. 838–839; doi:10.1126/science.367.6480.838

Everyone now has their Inner Neanderthal

For 20 years, we have known the full human genome. For 10 years the full content of Neanderthal DNA has been available, courtesy of Svante Paabo’s team at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany. The two were compared and suddenly every living person with a Eurasian ancestry learned that they had significant and functional bits of Neanderthal in their make-up: some beneficial, some not so good (see: Yes, it seems that they did… in Human evolution and migrations, May 2010). Then the Denisovan connection emerged for East Asians and original populations of Australasia. Africans seemed not to share such a privilege. But now it seems that they do, but as a result of a somewhat tortuous route (Lu Chen et al. 2020. Identifying and interpreting apparent Neanderthal ancestry in African individuals. Cell v. 180, p. 1–11; DOI: 10.1016/j.cell.2020.01.012).

Reconstruction of Neanderthal male

Lu and colleagues used a new approach to discover that 2500 people from five widespread subpopulations living in Africa carry in their DNA several million base-pairs of Neanderthal origin (about 0.3% of their genomes). This happened in two steps. The most recent resulted when ancient anatomically modern humans (AMH), who carried Neanderthal DNA as a result of repeated interbreeding, migrated back to Africa from Europe about 20 thousand years ago. But the modern Africans’ DNA also suggests that their ancestral Neanderthals had also interbred with a much earlier group of Africans who had left their home continent between 150 to 100 thousand years ago. The Neanderthals already carried sections of that earlier AMH genome. The relationship between modern humans and Neanderthals seems to have been a great deal more complex that previously thought.

The authors conclude, …  our data show that out-of-Africa and in-to-Africa dispersals must be accounted for when interpreting archaic hominin ancestry in contemporary human populations. It is notable that Neanderthal sequences have been identified in every contemporary modern human genome analyzed to date. Thus, the legacy of gene flow with Neanderthals likely exists in all modern humans, highlighting our shared history’. Palaeo-geneticists have also shown that a similarly complex social relationship may have characterised Neanderthals and Denisovans, where their ranges overlapped (see Neanderthal Mum meets Denisovan Dad in Human evolution and migrations, August 2018). It would come as no surprise to learn, eventually, that wherever different human groups crossed paths in the more distant past they engaged in similar practices, that is, they behaved humanly. Things have changed a bit in recorded history, when only a single human group has existed; perhaps a consequence of the emergence of what today passes for ‘economy’.

Watch Chris Stringer discussing his views on Neanderthal-AMH interactions

See also: Price, M. 2020. Africans, too, carry Neanderthal genetic legacy. Science, v. 367, p. 497; DOI: 10.1126/science.367.6477.497

Note added 14 February 2020

Several studies of DNA from living Africans have suggested introgression (interbreeding) of an even earlier archaic population into ancient AMH in Africa. Because this cannot be related to any known fossils, such as Homo erectus, such a population is known in palaeogenetic circles as a ‘ghost’. A new paper (Durvasula, A. & Sankararaman, S. 2020. Recovering signals of ghost archaic introgression in
African populationsScience Advances, v. 6, article eaax5097; DOI: 10.1126/sciadv.aax5097) suggests that two living groups from West Africa (Yoruba and Mende) derive 2 to 19% of their genetic ancestry from such a ‘ghost’ population. It seems that this archaic group diverged from the descent path of AMH before the split of Neanderthals and AMH. But when the Neanderthal-AMH event took place is uncertain, estimates ranging from 185 to 800 ka. This time uncertainty further obscures the genetic ‘trail’. Curiously, as far as I know non-Africans whose AMH ancestors were of African origin, show no sign of this particular ‘ghost’ among their forebears. That perhaps suggests that few if any West Africans engaged in ‘out-of-Africa’ migrations …

The last known Homo erectus

There are a lot of assumptions made about Homo erectus and, indeed, there is much confusion surrounding the species (see: various items in Human evolution and migrations logs for 2001, 2002, 2003 and several other years). For a start, the name derives from Eugene Dubois’s 1891 discovery of several hominin cranial fragments in sediments deposited by the Solo River in Java. Dubois was the first to recognise in ‘Java Man’ the human-ape ‘missing link’ about which Charles Darwin speculated in his The Descent of Man, and Selection in Relation to Sex (1871). Dubois named the beings Pithecanthropus (now Homo) erectus. Once the “multiregional” versus “out-of-Africa” debate about the origin of anatomically modern humans (AMH) emerged after a variety of H. erectus-like fossils had also turned up in Africa and Europe, as well as in East and SE Asia, ‘Java Man’ was adopted by the multiregionalists as ‘evidence’ for separate evolution of AMH in Asia. Such a view remains adhered to by a tenacious number of Chinese palaeoanthropologists, but by virtually no-one else.

Reconstruction of the Nariokotome Boy from the skeleton found in the Turkana Basin of Kenya (credit: Atelier Daynes/Science Photo Library)

The earliest of the African ‘erects’ were distinguished as H. ergaster, represented by the 1.6 Ma old, almost intact skeleton of Nariokotome Boy from the Turkana area of Kenya. In Africa the specific names ergaster and erectus often seem to be used as synonyms, whereas similar-looking fossils from Asia are almost always referred to as ‘Asian ­H. erectus’. Matters became even more confusing when the earliest human migrants from Africa to Eurasia were discovered at Dmanisi in Georgia (see; Human evolution and migrations logs for 2002, 2003, 2007, 2013). Anatomically they deviate substantially from both H. ergaster and Asian erectus – and from each other! – and at 1.8 Ma they are very old indeed. Perhaps as a palliative in the academic rows that broke out following their discovery, for the moment they are called Homo erectus georgicus; a sub-species. But, then, how can Asian H. erectus be regarded as their descendants. Yet anatomically erectus-like fossils are known in East and SE Asia from 1.5 Ma onwards.

There is another mystery. Homo ergaster/erectus in Africa made distinctive tools, typified by the bifacial Acheulian hand axe. Their tool kit remained substantially the same for more than a million years, and was inherited by all the descendants of H. erectus in Africa and Europe: by H. antecessor, heidelbergensis, Neanderthals and early AMH. Yet in Asia, such a technology has not been discovered at sites older than around 250 thousand years. Either no earlier human migrants into Asia made and carried such artefacts or stone tools were largely abandoned by early Asian humans in favour of those more easily made from woods, for instance bamboo.

In 1996 the youngest Solo River sediments that had yielded H. erectus remains in the 1930s were dated using electron-spin resonance and uranium-series methods. The results suggested occupation by ‘erects’ between 53 and 27 ka, triggering yet more astonishment, because fully modern humans had by then also arrived in Indonesia. Could anatomically modern humans have co-existed with a species whose origin went back to almost two million years beforehand? It has taken another two decades for this perplexing issue to be clarified – to some extent. The previous dates were checked using more precise versions of the original geochronological methods covering a wider range of sediment strata (Rizal, Y. et al. 2019. Last appearance of Homo erectus at Ngandong, Java, 117,000–108,000 years ago. Nature, published online; DOI:10.1038/s41586-019-1863-2). No AMH presence in Asia is known before about 80 ka, so can the astonishment be set aside? Possibly, but what is known for sure from modern and ancient DNA comparisons is that early modern human migrants interbred with a more ancient Asian group, the Denisovans. At present that group is only known from a site in Siberia and another in Tibet through a finger bone and a few molar teeth that yielded DNA significantly different from both living humans and ancient Neanderthals. So we have no tangible evidence of what the Denisovans looked like, unlike Asian H. erectus of whom there are many substantial fossils. Yet DNA has not been extracted from any of them. That is hardly surprising for the Indonesian specimens because hot and humid conditions cause DNA to break down quickly and completely. There is a much better chance of extracting genomes from the youngest H. erectus fossils from higher latitudes in China. Once that is achieved, we will know whether they are indeed erects or can be matched genetically with Denisovans.

See also:  Price, M. 2019. Ancient human species made ‘last stand’ 100,000 years ago on Indonesian island (Science)

Chewing gum and the genetics of an ancient human

The sequencing of DNA has advanced to such a degree of precision and accuracy that minute traces of tissue, hair, saliva, sweat, semen and other bodily solids and fluids found at crime scenes are able to point to whomever was present. That is, provided that those persons’ DNA is known either from samples taken from suspects or resides in police records. In the case of individuals unknown to the authorities, archived DNA sequences from members of almost all ethnic groups can be used to ‘profile’ those present at a crime. Likely skin and hair pigmentation, and even eye colour, emerge from segments that contain the genes responsible.

One of the oddest demonstrations of the efficacy of DNA sequencing from minute samples used a wad of chewed birch resin. Such gums are still chewed widely for a number of reasons: to stave off thirst or hunger; to benefit from antiseptic compounds in the resin and to soften a useful gluing material – resin derived by heating birch bark is a particularly good natural adhesive . Today we are most familiar with chicle resin from Central America, the base for most commercial chewing gum, but a whole range of such mastics are chewed on every inhabited continent, birch gum still being used by Native North Americans: it happens to be quite sweet. The chewed wad in this case was from a Neolithic site at Syltholm on the Baltic coast of southern Denmark (Jensen, T.Z.T. and 21 others 2019. A 5700 year-old human genome and oral microbiome from chewed birch pitch. Nature Communications v. 10, Article 5520; DOI: 10.1038/s41467-019-13549-9). The sample contained enough ancient human DNA to reconstruct a full genome, and also yielded fragments from a recent meal – duck with hazelnuts – and from several oral bacteria and viruses, including a herpes variety that is a cause of glandular fever. The sample also shows that the carrier did not have the gene associated with lactase persistence that allows adults to digest milk.

An artist’s impression of the gum chewing young woman from southern Denmark (credit: Tom Bjorklund)

The chewer was female and had both dark skin and hair, together with blue eyes; similar to a Mesolithic male found in a cave in Cheddar Gorge in SW England whose petrous ear bone yielded DNA. By no means all fossil human bones still carry enough DNA for full sequencing, and are in any case rare. Chewed resin is much more commonly found and its potential awaits wider exploitation, particularly as much older wads have been found. Specifically, the Danish woman’s DNA reveals that she did not carry any ancestry from European Neolithic farmers whose DNA is well known from numerous burials. It was previously thought that farmers migrating westward from Anatolia in modern Turkey either replaced or absorbed the earlier Europeans. By 5700 years ago farming communities were widespread in western Europe, having arrived almost two thousand years earlier. The blue-eyed, dark Danish woman was probably a member of a surviving group of earlier hunter gatherers who followed the retreat of glacial conditions at the end of the Younger Dryas ice re-advance about 11,500 years ago. The Syltholm site seems to have been occupied for hundreds of generations. Clearly, the community had not evolved pale skin since its arrival, as suggested by a once popular theory that dark skin at high latitudes is unable to produce sufficient vitamin-D for good health. That notion has been superseded by knowledge that diets rich in meat, nuts and fungi provide sufficient vitamin-D. Pale skins may have evolved more recently as people came to rely on a diet dominated by cereals that are a poor source of vitamin-D.

Why did anatomically modern humans replace Neanderthals?

Extinction of the Neanderthals has long been attributed to pressure on resources following the first influx into Europe by AMH bands and perhaps different uses of the available resources by the two groups. One often quoted piece of evidence comes from the outermost layer in the teeth of deer. Most ruminants continually replace tooth enamel to make up for wear, winter additions being darker than those during summer. Incidentally, the resulting layering gives away their age, as in, ‘Never look a gift horse in the mouth’! Deer teeth associated with Neanderthal sites show that they were killed throughout the year. Those around AMH camps are either summer or winter kills. The implication is that AMH were highly mobile, whereas Neanderthals had fixed hunting ranges whose resources would have been depleted by passing AMH bands. That is as may be, but another possibility has received more convincing support.

Neanderthal populations across their range from Gibraltar to western Siberia were extremely low and band sizes seem to have been small, even before AMH made their appearance. This may have been critical in their demise, based on considerations that arise from attempts to conserve threatened species today (Vaesen, K. et al. 2019. Inbreeding, Allee effects and stochasticity might be sufficient to account for Neanderthal extinction. PLoS One, v. 14, article e0225117; DOI: 10.1371/journal.pone.0225117). The smaller and more isolated groups are, the more likely they are to resort to inbreeding in the absence of close-by potential mates. There is evidence from Neanderthal DNA that such endogamy was practised. Long-term interbreeding between genetic relatives among living human groups is known to result in decreased fitness as deleterious traits accumulate. On top of that, very low population density makes finding mates, closely related or not, difficult (the Allee effect). A result of that is akin to the modern tendency of young people born in remote areas to leave, so that local population falls and becomes more elderly. The remaining elders face difficulties in assembling hunting and foraging parties; i.e. keeping the community going. Many Neanderthal skeletons show signs of extremely hard, repetitive physical effort and senescence; e.g. loss of teeth and evidence of having to be cared for by others. Both factors in small communities are exacerbated by fluctuating birth and death rates and changed gender ratios more than are those with larger numbers; i.e. random events have a far greater overall effect (stochasticity). Krist Vaesen and colleagues from the Netherlands use two modern demographic techniques that encapsulate these tendencies to model Neanderthal populations over  10,000 years.

By themselves, none of the likely factors should have driven Neanderthals into extinction. But in combination they may well have done so, even if modern humans hadn’t arrived around 40 ka. Completely external events, such as epidemics or sudden climate change, would have made little difference. Indeed the very isolation of Neanderthal bands over their vast geographic range would have shielded them from infection, and they had been able to survive almost half a million years of repeated climate crises. If their numbers were always small that begs the question of how they survived for so long. The authors suggest that they ran out of luck, in the sense that, finally, their precariousness came up against a rare blend of environmental fluctuations that ‘stacked the odds’ against them. It is possible that interactions, involving neither competition nor hostility, with small numbers of AMH migrants may have tipped the balance. A possibility not mentioned in the paper, perhaps because it is speculation rather than modelling, is social fusion of the two groups and interbreeding. Perhaps the Neanderthals disappeared because of hybridisation through choice of new kinds of mate. Some closely-related modern species are under threat for that very reason. Although individual living non-African humans carry little more than 3% of Neanderthal genetic material it has been estimated that a very large proportion of the Neanderthal genome is distributed mainly in the population of Eurasia. For that to have happened suggests that interbreeding was habitual and perhaps a popular option

See also: Sample, I. 2019. Bad luck may have caused Neanderthals’ extinction – study. (Guardian 27 November 2019)

Early human migrations in southern Africa

Comparing the DNA profiles of living people who are indigenous to different parts of the world has achieved a lot as regards tracing the migrations of their ancestors and amalgamations between and separations from different genetic groups along the way. Most such analyses have centred on alleles in DNA from mitochondria (maternal) and Y chromosomes (paternal), and depend on the assumption that rates of mutation (specifically those that have neither negative nor positive outcomes) in both remain constant over tens of thousand years and genetic intermixing through reproduction. Both provide plausible hypotheses of where migrations began, the approximate route that they took and the timing of both departures from and arrival at different locations en route. Most studies have focused on the ‘Out of Africa’ migration, which began, according to the latest data, around 80 ka ago. Arrival times at various locations differ considerably, from around 60 ka for the indigenous populations of Australia and New Guinea, roughly 40 ka for Europe and ~12 ka for the Americas. Yet an often overlooked factor is that not all migrating groups have descendants that are alive today. For instance, remains of anatomically modern humans (AMH)have been found in sediments in the Levant as old as 177 ka (see: Earliest departure of modern humans from Africa, January 2018), and between 170 to 210 ka in southern Greece (See: Out of Africa: The earliest modern human to leave). Neither have yielded ancient DNA, yet nor are their arrival times compatible with the ‘route mapping’ provided by genetic studies of living people. Such groups became extinct and left no traceable descendants, and there were probably many more awaiting discovery. Maybe these mysteries will be penetrated by DNA from the ancient bones, should that prove possible.

The recorded history of AMH within Africa began around 286 to 315 ka in Morocco (see: Origin of anatomically modern humans, June 2017) and their evolutionary development may have spanned much of the continent, judging by previously discovered fossils in Ethiopia and South Africa that are older than 200 ka. Again, ancient DNA has not been extracted from the oldest fossils; nor is that likely to be possible because the double helix breaks down quickly in hot and humid climates. Genetic data from living Africans are growing quickly. An additional 198 African mtDNA genomes reported recently have pushed up the total available for analysis, the bulk of them being from eastern and southern Africa (Chan, E.K.F. and 11 others 2019. Human origins in a southern African palaeo-wetland and first migrations. Nature, v. 575, p. 185-189; DOI: 10.1038/s41586-019-1714-1). The study focuses on data from the KhoeSan ethnic group, restricted to areas south of the Zambezi River, who speak a language with distinctive  click consonants. Some KhoeSan still practice a hunter-gatherer lifestyle. Previous genetic studies showed the KhoeSan to differ markedly from other inhabitants of southern Africa, and they are widely regarded as having inhabited the area for far longer than any other groups. A sign of this emerges from their mtDNA in a genetic lineage signified as L0. Comparing KhoeSan mtDNA with the wider genetic database allowed the researchers to plot a ‘family tree’. Measures of the degree of difference between samples push back the origin of L0 and the KhoeSan themselves to roughly 200 ka.

The Okavango Delta today during the wet season (Credit: Wikimedia Commons)

It turns out that the LO lineage has several variants, whose geographic distributions allow the approximate place of origin for the lineage and directions of later migration from it to be mapped. It seems that LO was originally indigenous to the modern Okavango Delta and Makgadikgadi salt flats of Botswana. People carrying the original (L0k) variant are estimated to have remained in the broad area for about 70 thousand years. During that time it was all lush, low-lying wetland around a huge, now vanished lake. The hydrology of the area was dramatically split by regional tectonic activity at around 60 ka. The lake simply evaporated to form the salt pan of the Makgadikgadi, leaving only the seasonal Okavango Delta as a destination for flood water. People carrying Lok stayed in the original homeland whereas other shifted. Migration routes to the northeast and towards the southwest and south are crudely mapped by the distribution of the other L0 variants among modern populations. They followed ‘green corridors’ between 130 and 110 ka, the collapse of the ecosystem leaving a small group of the founding population isolated from its descendants.

The paper claims that the former Botswana wetlands were the cradle of the first modern humans. Perhaps in southern Africa, but other, older AMH remains found far off and perhaps undiscovered elsewhere are more likely. But that can only be reconciled with the KhoeSan study by ancient DNA from fossils. Criticism of the sweeping claims in the paper has already been voiced, on these grounds and the study’s lack of data on paternal DNA or whole genomes from the sampled population.

See also: Gibbons, A. 2019. Experts question study claiming to pinpoint birthplace of all humans. Science (online); DOI: 10.1126/science.aba0155

Tracing hominin evolution further back

The earliest hominin known from Africa is Sahelanthropus tchadensis, announced in 2002 by Michel Brunet and his team working in 7 Ma old Miocene sediments deposited by the predecessor to Lake Chad in the central Sahara Desert. Only cranial bones were present. From the rear the skull and cranial capacity resembled what might have been regarded as an early relative of chimpanzees. But its face and teeth look very like those of an australopithecine. Sadly, the foramen magnum – where the cranium is attached to the spine – was not well preserved, and leg bones were missing. The position of the first is a clue to posture; forward of the base of the skull would suggest an habitual upright posture, towards the rear being characteristic of knuckle walkers. Some authorities, including Brunet, believe Sahelanthropus may have been upright, but others strongly contest that. The angle of the neck-and-head ball joint of the femur (thigh bone), where the leg is attached to a socket on the pelvis to form the hip joint is a clue to both posture and gait. The earliest clear sign of an upright, bipedal gait is the femur of a fossil primate from Africa – about a million years younger than Sahelanthropus, found in the Tugen Hills of Kenya. Orrorin tugenensis was described from 20 bone fragments, making up: a bit of the other femur, three hand bones; a fragment of the upper arm (humerus); seven teeth; part of the left and right side of a lower jawbone (mandible). Apart from the femur that retains a neck and head and signifies an upright gait, only the teeth offer substantial clues. Orrorin has  a dentition similar to humans apart from ape-like canines but significantly smaller in size – all known hominins lack the large canines, relative to other teeth. Despite being almost 2 Ma older than Ardipithecus ramidus, the first clearly bipedal hominin, Orrorin is more similar to humans than both it and Australopithecus afarensis, Lucy’s species.

Near-complete skeleton of Oreopithecus bambolii from Italy (credit: Wikipedia Commons)

DNA differences suggest that human evolution split from that of chimpanzees about 12 Ma ago. Yet the earlier Miocene stratigraphy of Africa has yet to provide a shred of evidence for earlier members of either lineage or a plausible last common ancestor of both. In 1872, a year after publication of Charles Darwin’s The Descent of Man parts of an extinct primate were found in Miocene sediments in Tuscany and Sardinia, Italy. In 1950 an almost complete skeleton was unearthed and named Oreopithecus bambolii (see Hominin evolution becoming a thicket, January 2013). Despite dozens of specimens having been found in different localities, the creature was largely ignored in subsequent debate about human origins, until 1990 when it was discovered that not only could Oreopithecus walk on two legs, albeit differently from humans, it had relatively small canine teeth and its hands were like those of hominins, capable of a precision grip. Dated at 7 to 9 Ma, it may lie further back on the descent path of hominins; but it lived in Europe not Africa. Now the plot has thickened, for another primate has emerged from a clay pit in Bavaria, Germany (Böhme, M. and 8 others 2019. A new Miocene ape and locomotion in the ancestor of great apes and humans. Nature, online publication; DOI: 10.1038/s41586-019-1731-0).

Bones from 4 Danuvius guggenmosi individuals. Note the diminutive sizes compared with living apes (Credit: Christoph Jäckle)

Danuvius guggenmosi lived 11.6 Ma ago and its fossilised remains represent four individuals. Both femurs and a tibea (lower leg), together with the upper arm bones are preserved. The femurs and vertebrae strongly suggest that Danuvius could walk on two legs, indeed the vertebral shapes indicate that it had a flexible spine; essential for balance by supporting the weight of the torso over the pelvis. It also had long arms, pointing to its likely hanging in and brachiating through tree canopies. Maybe it had the benefit of two possible lifestyles; arboreal and terrestrial. Its discoverers do not go that far, suggesting that it probably lived entirely in trees using both forms of locomotion in ‘extended limb clambering’. It may not have been alone, another younger European primate found in the Miocene of Hungary, Rudapithecus hungaricus, may also have had similar clambering abilities, as might have Oreopithecus.

There is sure to be a great deal of head scratching among palaeoanthropologists, now that three species of Miocene primate seem – for the moment – to possess  ‘prototype specifications’ for early entrants on the evolutionary path to definite hominins. Questions to be asked are ‘If so, how did any of them cross the geographic barrier to Africa; i.e. the Mediterranean Sea?’, ‘Did the knuckle-walking chimps evolve from a bipedal common ancestor shared with hominins?, ‘Did bipedalism arise several times?’. The first may not have been as difficult as it might seem (see Africa_Europe exchange of faunas in the Late Miocene, July 2013). The Betic Seaway that once separated Iberia from NW Africa, in a similar manner to the modern Straits of Gibraltar, closed during the Miocene after a ‘mild’ tectonic collision that threw up the Betic Cordillera of Southern Spain. Between 5.6 and 5.3 Ma there was a brief ‘window of opportunity’ for the crossing, that ended with one of the most dramtic events in the Cenozoic Era; the Zanclean Flood, when the Atlantic burst through what is now the Straits of Gibraltar cataclysmically to refill the Mediterranean .

See also: Barras, C. 2019. Ancient ape offers clues to evolution of two-legged walking. Nature, v. 575, online; Kivell, T.L. 2019. Fossil ape hints at how walking on two feet evolved. Nature, v. 575, online; DOI: 10.1038/d41586-019-03347-0

Life with the Neanderthals

From Robinson Crusoe’s discovery of Friday’s footprint on his desert island to Mary Leakey’s unearthing of a 3.6 Ma old trackway left by two adults and a juvenile of the hominin species Australopithecus afarensis at Laetoli in Tanzania, such tangible signs of another related creature have fostered an eerie thrill in whoever witnesses them. Other ancient examples have turned up, such as the signs of mud trampled by 800 ka humans (H. antecessor?) at Happisburgh, Norfolk, UK (see Traces of the most ancient Britons, February 2014). From a purely scientific standpoint, footprints provide key evidence of foot anatomy, gait, travel speed, height, weight, and the number of individuals who contributed to a trackway. At Le Rozel on the Cherbourg Peninsula in Normandy, France – about 30 km west of the D-Day landing site at Utah beach – Yves Roupin, an amateur archaeologist, discovered a footprint on the foreshore in the 1960s close to the base of a thick sequence of late-Pleistocene dune sediments exposed below a rocky cliff. Fifty years later, rapid onset of wind and tidal erosion threatened to destroy the site, so excavations and scientific analysis began. This involved excavation of thick overburden on an annual basis to expose as much of five footprint-bearing horizons as possible (about 90 m2).

Le Rozel
The Le Rozel excavation, with weighted plastic sheets to protect the site from erosion between visits (credit: Dominique Cliquet)

More and more prints emerged, each photographed and modelled in 3-D, with the best being preserved as casts using a flexible material, similar to that used by dentists (Duveau, J. eyt al. 2019. The composition of a Neandertal social group revealed by the hominin footprints at Le Rozel (Normandy, France). Proceedings of the National Academy of Sciences. 9 September 2019; DOI: 10.1073/pnas.1901789116). At the end of the excavation hundreds of prints had been found and recorded. They had been preserved in wet sand, probably deposited in an interdune pond. Luminescence dating of sand grains revealed that the footprints were produced around 80 ka ago, 35 ka before Europe was occupied by anatomically modern humans. Scattered around the site are numerous fossils of butchered prey animals, together with stone tools typical of Neanderthal technology.

Such a large number of footprints presented a unique opportunity to analyse the social structure of the Neanderthal group that produced them, for they came in many different sizes. During the very short period in which they were produced and buried by wind-blown sand, an estimated 10 to 13 individuals had crossed and re-crossed the site – there may have been more individuals who didn’t happen to cross the wet patch But the evidence suggests that children and adolescents, one of whom may have been as young as 2 years, predominated. Two or three with the biggest feet were probably adults as tall as 1.9 metres – about 20 cm taller that the average for modern human males. That is surprising for Neanderthals who are widely believed to have been more stocky. The fact that footprints occur in 5 horizons suggests that the band, or perhaps family, found the site to be good for occupation. Wider hypotheses are a little shaky. Did Neanderthals have large families? Does the predominance of children and adolescents indicate that they died young? But perhaps children stayed close to habitations with just a few ‘minders’, while other adults went off hunting and foraging. Were the kids playing?