The ancestry of our opposable thumbs

Since the appearance of smart phones and the explosion of social media our thumbs have found a new niche; typing while holding a mobile. At a desktop keyboard, most of us don’t use thumbs very much, unless we have mastered fast touch typing, but for a huge variety of manual tasks thumbs are essential. The first makers of sophisticated stone tools must have been able to grip between fingers and thumb to manipulate the materials from which they were made and to perform the various stages in creating a razor sharp edge. To do that, as most of us are aware, the tip of the thumb must be capable of touching the tips of all four fingers; an opposable thumb is essential for the ‘precision grip’. Being able to tell when opposable thumbs evolve depends, of course, on finding hand-bone fossils. Being made of many bones disarticulated hands are a lot more fragile than long bones or those of the skull. Complete fossil hands are rare, as are feet, but a number have been found more or less complete. Whichever hominin had evolved opposable thumbs, their potential would have given them a considerable advantage over those that hadn’t.

The main muscles that control the movements of modern human fingers and thumb (Credit: Wikipedia)

Simply comparing the shapes of fossilised bones of fingers and thumbs with those of modern humans and other living primates has, so far, not proved capable of resolving with certainty which hominin groups either did or did not have opposable thumbs. The key lies in the muscles that operate them. It has become commonplace to reconstruct faces and even whole bodies from fairly complete skeletal remains by modelling musculature from the positioning and shape of the points of attachment of muscles to bone. But that become increasingly difficult for the small-scale and intricate attachments in hands. The critical muscle for opposable thumbs is known as the Opponens pollicis (the Latin for thumb is Digitus pollex); a small triangular muscle that operates in conjunction with three others (with pollicis in their Latin names).

Fotios Karakostis and six colleagues from German, Swiss and Greek universities have devised software that can model muscles in 3-D (F.A. Karakostis et al. 2021. Biomechanics of the human thumb and the evolution of dexterityCurrent Biology, v.31,  online; DOI: 10.1016/j.cub.2020.12.041). Based on the anatomy of human and chimpanzee hand muscles and the positions of their attachment to individual bones, they have been able to establish a series of parameters that clearly distinguish the morphological and probably functional characteristics of the thumbs of these living primates. Complete sets of thumb bones from four Neanderthal skeletons show that they were significantly, but only slightly, different from anatomically modern humans. Those from three species of Australopithecus (africanus, sediba and afarensis) lie between ours and chimps’, with significantly closer affinity to chimpanzees. It seems that australopithecines of whatever age were not equipped with opposable thumbs and were possible tool producers and users with the very limited capabilities of modern chimps; holding, pounding and poking. A single set of hominin thumb bones from about two million years ago that were found in the famous Swartkrans Cave in South Africa show just as close affinity in thumb opposability to humans as do Neanderthals. So at 2 Ma there was a hominin species sufficiently dextrous to make and use sophisticated tools. The problem is, the bones are not directly associated with others and have been ascribed by different authors either to H. habilis or Paranthropus robustus. Interestingly, this paranthropoid has also been suggested (controversially) to have been the first known hominin to use fire, and it also used digging sticks. No one has ever suggested that the genus Homo descended from a paranthropoid ancestor or vice versa; these massively jawed beings did coexist with early humans in East Africa for over a million years. The other hominin who left hands in the geological record was Homo naledi; a controversial species because it was found in a barely accessible cave chamber, and took a while to date. This context gave rise to the notions that it was the direct ancestor of humans and that it buried its dead in a special place. However, it turned out to be relative recent, at about 280 ka (see: Homo naledi: an anti-climax; May 2017). Homo naledi does seem to have had opposable thumbs, but there is no associated evidence to suggest either tool making or use.

Fascinating as the methodology outlined by Karakostis et al. is, their findings do not take early human capabilities very much further than what is already known. Tools were made and used as far back as 3.3 Ma ago, and we know that H. habilis was doing this by about 2.6 Ma; i.e. long before the first evidence for opposable thumbs, and who had them first is uncertain. What is clear is that sophisticated tools, such as the bifacial Acheulian artifacts whose manufacture demands great dexterity, only appeared after the potential for nimble dexterity (about 1.8 Ma). The same goes for the first migration out of Africa, at about the same time, which demanded resourcefulness that may have sprung from the ability to manipulate natural materials effectively and carefully

See also: Handwerk B. 2012. How dexterous thumbs may have helped shape evolution two million years ago. (Smithsonian Magazine, 28 January 2021); Bower, B. 2021. Humanlike thumb dexterity may date back as far as 2 million years ago. (Science News, 28 January 2021)

A protein clue to H. antecessor’s role in human evolution

Homo_antecessor child
Forensic reconstruction of the remains of a Homo antecessor child from Gran Dolina Cave in northern Spain (credit Élisabeth Daynès, Museo de la Evolución, Burgos, Spain)

The older a fossil, no matter how well preserved it is, the less chance it has to contain enough undegraded DNA for it to be extracted and sequenced using the most advanced techniques. At present the oldest fossil DNA not to have passed its ‘sell-by’date is that of a 560 to 780 thousand year-old horse’s legbone found in Canadian permafrost. For human remains the oldest mtDNA is that of a ~430 ka individual from the Sima de los Huesos in northern Spain (see: Mitochondrial DNA from 400 thousand year old humans; Earth-logs December 2013). But there is another route to establishing genetic relatedness from the amino-acid sequences of proteins recovered from ancient individuals (see: Ancient proteins: keys to early human evolution?). Fossil teeth have proved to be good repositories of ancient protein and are the most commonly found hominin fossils.

A key species for unravelling the origins of the three most recent human groups (ourselves, Neanderthals and Denisovans) is thought to be Homo antecessor who inhabited the Gran Dolina Cave in the Atapuerca Mountains in northern Spain between about 1.2 Ma and 800 ka ago (see: Human evolution: bush or basketwork? Earth-logs, January 2014). Palaeoanthropologists excavated 170 skeletal fragments from six individuals in the most productive layer at Gran Dolina. Incomplete facial bones suggest a ‘modern-like’ face, although the remains as a whole are insufficient to reconstruct the oldest Europeans with sufficient detail to place them in anatomical relation to the younger groups. But there are several teeth. One of them, a permanent molar, has yielded informative proteins (Welker, F. and 26 others 2020. The dental proteome of Homo antecessor. Nature, v. 580, p. 235-238; DOI: 10.1038/s41586-020-2153-8) and has been dated to between 772 to 949 ka.

Amino acids in the dental proteins, sequenced using mass spectrometry, were compared with those of other hominins. Because protein sequences are coded by an animal’s genome they are a ‘proxy’ for DNA. The outcome is that the Gran Dolina proteins are roughly equally related to Denisovans, Neanderthals and ourselves, suggesting that, although the younger three groups are closely related, H. antecessor is an ‘outlier’. Being significantly older, it is likely to be the common ancestor of all three. Another species with close anatomical affinities is H. heidelbergensis (700 to 300 ka) found in Africa as well as in Europe. Its mtDNA (see: Mitochondrial DNA from 400 thousand year old humans; Earth-logs December 2013) matches that of Denisovans better than it does Neanderthals, yet without protein and full-genome analysis all that can be concluded is that it may be an intermediary between H. antecessor and the well known interbreeding triad of more recent times.

We are getting closer to a documented web of interrelationships between humans in general whose time span from 2 Ma ago is now well established. The remaining genetic link to be documented is that to H. erectus, the longest lived and most travelled of all ancient humans. Frido Welker and co-workers also had a shot at the proteomics of one of the first humans known to have migrated from Africa, using an isolated, presumably H. erectus, molar found at the 1.77 Ma site at Dmanisi in the Caucasus foothills of Georgia. Although inconclusive in placing that precociously intrepid group firmly in the human story, the fact that dental proteins were discovered is cause for optimism.

See also: Campbell, M. 2020. Protein analysis of 800,000-year-old human fossil clarifies dispute over ancestors (Technology Networks, 1 April 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

Ancient proteins: keys to early human evolution?

A jawbone discovered in a Tibetan cave turned out to be that of a Denisovan who had lived and died there about 160,000 years ago (see: Denisovan on top of the world; 6 May, 2019). That discovery owed nothing to ancient DNA, because the fossil proved to contain none that could be sequenced. But the dentine in one of two molar teeth embedded in the partial jaw did yield protein. The teeth are extremely large and have three roots, rather than the four more common in modern, non-Asian humans, as are Denisovan teeth from in the Siberian Denisova Cave. Fortunately, those teeth also yielded proteins. In an analogous way to the genomic sequencing of nucleotides (adenine, thymine, guanine and cytosine) in DNA, the sequence of amino acids from which proteins are built can also be analysed. Such a proteomic sequence can be compared with others in a similar manner to genetic sequences in DNA. The Tibetan and Siberian dentine proteins are statistically almost the same.

Triple helix structure of collagen, colour-coded to represent different amino acids (credit: Wikipedia)

At present the most ancient human DNA that has been recovered – from an early Neanderthal in the Sima de los Huesos in Spain – is 430,000 years old (see: Mitochondrial DNA from 400 thousand year old humans; December 2013). Yet it is proving difficult to go beyond that time, even in the cool climates that slow down the degradation of DNA. The oldest known genome of any animal is that of mtDNA from a 560–780 thousand year old horse, a leg bone of which was extracted from permafrost in the Yukon Territory, Canada. The technologies on which sequencing of ancient DNA depends may advance, but, until then, tracing the human evolutionary journey back beyond Neanderthals and Denisovans seems dependent on proteomic approaches (Warren, M. 2019. Move over, DNA: ancient proteins are starting to reveal humanity’s history. Nature, v. 570, p. 433-436; DOI: 10.1038/d41586-019-01986-x). Are the earlier Homo heidelbergensis and H. erectus within reach?

It seems that they may be, as might even earlier hominins. The 1.8 Ma Dmanisi site in Georgia, now famous for fossils of the earliest humans known to have left Africa, also yielded an extinct rhinoceros (Stephanorhinus). Proteins have been extracted from it, which show that Stephanorhinus was closely related to the later woolly rhinoceros (Coelodonta antiquitatis). Collagen protein sequences from a 3.4 Ma camel preserved in the Arctic and even from a Tanzanian 3.8 Ma ostrich egg shell show the huge potential of ancient proteomics. Most exciting is that last example, not only because it extends the potential age range to that of Australopithecus afarensis but into tropical regions where DNA is at its most fragile. Matthew Warren points out potential difficulties, such as the limit of a few thousand amino acids in protein sequences compared with 3 million variants in DNA, and the fact that the most commonly found fossil proteins – collagens –  may have evolved very little. On the positive side, proteins have been detected in a 195 Ma old fossil dinosaur. But some earlier reports of intact diosaur proteins have been questioned recently (Saitta, E.T. et al. 2019. Cretaceous dinosaur bone contains recent organic material and provides an environment conducive to microbial communities. eLife, 8:e46205; DOI: 10.7554/eLife.46205)


Multiregional human evolution in Africa

Africa is not only a large continent, but is subdivided into many different climatic zones and ecosystems and these have changed drastically over the last 2 Ma. It is further subdivided by terrain features, such as the courses of major rivers, large plateaus, tectonic rift systems and the mountains that frequently define their flanks. Getting around Africa is not easy today, was more difficult before modern transport, and many geomorphic provinces may have been mutually inaccessible in the distant past. For instance, the Sahara Desert forms a major barrier to travellers on foot because access to surface water is non-existent except at widely spaced oases. Without boats or rafts the Nile and Congo cannot be crossed for a thousand miles or more. Migration was perhaps a very rare event outside of periods of widespread humid climates or when great environmental stress forced people either to move or perish. Despite these physical and ecological divisions and barriers palaeoanthropologists have, until recently, tended to regard the evolution of Homo sapiens and earlier human and hominin species as having occurred within single populations: a linear view forced on them by scanty fossil remains and limited methodologies. Logically, when human numbers were small Africa probably had several isolated population Physical isolation would have engendered genetic isolation in which our ancestors evolved for tens of thousand years.

Anatomically modern human (AMH) remains found at Jebel Irhoud in Morocco turned out to be 315 ka old, displacing those from Ethiopia (190 ka) as the earliest known examples of AMH. Several more archaic H. sapiens fossils have turned up in southern Africa and as far afield as the Middle East, suggesting that the early evolution of AMH was in an Africa-wide context rather than in one area – the rift system of Ethiopia and Kenya – from which a new species radiated outwards. This breadth of finds has encouraged Eleanor Scerri of Oxford University and her many international colleagues to resurrect what was once a widely discarded hypothesis; a multiregional model of modern human origins, originally proposed to have arisen from pre-sapiens groups in Eurasia by Milford Wolpoff but which was sunk once genetic connections among living humans turned out to be rooted in Africa. (Scerri, E.M.L. and 22 others 2018. Did our species evolve in subdivided populations across Africa, and why does it matter? Trends in Ecology & Evolution, v. 33, p. 582-594; (PDF) doi: 10.1016/j.tree.2018.05.005). Scerri et al’s model is sited in Africa and the paper’s authors include several leading palaeoanthropologists who once opposed multiregionalism and established the Recent African Origin hypothesis on the back of the early genetic data.

early homo
Different early AMH cranium shapes: left Jebel Irhoud, Morocco (315 ka), right Qafzeh, the Levant (85 ka) (credit: Scerri et al, 2018; Figure 1)

From region to region in Africa, the oldest AMH crania show significant differences from each other, but within a distinct combination of features that clearly distinguish us from our fossil relatives and ancestors, such as Homo heidelbergensis from Zimbabwe and the primitive-looking H. naledi found in a South African cave in 2015. Improved dating now shows that the Zimbabwean H. heidelbergensis and H.naledi remains are roughly the same age as the Jebel Irhoud AMH specimens. The first has long been held as the progenitor of AMH and descended from H. antecessor, perhaps the common ancestor for AMH, Neanderthals and Denisovans about 700 ka ago. The three human species cohabited Africa early in the evolutionary history of AMH. It is now abundantly clear from ancient and modern genomes that AMH, Neanderthals and Denisovans interbred in Eurasia. The proximity in time and space of earlier African AMH to two more ancient human species opens up a similar possibility earlier in the emergence of all living humans. There is evidence for that too: Yoruba people living in West Africa, whose genomes have been analysed, carry up to 8% of genetic ancestry that originated in an unidentified ancient population that was non-sapiens. At present, DNA analysis with the same high precision and information content from other living Africans has not been performed, and deterioration of ancient DNA in African climates has so far thwarted genomic studies of ancient African fossils.

The new view of our origins points to repeated hybridisation involving other coexisting human species, as well as evolution in isolation, from the outset. It continued through later times while Neanderthals and Denisovans survived. Even recent human genetic history is peppered with intermingling of a great variety of migrants passing through all the habitable continents. Another issue: In the earliest times, were cultures exchanged as well as genes? The first appearance of AMH coincides with that of a new stone technology (Levallois technique), moving away from the earlier dominance by handaxes towards more delicate, leaf-shaped points, that characterise the African Middle Stone Age. Similar techniques reached Europe with the Neanderthals. Was this an invention of the earliest AMH or a joint venture?

You can find an excellent review of these issues in the September 2018 issue of Scientific American (Wong, K. 2018. Last hominin standing.  Scientific American, v. 319(3), p. 56-61) along with several other articles on human evolution.

A revised and updated edition of Steve Drury’s book Stepping Stones: The Making of Our Home World can now be downloaded as a free eBook

Early modern humans in Sumatra before the Toba eruption

In late July 2017 news emerged that modern humans first reached Australia at least 65 thousand years ago. Confirming that the date of departure from Africa to end up in SE Asia and Australasia was  considerable earlier than previously believed, deposits in Sumatra that contain remains of early Home sapiens have yielded even older ages (Westaway, K.E. and 22 others 2017. An early modern human presence in Sumatra 73,000–63,000 years ago. Nature v. 548 online; doi:10.1038/nature23452). This resulted from a re-examination of material from the Padang Caves first excavated more than a century ago by Eugène Dubois, famous for his discovery in Java of the first H. erectus remains. A richly fossiliferous breccia in the Lida Ajer cave yielded a fauna characteristic of a rainforest biome and included two teeth that Dubois considered to be human. Several later palaeontologists confirmed his identification as have hominin specialists in the present Australian-Indonesian-American-British-Dutch-German team. The fossil assemblage has long suggested great antiquity for the site, but only now has it been dated precisely. The dating employed three methods: optically stimulated luminescence dating of quartz grains from the breccia (85±25 to 62±5  ka); uranium-series dating of speleothem including fragments of hollow ‘soda-straw’ stalactites(84±1 to 71±7 ka); uranium-series dating of gibbon and orangutan teeth found together with the human teeth (86±13 to 76±7 ka). Statistical analysis of the age data suggests 73 to 63 ka for the fauna, with a maximum age for deposition of the breccia of 84±1 ka.

Satellite image of Lake Toba, the site of a VE...
Satellite image of Lake Toba in NW Sumatra (at centre), the site of the largest volcanic eruption during the history of human evolution ~71,600 years ago (credit: Wikipedia)

Stone tools which may have been carried by anatomically modern humans into the area have previously been used to suggest a minimum date of the arrival of migrants, though they may have been carried by ­H. erectus. Remarkably, such tools have been found beneath a thick bed of volcanic ash found throughout southern Asia and in Indian Ocean sediment cores. This has been dated at 71.6 ka and represents the explosive collapse of the caldera now containing Lake Toba in NW Sumatra that was the largest volcanic event in the entire history of the genus Homo. The new age data from Lida Ajer suggests that modern humans were present in its vicinty before the eruption, a view also supported by ‘molecular-clock’ dating of the range of mitochondrial DNA carried by living SE Asian people (79 to 75 ka). So, despite the stupendous magnitude of the Toba eruption is seems likely that some of the migrants survived.  Together with the dating of the earliest Australians the Sumatran evidence is at odds with the view, widely held by palaeoanthropologists, that the ‘Out of Africa’ exodus began by crossing the Straits of Bab el Mandab between 74 and 58 ka when global sea-level fell markedly during marine oxygen-isotope Stage 4 (MIS4). A problem with that hypothesis has been that climatic and ecological conditions in southern Asia during MIS4 were unfavourable. But is seems that modern humans were already there and capable of adapting to both the climate shift and to the devastation undoubtedly caused by Toba.

Stepping Stones eBook


A revised and updated edition of Stepping Stones: The Making of Our Home World by Steve Drury, first published in 1999, has been released as a free eBook on the book’s web site The revision incorporates the hundreds of commentaries on geoscientific advances written since 2000 by Steve for earth-pages. It is a personal view of the evolution of the Earth System and the emergence of humanity from it. First published by Oxford University Press, Stepping Stones was widely acclaimed by  fellow Earth scientists and general readers.

Homo naledi: an anti-climax

In September 2015 a barrage of publicity announced the remarkable unearthing of the remains of 15 diminutive hominins, dubbed Homo nadeli, from the floor sediments of an almost inaccessible South African cave, part of the equally hyped ‘Cradle of Humankind’ UNESCO World Heritage Site near Johannesburg. An international team of lithe women speleo-archaeologists was recruited for the excavation, for which the original discoverers were incapably burly. The remains included numerous examples of still articulated intricate bones, such as those of feet and hands, and none show signs of dismemberment by large scavengers. Indeed the discovery chamber was so far from the cave entrance that such animals probably were unaware of their presence. These features and the sheer complexity of the system strongly suggested that cadavers had been deliberately taken to the chamber; implying that the deep penetration had been accomplished using fire-brand illumination. What seized the headlines was the possibility of ritual burial, although sanitary disposal or panicked refuge from predators seem equally, if not more likely.

Embed from Getty Images

Lee Burger and the reconstructed skull of Homo naledi

Now yet more fossils have been reported from a separate chamber at a crawling distance about 150 m away from the original but closer to the system’s main entrance (~85 m). These add at least other 3 individuals to the H. nadeli association, with sufficient similarity to indicate that all 18 belong to H. naledi. This wealth of detail enabled the team of authors (Hawks, J. and 37 others 2017. New fossil remains of Homo naledi from the Lesedi Chamber, South Africa. eLife, v. 6, online; to perform a detailed comparative anatomic analysis of the species. The results are a mosaic, showing some post-cranial affinities with australopithecines, H. habilis, H.floresiensis, H. erectus, Neanderthals and anatomically modern humans, and others, such as the hands and shoulders, that are not well matched with other hominins. Their crania show a similar broad spectrum of resemblances, and as regards dentition they are distinctly primitive. They are also on the small-brained side of the hominin clade. Despite the astonishing abundance of fossil material, not a single artifact was found in the cave system, despite the apparent similarity of its hands to those of ourselves and Neanderthals.

With plenty of scope for speculation, H. nadeli remains enigmatic. The big question looming over the 2015 announcement of the species was its age, the discovers suggesting about 2 Ma, and placing on the direct line of human descent. On the same day as the fossil description there appeared a multi-method dating analysis (Dirks, P.H.G.M. and 19 others 2017. eLife, v. 6, online;, which showed that with little doubt that the H. nadeli association was deposited between 236 ka and 335 ka; around the time when anatomically modern humans first emerged and stone tools had undergone a >2 Ma technological evolution. To me, the only sensible conclusion at present is that H. nadeli is another addition to the 6 species living and in some cases coexisting across the late Pleistocene world, and that expansion of ideas beyond that must await DNA analysis; a definite possibility considering the age of the fossils, their seemingly good preservation in a relatively dry cave system and the new possibility of cave soils as well as bones yielding genetic materials. The leader of the research team, Lee Berger of the University of the Witwatersrand now maintains, together with four other members of the research team, that H. nadeli may be a coelacanth-like survivor of Homo’s earliest diversification and that ‘we cannot exclude that this lineage was responsible for the production of Acheulean or Middle Stone Age tool industries’.

Barras, C. 2017. Homo naledi is only 250,000 years old – here’s why that matters. New Scientist, 6 May 2017 Issue

Sample, I. 2017. New haul of Homo naledi bones sheds surprising light on human evolution. The Guardian, 9 May 2017

Human penis bone lost through monogamy?

The baculum or penis bone is arguably the most variable of mammalian bones, present in some species but not others. Among those in which it does occur the baculum varies enormously in shape, length and breadth relative to body size. This makes it likely to have been subject to the most divergent evolution among mammals. Yet its evolution has remained somewhat puzzling until recently. Observation has shown that the width of the baculum in male house mice is positively correlated with reproductive success. So one factor in the bone’s evolution may be postcopulatory sexual selection: female mice seem to favour males well endowed in this department once they have mated with them, a notion supported by careful laboratory experimentation. The physical role of the penis bone is to support and protect the penis during sexual intercourse. Sturdy dimensions are increasingly efficacious the longer the duration and the greater the frequency of copulation, particularly among polygamous and seasonally breeding species. They also tend to delay or inhibit a female mating with another male after copulation.

Walrus baculum, approximately 22 inches long
Walrus baculum, approximately 0.6 metres long (credit: Wikipedia)

Matilda Brindle and Christopher Opie of University College London have applied advanced phylogenetic statistical analysis to data on the dimensions of penis bones among 2000 mammal species (Brindle, M. & Opie, C. 2016. Postcopulatory sexual selection influences baculum evolution in primates and carnivores. Proceedings of the Royal Society, B, v. 283, doi: 10.1098/rspb.2016.1736) and suggest that the baculum first evolved in mammals between 145 to 95 Ma ago, earlier mammals likely having no penis bone. Ancestral primates and carnivorous mammals, however, were so endowed. Yet some mammalian species have lost the baculum.  Among the primates human males do not have one whereas male chimpanzees and bonobos, with which we share a last common ancestor, do: both are boisterously promiscuous whereas humans are pair-bonded to a large degree.

The issue of polygamy versus monogamy among human ancestors, and when the latter emerged, continues to exercise palaeoanthropologists. The former in other living primates is often associated with a marked contrast in size between males and females – sexual dimorphism. The earliest hominins, such as species of Australopithecus, did exhibit such dimorphism whereas species of Homo show significantly less size contrast, which some have taken to mark the emergence of pair-bonding amongst members of the earliest human species to be passed on to their successors. Another indicator of competitiveness among primate males for females, and their dominance over the latter, is the near universal possession of large canine teeth among males of polygamous primates; an odd feature for species whose diet is dominantly and often exclusively vegetarian. Not only do living humans not have prominent canines, neither do any known fossil hominins. Despite the views of a small minority of anthropologists who demand that modern human females won social parity with males only in the last 100 thousand years, only to lose it following the Neolithic ‘revolution’, the physical evidence suggests that a trend towards that emerged with other distinct characteristics of hominins and concretised in early Homo. An assiduous search for fossil hominin penis bones may yet reveal the moment of monogamy.

Tree-climbing australopithecines

We know that Lucy, the famous Australopithecus afarensis, could climb trees because her many bone fractures show that she fell out of a tree to her death. But that does not mean her species was an habitual tree-climber: plenty of modern humans fall to their deaths from trees, cliffs and the like. But the issue seems to have been resolved by using X-ray tomography of Lucy’s limb bones (Ruff, C.B. et al. 2016. Limb bone structural proportions and locomotor behaviour in A.L. 288-1 (“Lucy”).  PLOS ONE v. 11, e0166095. doi:10.1371/journal.pone.0166095) during the skeleton’s triumphal series of exhibits in the US.

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The authors, including two of those who showed that Lucy died after a fall using similar data, compared the digital 3-D models of her surviving arm- and leg bones with those of other hominins and living primates, estimating their relative strengths at different positions. Lucy was probably stronger in the arm than in the leg, but not to the same degree as chimpanzees. This is a feature that would significantlyassist climbing , but her bipedal locomotion on the ground would have been only slightly different from that of later Homo species. If anything, her strength relative to size would have been greater than ours, perhaps reflecting less reliance on tools for getting food and defending herself. But almost certainly Australopithecus afarensis habitually spend more time in trees, perhaps foraging and as a defence against predation, especially at night.

The new data for Lucy allows palaeoanthropologists to better judge the capabilities of other hominins. Interestingly Homo habilis, the earliest of our genus, may have had similar habits. But later species, beginning with H. erectus/ergaster, were as Earth-bound as we are. This suggests a shift in hominin ecology from an early and probably long history of semi-arboreal behavior until humans became masters of their terrain about 1.9 Ma ago, probably through their invention of better tools and the controlled use of fire.

Read more about human evolution here and here