Tag Archives: human evolution

Genus Homo pushed back nearly half a million years

Bill Deller, a friend whose Sunday is partly spent reading the Observer and Sunday Times from cover to cover, alerted me to a lengthy article by Britain’s doyen of paleoanthropologists Chris Stringer of the Natural History Museum. (Stringer, C. 2015. First human? The jawbone that makes us question where we’re from. Observer, 8 March 2015, p. 36). His piece sprang from two Reports published online in Science that describe about 1/3 of a hominin lower jaw unearthed – where else? – in the Afar Depression of Ethiopia. The discovery site of Ledi-Geraru is a mere 30 km from the most hominin-productive ground in Africa: Hadar and Dikika for Australopithecus afarensis (‘Lucy’ at 3.2 Ma and ‘Selam’ at 3.3 Ma, respectively); Gona for the earliest-known stone tools (2.6 Ma); and the previously earliest member of the genus Homo, also close to Hadar.

On some small objects mighty tales are hung, and the Ledi-Geraru jawbone and 6 teeth is one of them. It has features intermediate between Australopithecus and Homo, but more important is its age: Pliocene, around 2.8 to 2.75 Ma (Villmoare, B. And 8 others. Early Homo at 2.8 Ma from Ledi Geraru, Afar, Ethiopia. Science Express doi: 10.1126/science.aaa1343). The sediments from which Ethiopian geologist Chalachew Seyoum, studying at Arizona State University, extracted the jawbone formed in a river floodplain. Other fossils suggest open grassland rich with game, similar to that of the Serengeti in Tanzania, with tree-lined river courses. These were laid down at a time of climatic transition from humid to more arid conditions, that several authors have suggested to have provided the environmental stresses that drove evolutionary change, including that of hominins (DiMaggio, E.N. and 10 others 2015. Late Pliocene fossiliferous sedimentary record and the environmental context of early Homo from Afar, Ethiopia. Science Express doi: 10.1126/science.aaa1415).

Designating the jawbone as evidence for the earliest known member of our genus rests almost entirely on the teeth, and so is at best tentative awaiting further fossil material. The greatest complicating factor is that the earliest supposed fossils of Homo (i.e. H. habilis, H rudolfensis and others yet to be assigned a species identity) are a morphologically more mixed bunch than those younger than 2 Ma, such as H. ergaster and H. erectus. Indeed, every one of them has some significant peculiarity. That diversity even extends to the earliest humans to have left Africa, found in 1.8 Ma old sediments at Dmanisi in Georgia (Homo georgicus), where each of the 5 well-preserved skulls is unique.  The Dmanisi hominins have been likened to the type specimen of H. habilis, but such is the diversity of both that is probably a shot in the dark.

English: Cast replica of OH 7, the type specim...

Replica of OH 7, the deformed type specimen of Homo habilis. (credit: Wikipedia)

Coinciding with the new Ethiopian hominin papers a study was published in Nature the same week that describes how the type specimen of H. habilis (found, in close association with crude stone tools and cut bones, by Mary and Lewis Leakey at Olduvai Gorge, Tanzania in 1960) has been digitally restored from its somewhat deformed state when found (Spoor, F. et al. 2015. Reconstructed Homo habilis type OH 7 suggests deep-rooted species diversity in early Homo. Nature, v. 519, p. 83-86, doi:10.1038/nature14224). The restored lower jaw and teeth, and part of its cranium, deepened the mysterious diversity of the group of fossils for which it is the type specimen, but boosts its standing as regards probable brain size from one within the range of australopithecines to significantly larger –~750 ml compared with <600 ml – about half that of modern humans. The habilis diversity is largely to do with jaws and teeth: it is the estimated brain size as well as the type specimen’s association with tools and their use that elevates them all to human status. Yet, the reconstruction is said by some to raise the issue of a mosaic of early human species. The alternative is an unusual degree of shape diversity (polymorphism) among a single emerging species, which is not much favoured these days. An issue to consider is: what constitutes a species? For living organisms morphological similarity has to be set against the ability for fertile interbreeding. Small, geographically isolated populations of a single species often diverge markedly in terms of what they look like yet continue to be interfertile, the opposite being convergence in form by organisms that are completely unrelated.

Palaeontologists tend to go largely with division on grounds of form, so that when a specimen falls outside some agreed morphological statistics, it crosses a species boundary. Set against that the incontrovertible evidence that at least 3 recent human species interbred successfully to leave the mark in all non-African living humans. What if the first humans emerging from, probably, a well-defined population of australopithecines continued to interbreed with them, right up to the point when they became extinct about 2 Ma ago?

On a more concrete note, the Ledi Geraru hominin is a good candidate for the maker of the first stone tools found ‘just down the road’ at Gona!

Convincing, indirect evidence for early toolmakers

A surprising number of animals pick up items from their surroundings and use them, mainly to get at otherwise inaccessible foodstuffs. What sets humans apart from such tool users is that we make them and for a long time part of our repertoire has been tools used to make other tools; so-called ‘machine tools’. An example is a piece of antler used to pressure-flake flint to give a stone blade a better edge, a more recent one is the increasing use of robots on assembly lines. Making a tool is impossible for a bird with only its beak and ill-adapted feet, while even a chimpanzee lacks various forms of grip needed for precisely directed force and manipulation. It was Frederick Engels who first focussed on the importance of the hand being freed to evolve the capacity for manual labour by the permanent adoption of an upright posture and gait, in his essay The Part Played by Labour in the Transition from Ape to Man written in 1876.

The earliest tools known turned up in 2.6 Ma old sediments at Gona in NE Ethiopia, while evidence for tool use is well accepted from cracked and sliced bones found in sediments dated at 2.5 Ma from Bouri in the same region. In neither case can the finds be tied to fossil remains of the makers and users, the earliest direct link emerging from famous Olduvai Gorge in western Tanzania, where crude Oldowan tools and worked bones occur with incomplete remains of a hominin, dubbed Homo habilis (‘handy man’) because of this association. Somewhat more controversial are bones that show cuts and scrape marks plus signs of having been cracked open that were found in a 3.4 Ma context at Dikika, also in Ethiopia, within the same sedimentary horizon as the young Australopithecus afarensis known as Selam (‘Hello’). The Dikika material is little different from 0.9 to 1.2 Ma younger bones at Bouri and Olduvai: the controversy seems to stem more from its much greater age and association with hominins deemed by some to have been incapable of creating tools.

English: Main division on the (right) human hand.

Bone structure of the (right) human hand. (credit: Wikipedia)

An entirely novel approach to the issue of the first tools and their makers, which with little doubt would have tickled Engels no end, is a careful anatomical and physiological examination of fossil hominin hand bones in comparison with those of chimps and living humans (Skinner, M.M. et al. Human-like hand use in Australopithecus africanus. Science, v. 347, p. 395-399). The bones being scrutinized are the five metacarpals that form the links in the palms from muscles of the forearm to finger and thumb movements and thus to various kinds of grip. In humans there are a host of ways of gripping objects from the precision of opposed thumb and finger pinching, especially that using the forefinger, to the squeezing power grip that wraps thumb and all fingers around an object and makes a fist. The best a chimp can do is grabbing a branch, to which its knuckle-walking hands are well adapted. The tips of the metacarpals are mechanically loaded according to the types of grip used repeatedly in life and that works to modify the physical density of the tips’ spongy bone tissue in patterns that vary according to habitual usage of the hand and its digits. This new approach is reputedly far more diagnostic than the actual shape of metacarpal bones, and requires high-resolution CT scanning.

Known early human and Neanderthal tool-makers show very similar patterns: in fact they suggest far more heavy loading through various kinds of grip than the metacarpals of humans from the modern period. In 1.8 to 3.0 Ma old A. africanus and Paranthropus robustus (a gorilla-like but bipedal australopithecine) from South Africa metacarpals suggest that both were habitually using a tree-climbing grip, much as chimpanzees do, but more closely resembled modern human and Neanderthal committed tool users. Both were certainly capable of using forceful precision grips to make and use tools up to 0.5 Ma earlier than the date of the earliest known tools. So far the technique has not been applied to the palm bones of earlier hominins such as A. afarensis (2.9-3.9 Ma) and Orrorin tugenensis (~6 Ma). Despite the suggestion of tool-making capability­, agreeing that it did take place in non-Homo hominins must await finds of tools, as well as signs of their use, in close association with fossil remains of their makers. The Dikika association is simply not enough. Yet, some bipedal being must have made tools before the date of the earliest ones (~2.6 Ma) discovered at Gona. Look at it this way: it is a lucky archaeologist who discovers every piece of evidence for a fundamental social change at one site. The fact that, by definition, the vast bulk of Pliocene and Pleistocene sediments that may contain the key evidence is either buried by younger material or was a victim of erosion, means that the chance of resolving the origin of the fundamental feature of human behaviour is tiny. The chance that scientists will continue looking is astronomically higher.

Are modern humans ‘domesticated’?

While animals, especially dogs, underwent domestication the deliberate or unconscious human choice of favoured physiological and behavioural traits produced distinct differences between the ancestral species and the ‘breeds’ with which we are now familiar. In general domestication has resulted in dogs with reduced jaws and flatter faces, lower aggression, especially in the case of males, and reduced stressfulness in the company of humans and other tame dogs compared with their wolf ancestors. It is widely accepted that cats have ‘tamed themselves’ through the adoption of lifestyles associated with the benefits of close association with human communities, which have resulted in similar adaptations to those in more deliberately domesticated dogs. It is beginning to dawn on anthropologists that human social evolution may unwittingly have affected the course of our own evolution. Tighter social bonding among growing sizes of communities as brain capacity increased and the behavioural and cognitive attributes needed for that have been summarised recently by a group associated with the Social Brain hypothesis of Robin Dunbar of Oxford University, UK (Gamble, C., Gowlett, J. & Dunbar, R.I.M. 2014. Thinking Big: How the Evolution of Social Life Shaped the Human Mind. ISBN-13: 978-0500051801;Thames and Hudson: London).

It was Charles Darwin who first speculated that ‘Man in many respects may be compared with those animals which have long been domesticated’. But to what extent does the hominin fossil record support such a view? Collaborators from Duke University and the University of Iowa, USA, have set out to analyse physiological changes over the last 200 ka that may be explained in this way (Cieri, R.L. et al. 2014. Craniofacial feminization,social tolerance and the origins of behavioural modernity. Current Anthropology, v. 55, p. 419-443. Includes discussion and responses). They used the degree of projection of brow ridges, facial shape and cranial volume from 3 groups of Homo sapiens remains: skulls older than 80 ka (13 specimens); spanning 38 to 80 ka (41) and from recent humans (1367). They found that brow ridges shrank significantly over the last 80 thousand years, faces shortened and cranial capacity decreased, especially among males. This resulted in a convergence in appearance between males and females, which the authors attributed to general lowering of testosterone and stress hormone levels through selection for greater social tolerance: akin to similar physiognomic changes in domesticated dogs which DNA analyses have shown to be been linked with modification of genes associated with aggression regulation. The finding among dogs suggests that their domestication is accomplished by slower development; i.e. young animals are naturally less fearful and have a greater tendency to taming. This delayed development from foetus to adulthood, with retention in mature individuals of juvenile characteristics, is known as neoteny, and affects all manner of adult characteristics, including coloration, snout length and the adrenal glands: as adult dogs now more resemble wolf pups, so human adults are more like young chimps than elders. At a conference where Cieri et al.’s results were presented, it was observed that hunter gatherer bands are intolerant, to the point of capital punishment, of wife stealers, murderers and seriously dishonest men, whereas such reactions fall off significantly among members of larger social groups involved in agriculture and urban life. Such modern behavioural patterns tally with brow ridge, face length and cranial capacity, perhaps linked with selection for personalities more attuned to cooperation.

English: comparison of Neanderthal and Modern ...

Comparison of Neanderthal and Modern human skulls from the Cleveland Museum of Natural History (credit: Wikipedia)

Although earlier human species, such as H. neanderthalensis, heidelbergensis and erectus had significantly different skull anatomy, each had prominent brow ridges that, on this account, may signify both greater exposure to testosterone and less social tolerance, and smaller group sizes. But, so far, analysis of the Neanderthal genome has not led to publication of any comments about testosterone or stress-hormone related genes. However, a clear strand of discussion is developing around evidence rather than mere speculation about psychological/cognitive aspects of human evolution that challenges the old-style ‘what-you-see-is-what-there-was’ (WYSWTW) archaeological dogma: a dialectic of social and biological relationships.

Human evolution news

Since discovery of its fossilised remains in Liang Bua cave on the Indonesian island of Flores was discovered in 2004 the diminutive Homo floresienesis, dubbed the ‘hobbit’ by the media, has remained a popular news item each time controversies surrounding it have flared. To mark the tenth anniversary  of its publication of a paper describing the remains Nature has summarised the recollections of many of those involved in trying to understand the significance of H. floresiensis (Callaway, E. 2014. Tales of the hobbit. Nature, v. 514, p. 422-426). Two main schools of thought continue in dispute, one holding that it is anatomically so different from anatomically modern humans and earlier members of the genus Homo that it constitutes a new species, despite its youngest member dating back only 18 ka, the other that it is H. sapiens, its tiny size having resulted from some kind of genetic disorder, such as microcephaly or Down’s syndrome. There have been so many attempts to expunge the idea of such an odd fossil cohabiting an island with fully modern humans yet being a different and perhaps extremely archaic species that such an outlook itself seems somewhat pathological.

English: Homo floresiensis, replica Deutsch: H...

Replica of the Homo floresiensis skull from Liang Bua cave, Flores, Indonesia (credit: Wikipedia)

The evidence presented to force H. floresiensis into a deformed human mould has never been convincing, and the best way of combating that view is to document from a ‘non-combatant ‘standpoint the many ways in which its anatomy differs from ours and how it might have arisen; a job to which Chris Stringer of the Museum of Natural History in London is amply qualified (Stringer, S. 2014. Small remains still pose big problems. Nature, v. 514, p. 427-429). He, like the original discoverers, feels this is a case of evolution of small stature due to a limited population being isolated for a long time on a relatively small island, which is just what happened to elephants that colonised Flores to become the pigmy Stegodon that H. floresiensis seemingly hunted. These tiny Flores dwellers (adults were about 1 m tall) used fire and made tools, similar ones dating as far back as ~1 Ma. Stringer mentions the possibility of first human colonisation about that time by Asian H. erectus but also the view that if it happened once there may have been several waves of immigration to Flores. The unusual ‘hobbit’ anatomy is not restricted to tiny size and a small skull and brain cavity (400 cm3), but includes odd hips, wrist bones, shoulder joint and collar bone. In fact the remains bear as much or more resemblance to australopithecines like ‘Lucy’ (3.2 Ma) than to other members of our genus, even H. erectus that has been proposed as its possible ancestor. Could they be far-travelled descendants of the 1.8 Ma old H. georgicus from Dmanisi in Georgia? More fossils clearly need to be found, and Stringer raises the possibility of the search being widened to other islands east of Java, such as Sulawesi, the Philippines and Timor. He hints that in such a tectonically active region tsunamis may have led to animals and humans saving themselves and then being current dispersed on rafts of broken vegetation, rather like some survivors of the 2004 Indian Ocean tsunami who ended up 150 miles from their homes by such a means.

Another story that is set to ‘run and run’ is that of ‘alien’ DNA in the human genome and productive relations between early out-of-Africa migrants with Neanderthals, Denisovans and perhaps yet a mysterious, earlier human species. The oldest (45 ka) anatomically modern human genome sequence so far charted is from a leg bone found by a mammoth-ivory prospector in Siberian permafrost (Fu, Q. and 27 others 2014. Genome sequence of a 45,000-year-old modern human from western Siberia. Nature, v. 514, p. 445-449). Like a great many living non-Africans this individual carried about 2 % Neanderthal DNA, but unlike living people the 45 ka genome has it in significantly longer segments. That allowed the authors to re-estimate the timing of the genetic flow from Neanderthals into the individual’s ancestors. Previous estimates from living DNA geve the possibility of that being between 37-86 ka, but this closer data suggests that it happened between 7 to 13 ka before the date of the fossil femur, i.e. narrowing it down to between 52 and 58 ka closer to the widely suggested time of African exodus around 60 ka (but see an Earth Pages item from September 2014)

More on human evolution here and here

Human evolution: bush or basketwork?

Analysis of DNA from ancient humans has revealed its power decisively in the last few years, and especially at the beginning of 2014 with publication of the sixth full genome of an individual who was not an anatomically modern human (Prüfer, K. and 44 others 2014. The complete genome sequence of a Neanderthal from the Altai Mountains. Nature, v. 505, p. 43-49). The newly sequenced material came from a toe bone found in the Denisova Cave in the Altai Mountains of southern Siberia; the same location made famous in 2010 by genetic evidence for unknown late hominins, the Denisovans . The bone occurred in the same layer of cave sediment, dated at 50.3 ka, which yielded the Denisovan finger bone, but from a lower sublayer. So there is no firm evidence that both groups cohabited the cave.

The genome reveals that the individual was female and related to the three Neanderthals from Croatia and another infant Neanderthal from the Caucasus, also analysed previously by Svante Pääbo’s team at the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany (Note that the toe-bone team also includes co-workers from US, Chinese, Austrian, French and Russian institutions). The closest statistical link is to the Caucasian infant Neanderthal’s DNA. Interestingly, it proved possible to demonstrate that the Siberian Neanderthal woman was from a population that was clearly inbred, her parents having been related at the level of half siblings. Her mtDNA shows that she shared a common ancestor with all 6 Neanderthals from whom mtDNA has been analysed.

Comparing genomes from the single Denisovan, the 5 Neanderthals and living humans from sub-Saharan Africans gives an estimated 550 to 765 ka time of divergence of a population leading to anatomically modern humans from the progenitors of Neanderthals and the Denisovan. The Neanderthal-Denisovan split was roughly 380 ka ago. It was already known that non-African living humans contain genetic evidence for past interbreeding with Neanderthals and that some people in Asia, Australia, Melanesia and the Philippines had acquired genes from Denisovans. More refined comparisons now show Oceanians to have 3 to 6% Denisovan make-up with Asians in general sharing 0.2%. Neanderthal to modern non-African gene flow is now estimated at between 1.5 and 2.1%, with Asians and Native Americans being at the high end.  Neanderthals and Denisovans also interbred, but only at the level of about 0.5% inheritance. However, that genetic sharing involved DNA regions known to confer aspects of immunity and sperm function, that also made their way into living non-African humans.

Since the common ancestor of Neanderthals and Denisovans left Africa long before modern humans appeared on the scene it would be expected that living Africans’ genomes would show the same level of similarity with both the now extinct groups, if all three originally shared a common ancestor. A surprising outcome from comparison of Neanderthal and Denisovan genomes with those of living sub-Saharan Africans is that there is a significant bias towards Neanderthal rather than Denisovan comparability.  There are three possibilities for this bias. After the Neanderthal-Denisovan split the former group may have continued to interbreed with the group leading to modern Africans (and indeed to modern non-Africans): that would require Neanderthal genetics to have originated in Africa before they migrated to Eurasia. Secondly, the gene flow could have been from the ancestors of modern humans to Neanderthal progenitors, making descendant Neanderthals more like modern humans. Prüfer et al. suggest that the evidence is less supportive of both and weighs towards a third possibility; that the Denisovans interbred with an unknown contemporary hominin, whose genetic make-up was yet more different from that of all three known groups of the late Pleistocene and therefore their common ancestor . This may have been Homo antecessor or possibly H. erectus who survived until as late as 20 ka in SE Asia.

Family tree of the four groups of early humans living in Eurasia 50,000 years ago and the gene flow between the groups due to interbreeding. Image credit: Kay Prüfer et al.

Family tree of the four groups of early humans living in Eurasia 50,000 years ago and the gene flow between the groups due to interbreeding. Image credit: Kay Prüfer et al.

As other commentators  on the paper (Birney, E. & Pritchard J.K. 20113. Four makes a party. Nature, v. 505, p. 32-34)  have observed, ‘…Eurasia during the late Pleistocene was an interesting place to be a hominin, with individuals of at least four quite diverged groups living, meeting and occasionally having sex.’ All this arises quite convincingly from the genetics of only 7 ancient individuals, to show that it may no longer be appropriate to consider human evolution as a tree or a bush linking permanently separated species. Either it is the history of a single, polymorphic species – remains of 1.7 Ma old Homo georgicus show clear evidence of such polymorphism – or a better metaphor for human development is an interwoven basket or twine. Rumour has it that attempts are being made to sequence an H. antecessor dated at 900 ka from Gran Dolina Cave in the Atapuerca Mountains in Northern Spain: as they say, ‘Watch this space’!

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Mitochondrial DNA from 400 thousand year old humans

The Sima de los Huesos (‘pit of bones’) site in the cave complex of Atapuerca in northern Spain has yielded one of the greatest assemblages of hominin bones. Well-preserved remains of at least 28 individuals date to the Middle Pleistocene (>300 ka). Anatomically the individuals have many Neanderthal-like features but also show affinities with earlier Homo heidelbergensis, who is widely considered to be the common ancestor for anatomically modern humans and Neanderthals, and perhaps also for the mysterious Denisovans. Most palaeoanthropologists have previously considered this Atapuerca group to be early Neanderthals, divergent from African lineages because they migrated to and became isolated in Europe.

English: Cranium 5 is one of the most importan...

Human cranium from the Sima de los Huesos, Atapuerca mountains (Spain). (credit: Wikipedia)

The riches of the Sima de los Huesos ossuary made it inevitable that attempts would be made to extract DNA that survived in the bones, especially as bear bones from the area had shown that mtDNA can survive more than 4300 ka. There has been an air of expectancy in hominin-evolution circles, and indeed among the wider public, since rumours emerged that the famous Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany had initiated genetic sequencing under the direction of Svante Pääbo: perhaps another ‘scoop’ to add to their reconstructing the first Neanderthal and Denisovan genomes. The news came out in the 5 December 2013 issue of Nature, albeit published on-line (Meyer, M. and 10 others 2013. A mitochondrial genome sequence of a hominin from Sima de los Huesos, Nature, v. 504; doi:10.1038/nature12788) with a discussion by Ewan Callaway (Callaway, E. 2013. Hominin DNA baffles experts Nature, v. 504, p. 16-17).

The bafflement is because the mtDNA from a femur of a 400 ka  individual does not match existing Neanderthal data as well as it does that of the Denisovan from Siberia by such a degree that the individual is an early Denisovan not a Neanderthal. Northern Spain being thousands of kilometres further west than the Denisova cave heightens the surprise.  Indeed, it may be on a lineage from an earlier hominin that did not give rise to Neanderthals. The full Neanderthal and Denisovan genomes suggest that they shared a common ancestor up to 700 ka ago. So the Sima de los Huesos individual presents several possibilities. It could be a member of an original population of migrants from Africa that occupied wide tracts of Eurasia, eventually to give rise to both Neanderthals and Denisovans. That genetic split may have arisen by the female line carrying it not surviving into populations that became Neanderthals – mtDNA is only present in the eggs of mothers. Mind you, that begs the question of who the Neanderthal females were. Another view is that the Sima de los Huesos individual may be descended from even earlier H. antecessor, whose 800 ka remains occur in a nearby cave. Pääbo’s team have even suggested that Denisovans interbred with a mysterious group: perhaps relics of the earlier H. antecessor colonists.

Established ideas of how humans emerged, based on bones alone and very few individuals to boot, are set to totter and collapse like a house of cards. Interbreeding has been cited three times from DNA data: modern human-Neanderthal; modern human-Denisovan and Denisovan with an unknown population. Will opinion converge on what seems to be obvious, that one repeatedly errant species, albeit with distinct variants, has been involved from far back in the human evolutionary journey?  There seems only one avenue to follow for an answer, which is to look for well preserved H. heidelbergensis. H. antecessor and H. erectus remains and apply ever improving techniques of genetic retrieval. Yet there is a chance that stretches of ancient DNA can be teased out of younger fossils.

Early humans could probably kill at a distance

It is always refreshing when physical anthropologists perform experiments as well as pondering on bones. It turns out that examining the bio-mechanics of college baseball players can provide useful clues about where in fossil anatomy to look for signs of potential big-game hunters. Anyone who can hurl a baseball, or one of the smaller but much harder red ones preferred by non-Americans, at speeds exceeding 100 kph could in all likelihood bring down a substantial prey animal with a rock and even more so with a spear. At the heart of an important examination of what our forebears might have done to get a meaty meal (Roach, N.T. et al. 2013. Elastic energy storage in the shoulder and the evolution of high-speed throwing in Homo. Nature, v. 498, p. 483-486) is a US-Indian team’s sophisticated study of college baseball players’ throwing action using high-speed video, radar and precise timing techniques.

Matt Kata throwing

Matt Kata throwing for the Houston Astros (Photo credit: Wikipedia)

It seems that there are several physiological phases in demon ball throwing: rotation of the torso; rotation flexion and extension of the shoulder; flexion and extension of the elbow; and wrist extension. All of these contribute to acceleration of the ball before release. While the thrower steps forward the arm is cocked so that ligaments, tendons and muscles crossing the shoulder become stretched, thereby storing energy. During the acceleration phase the bend in the elbow is snapped straight adding yet more power. Readers should note the difference between this action and that of a bowler in cricket, where the elbow snap is banned on pain of severe penalty and public humiliation of the bowler who ‘chucks’. Since a fast bowler also adds energy by running into the crease, this is a humanitarian aspect of the Rules of cricket, although several legal West Indian bowlers of the past 40 years are still remembered with terror by their batsmen contemporaries. No such stricture is placed on the baseball pitcher who has no run-up.

These observations focus attention on the structure of shoulder and elbow, yielding a robust means of predicting how fast throwers with different configurations may have thrown objects. Chimpanzees make poor players of ball games, although they will throw the odd stick, but just for aggressive show. The same goes for the earliest hominins for which we have suitable fossil material: australopithecines may occasionally have eaten carrion but they couldn’t throw rocks or spears with enough force to bring down anything and their throwing range would have been pathetic. Not so Homo erectus! They were well equipped in the hurling department and could, were they so inclined, have hunted equally as well as modern humans. Interestingly, earlier hominins had some of the physiological necessities of decent throwing, but not all of them. So it seems that the full combination emerged in the evolution of our own genus around 2 Ma ago,

This is in contrast to a view held by some anthropologists, such as Christopher Boehm of the University of Southern California, that big game hunting using projectile weapons emerged only with anatomically modern humans after 250 ka, and most likely only reached its acme 45 ka ago. That assumption, at least by Boehm, is central to notions of how social activities centred on meat sharing may have helped evolve morals, such as altruism and shame (see Boehm, C. 2012. Moral Origins: The Evolution of Virtue, Altruism and Shame. Basic Books, New York). That H.erectus would have been able to harness sufficient energy to kill at a distance casts doubt on such assertions. Mere foraging does not require throwing-capable physiology, so how it evolved in early humans with neither the inclination nor bodies to at least begin throwing projectiles at potential prey is something that school might consider.

 

Could the Toba eruption have affected migrating humans?

Around 73 thousand years ago a supervolcano in Sumatra erupted on a scale unprecedented in the last 2 million years. It left a 100 by 30 km elliptical caldera now occupied by Lake Toba, and explosively ejected 2800 of magma, about 800  km3 falling as ash as far afield as the Greenland ice cap. Although ice-core records show little if any sign of associated climate change in polar regions, the vast amount of ash and sulfate aerosols blasted into the stratosphere must have had some ‘global winter’ effect. Large areas of South Asia were blanketed by thick beds of ash. Human migration from Africa into Eurasia was probably underway at the time, indeed stone tools are found directly beneath and above the Toba ash in southern India and Malaysia. Some palaeoanthropologists have seen the stresses imposed by the Toba eruption as possible means of reducing the entire human population to a mere few thousand: a genetic ‘bottleneck’ that could have led to rapid evolution among surviving generations that may have shaped changes in human behaviour and culture.

Landsat image of Lake Toba, the largest volcan...

Landsat image of Lake Toba, the largest volcanic crater lake in the world. (credit: Wikipedia)

There is a widening range of views on the climate changes that may have followed Toba. It has even been suggested that global mean surface temperature fell by as much as 10°C (Robock, A. et al. 2009. Did the Toba volcanic eruption of ∼74 ka B.P. produce widespread glaciation? Journal of Geophysical Research: Atmospheres, v. 114, DOI: 10.1029/2008JD011652), although not so far as to produce a worldwide glacial surge but sufficient to devastate vegetation. This bleak look back to a critical point in human affairs resulted from modeling of the effects of a global reflective cloud of ash and sulfate. A later modeling study factored in particle and aerosol sizes (Timmreck, C. et al. 2010. Aerosol size confines climate response to volcanic super-eruptions. Geophysical Research Letters, v. 37, doi:10.1029/2010GL045464) to give a less dramatic, but still severe maximum global cooling due to Toba of ~3.5°C.

The focus has now shifted from modelling to a more direct look at the environmental effects of the Toba super-eruption, preserved in sediments beneath Lake Malawi in southern Africa (Lane, C.S. et al. 2013. Ash from the Toba supereruption in Lake Malawi shows no volcanic winter in East Africa at 75 ka. Proceedings of the National Academy of Science, v. 110, doi/10.1073/pnas.1301474110). The sediments contain a thin ash layer that is very different from those produced by East African Rift volcanism but chemically and texturally similar to the Toba ash from the Indian Ocean and India. The sediments, diatom fossils and chemical biomarkers immediately above the ash show little sign of a significant temperature fall. At most it records a 1.5°C fall, and the authors conclude little chance of a human genetic bottleneck among Africans living at the time.

There is clearly a conflict between results of modeling and real-world climatic data, which is interesting in its own right. But the Malawi findings do not rule out ‘bottlenecks’ resulting from severe stress in South Asia where the ash itself would have severely affected game and vegetation for long enough to face migrating human bands with the prospect of starvation. Obviously, some survived to move on and to leave their tools behind on top of the Toba Ash.

Hybridisation in human evolution

A press release from the Max Planck Institute for Evolutionary Anthropology in Leipzig, Germany, announces the completion of a genome from a third Neanderthal individual and its release to other anthropological researchers. Using a toe bone found in the same Siberian cave as the finger bone used to reconstruct the genome of a Denisovan, the new analysis is by far the most precise obtained from Neanderthal remains. For the first time it is possible to distinguish copies of the genes inherited by the individual from both parents. In that regard its quality is as good or even better than genomes from present-day humans.  Svante Pääbo, lead scientist at the Institute, hopes that the team will now be able to more deeply penetrate aspects of the history of Neanderthals and Denisovans – the Denisovan genome is of a similar quality – and of the genetic divergence of anatomically modern humans from the common ancestors of all three.

The data release coincided with a review of genetic evidence for interbreeding between early Homo sapiens and other species (Hammer, M.F. 2013. Human hybrids. Scientific American, v. 308 (May 2013), p. 52-57). Michael Hammer of the University of Arizona begins by comparing the main hypotheses for the evolution of fully modern humans. The Out-of-Africa model involves modern people of African origin completely replacing all other human species in and outside Africa. Multi-regional evolution posits archaic populations  originally living in and outside Africa being  gradually assimilated by migration and interbreeding that transferred modern traits everywhere yet retained some regionally distinct features of the archaic groups.

1: 1=Homo sapiens 2=Neanderthals 3=Early Homin...

Modern human migration out of and within Africa relative to the domains of coeval archaic humans 1 = modern humans 2 = Neanderthals 3 = other archaic humans (credit: Wikipedia)

The first model clearly has to be modified as evidence accumulates for some degree of hybridisation with archaic groups outside Africa. The second of the two pre-genome ideas seemed to be rendered obsolete by the DNA evidence for significant interbreeding between early immigrants from Africa and Eurasian and Asian populations of earlier archaic migrants – Neanderthals and Denisovans respectively – whereas modern Africans show no sign of recent contact with these archaic groups. However, not all regions of the genome have been examined for signs of more universal hybridisation.

Hammer cites a 2005 study of DNA sequences in a non-functional region of the X chromosome that pointed towards its origin as far back as 1.5 Ma and entry into the modern genome in East Asia from a species of Homo that had entered the region far earlier than Neanderthals or Denisovans (perhaps Homo erectus). There is similar evidence for fertile interbreeding of modern humans with an archaic species in Africa. Together with the evidence for a degree of Neanderthal-modern interbreeding in the Middle East around 80 to 50 ka, some of whose descendants destined to reach Australasia interbred with Denisovans, probably further to the east, such reports clearly indicate a significant role for hybridisation.

As the source of all human species, Africa had the greatest chance of several of them living close-by at any one time and thus of interbreeding. Hammer and colleagues at the University of California, San Francisco report a 2 percent contribution of genetic material in three sub-Saharan modern populations from archaic humans split-off from them around 700 ka and recombined in moderns at about 35 ka. By chance Albert Perry, an African-American who chose to be genetically profiled commercially, found himself the possessor of a never-before recorded DNA variant in his Y chromosome. It was shown to have branched off the modern genetic tree almost 350 ka ago. His overall Y-chromosome DNA match was with men who live in a small area of Cameroon. Further complicating matters is evidence for a small Neanderthal component in the DNA of Maasai people living in East Africa.

Though still unpublished, fossil evidence unearthed in Nigeria and the Democratic Republic of the Congo of humans with cranial characteristics that bear both modern and archaic features. These are not early moderns but date back to about 13 ka. They imply either that there were still archaic humans cohabiting with moderns recently, or regular interbreeding had been going on for millennia further back in time. Hybridisation is emerging as a complicating factor in human evolution, and possibly one of great importance. It may have conferred immunity to pathogens endemic in new territories entered by modern migrants from Africa, and who is to say what other aspects of fitness? The once favoured Replacement model is looking shaky and will be refuted if more evidence emerges of viable hybridisation between various archaic humans and new arrivals from Africa. The African modern genetic pattern may dominate but the ‘old ones’ maintain a genetic foothold, despite their extinction. It always has to be borne in mind that all the modern genetic lines that emerged from Africa since about 100 ka probably did not survive either: those that did may have done so because they combined with significant traces of humans of much greater antiquity and owe their continuity to that legacy.

More on Neanderthals, Denisovans and anatomically modern humans

Further support for Homo floresiensis (the ‘hobbit’)

English: Cave where the remainings of ' where ...

Liang Bua cave on Flores, Indonesia where fossils of Homo floresiensis were discovered in 2003 (credit: Wikipedia)

When they were first discovered in Liang Bua cave on the Indonesian island of Flores diminutive hominin remains sparked off a heated debate. Part of the reason for dispute was the age of the deposit in which they were found (18 to 850 ka), so young that it indicated possible cohabitation on the island with anatomically modern humans. On the one hand, the finders claimed that they represented a previously unknown hominin species. Other specialists considered that the tiny size (adults no taller than about a metre with brain capacity around that of australopithecines) indicated some congenital  dwarfism.

Homo floresiensis (the "Hobbit")

Homo floresiensis skull (credit: Wikipedia)

In the 9 years since the remains came to light, several anatomically features have been cited to support the view of a distinct hominin species: their lack of a chin and different arm and shoulder anatomy, which H. floresiensis shares with H. erectus and H. georgicus. The fossils are associated with simple stone tools and bones of a variety of prey animals that show cut marks and charring, suggesting that cooking was part of these hominins’ lifestyle; despite having small brains they were not unintelligent.

Substantial remains of nine or more individuals have been unearthed so that anatomical detail is almost complete. In 2007 details were published of three well-preserved wrist bones from the original find. They too were sufficiently different from modern and Neanderthal humans to warrant confirmation that H. floresiensis is indeed a distinct hominin species. Further work on wrist bones from other individuals has now more or less put the seal on this identity (Orr, C.M. et al. 2013. New wrist bones of Homo floresiensis from Liang Bua (Flores, Indonesia). Journal of Human Evolution, v. 64, p. 109-129), the authors  concluding that ‘The pattern of morphology … supports H. floresiensis as a valid taxon and refutes the hypothesis that these specimens represent modern humans with some kind of pathology or growth disturbance’. They take matters further by suggesting that their lineage was established before divergence of modern humans and Neanderthals. As with the shoulder morphology that of their wrists would have somewhat hindered tool-making dexterity, but nonetheless they did make tools.