Tag Archives: Homo

Multiple invention of stone tools

Steadily, the record of stone tools has progressed further back in time as archaeological surveys have expanded, especially in East Africa (Stone tools go even further back, May 2015). The earliest known tools – now termed Lomekwian – are 3.3 million years old, from deposits in north-western Kenya, as are cut-marked bone fragments from Ethiopia’s Afar region. There is no direct link to their makers, but at least six species ofAustralopithecus occupied Africa during the Middle Pliocene. Similarly, there are various options for who made Oldowan tools in the period between 2.6 and 2.0 Ma, the only known direct association being with Homo habilis in 2.0 Ma old sediments from Tanzania’s Olduvai Gorge; the type locality for the Oldowan.

The shapes of stone tools and the manufacturing techniques required to make them and other artefacts, are among the best, if not the only, means of assessing the cognitive abilities of their makers. A new, detailed study of the shapes of 327 Oldowan tools from a 2.6 Ma old site in Afar, Ethiopia has revealed a major shift in hominin working methods (Braun, D.R. and 17 others 2019. Earliest known Oldowan artifacts at >2.58 Ma from Ledi-Geraru, Ethiopia, highlight early technological diversity. Proceedings of the National Academy, v. 116, p. 11712-11717; DOI: 10.1073/pnas.1820177116). The sharp-edged tools were made by more complex methods than the Lomekwian. Analysis suggests that they were probably made by striking two lumps of rock together, i.e. by a deliberate two-handed technique. On the other hand, Lomekwian tools derived simply by repeatedly bashing one rock against a hard surface, not much different from the way some living primates make rudimentary tools. But the morphology of the Ledi-Geraru tools also falls into several distinct types, each suggesting systematic removal of only 2 or 3 flakes to make a sharp edge. The variations in technique suggest that several different groups with different traditions used the once lake-side site.

Various 2.6 Ma old Oldowan stone tools from Ledi-Geraru, Ethiopia (credit: Braun et al., 2019)

Ledi-Geraru lies about 5 km from another site dated about 200 ka earlier than the tools, which yielded a hominin jawbone, likely to be from the earliest known member of the genus Homo. A key feature that suggested a human affinity is the nature of the teeth that differ markedly from those of contemporary and earlier australopithecines. It appears that the tools are of early human manufacture. The ecosystem suggested by bones of other animals, such as antelope and giraffe was probably open grassland – a more difficult environment for hominin subsistence. The time of the Lomekwian tools was one of significantly denser vegetation, with more opportunities for gathering plant foods. Perhaps this environmental shift was instrumental in driving hominins to increased scavenging of meat, the selection pressure acting on culture to demand tools sharp enough to remove meat from the prey of other animals quickly, and on physiology and cognitive power to achieve that.

See also: Solly, M. 2019. Humans may have been crafting stone tools for 2.6 million years (Smithsonian Magazine)

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!

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