John S. Compton

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Update #4 to Human Origins: How diet, climate and landscape shaped us

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Australia is home to some of our species’ most ancient roots outside of Africa. When the first of our human (Homo) lineage arrived in Australia has long been debated. Although early members of our lineage (Homo erectus, for example) were living nearby on the island of Java as early as 1.7 million years ago, it was only much later that humans entered Australia. Dated archaeological sites suggest that our species, Homo sapiens was the first to arrive by around 50 thousand years ago (50 ka) and that we had become widespread throughout the continent by 45 ka (Hamm and others, 2016). This chronology generally fits with the proposed timing of the ‘Great Expansion,’ when our species rapidly spread out of Africa around 60 to 50 ka and effectively conquered the world. Our species had earlier forays out of Africa prior to the Great Expansion, but these never appear to have amounted to much. In my book Human Origins I refer to those who left Africa prior to the Great Expansion as anatomically modern humans (AMHs) and those that left as part of the Great Expansion as people having cultures on a par with modern hunter-gatherers. Our species evolved in Africa 200 to 160 ka, but appears to have only slowly acquired cultures equivalent to modern hunter-gatherers between 100 and 70 ka while in Africa. This may explain, in part, why earlier exits were relatively unsuccessful: the Great Expansion had to wait until modern hunter-gatherers had emerged in Africa and their movement beyond Africa, in turn, had to wait until the Sahara-Arabian Desert became sufficiently green to let them out 60 to 50 ka. However, a recent study by Clarkson and others (2017) proposes that modern people had arrived in Australia by at least 59.3 ka and possibly as early as 70 ka. Such an early arrival of people in Australia would be at odds with the Great Expansion scenario.

photo of front cover of the book Human Origins by John Compton

Going global: possible pathways and timing of the Great Expansion, when modern people conquered the world.

The study by Clarkson and others revisited and expanded upon earlier work carried out at the Madjedbebe rock shelter located in northern Australia. From the lowest and oldest layers of human occupation they recovered over 10,000 artefacts that, in addition to an in place hearth, included many stone flakes, ground ochre (associated with mica), edge-ground hatchets and a grinding stone. The age of the deposit was determined by optically stimulated luminescence (OSL), a method by which the measured luminescence of many individual sand grains is related to the time it took for them to accumulate their luminescence from exposure to radioactivity in the surrounding sediment. The artefacts and their associated OSL ages establish that the sequence of layers at the site has not been significantly disturbed or mixed and indicate, quite convincingly, that our species was in Australia as early as 65±5 ka. Such an early arrival of modern hunter-gatherers is inconsistent with all other archaeological evidence indicating that the peopling of the Eurasian continent occurred between 55 and 45 ka.

 

A critical question is who were these first Australians? If they were part of one of the earlier waves of our species (AMHs) that expanded out of Africa between 130 and 80 ka, then there is no problem with the timing of their arrival. Instead, the potential problem is with their cultural artefacts, which appear to be inconsistent with those associated with earlier waves of our species out of Africa. On the other hand, if they were part of the exodus from Africa of modern hunter-gatherers that successfully filled the world, then the problem lies not with their artefacts but with the time of their arrival, which would appear too early relative to other sites. At this stage, the best explanation appears to be that the first to arrive were AMHs from an earlier exodus out of Africa that had developed some cultural artefacts similar to modern hunter-gatherers. Despite their cultural artefacts, these early arrivals in Australia, as elsewhere, appear to have had a limited presence in the archaeological record and were largely replaced by the later arrival of modern hunter-gatherers by around 50 ka as part of the Great Expansion.

 

The authors of the paper argue that those living at the Madjedbebe rock shelter were behaviourally modern people based on the presence of an edge-ground hatchet, a grinding stone and ochre (a red coloured pigment) mixed with highly reflective mica flakes. The use of ochre dates back to our predecessor species, already widespread in Africa by 230 ka. However, these earlier ochre occurrences are not associated with highly reflective mica flakes, for which the Madjedbebe site is the earliest known example. Ground hatchets and a grinding stone at the site may also be the earliest occurrences known, but the lack of fine stone tools (such as backed microlithic stone tools for spear or bow and arrow), along with art or jewellery (micaceous ochre aside) suggest that these earliest arrivals were perhaps not yet in possession of a complete modern hunter-gatherer culture.

 

Interpreting the significance of these artefacts is difficult because cultures evolve over time and can be lost. Loss of cultures may result if specific cultural items are no longer needed or if groups are too small to sustain them. There is evidence that humans living in Southeast Asia developed different cultures to those elsewhere in Eurasia, with a lack of stone tools in particular. The lack of stone tools may reflect their use of wood (bamboo), which is much less likely to be preserved than stone tools. There is evidence that those associated with the Great Expansion were innovative, using fire ash to detoxify yams and tree nuts by 46 ka in Borneo, as well as artistic, making some of the earliest cave paintings 40 ka on Sulawesi (Aubert and others, 2014). What the archaeological record suggests in Australia is that the first to cross over did not arrive with the typical modern hunter-gatherer toolkit but managed to reinvent it over time, with bone points appearing by 40 ka and backed microliths by 30 to 20 ka (Hamm and others, 2016).

 

If Homo erectus was already in Java by 1.7 Ma, why did it take so long for humans to cross over to Australia? The difficulty in reaching Australia is that it requires crossing over water to get there. This is true today as well as in the past when many of the islands were joined into one large landmass during periods of lowered sea level when ice sheets built up in the Northern Hemisphere. Homo erectus survived in Southeast Asia until around 300 ka. Some among them managed to cross narrow, calm seaways to reach the island of Flores, where they underwent island dwarfism to become the one-metre tall ‘hobbit’ (Homo floresiensis) who lived there until around 50 ka. But crossing the larger seaways required to reach New Guinea – Australia (collectively called Sahul) was apparently too great for earlier members of our lineage to manage. There is evidence that our species was living in Sumatra sometime between 78 and 58 ka, but unfortunately no cultural items were found associated with the fossil evidence (Westaway and others, 2017). Perhaps these were AMHs for an earlier exit and among those who the first to successfully navigate their way as far as Australia.

 

Humans could expand into Southeast Asia when sea level was lower (pale blue areas of Sunda) but had to island-hop their way through Wallacea to reach the connected landmasses of New Guinea and Australia (Sahul).

Therefore, it would appear that the first arrivals in Australia were AMHs from an earlier exit out of Africa. As elsewhere in Eurasia, the AMHs who arrived in Australia had a relatively subdued impact and were largely displaced by the later arrival of modern hunter-gatherers as part of the Great Expansion who reached Australia by around 50 ka. This Great Expansion scenario is supported by genetic studies which indicate that all people today outside of Africa descend from a single population that exited out of Africa and that they acquired DNA from intermingling with Denisovans and Neanderthals along the way between 53 to 45 ka. DNA studies of indigenous Australians (aborigines) indicate that those who arrived with the Great Expansion rapidly colonized the continent by 45 ka (Tobler and others, 2017). To whatever extent earlier expansions of our species may have taken place prior to the Great Expansion, none feature strongly in either the archaeological or genetic evidence, their existence appears to have been swamped out by the rapid peopling of the world during the Great Expansion. However, the earlier forays of our species out of Africa were perhaps not completely erased, with some genetic studies suggesting that a few percent of the DNA from these earlier expansions survives in modern populations (Pagani and others, 2016).

 

 

Further reading

Aubert, M., and others, 2014. Pleistocene cave art from Sulawesi, Indonesia. Nature 514, 223-227. doi:10.1038/nature13422

 

Clarkson, C., and others, 2017. Human occupation of northern Australia by 65,000 years ago. Nature 547, 306-310. doi:10.1038/nature22968

 

Gibbon, A., 2017. The first Australians arrived early. Science 357, 238-239. doi: 10.1126/science.357.6348.238

 

Hamm, G., and others, 2016. Cultural innovation and megafauna interaction in the early settlement of arid Australia. Nature 539, 280-283. doi:10.1038/nature20125

 

Marean, C.W., 2017. Early signs of human presence in Australia. Nature 547, 285-287.

 

Pagani, L., and others, 2016. Genomic analyses inform on migration events during the peopling of Eurasia. Nature 538, 238-241. doi:10.1038/nature19792

 

Tobler, R., and others, 2017. Aboriginal mitogenomes reveal 50,000 years of regionalism in Australia. Nature 544, 180-184. doi:10.1038/nature21416

 

Westaway, K.E., and others, 2017. An early modern human presence in Sumatra 73,000–63,000 years ago. Nature 548, 322-325. doi:10.1038/nature23452

 

 

© John S. Compton (www.johnscompton.com)

 

The first humans in America

 

Update #3 to Human Origins: How diet, climate and landscape shaped us

 

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A recent study presents evidence that members of our human (Homo) lineage were in North America around 130 thousand years ago (Holen and others, 2017; Wade, 2017a). This is a shocking claim because it is more than 100 thousand years before the previously established timing of 14 thousand years ago for when humans first entered the Americas. This latest report is not the first to argue for a much earlier presence of humans in the Americas, but it provides by far the most compelling and best dated evidence yet.

 

photo of front cover of the book Human Origins by John Compton

The evidence consists of scattered mastodon bones lying immediately adjacent to several large stones. The bed in which the bones and stones occur were recovered from a 12-m-thick succession of river deposits discovered at a construction site near San Diego, California. The mastodon is a distant relative of elephants and was common in North America (along with the woolly mammoth and other large animals) up until people hunted them to extinction by around 12 thousand years ago. The authors of the study argue that the physical damage of some mastodon bones and associated stones indicate that the stones were used as hammerstones and anvils to break open the large mastodon bones. Breaking of the bones was done most likely to access the oozing, nutrient-rich marrow inside. The large stones were locally sourced but, oddly, none was modified or shaped in any way by removal of smaller stone flakes. Although no human bones were found at the site, it is assumed that humans were responsible because no other animal capable of smashing large mastodon bones with large stones is known to have lived at this time in the Americas.

Many of the large animals shown here (including the mastodon, upper right) became extinct soon after people arrived in North America 14 thousand years ago (human with spear for scale).

The site was determined to be 130.7 ± 9.4 thousand years old based on the decay of the radioactive element uranium contained within the bones. There was no organic carbon left in the bones to date using radiocarbon methods and the application of optically stimulated luminescence (OSL) indicated that the sediment at the site was deposited at least 60 thousand years ago. The age uncertainty of plus/minus nearly ten thousand years reflects, in part, the model assumptions made in using the uranium-series disequilibrium method. However, the uranium-derived ages appear to be robust and indicate that the deposit most likely formed sometime between 140 and 120 thousand years ago.

 

The lack of human bones, as well as any other stone tools or cultural artefacts besides the hammerstones and anvils, make it difficult to say which member of our human lineage was active at the site. It is certainly conceivable that members of our lineage living in Eurasia may have crossed over to North America when the Beringia land bridge was exposed. The Beringia land bridge today is flooded by the Bering Sea, but the lowering of sea level at times in the past was sufficient to expose the Bering Sea as a land bridge connecting Eurasia and North America. For example, the Beringia land bridge could have been crossed roughly 134 to 131 thousand years ago, within the age window of the mastodon site. Any humans living in eastern Siberia at that time may have made the journey across on foot without necessarily making use of boats.

 

Siberia was connected to North America periodically when sea level was lowered by major ice build up. Humans living in Siberia could have crossed over to North America either by boat along a coastal route or on foot overland through ice-free corridors.

The highly successful crossing 14 thousand years ago was part of the Great Expansion of behaviourally modern people who left Africa around 60 to 50 thousand years ago. There are numerous archaeological sites that show modern people had entered and become widespread throughout the Americas, reaching the southern coast of Chile by around 14 thousand years ago. The timing of initial entry into the Americas is thought to be mostly determined by when people living in the Far East and eastern Siberia could cross over the Beringia land bridge connecting Eurasia and North America during the Last Glacial Maximum when sea level was lowered in response to the build-up of major ice sheets. Passage into North America from Beringia was delayed until the large ice sheet blocking the way had started to melt back with the onset of warmer climates 18 to 14 thousand years ago. There was a relatively brief window to pass over the land bridge before it was flooded by the rising sea as the ice sheets quickly melted. Most are sceptical that humans had crossed over before 14 thousand years ago, with the current debate centred on when people crossed over, where they came from and whether they travelled by canoe along a coastal route or overland on foot through ice-free corridors that opened as the large Laurentide ice sheet melted back (Wade, 2017b).

 

It is not too far-fetched that some of our ancestors living in Eurasia might have crossed over to the Americas much earlier than the well-documented crossing of people by 14 thousand years ago. This is because the Beringia land bridge was repeatedly exposed as Earth cycled through glacial and interglacial periods over the last million years. Any of our ancestors adapted to living at relatively high latitudes may have inadvertently crossed over Beringia in pursuit of game and, once across, they could expand and fill the virgin American landscapes.

 

Sea-level cycles over the last million years and the periodic exposure of the Beringia land bridge in the transition from glacial to interglacial periods when animals (including humans) may have crossed over to North America (brown columns). The mastodon archaeological site reported from southern California implies humans crossed over sometime prior to 134 to 131 thousand years ago during the MIS 6 glacial to MIS 5 interglacial transition (third brown column on the right) when Neanderthals and Denisovans, but probably not our species (Homo sapiens) or the ‘hobbit’ (H. floresiensis), were living at high latitudes in Eurasia.

Could it have been our species, Homo sapiens, who crossed over? This seems unlikely because, although our species had appeared in Africa by around 200 to 160 thousand years ago, the earliest evidence of when we left Africa is 131 to 113 thousand years ago (MIS 5). After crossing over, our species appears to have largely been confined to the Middle East region. There is, as yet, no evidence that they had expanded into Siberia as early as when the Beringia land bridge to the Americas was exposed 134 to 131 thousand years ago. So, if not our species, then what other member of our lineage may have crossed over prior to 140 to 120 thousand years ago?

 

We know that Neanderthals, Denisovans and the ‘hobbit’ (Homo floresiensis) were all living in Eurasia at this time. Homo erectus was widespread throughout Eurasia even earlier, but does not appear to have lived at high enough latitudes, above 40°N, to have crossed over the Beringia land bridge located at latitudes above 55°N. The ‘hobbit’ is only known from the Indonesian island of Flores where it lived from 700 up until 50 thousand years ago, whereas Neanderthals and Denisovans are known to have lived at high latitudes, including Siberia. However, it is unclear which of the two crossed over the Beringia land bridge because the only stone tools (hammerstones and anvils) yet to be recovered from the site are not shaped in any way. Almost all contemporaneous stone tools documented in Eurasia (and Africa) were intentionally shaped by the removal of stone flakes. The lack of shaped stone tools is an unusual, and difficult to comprehend, aspect of the mastodon site.

 

Another surprising aspect about the mastodon site besides its lack of shaped stone tools, is that there has been so little convincing evidence of an earlier human presence in the Americas before now. If humans did managed to cross over, then it is predicted that they would have rapidly spread into the virgin landscapes, landscapes never before occupied by humans and full of large animals relatively easy to hunt. The Americas are two enormous continents no more difficult for humans to thrive in than Eurasia. So why are traces of humans living there so difficult to see in comparison to the record in Eurasia? Has the abundance of archaeological sites younger than 14 thousand years old obscured older, less abundant evidence? Perhaps if people dig a bit deeper and consciously look for it, an older record of humans in the Americas will be revealed. Now that the first compelling site has been discovered, perhaps more will follow.

 

Further reading

Holen, S. R., and others, 2017. A 130,000-year-old archaeological site in southern California, USA. Nature 544, 479–483. doi:10.1038/nature22065

 

Wade, L., 2017a. Claim of very early humans in Americas shocks researchers. Science 356, 361. doi: 10.1126/science.356.6336.361

 

Wade, L., 2017b. On the trail of ancient mariners. Science 357, 542-545.

doi: 10.1126/science.357.6351.542

 

© John S. Compton (www.johnscompton.com)

 

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Surprisingly young age of the Homo naledi fossil site

 

Update #2 to Human Origins: How diet, climate and landscape shaped us

 

photo of front cover of the book Human Origins by John Compton

John S. Compton

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The age of the fossil-rich Homo naledi fossil site within the Rising Star cave system in the Cradle of Humankind World Heritage Site near Johannesburg, South Africa was frustratingly unknown until recently. The exceptionally large number of fossil bones scattered on the cave floor proved difficult to date, but an international team of scientists has applied several dating techniques (optically stimulated luminescence, uranium/thorium and palaeomagnetism) to constrain the age of the deposit to between 414 and 236 thousand years old (Dirks and others, 2017). This age is supported by the estimated age of several fossil teeth from the deposit, independently dated to between 335 and 183 thousand years old using a combination of uranium series and electron spin resonance (US-ESR) methods. The US-ESR methods directly date the fossils, but rely on a number of complex model assumptions that result in a large amount of uncertainty. Although not terribly precise, these ages are considered fairly robust and are surprisingly young for fossils whose features (or traits) suggest that they are much older. Prior to the age determinations, many estimated that the fossils, given their mix of australopith and early Homo features, would date to around when our genus Homo first appeared in the fossil record between 3 and 2 million years ago (Ma). How, then, is such a young age explained for fossils that retain so many old features?

 

A mosaic of features

One of the striking aspects of the Homo naledi fossils is the odd mix or mosaic of features they display. The amazingly rich fossil find from the Rising Star cave system includes a total of over 1500 bones from the Dinaledi Chamber alone (Berger and others, 2015), with more bones recently discovered in the separate Lesedi Chamber (Hawks and others, 2017). All of the bones recovered from both caves are considered to belong to Homo naledi and comprise a minimum of 15 individuals. Such a large number of bones provides a fairly complete skeleton of Homo naledi who stood 1.4-1.6 m high and weighed 40-55 kg. One of the most notable features is the small size of the skull, having an interior volume (endocranial capacity) of between 460 and 610 cc. This range in volume is based on three skulls and is intermediate between the mean skull size of the australopiths and the earliest Homo species for which skulls are available that date to around 2 Ma. Such a small skull suggested that Homo naledi represented one of the earliest members of our genus Homo, which branched from the australopiths around 2.8 to 2.3 Ma, based on fossil teeth and jaws from East Africa (Villmoare and others, 2015). In contrast, its foot shares many features similar to ours. Comparing Homo naledi to other species is difficult to do because for most other species there are simply not enough fossil bones to compare. However, overall, Homo naledi much more closely resembles early Homo (H. habilis and H. erectus) than it does us (H. sapiens) or our predecessor species (‘archaic’ H. sapiens). The close resemblance to early Homo suggested to many that the age of the deposit would be similar in age to when early Homo appeared circa 2.5 Ma and not the reported age of less than 0.5 Ma.

 

Image from Hawks and others (2017)
image from Hawks and others (2017)

Skull of Homo naledi (LES1) from the Lesedi Chamber (left; scale bar 5 cm) and the digital reconstruction of the endocranial volume of 610 cc, scale sphere is 10 mm (Hawks and others, 2017).

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The most likely explanation is that Homo naledi represents one of the earliest members of our Homo lineage and that it managed to retain many features of early Homo up until at least 414 to 236 thousand years ago. While retaining many of its early features, it also appears to have acquired features that closely resemble later features, which either evolved independently (convergent evolution) or were acquired through interbreeding (hybridization) with other, later-evolved Homo species (but probably not our species, which only appeared by around 200 to 150 thousand years ago).

 

What is remarkable is that Homo naledi persisted for so long in a region occupied by other, later-evolved Homo species. For example, the skull from the Florisbad fossil site is thought to represent our predecessor species and was likely contemporary with Homo naledi, the two living within several hundred kilometres of one another. One possible explanation of their co-existence is that they occupied distinctly different habitats. Although their feet and aspects of their hands are similar to ours, Homo naledi’s fingers are curved. Curved fingers suggest that they were adapted for living in and moving about in trees. Therefore, they may have resided within heavily treed habitats, such as forest canopies, whereas our predecessor species was living primarily in more open grassland and savannah habitats. Although such niche partitioning may explain the co-existence of different species of Homo, it is remarkable that a species with so many early features, including such a small brain, managed to survive until just prior to when our species Homo sapiens evolved onto the scene.

 

Hangers on

Homo naledi is not the only member of our Homo lineage who managed to persist over such a long period of time. Homo erectus, for example, managed to survive in Asia long after they had become locally extinct in Africa, surviving up until around 300 thousand years ago in China and Java. And, in many respects, the suite of unusual features of Homo naledi reflect those of Homo floresiensis, the ‘hobbit”, which also retains features of early Homo (H. habilis and H. erectus) and lived up until just 50 thousand years ago on the island of Flores (Sutikna and others, 2016). The persistence of H. floresiensis on the somewhat remote island of Flores seems more plausible than a group within the African continent, but perhaps this reflects the fact that groups in Africa were adapted to specific habitats that effectively isolated them from other groups. Population densities were likely low and, along with the diversity of habitats, may have facilitated the survival of earlier groups for long periods of time. More recently evolved species might have been widely dispersed in part because of their big brain, sophisticated tools and control of fire, but were perhaps spread thinly enough over the landscape to have permitted pockets of earlier evolved groups to hang on. What the hobbit and Homo naledi seem to indicate is that among the more recently evolved members of our lineage, older groups managed to persist, either within distinct habitat or niche holdouts  ̶  perhaps culturally as much as physically isolated from other groups. The fossil record is so limited it may have hidden from us or we may have tended to underestimate the amount of variability in features, such as brain and body size that existed in the past.

 

Homo naledi culture?

Typically an archaeological site has hundreds to thousands of stone artefacts but very few if any fossil bones of those who made the artefacts. And whether or not any bones of the makers of the stone tools are found, it is common to find the bones of other animals at many archaeological sites. Hence, the Rising Star cave sites present the most unusual case: many bones of Homo naledi not in association with any stone tools or other cultural artefacts, nor any other large animal bones. Hence, although we know a lot about what Homo naledi looked like, we have very little idea of what they made or what other animals they lived among.

 

This lack of context makes it difficult to know much about the habitat in which they lived and how they lived. They most clearly did not live in the deep caves where they ended up as fossils. It is conceivable that they fell into or were washed down into the caves through surface openings connected to deep cave chambers. But in that case we would expect other large animals to have fallen in or been washed into the caves along with them. Some have proposed that they were intentionally disposed of into cave openings and that this disposal may indicate a type of ritual burial (Berger and others, 2017). Ritual burial is a cultural behaviour that has so far only been associated with our species, with the earliest hints (mortuary defleshing) dating to around 160 thousand years ago, and proper burials with grave goods not until around 100 thousand years ago. It is possible they disposed of their dead into the caves as a form of good housekeeping, but it is hard to imagine that small-brained Homo naledi had the mental ability to practice ritual burials, and evidence to substantiate ritual disposal into the caves remains lacking.

 

The complete absence of stone tools makes it difficult to know what stone tools, if any, were used by Homo naledi. They possess the wrists and hands capable of making and using stone tools, but did they? The age of the site falls within the Middle Stone Age (MSA), a time of regionally diverse stone tool industries throughout Africa. Some have suggested that Homo naledi may have been the maker of some of these stone tools (Berger and others, 2017), but so far there is no direct evidence to support this idea. The appearance of stone tool use has been dated as far back as 3.3 Ma and associated with australopiths having brains similar to or smaller sized than Homo naledi, but these early stone tools are a far cry from the diversity and sophistication of MSA stone tools that existed throughout much of Africa by 300 thousand years ago. Hopefully future finds of Homo naledi in association with cultural artefacts will shed some light on where and how they lived.

A 3.3 Ma Lomekwian stone tool (left; Harmand and others, 2015) is a far cry from regionally diverse MSA stone tools throughout Africa 300-100 thousand years ago (right).

Further Reading

 

Berger. L., and others, 2017. Homo naledi and Pleistocene hominin evolution in subequatorial Africa. eLife 2017;6:e24234. DOI: 10.7554/eLife.24234

 

Berger, L., and others, 2015. Homo naledi, a new species of the genus Homo from the Dinaledi Chamber, South Africa. eLife 2015;4:e09560. DOI: 10.7554/eLife.09560

 

Dirks, P., and others, 2017. The age of Homo naledi and associated sediments in the rising star Cave, South Africa. eLife 6:e24231. doi: 10.7554/eLife.24231

 

Harmand, S., and others, 2015. 3.3-million-year-old stone tools from Lomekwi 3, West Turkana, Kenya. Nature 521, 310–318.

 

Hawks, J., and others, 2017. New fossil remains of Homo naledi from the Lesedi Chamber, South Africa. eLife 6:e24232. DOI: 10.7554/eLife.24232

 

Sutikna, T., and others, 2016. Revised stratigraphy and chronology for Homo floresiensis at Liang Bua in Indonesia. Nature 532, 366–369.

 

Villmoare, B., and others, 2015. Early Homo at 2.8 Ma from Ledi-Geraru, Afar, Ethiopia. Science 347, 1352–1355.

New ages from Jebel Irhoud, Morocco

 

Update #1 to Human Origins: How diet, climate and landscape shaped us

 

John S. Compton

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The date

New fossil finds and a better constrained age for the Jebel Irhoud fossil site in Morocco shed new light on our evolution. The new ages indicate that the site is approximately 315 thousand years old (315 ka), or nearly twice as old as the previous estimated age of 160 ka for the site. The new, older age was obtained by applying the thermoluminescence dating method, which estimates the amount of time lapsed since stone flint tools found from the same layer as the fossil bones had been heated by fires made by those living at the site. The thermoluminescence method measures the amount of light given off by the stones tools (the amount they luminesce) as they are heated to higher temperatures. The idea is that the fires at the site were hot enough to completely erase any luminescence trapped in the stones. Once cooled, and so long as they were never exposed to the heat of a fire again, the stones gradually accumulated stored luminescence through time from natural radiation received from the surrounding sediment. The present-day background radiation dose of the deposit was measured from where the stone tools were recovered and was assumed to have remained more or less constant back through time. Heating of the stone flints in the lab causes the stored luminescence to be released, and the amount released could be measured. Dividing the total released luminescence by the radiation dose rate measured at the site allows an age to be calculated.

 

Luminescence dating, both thermoluminescence and optically stimulated luminescence (or OSL), has revolutionized our ability to date archaeological sites beyond what was previously limited to the last 50 thousand years by the radiocarbon method. The drawback to luminescence dating is that it too has limits in how far back it can reach because the amount of luminescence that can accumulate is limited. Once the stones have acquired maximum luminescence they can acquire no more, making it difficult to apply the method to samples older than 200 ka. So, the new age determined at Jebel Irhoud by thermoluminescence in excess of 300 ka is remarkable.

 

The other drawback with the thermoluminescence method is that the dates are not nearly as precise as those obtained by the radiocarbon dating method. The weighted average age of 14 samples from the fossil-bearing layer at Jebel Irhoud is 315 ka, but this age is associated with a fairly large uncertainty, with a 68% probability that the age of the site is between 349 ka and 281 ka, and a 95% probability that the age of the site is between 383 ka and 247 ka. The age range of 383 ka to 247 ka is consistent with both the range in age of a human tooth recovered from the site determined by a different method known as electron spin resonance (a far more complicated method than thermoluminescence that I will not try to explain here), and the known age ranges of fossil animals found at the site. Therefore, although not terribly precise, the new age is a huge improvement on previous ones for the site and, as we shall see, the new age provides valuable insights into the interpretation of the Jebel Irhoud fossils and their possible connection to other fossil sites in Africa.

 

The fossils

The Jebel Irhoud site has yielded a rich human fossil assemblage comprising skulls, jaws, teeth, leg and arm bones from at least five individuals, including a child and adolescent. Although the original location of all the fossils is not known, they are all assumed to have come from the same layer at the site associated with the new date of circa 315 ka. The face, jaw and teeth are considered to be similar to our own, although the jaw and teeth are unusually large, and the size of the browridge is variable. The most significant difference appears to be in the shape of the skull, being lower in height and more elongate (less bulbous) than ours (see figure below). The features of the skull resemble those of other skulls of similar age, such as the 260 ka skull from the Florisbad site in South Africa. These differences in the shape of the skull suggest that those living at Jebel Irhoud, and more broadly throughout Africa from around 330 to 230 ka, were not yet fully us (Homo sapiens).

 

The line of descent in our Homo lineage is thought to be from early Homo species (Homo habilis, for example) to Homo erectus to Homo heidelbergensis to our predecessor species, and finally, to us Homo sapiens. As I discuss in my book, the range of variations within a species and the complexity of speciation in a large continent such as Africa, makes it difficult from the limited number of fossils available to delineate species clearly. Some experts, the ‘lumpers’, view the changes in our lineage as gradational and refer to the Jebel Irhoud fossils as ‘archaic’ Homo sapiens. Other experts are ‘splitters’ and argue for stepwise evolution of distinct species. I believe there are enough physical differences and culture differences (see below) to support an intermediary species in the evolution of H. heidelbergensis to H. sapiens, an intermediary species that would take the place of the lumpers’ archaic Homo sapiens. However, this intermediary species lacks a generally accepted species name, and so I refer to it as our ‘predecessor’ species.

 

To my view, the new age indicates that the fossils at Jebel Irhoud represent some of the earliest members of our predecessor species and not, as implied by many of the news stories, that our species Homo sapiens now has an age range that extends back to around 300 thousand years. The oldest yet recovered fossils of our species Homo sapiens, ones that represent anatomically modern humans (AMHs) remain those found in East Africa that date to between 200 ka and 150 ka. The earliest members of our species most probably descended from our predecessor species, who were widespread throughout Africa by around 280 ka as represented by the fossil skulls found at Jebel Irhoud in Morocco, Florisbad in South Africa and Laetoli and Ileret in East Africa.

 

 

human skulls from Florisbad, Laetoli and Jebel Irhoud

Fossil skulls dated between 315 and 260 thousand years old that are possibly representative of our predecessor species intermediate between H. heidelbergensis and H. sapiens (from left to right: Florisbad, Laetoli (Smithsonian Institution) and Jebel Irhoud (Natural History Museum, London)).

 

Herto skull   Herto Skull side view   modern human skull, side view  modern human skull, front view

Fossil skulls of our species Homo sapiens from 160 thousand years ago (Herto, two images on left; photos by David Brill (humanoriginsphotos.com)) and modern (two images on the right). Our species is largely defined by its bulbous shaped skull.

 

 

Cultural differences

The arrival of our predecessor species is associated with the major transition from the Earlier Stone Age to the Middle Stone Age (MSA), a transition marked by smaller, more regionally-diverse stone tools. Jebel Irhoud is now the oldest site known having a direct association of fossils with MSA tools. MSA tools are widespread throughout Africa from around 300 ka to 230 ka, but most often not associated with human fossil remains. Some of the MSA stone points recovered from Ethiopia have been interpreted from their edge damage to have been used as thrown spears by at least 279 ka. The stone points from Jebel Irhoud have not yet been interpreted as having been thrown as spears (javelins). Those living at Jebel Irhoud were competent hunters based on the animal bones found. It may be that they made effective use of sharpened wooden spears without stone armatures, something their predecessor Homo heidelbergensis was doing.

 

 

Obsidian spear points from Ethiopia  San throwing a spear

Obsidian projectile spear tips from Ethiopia dating to at least 279 ka (Sahle et al., 2013) and a photo of a !Kung San throwing a spear (photo courtesy of Neil Roach).

 

The presence of burnt bones and charcoal suggests they had control of fire. However, they do not appear to have used fire to intentionally heat the stones (pyrotechnology). The earliest evidence for intentional heating of stones to improve their workability dates from 164 ka at the Pinnacle Point site in South Africa, presumably made by H. sapiens. Heating was probably not necessary in the case of the raw stone material available at Jebel Irhoud and the heating of about a third of the stone tools there was likely because they inadvertently ended up beneath where later fires were made.

 

The other cultural artefact to appear for the first time in the MSA is ochre, an iron-rich rock used for, among other things, symbolic body painting based on the specific collection of the reddest coloured stones. The earliest use of ochre is in East Africa, but no ochre has been reported from the Jebel Irhoud site. The absence of thrown stone-tipped spears and ochre at Jebel Irhoud may indicate that these cultural innovations were only developed later among groups of our predecessor species. Alternatively, these cultural items may have been present among groups in sub-Saharan Africa but were lost by groups too small to sustain these cultures after they had expanded into North Africa where soon became isolated from other groups.

 

ochre stones   Red ochre ground into a powder

Ochre stones and grinding ochre into a red powder.

 

 

Speciation events

Do the fossils at Jebel Irhoud suggest that the geographical region of origin of our predecessor species was North Africa? I argue in my book that the Maghreb, located at the northernmost tip of Africa was a potential geographical region of origin for species within our human lineage. This was based primarily on the periodic isolation of the Maghreb, separated from the rest of Africa and Eurasia by the Sahara-Arabian Desert and the Mediterranean Sea. Relatively small groups living in isolation in the Maghreb over long periods of time may have evolved away from other groups. These substantial physical barriers would have made exchange highly unlikely, except during relative brief periods when the Sahara-Arabian Desert ‘greened’ by receiving enough rainfall for the transformation of the desert into grassland and lakes.

 

These greening events may have made it possible for the periodic mixing of previously isolated groups living in the Maghreb, sub-Saharan Africa and Eurasia. We know the Sahara-Arabian Desert greened most recently 9 to 6 ka, with large impacts on the movement and interaction of people throughout the region. There is also good evidence of widespread greening 130 to 120 ka go associated with the movement of our species out of Africa into the Levant, and possibly into North Africa as indicated by the first appearance there of distinctive Aterian stone tools. What the groups represented by Jebel Irhoud fossils had evolved into by the time of the 130 ka greening event and to what extent they may have intermingled with Homo sapiens coming from sub-Saharan Africa is unknown. There were other greening events, such as the one associated with when modern people left Africa as part of the Great Expansion 60 thousand years ago, but the timing and extent of past greening events remains poorly known.

The interglacial period that occurs within the dated range of the Jebel Irhoud fossil site of 383-247 ka that is most likely to have had a significant greening event is Marine Isotope Stage (MIS) 9  ̶̶   roughly 330 ka. The fossils at Jebel Irhoud may represent early members of our predecessor species who evolved there among small, isolated populations of H. heidelbergensis and then spread out into the African continent with the greening of the Sahara associated with the MIS 9 interglacial period. Alternatively, our predecessor species may have evolved from H. heidelbergensis somewhere in sub-Saharan Africa and moved into the Maghreb during the MIS 9 or another greening event. Unfortunately, the age resolution of archaeological sites and number of sites are not sufficient to determine at this stage where in Africa our predecessor species evolved.

 

 Climate and human evolution: relation of glacial cycles to our evolution (from JS Compton's book Human Origins)

Ochre, javelins and diverse Middle Stone Age regional cultures are associated with the evolution of our predecessor species from H. heidelbergensis in Africa. The age range of 383-247 ka of the Jebel Irhoud site places it around the time of the MIS 9 Interglacial period when a greening of the Sahara may have allowed movement of groups living in North Africa and sub-Saharan Africa.

 

What we know from the dated MSA stone tool assemblages in North Africa, East Africa Rift Valley and South Africa is that our predecessor species was widespread throughout Africa by around 280 ka, but from which of these areas they evolved into us Homo sapiens remains unresolved. In my book I suggest that the currently available information favours South Africa as our species’ geographical region of origin  ̶̶  check it out and see if you agree.

 

Further Reading

Richter, D. et al., 2017. The age of the Jebel Irhoud (Morocco) hominins and the origins of the Middle Stone Age. Nature http://www.nature.com/doifinder/10.1038/nature22335.

 

Hublin, J.-J. et al., 2017. New fossils from Jebel Irhoud, Morocco and the pan-African origin of Homo sapiens. Nature http://dx.doi.org/10.1038/nature22336.

 

Stringer, C. and Galway-Witham, J., 2017. On the origin of our species. Nature 456, 212-214.

 

Gibbons, A., 2017. Oldest members of our species discovered in Morocco. Science 356, 993-994. [doi: 10.1126/science.356.6342.993]

 

Compton, J.S., 2016. Human Origins, How diet, climate and landscape shaped us. Earthspun Books, www.johnscompton.com.

 

Sahle Y., Hutchings W.K., Braun D.R., Sealy J.C., Morgan L.E., et al., 2013. Earliest stone-tipped projectiles from the Ethiopian Rift date to >279,000 years ago. PLoS ONE 8(11): e78092. doi:10.1371/journal.pone.0078092.

 

 

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©John S. Compton (www.johnscompton.com)

© John S. Compton