tag:theconversation.com,2011:/us/topics/uranium-dating-62615/articlesUranium dating – The Conversation2021-11-08T21:58:46Ztag:theconversation.com,2011:article/1713912021-11-08T21:58:46Z2021-11-08T21:58:46ZLand ahoy: study shows the first continents bobbed to the surface more than 3 billion years ago<figure><img src="https://images.theconversation.com/files/430723/original/file-20211108-48235-1iw24v8.png?ixlib=rb-1.1.0&rect=10%2C55%2C2446%2C1575&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption"></span> <span class="attribution"><span class="license">Author provided</span></span></figcaption></figure><p>Most people know that the land masses on which we all live represent just 30% of Earth’s surface, and the rest is covered by oceans. </p>
<p>The emergence of the continents was a pivotal moment in the history of life on Earth, not least because they are the humble abode of most humans. But it’s still not clear exactly when these continental landmasses first appeared on Earth, and what tectonic processes built them.</p>
<p>Our research, <a href="https://www.pnas.org/cgi/doi/10.1073/pnas.2105746118">published</a> in Proceedings of the National Academy of Sciences, estimates the age of rocks from the most ancient continental fragments (called cratons) in India, Australia and South Africa. The sand that created these rocks would once have formed some of the world’s first beaches.</p>
<p>We conclude that the first large continents were making their way above sea level around 3 billion years ago – much earlier than the 2.5 billion years estimated by previous research.</p>
<h2>A 3-billion-year-old beach</h2>
<p>When continents rise above the oceans they start to erode. Wind and rain break rocks down into grains of sand, which are transported downstream by rivers and accumulate along coastlines to form beaches. </p>
<p>These processes, which we can observe in action during a trip to the beach today, have been operating for billions of years. By scouring the rock record for signs of ancient beach deposits, geologists can study episodes of continent formation that happened in the distant past.</p>
<p>The Singhbhum craton, an ancient piece of continental crust that makes up the eastern parts of the Indian subcontinent, contains several formations of ancient sandstone. These layers were originally formed from sand deposited in beaches, estuaries and rivers, which was then buried and compressed into rock.</p>
<p>We determined the age of these deposits by studying microscopic grains of a mineral called zircon, which is preserved within these sandstones. This mineral contains tiny amounts of uranium, which very slowly turns into lead via radioactive decay. This allows us to estimate the age of these zircon grains, using a technique called <a href="https://en.wikipedia.org/wiki/Uranium%E2%80%93lead_dating">uranium-lead dating</a>, which is well suited to dating very old rocks.</p>
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<a href="https://images.theconversation.com/files/430725/original/file-20211108-10108-1cbfduv.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Sandstone and zircon grains" src="https://images.theconversation.com/files/430725/original/file-20211108-10108-1cbfduv.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/430725/original/file-20211108-10108-1cbfduv.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=348&fit=crop&dpr=1 600w, https://images.theconversation.com/files/430725/original/file-20211108-10108-1cbfduv.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=348&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/430725/original/file-20211108-10108-1cbfduv.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=348&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/430725/original/file-20211108-10108-1cbfduv.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=438&fit=crop&dpr=1 754w, https://images.theconversation.com/files/430725/original/file-20211108-10108-1cbfduv.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=438&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/430725/original/file-20211108-10108-1cbfduv.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=438&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Left: sandstone formations (with ruler for scale); right: microscopic images of zircon grains.</span>
<span class="attribution"><span class="license">Author provided</span></span>
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<p>The zircon grains reveal that the Singhbhum sandstones were deposited around 3 billion years ago, making them some of the oldest beach deposits in the world. This also suggests a continental landmass had emerged in what is now India by at least 3 billion years ago. </p>
<p>Interestingly, sedimentary rocks of roughly this age are also present in the oldest cratons of Australia (the Pilbara and Yilgarn cratons) and South Africa (the Kaapvaal Craton), suggesting multiple continental landmasses may have emerged around the globe at this time.</p>
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Read more:
<a href="https://theconversation.com/whats-australia-made-of-geologically-it-depends-on-the-state-youre-in-83575">What's Australia made of? Geologically, it depends on the state you're in</a>
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<h2>Rise above it</h2>
<p>How did rocky continents manage to rise above the oceans? A unique feature of continents is their thick, buoyant crust, which allows them to float on top of Earth’s mantle, just like a cork in water. Like icebergs, the top of continents with thick crust (typically more than 45km thick) sticks out above the water, whereas continental blocks with crusts thinner than about 40km remain submerged.</p>
<p>So if the secret of the continents’ rise is due to their thickness, we need to understand how and why they began to grow thicker in the first place. </p>
<p>Most ancient continents, including the Singhbhum Craton, are made of granites, which formed through the melting of pre-existing rocks at the base of the crust. In our research, we found the granites in the Singhbhum Craton formed at increasingly greater depths between about 3.5 billion and 3 billion years ago, implying the crust was becoming thicker during this time window. </p>
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<a href="https://images.theconversation.com/files/430727/original/file-20211108-9872-18vgwir.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="Granite formation with pen for scale." src="https://images.theconversation.com/files/430727/original/file-20211108-9872-18vgwir.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/430727/original/file-20211108-9872-18vgwir.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=728&fit=crop&dpr=1 600w, https://images.theconversation.com/files/430727/original/file-20211108-9872-18vgwir.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=728&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/430727/original/file-20211108-9872-18vgwir.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=728&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/430727/original/file-20211108-9872-18vgwir.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=914&fit=crop&dpr=1 754w, https://images.theconversation.com/files/430727/original/file-20211108-9872-18vgwir.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=914&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/430727/original/file-20211108-9872-18vgwir.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=914&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Granites are some of the least dense and most buoyant types of rock (pen included for scale).</span>
<span class="attribution"><span class="license">Author provided</span></span>
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<p>Because granites are one of the least dense types of rock, the ancient crust of the Singhbhum Craton would have become progressively more buoyant as it grew thicker. We calculate that by around 3 billion years ago, the continental crust of the Singhbhum Craton had grown to be about 50km thick, making it buoyant enough to begin rising above sea level.</p>
<p>The rise of continents had a profound influence on the climate, atmosphere and oceans of the early Earth. And the erosion of these continents would have provided chemical nutrients to coastal environments in which early photosynthetic life was flourishing, leading to a <a href="https://www.sciencedirect.com/science/article/abs/pii/S0012821X16307117">boom in oxygen production</a> and ultimately helping to create the <a href="https://www.pnas.org/content/118/33/e2107511118">oxygen-rich atmosphere</a> in which we thrive today.</p>
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Read more:
<a href="https://theconversation.com/the-floor-is-lava-after-1-5-billion-years-in-flux-heres-how-a-new-stronger-crust-set-the-stage-for-life-on-earth-151276">The floor is lava: after 1.5 billion years in flux, here's how a new, stronger crust set the stage for life on Earth</a>
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<p>Erosion of the early continents would have also helped in sequestering carbon dioxide from the atmosphere, leading to global cooling of the early Earth. Indeed, the earliest glacial deposits also happen to <a href="https://www.sciencedirect.com/science/article/pii/S0012825220303445#bb0765">appear in the geological record</a> around 3 billion years ago, shortly after the first continents emerged from the oceans.</p><img src="https://counter.theconversation.com/content/171391/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Priyadarshi Chowdhury receives funding from Australian Research Council Grant No FL160100168. </span></em></p><p class="fine-print"><em><span>Jack Mulder receives funding from Australian Research Council grant FL160100168</span></em></p><p class="fine-print"><em><span>Oliver Nebel receives funding from the Australian Research Council Grant No DP180100580. </span></em></p><p class="fine-print"><em><span>Peter Cawood receives funding from Australian Research Council grant FL160100168</span></em></p>Dating of rocks that once formed some of the world’s first beaches suggests the first large continents grew large enough to rise above sea level roughly 3 billion or so years ago.Priyadarshi Chowdhury, Postdoctoral research fellow, Monash UniversityJack Mulder, Research Associate, The University of QueenslandOliver Nebel, Associate Professor, Monash UniversityPeter Cawood, Professor and ARC Laureate Fellow, Monash UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1550472021-03-31T15:20:23Z2021-03-31T15:20:23ZAncient southern Kalahari was more important to human evolution than previously thought<figure><img src="https://images.theconversation.com/files/383946/original/file-20210212-13-vbm8va.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Ga-Mohana Hill in South Africa's Northern Cape province.</span> <span class="attribution"><span class="source">Benjamin Schoville</span></span></figcaption></figure><p>The Kalahari is a <a href="https://www.britannica.com/place/Kalahari-Desert">huge expanse of desert</a> in southern Africa, stretching across Botswana and into the northernmost part of South Africa’s Northern Cape province. </p>
<p>It’s in the Northern Cape that we studied and described a new archaeological site, Ga-Mohana Hill, for research <a href="https://www.nature.com/articles/s41586-021-03419-0">just published in Nature</a>.</p>
<p>Our international team, made up of researchers from South Africa, Canada, the UK, Australia and Austria, has <a href="https://theconversation.com/ancient-eggshells-and-a-hoard-of-crystals-reveal-early-human-innovation-and-ritual-in-the-kalahari-154191">found evidence</a> for complex symbolic behaviours 105,000 years ago. </p>
<p>Humans use symbols as a shortcut to communicate important ideas. Identifying the ancient roots of symbolism is limited to what preserves over time. Large calcite crystals from several kilometres away were found in the cave alongside stone tools. Why the crystals were brought there is unknown; they are not modified and do not seem to have a functional purpose. They may have been collected for their aesthetic properties, or included in ritual activities. </p>
<p>Crystals are collected by many people around the world to this day for ritual purposes. Early humans bringing crystals into Ga-Mohana suggests innovation in how people interacted with each other and their environment.</p>
<p>But such ancient innovation didn’t occur in a bubble: there is context to when and where innovation occurs. What brought people there in the first place, at that time, to begin using those tools and collecting those crystals?</p>
<p>Reconstructing past environments allows us to understand this context. And so, a major part of our research centred on working out what the area’s climate was like 105,000 years ago. To do so, we looked at Ga-Mohana’s rocks.</p>
<p>The southern Kalahari is often considered too arid to be important for human evolution. Our work contradicts the idea of an arid and empty interior. At some points, Ga-Mohana was much wetter than today, with pools of standing water and waterfalls tumbling down the hillside. The fact that the climate was very different then opens up possibilities about why this previously under-appreciated region must have played an important role in our species’ evolutionary history. </p>
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<a href="https://images.theconversation.com/files/384949/original/file-20210218-22-1ff8yod.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/384949/original/file-20210218-22-1ff8yod.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/384949/original/file-20210218-22-1ff8yod.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=337&fit=crop&dpr=1 600w, https://images.theconversation.com/files/384949/original/file-20210218-22-1ff8yod.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=337&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/384949/original/file-20210218-22-1ff8yod.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=337&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/384949/original/file-20210218-22-1ff8yod.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=423&fit=crop&dpr=1 754w, https://images.theconversation.com/files/384949/original/file-20210218-22-1ff8yod.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=423&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/384949/original/file-20210218-22-1ff8yod.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=423&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
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<span class="caption">Survey team from University of Cape Town and University of Queensland.</span>
<span class="attribution"><span class="source">Jayne Wilkins</span></span>
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<p>Archaeological and geological fieldwork allowed us to piece together this story. </p>
<h2>The rocks</h2>
<p>Some kinds of rocks preserve traces of the past environment. The Ga-Mohana hillside is draped in deposits called <a href="https://www.howitworksdaily.com/how-are-tufa-towers-formed/">tufa</a>; these form from water leaking out of cracks in the bedrock. This occurs when underground aquifers are recharged with rain water and begin to overflow. Over time, these waters precipitate calcium carbonate and form tufa. </p>
<p>The tufa system is no longer active, apart from small drips during the rainy season. But the fossil tufas represent periods in the past when there was more water available. Similar structures are growing today at places like <a href="https://youtu.be/IowL8FebJ5I">Sitting Bull Falls, New Mexico in the US</a>. Knowing when the tufas formed at Ga-Mohana tells us when it was wetter there.</p>
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<figcaption><span class="caption">Fossil tufas occur in several parts of the world.</span></figcaption>
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<p>To find out how old the tufas are and when these wet periods occurred, we used a method called <a href="https://www.futurelearn.com/info/courses/human-fossils/0/steps/49021">uranium-thorium dating</a>. Uranium is radioactive, meaning that it decays at a constant rate over time and produces ‘daughter’ elements; thorium is one of them. When tufa forms, uranium is ‘locked’ into the crystal structure and begins to decay to produce thorium. </p>
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<span class="caption">Jess von der Meden standing next to large tufa flow at Ga-Mohana.</span>
<span class="attribution"><span class="source">Robyn Pickering</span></span>
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<p>The uranium-thorium system acts like a clock that starts when the tufa is formed. By precisely measuring how much uranium and thorium is in the tufa today, we use the known decay rate to calculate when the ‘clock’ started. This method is <a href="https://theconversation.com/how-we-calculated-the-age-of-caves-in-the-cradle-of-humankind-and-why-it-matters-104856">routinely applied</a> to cave deposits like stalagmites and flowstones but has not been used very much on tufa. </p>
<p>This is because dating tufas is not straightforward. Unlike protected caves, tufa forms in the open where sunlight, dust, and debris can contaminate the ages. It took several years of dedicated work to get around these problems: we chose the tufa samples in the field with care and used a sensitive laser to make images of the layers with the most uranium present. We could then target these layers for dating. This provided a real breakthrough.</p>
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Read more:
<a href="https://theconversation.com/how-we-calculated-the-age-of-caves-in-the-cradle-of-humankind-and-why-it-matters-104856">How we calculated the age of caves in the Cradle of Humankind -- and why it matters</a>
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<p>In the end, we dated two layers from an ancient tufa waterfall to between 110,000 and 100,000 years old. This means that fresh water was flowing down the hillside at exactly the same time that people were living at the shelter. Such wet conditions at this time were unexpected, so we wanted to know what caused such a large increase in water to begin with.</p>
<p>To understand the reasons for the region being much wetter 105,000 years ago than it is today, we looked at how climate processes influence modern rainfall there. </p>
<p>We did this by comparing historical rainfall records to current major climate drivers. We then looked back into the past and used data from an ocean core (deep sea sediments drilled out of the ocean floor which record changes in the earth’s ocean and climate). These data show that parts of the Indian Ocean were warmer around 105,000 years ago. Climate systems are complex, but basically this would have increased the amount of rain in the southern Kalahari, filling the aquifer, and causing the build up of the tufa during this time period. </p>
<h2>An important role</h2>
<p>People were drawn to Ga-Mohana for many reasons. Surface water would have been one. The many ostrich eggshell fragments we also found were probably used as water carriers 105,000 years ago. Perhaps these were being filled with water as it flowed down the hillside. One possibility is that water carriers allowed our ancestors to travel further distances. </p>
<p>There is still more to be learned from Ga-Mohana, its artefacts and its rocks. This will allow scientists to understand the role this space played in human evolutionary history better.</p><img src="https://counter.theconversation.com/content/155047/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Benjamin Schoville receives funding from the University of Queensland, National Geographic, the Tswalu Foundation, and the Centre for Excellence in Palaeoscience. </span></em></p><p class="fine-print"><em><span>Jessica von der Meden receives funding from the National Research Foundation of South Africa and the NRF Centre of Excellence in Palaeosciences.</span></em></p><p class="fine-print"><em><span>Robyn Pickering receives funding from The University of Cape Town, the National Research Foundation and the NRF Centre of Excellence in Palaeosciences.</span></em></p><p class="fine-print"><em><span>Wendy Khumalo receives funding from the University of Cape Town, the NRF Centre of Excellence in Palaeosciences, and the South. African National Antarctic Programme.</span></em></p>People were drawn to Ga-Mohana for many reasons. Surface water was likely among them.Benjamin Schoville, Senior Lecturer in Archaeology, The University of QueenslandJessica von der Meden, PhD candidate, University of Cape TownRobyn Pickering, Senior lecturer, University of Cape TownWendy Khumalo, Student, Department of Geological Sciences, University of Cape TownLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1266612019-11-13T19:01:08Z2019-11-13T19:01:08ZClimate change fueled the rise and demise of the Neo-Assyrian Empire, superpower of the ancient world<figure><img src="https://images.theconversation.com/files/301411/original/file-20191113-37401-1f01jrx.jpg?ixlib=rb-1.1.0&rect=0%2C3%2C2402%2C1938&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Ashurbanipal, last major ruler of the Assyrian Empire, couldn't outrun the effects of climate change.</span> <span class="attribution"><span class="source">British Museum</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>Ancient Mesopotamia, the fabled land between the Tigris and the Euphrates rivers, was the command and control center of the <a href="http://www.en.uni-muenchen.de/news/newsarchiv/2018/mooc_radner.html">Neo-Assyrian Empire</a>. This ancient superpower was the largest empire of its time, lasting from 912 BC to 609 BC in what is now modern Iraq and Syria. At its height, the Assyrian state stretched from the Mediterranean and Egypt in the west to the Persian Gulf and western Iran in the east.</p>
<p>Then, in an astonishing reversal of fortune, the Neo-Assyrian Empire plummeted from its zenith (circa 650 BC) to <a href="https://www.eisenbrauns.org/books/titles/978-1-57506-754-4.html">complete political collapse</a> within the span of just a few decades. What happened?</p>
<p>Numerous theories attempt to explain the Assyrian collapse. Most researchers attribute it to imperial overexpansion, civil wars, political unrest and Assyrian military defeat by a coalition of Babylonian and Median forces in 612 BC. But exactly how these two small armies were able to annihilate what was then the most powerful military force in the world has <a href="https://ucl.rl.talis.com/items/DDBEC935-1F16-2C7B-7370-3D5BED633425.html">mystified historians and archaeologists</a> for more than a hundred years.</p>
<p>Our new research published in the journal Science Advances <a href="https://advances.sciencemag.org/content/5/11/eaax6656">sheds light on these mysteries</a>. We show that climate change was the proverbial double-edged sword that first contributed to the meteoric rise of the Neo-Assyrian Empire and then to its precipitous collapse.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/301416/original/file-20191113-37425-u6r88t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/301416/original/file-20191113-37425-u6r88t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/301416/original/file-20191113-37425-u6r88t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=385&fit=crop&dpr=1 600w, https://images.theconversation.com/files/301416/original/file-20191113-37425-u6r88t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=385&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/301416/original/file-20191113-37425-u6r88t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=385&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/301416/original/file-20191113-37425-u6r88t.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=484&fit=crop&dpr=1 754w, https://images.theconversation.com/files/301416/original/file-20191113-37425-u6r88t.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=484&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/301416/original/file-20191113-37425-u6r88t.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=484&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">An artist’s vision of the interior of an Assyrian palace, based on drawings made in 1849 by Austen Henry Layard on the site of 19th century excavations.</span>
<span class="attribution"><span class="source">New York Public library digital collections</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Booming right up to an unexpected bust</h2>
<p>The Neo-Assyrian state was an economic powerhouse. Its formidable <a href="https://www.ancient.eu/Assyrian_Warfare/">war machine</a> boasted a large standing army with cavalry, chariots and iron weaponry. For over two centuries, the mighty Assyrians waged relentless military campaigns with ruthless efficiency. They conquered, plundered and subjugated major regional powers across the Near and Middle East, as each Assyrian king tried to outshine his predecessor.</p>
<p><a href="http://etc.ancient.eu/photos/assyrian-lion-hunting-british-museum/">Ashurbanipal</a>, the last great king of Assyria, ruled this vast empire from the <a href="https://www.britishmuseum.org/collection/galleries/assyria-nineveh">ancient city of Nineveh</a>, the ruins of which lie across the Tigris River from modern Mosul, Iraq. Nineveh was a sprawling metropolis of unprecedented size and grandeur filled with temples and palace complexes, with exotic gardens that were watered by an extensive system of canals and aqueducts.</p>
<p>And then it all ended within just a few years. Why?</p>
<p>Our research group wanted to investigate climate conditions over the few centuries when the Neo-Assyrian Empire took hold and then eventually collapsed. </p>
<h2>Building a picture of climate 2,600 years ago</h2>
<p>For clues about rainfall patterns over northern Mesopotamia, we turned to Kuna Ba cave, located near Nineveh.</p>
<p>Our colleagues collected samples from the cave’s stalagmites. These are the cone-like structures that point upward from the cave floor. They grow slowly, from the ground up, as rainwater drips down from the cave ceiling, depositing dissolved minerals.</p>
<figure class="align-right zoomable">
<a href="https://images.theconversation.com/files/301570/original/file-20191113-77326-1fwa01o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/301570/original/file-20191113-77326-1fwa01o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=237&fit=clip" srcset="https://images.theconversation.com/files/301570/original/file-20191113-77326-1fwa01o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=916&fit=crop&dpr=1 600w, https://images.theconversation.com/files/301570/original/file-20191113-77326-1fwa01o.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=916&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/301570/original/file-20191113-77326-1fwa01o.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=916&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/301570/original/file-20191113-77326-1fwa01o.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=1151&fit=crop&dpr=1 754w, https://images.theconversation.com/files/301570/original/file-20191113-77326-1fwa01o.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=1151&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/301570/original/file-20191113-77326-1fwa01o.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=1151&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The layers of a stalagmite record the climate conditions of the time when they were created.</span>
<span class="attribution"><span class="source">Ashish Sinha</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>The rainwater naturally contains heavy and light isotopes of oxygen – that is, atoms of oxygen that have different numbers of neutrons. Subtle variations in the oxygen isotope ratios can be sensitive indicators of climatic conditions at the time the rainwater originally fell. <a href="https://earthobservatory.nasa.gov/features/Paleoclimatology_Speleothems">As stalagmites grow</a>, they lock into their structure the oxygen isotope ratios of the percolating rainwater that seeps into the cave.</p>
<p>We painstakingly pieced together the climatic history of northern Mesopotamia by carefully drilling into stalagmites, across their growth rings, which are similar to those of trees. In each sample, we measured the oxygen isotope ratios to build a timeline of how conditions changed. That told us the order of events but didn’t tell us the amount of time that elapsed between them.</p>
<p>Luckily, the stalagmites also trap uranium, an element that’s ever-present in trace amounts in the infiltrating water. Over time, uranium decays into thorium at a predictable pace. So the dating experts on our research team made scores of high-precision <a href="https://doi.org/10.1016/B978-0-12-814124-3.00128-X">uranium-thorium measurements</a> on stalagmite growth layers.</p>
<p>Together these two kinds of measurements let us anchor our climate record to precise calendar years.</p>
<h2>Unusual wet period, then massive drought</h2>
<p>Now a direct comparison of the stalagmite climate record with the historical and archaeological records from the region was possible. We wanted to place the key events of Neo-Assyrian history into the long-term context of our climate reconstruction.</p>
<p>We found that the most significant expansion phase of the Neo-Assyrian state occurred during a two-centuries-long interval of anomalously wet climate, as compared with the previous 4,000 years. Called a megapluvial period, this time of unusually high rainfall was immediately followed by megadroughts during the early-to-mid-seventh century BC. These ancient dry conditions were as severe as recent droughts in Iraq and Syria but lasted for decades. The period marking the collapse of the Neo-Assyrian Empire occurred well within this time frame.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/301568/original/file-20191113-77315-fxb77e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/301568/original/file-20191113-77315-fxb77e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/301568/original/file-20191113-77315-fxb77e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=517&fit=crop&dpr=1 600w, https://images.theconversation.com/files/301568/original/file-20191113-77315-fxb77e.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=517&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/301568/original/file-20191113-77315-fxb77e.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=517&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/301568/original/file-20191113-77315-fxb77e.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=650&fit=crop&dpr=1 754w, https://images.theconversation.com/files/301568/original/file-20191113-77315-fxb77e.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=650&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/301568/original/file-20191113-77315-fxb77e.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=650&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">The Neo-Assyrian Empire rose during an unusual time of wet climate and collapsed soon after conditions swung to unusual dryness.</span>
<span class="attribution"><span class="source">Ashish Sinha</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>Mindful of the caveat that correlation doesn’t imply causation, we were interested in how this wild climate swing – an unusually rainy period that ended in drought – could have influenced an empire.</p>
<p>While the Neo-Assyrian state was huge in its final few decades, its economic core was always confined to a rather small region. This relatively <a href="https://www.ucl.ac.uk/sargon/essentials/countries/centralassyria/">small area</a> in northern Mesopotamia served as a primary source of agricultural revenues and powered Assyrian military campaigns.</p>
<p>We argue that nearly two centuries of unusually wet conditions in this otherwise semi-arid region allowed for agriculture to flourish and energized the Assyrian economy. The climate acted as a catalyst for the creation of a dense network of urban and rural settlements in the unsettled zones that previously hadn’t been able to support farming.</p>
<p>Our data show the wet period abruptly ended and the pendulum swung the other way. In the grips of recurring megadroughts, the Assyrian core and its hinterlands would have been engulfed within a “<a href="https://doi.org/10.1007/s10963-014-9072-2">zone of uncertainty</a>” – a corridor of land where the rainfall is highly erratic and any rain-fed agriculture comes with a large risk of crop failure.</p>
<p>Repeated crop failures likely exacerbated the political unrest in Assyria, crippled its economy and empowered the adjacent rival states. </p>
<h2>Uncertain climate, unsustainable growth</h2>
<p>Our findings have current-day implications.</p>
<p>In modern times, the same region that once constituted the Assyrian core has been repeatedly struck by multiyear droughts. The catastrophic <a href="https://doi.org/10.1073/pnas.1421533112">drought of 2007–2008</a> in northern Iraq and Syria, the most severe in the past 50 years, led to cereal crop failures across the region.</p>
<p>Droughts like this one offer a glimpse of what Assyrians endured during the mid-seventh century BC. And the collapse of the Neo-Assyrian Empire offers a warning to today’s societies.</p>
<p>Climate change is here to stay. In the 21st century, people have what Neo-Assyrians did not: the benefit of hindsight and plenty of observational data. Unsustainable growth in politically volatile and water-stressed regions is a time-tested recipe for disaster.</p>
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<p class="fine-print"><em><span>Ashish Sinha receives funding from the National Science Foundation. </span></em></p><p class="fine-print"><em><span>Gayatri Kathayat receives funding from National Science Foundation of China.</span></em></p>What caused the rise and then collapse 2,600 years ago of this vast empire centered on Mesopotamia? Clues from a cave in northern Iraq point to abrupt climate change.Ashish Sinha, Professor of Earth and Climate Sciences, California State University, Dominguez HillsGayatri Kathayat, Associate Professor of Global Environmental Change, Xi'an Jiaotong UniversityLicensed as Creative Commons – attribution, no derivatives.tag:theconversation.com,2011:article/1069142018-11-19T16:11:44Z2018-11-19T16:11:44ZNew dates for ancient stone tools in China point to local invention of complex technology<figure><img src="https://images.theconversation.com/files/246063/original/file-20181117-194516-nzuo6m.jpg?ixlib=rb-1.1.0&rect=8%2C0%2C1857%2C1362&q=45&auto=format&w=496&fit=clip" /><figcaption><span class="caption">Several of the newly identified stone tools – unearthed from a museum collection.</span> <span class="attribution"><span class="source">Hu Yue</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span></figcaption></figure><p>You probably think of new technologies as electronics you can carry in a pocket or wear on a wrist. But some of the most profound technological innovations in human evolution have been made out of stone. For most of the time that humans have been on Earth, they’ve chipped stone into useful shapes to make tools for all kinds of work. </p>
<p><a href="https://doi.org/10.1038/s41586-018-0710-1">In a study just published in Nature</a>, we’ve dated a distinctive and complex method for making stone tools to a much earlier timeframe in China than had previously been accepted. Archaeologists had thought that artifacts of this kind had been carried into China by groups migrating from Europe and Africa. But our new discovery, dated to between 170,000 and 80,000 years ago, suggests that they could have been invented locally without input from elsewhere, or come from much earlier cultural transmission or human migration.</p>
<p>Several different species of humans lived on Earth at this time, including modern ones like us. But we haven’t found any human bones from this site, so don’t know which species of human made these tools. </p>
<p>These Chinese artifacts provide one more piece of evidence that changes the way we think about the origin and spread of new stone tool technologies. And intriguingly we made our discovery based on artifacts that had been excavated decades ago.</p>
<h2>New technology among old stones</h2>
<p>Archaeologists have identified <a href="https://en.wikipedia.org/wiki/Stone_tool">five modes</a> humans have used to make stone tools over the last 3 million years. Each mode is represented by a new stone tool type that is dramatically different from what came before. The appearance of each new mode is also marked by a big increase in the number of steps needed to make the new tool type. </p>
<p>One of these modes, Mode III, also called Levallois, is at the center of several big debates about human evolution. Levallois tools are the defining features of the archaeological period referred to as the Middle Paleolithic, or Africa’s Middle Stone Age. They are the result of a set of very specific steps of chipping a piece of stone to create similar-sized tools suitable to be shaped for a variety of purposes. These steps are remarkable because they are a <a href="https://doi.org/10.1006/jasc.2000.0594">much more efficient way</a> to produce lots of useful cutting tools, with minimal wasted stone, compared to earlier technologies. </p>
<p>One of these debates is whether Mode III tools were invented in one place and then spread out, or independently invented in several different locations. Since the world’s <a href="https://doi.org/10.1038/nature22336">oldest securely dated Levallois tools</a> have been found in north Africa from around 300,000 years ago, it’s possible they spread out from there, carried by groups of early humans migrating across Europe and into Asia. On the other hand, finds of similarly early <a href="https://doi.org/10.1126/science.1256484">Levallois tools in Armenia</a> <a href="https://doi.org/10.1038/nature25444">and India</a> support the idea of independent inventions of the technology outside of Africa. </p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/246064/original/file-20181117-194513-wnyzlu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/246064/original/file-20181117-194513-wnyzlu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/246064/original/file-20181117-194513-wnyzlu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=481&fit=crop&dpr=1 600w, https://images.theconversation.com/files/246064/original/file-20181117-194513-wnyzlu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=481&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/246064/original/file-20181117-194513-wnyzlu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=481&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/246064/original/file-20181117-194513-wnyzlu.jpg?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=605&fit=crop&dpr=1 754w, https://images.theconversation.com/files/246064/original/file-20181117-194513-wnyzlu.jpg?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=605&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/246064/original/file-20181117-194513-wnyzlu.jpg?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=605&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Levallois tools were a leap forward in technology, a new, efficient way to create tools that could cut, scrape, chop and make other types of tools.</span>
<span class="attribution"><span class="source">Hu Yue</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<h2>Changing the chronology in China</h2>
<p>In China it has been hard to find evidence of Mode III tools until <a href="https://doi.org/10.1016/j.quaint.2012.07.020">relatively late in the Palaeolithic period</a>, approximately 30,000 to 40,000 years ago. That’s concurrent with when Mode IV (blade tools) appear there. Ancient people in China appeared to leap from Mode II (stone hand axes) to Mode III and IV at the same time. This suggests that Levallois tools appeared in China when modern humans migrated in and brought these new technologies with them around 30,000 to 40,000 years ago.</p>
<p>Our results support a different story for the origin of Levallois tools in China. At Guanyindong Cave in <a href="https://www.google.com/maps/place/Guiyang,+Guizhou,+China/@26.8485833,102.1052778,6z/data=!4m13!1m7!3m6!1s0x36bf67eaae6dd399:0xec9c1d7600abd55e!2sGuizhou,+China!3b1!8m2!3d26.8429645!4d107.2902839!3m4!1s0x36bf67c355ec1b37:0x173381c0d8316455!8m2!3d26.6474587!4d106.6305542">Guizhou Province</a> in south-central China, we found Mode III tools in layers dated to around 170,000 and around 80,000 years ago. This puts them well before Mode IV tools, and at around the same time that Levallois were the main tools used in Europe and Africa. </p>
<p>One major implication of our new early ages from Guanyindong Cave is that the appearance of Levallois tools in China is no longer tied to the arrival of modern humans and Mode IV tools 30,000 to 40,000 years ago. Instead, Levallois tools could have been invented locally in China – maybe by a different human species. Another possibility is that they were introduced by a much earlier migration, perhaps by the people whose <a href="https://doi.org/10.1038/nature.2015.18566">teeth have been found in a cave in Daoxian</a>, <a href="https://www.google.com/maps/place/Hunan,+China/@22.0022602,102.1170918,5.05z/data=!4m5!3m4!1s0x3420ba186987384d:0xcc21910be4ae2ce5!8m2!3d27.6252995!4d111.8568586">Hunan Province</a>, who lived between 80,000 and 120,000 years ago.</p>
<p><iframe id="K0382" class="tc-infographic-datawrapper" src="https://datawrapper.dwcdn.net/K0382/2/" height="400px" width="100%" style="border: none" frameborder="0"></iframe></p>
<h2>Going back to Guanyindong Cave</h2>
<p>Our discovery is a little unusual because we didn’t do any major new excavations. All of the stone tools we studied had been <a href="https://doi.org/10.4116/jaqua.28.243">excavated from Guanyindong Cave</a> in the 1960s and 1970s. Since that time Guanyindong has been famous as one of the most important Paleolithic sites in South China because of the relatively large number of stone tools found there.</p>
<p>Most are stored at the <a href="http://english.ivpp.cas.cn/">Institute of Vertebrate Paleontology and Paleoanthropology</a> in Beijing, and our team spent a lot of time carefully inspecting each tool to identify the traces that reveal how it was made. It was during this painstaking analysis of the museum specimens that we encountered a few dozen Levallois tools among the thousands of artifacts in the collection. </p>
<p>During the previous excavations at Guanyindong Cave, researchers had used <a href="https://en.wikipedia.org/wiki/Uranium%E2%80%93thorium_dating">uranium-series methods</a> to date fossils found in the sediments. This technique relies on the <a href="https://www.thoughtco.com/definition-of-isotopes-and-examples-604541">radioactive decay of tiny amounts of uranium</a> that collects in bone shortly after it is buried to come up with an age range for its burial. But it’s hard to precisely determine the true age of bone using this method. At Guanyindong these uranium-series ages span a wide range, from <a href="https://europepmc.org/abstract/cba/527068">50,000 to 240,000 years ago</a>. Also, the association between the dated fossil pieces and the stone artifacts was not recorded in detail. These problems meant that we couldn’t work out what layers the dated fossils came from, and if they were close to any of the Levallois stone tools.</p>
<p>Using only information available from the previous excavation, we couldn’t be sure of the exact age of the Levallois tools in the museum. The dates were important to nail down, because if they were older than 30-40,000 years, then they could be the earliest Levallois tools found in China.</p>
<figure class="align-center zoomable">
<a href="https://images.theconversation.com/files/246062/original/file-20181117-194497-13f9kch.png?ixlib=rb-1.1.0&q=45&auto=format&w=1000&fit=clip"><img alt="" src="https://images.theconversation.com/files/246062/original/file-20181117-194497-13f9kch.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&fit=clip" srcset="https://images.theconversation.com/files/246062/original/file-20181117-194497-13f9kch.png?ixlib=rb-1.1.0&q=45&auto=format&w=600&h=413&fit=crop&dpr=1 600w, https://images.theconversation.com/files/246062/original/file-20181117-194497-13f9kch.png?ixlib=rb-1.1.0&q=30&auto=format&w=600&h=413&fit=crop&dpr=2 1200w, https://images.theconversation.com/files/246062/original/file-20181117-194497-13f9kch.png?ixlib=rb-1.1.0&q=15&auto=format&w=600&h=413&fit=crop&dpr=3 1800w, https://images.theconversation.com/files/246062/original/file-20181117-194497-13f9kch.png?ixlib=rb-1.1.0&q=45&auto=format&w=754&h=520&fit=crop&dpr=1 754w, https://images.theconversation.com/files/246062/original/file-20181117-194497-13f9kch.png?ixlib=rb-1.1.0&q=30&auto=format&w=754&h=520&fit=crop&dpr=2 1508w, https://images.theconversation.com/files/246062/original/file-20181117-194497-13f9kch.png?ixlib=rb-1.1.0&q=15&auto=format&w=754&h=520&fit=crop&dpr=3 2262w" sizes="(min-width: 1466px) 754px, (max-width: 599px) 100vw, (min-width: 600px) 600px, 237px"></a>
<figcaption>
<span class="caption">Bo Li and Hu Yue collecting sediment samples from the same layers the stone tools had been in, in order to redate them.</span>
<span class="attribution"><span class="source">Weiwen Huang</span>, <a class="license" href="http://creativecommons.org/licenses/by-nd/4.0/">CC BY-ND</a></span>
</figcaption>
</figure>
<p>To uncover the true age of these Levallois tools, we made several trips to the cave to collect new samples for dating. It was challenging to find a suitable location to get the samples because the previous excavations didn’t leave much behind and much of the site was covered with thick vegetation.</p>
<p>We collected our new sediment samples from places where artifacts were still visible in the wall of the excavation, so we could be sure of a close connection between our samples and the stone tools. Essentially we were trying to collect new dirt from the spots where the museum artifacts had originally been excavated. The plan was then to test the samples with more advanced dating techniques than had originally been available.</p>
<h2>Analyzing new samples to date old artifacts</h2>
<p>Back in the lab, we analyzed the samples using single-grain <a href="https://www.thoughtco.com/luminescence-dating-cosmic-method-171538">optically stimulated luminescence</a> methods. This technique can identify how much time has passed since each individual grain was last exposed to the sun. Dating many individual grains in a sample is important because it can tell us if tree roots, animals or insects have mixed younger sediments down into older ones. After we identified and removed intrusive younger grains, we found that one layer of artifacts dated to about 80,000 years ago. We dated a lower layer to about 170,000 years ago. Our museum work had identified Levallois tools in both of these layers. </p>
<p>With the combination of careful inspection of the museum collection, new fieldwork to collect samples, and a new laboratory method of dating the site, we had uncovered a surprising and important result. These Levallois tools are much older than those from any other sites in East Asia. This suggests a more widespread geographic distribution of Levallois prior to the dispersal of modern humans out of Africa and Europe into Asia.</p>
<p>One reason why it has been so hard to find evidence of the technique in China until now is that the number of people in East Asia during the Palaeolithic might have been much smaller than in the West. Small, <a href="https://doi.org/10.1016/j.quaint.2008.12.001">low-density populations with weak and irregular patterns of social activity</a> might make it hard for new technologies to spread and persist over a long time.</p>
<p>We don’t know what species of human made the tools at Guanyindong because we haven’t found any bones. Whoever they were, they had similar skills to people living in the West at the same time. They appear to have independently discovered the Levallois strategy in China at the same time people were making extensive use of it in Europe and Africa.</p><img src="https://counter.theconversation.com/content/106914/count.gif" alt="The Conversation" width="1" height="1" />
<p class="fine-print"><em><span>Ben Marwick receives funding from the Australian Research Council. </span></em></p><p class="fine-print"><em><span>Bo Li receives funding from Australian Research Council.</span></em></p><p class="fine-print"><em><span>Hu Yue receives funding from the University of Wollongong.</span></em></p>A fresh look at museum artifacts fills in a gap in the Asian archaeological record and refutes the idea that an advanced technique was imported from the West by early modern humans.Ben Marwick, Associate Professor of Archaeology, University of WashingtonBo Li, Principal Research Fellow in Archaeological Science, University of WollongongHu Yue, Postgraduate Student in Earth and Environmental Sciences, University of WollongongLicensed as Creative Commons – attribution, no derivatives.