Sumerian city-states engaged in irrigation, which raised the water-tables. When the water table in those waterlogged soils reached the surface, the soils turned white with salt, especially with the high evaporation of those hot lands, and it would no longer support crops. The only solution was to stop irrigating and let the land go fallow as the water table fell, but the population pressures did not allow for it, so the process inexorably created saline soils, silt-filled canals due to upland deforestation, and today those Sumerian cities are all buried in silt in a desert. Eridu was a seashore city, and today its ruins lie more than 200 kilometers inland. But before silt and salt wrecked that civilization, many seminal inventions appeared. The appeared in . Gravity took a ship downstream, and wind power helped it move back upstream.
In the Fertile Crescent today, the ruins of hundreds of early cities are in their self-made deserts, usually buried under the silt of the erosion of exposed forest soils. As the Mediterranean Sea’s periphery became civilized, the same pattern was repeated; forests became semi-deserts and early cities were buried under silt. Before the rise of civilization, a forest ran from Morocco to Afghanistan, and only about 10% of the forest that still existed as late as 2000 BCE still remains. Everyplace that civilization exists today has been dramatically deforested. Humanity has since agriculture began. The only partial exceptions are places such as Japan, but they regenerated their forests by importing wood from foreign forests. North America and Asia have been supplying Japan with wood for generations. As civilizations wiped themselves out with their rapaciousness, some people were aware enough to lament what was happening, but they were a small minority. Usually lost in the anthropocentric view was the awesome devastation inflicted on other life forms. was only a prelude. Razing a forest to burn the wood and raise crops destroyed an entire ecosystem for short-term human benefit and left behind a lifeless desert when the last crops were wrenched from depleted soils. In the final accounting, the damage meted out to Earth’s other species, not other humans, may be humanity’s greatest crime. Humanity is the greatest destructive force on Earth since the , and our great task of devastating Earth and her denizens may be .
, near , had indicators of developing class systems, and male/female differences in diet. Cattle seem to have been in the vicinity, and is also where . Many still grow wild in the region. The apple may have been the first domesticated tree fruit, and . Early on, people also began to domesticate fiber-producing plants, and flax was among the . Fiber crops often competed with food crops for field space, especially when foreign conquerors reoriented that subject population’s efforts, which led to starvation in the subject population. A recent example is when , and opium instead of food, and Bengal had a .
about 10 kya. Pigs were first semi-domesticated in the Fertile Crescent , and were independently domesticated in China about eight kya. Combining domesticated plants and animals appeared fairly early. Farmers realized that animal manure could fertilize crops, so the close association of pastures and cropland became a standard feature of Fertile Crescent civilizations. Early domestic animals were all herd animals, and humans replaced herd leadership. Since are , their understanding of herd behaviors probably made their efforts more successful.
Those two early settlements may have been abandoned partly due to those climate events, but they would have also deforested their hinterlands and desertified the region, and the settlements were permanently abandoned. In the Jordan Valley, settlements were abandoned at the same time, which is thought to be because a thousand years of agricultural settlements eroded and deforested the land, and sufficient crops could no longer be grown. That pattern of population growth and apparent overtaxing of the environment was common all across the Fertile Crescent around eight-to nine kya, and populations migrated away from the first settlements in search of new lands to exploit, and animal herding became a more commonplace method of sustenance. Environmentally harmful practices combined with droughts destroyed many civilizations in the millennia after those early abandonments, including the , , and civilizations.
A water molecule containing (the most common oxygen isotope) will be lighter than a water molecule containing (both are stable isotopes), so it takes less energy to evaporate an oxygen-16 water molecule than an oxygen-18 water molecule. Also, after evaporation, oxygen-18 water will tend to fall back to Earth more quickly than oxygen-16 water will, because it is heavier. As a consequence, air over Earth’s poles will be enriched in oxygen-16 – the colder Earth’s surface temperature, the less oxygen-18 will evaporate and be carried to the poles – and . Also, the oxygen-isotope ratio in fossil shellfish (as their life processes prefer the lighter oxygen isotope) has been used to help determine ancient temperatures. During an ice age, because proportionally more oxygen-16 is retained in ice sheets and does not flow back to the oceans, the ocean’s surface becomes enriched in oxygen-18 and that difference can be discerned in fossil shells. Sediments are usually laid down in annual layers, and in some places, such as the off of Venezuela's coast, undisturbed sediments have been retrieved and analyzed, which has helped determine when ice sheets advanced and retreated during the present ice age.
Mass spectrometers have been invaluable for assigning dates to various rocks and sedimentary layers, as radioactive isotopes and their daughter isotopes are tested, including , , , and . Also, the ratios of elements in a sample can be determined, which can tell where it originated. Many hypotheses and theories have arisen, fallen, and been called into question or modified by the data derived from those increasingly sophisticated methods, and a few examples should suffice to give an idea of what is being discovered.
The moon rocks retrieved by astronauts are still being tested, as new experiments and hypotheses are devised. In 2012, which resulted from testing moon rocks for the ratios (both are stable isotopes), and it has brought into question the hypothesis that the Moon was formed by a planetary collision more than four billion years ago. The titanium ratio was so much like Earth’s that a collision with Earth forming the Moon has been questioned (as very little of the hypothesized colliding body became part of the Moon). The collision hypothesis will probably survive, but it may be significantly different from today’s hypothesis. , as well as , and their ages confirm that geologists have derived, and meteorite dates provide more evidence that our .
In the Western Hemisphere, the and civilization collapses of around a thousand years ago, or the Mississippian civilization collapse of 500 years ago, have elicited a great deal of investigation. From New Age ideas that the Anasazi and Mayan peoples “ascended” to the Eurocentric conceit that the was European in origin, many speculations arose that have been disproven by the evidence. It is now known that the Anasazi and Mayan culture collapses were influenced by epic droughts, but that was only the proximate cause. The ultimate cause was that those civilizations were not energetically sustainable, and the unsustainable long before Europeans invaded North America. The Anasazi used logs to build their dwellings that today . Scientists have used strontium ratios in the wood to determine where the logs came from, as well as dating the wood with and analyzing , and a sobering picture emerged. The region was already arid, but agriculture and deforestation desertified the region around , which was the heart of Anasazi civilization. When Anasazi civilization collapsed, at Chaco Canyon they were hauling in timber from mountains more than 70 kilometers away (the strontium ratios could trace each log from the particular mountain that it came from). When the epic droughts delivered their final blows, Anasazi civilization collapsed into a morass of starvation, warfare, and cannibalism, and the forest has yet to begin to recover, nearly a millennium later.
Another major advance happened in the late 20th century: the ability to analyze DNA. was discovered in 1953. In 1973, . In 2003, . was accomplished in 2005, for orangutans in 2011, and for in 2012. The comparisons of human and great ape DNA have yielded many insights, but the science of DNA analysis is still young. What has yielded far more immediately relevant information has been studying human DNA. The have been identified. Hundreds of falsely convicted Americans have been released from prison, and nearly 20 from , due to Human DNA testing has provided startling insights into humanity's past. For instance, in Europe it appears that after the ice sheets receded 16,000 to 13,000 years ago, , and for all the bloody history of Europe over the millennia since then, there have not really been mass population replacements in Europe by invasion, migration, genocide, and the like. Europeans just endlessly fought each other and honed the talents that helped them conquer humanity. There were , but other than hunter-gatherers being displaced or absorbed by the more numerous agriculturalists, there do not appear to be many population replacements. In 2010, suggested that male farmers from the Fertile Crescent founded the paternal line for most European men as they mated with the local women. DNA testing has demonstrated that all of today’s humans are , of whom a few hundred and conquered Earth. The , as well as genomes of other extinct species, and for a brief, exuberant moment, some scientists thought that , -style. Although dinosaur DNA is unrecoverable, organic dinosaur remains been recovered, and even some proteins have been sequenced, which probably no scientist believed possible in the 1980s.
Carbon has two primary stable isotopes: and . is the famous unstable isotope used for dating recently deceased life forms, but testing carbon’s stable isotopes has yielded invaluable information. is the backbone of all of life’s structures, and life processes often have a preference for using carbon-12, which is lighter than carbon-13 and hence take less energy to manipulate. Scientists have been able to test rocks in which the “fossils” are nothing more than smears and determine that those smears resulted from life processes, as there is more carbon-12 in the smear than carbon-13 than would be the case if life was not involved. This has also helped date the earliest life forms. Life’s preference for lighter isotopes is evident for other key elements such as sulfur and nitrogen, and scientists regularly make use of that preference in their investigations.