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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.
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Few studies have been performed on the relationships between energy, brains, and sleep, but a recent one found that sleep seems to be .Larger brains had to confer immediate advantages or else they would not have evolved, especially as energy-demanding as they are. Evolutionary pressures ensure that there is no cost without an immediate benefit. As humans have demonstrated, intelligence combined with manipulative ability led to a domination of Earth that no other organism ever achieved. Humans weigh about 50% more than chimpanzees, but have brains three times the size. A human brain comprises about 2% of the body’s mass, but uses nearly 20% of its energy at rest. Growing an energy-demanding organ was funded with the coin of energy. How did protohumans manage it?There are a number of possible solutions to obtaining the energy to fuel the growing protohuman brain, and they all fall under these categories: Studies have shown that humans and chimpanzees have the same basal metabolism, so the first possibility is considered very unlikely in our ancestors, although large brains in general . The subject of reducing energy output has an intriguing hypothesis: bipedal motion allowed humans to move by using less energy than our pre-bipedal ancestors. Human bipedal locomotion requires only a quarter of the energy that chimpanzee locomotion does, and chimps use about a quarter of their metabolism walking, although whether this was a key evolutionary event is controversial. Even though protohumans would have taken advantage of bipedal walking to range farther than chimps (humans can average 11 miles a day, while chimps can only achieve six), thereby using a relatively larger proportion of their energy on locomotion; bipedal locomotion energy savings alone might largely account for the growing brain’s energy needs. was developed to account for the required energy, which proposed that energy to fuel the growing brain came from reducing digestion costs, which was initially provided by eating more meat. and can digest cellulose while humans cannot. The human digestive tract is only about 60% of the size expected for a primate of our size. Human guts are far smaller than chimp and especially gorilla guts, which process all of that low-calorie foliage. Chimp and gorilla rib cages flare outward from top-to-bottom, like a dress, as did australopithecine rib cages, to accommodate large guts, as . When chimpanzees eat meat, they put large, tough leaves in their mouths. That helps them overachieve as meat eaters, as their teeth and jaws are poorly adapted for chewing meat. Mountain gorillas eat no meat at all. In the wild, great apes spend about half of their day chewing. Chimpanzees are the most carnivorous great ape, and although meat is the greatest treasure in chimpanzee societies, they often stop eating meat after chewing it for an hour or two and revert to fruit and other softer foods if they can get it. Chimpanzees when their staple, fruit, is scarce. Chimps have been seen killing monkeys, eating their organs, and then abandoning the carcasses to find more monkeys to kill. Organ meats and intestines are far easier to chew, and a poor meat chewer like a chimpanzee prefers soft meats. Just as chimpanzees prefer soft meats, predators will eat soft organs first and leave the tougher muscle for later, if they eat it at all. It depends on how plentiful the available flesh is, but the pattern across all predator groups is clear: eat the best, first, and leave the lesser quality foods to the end or let scavengers have them. It will always be a cost/benefit decision. All things being equal, the less time and energy needed to eat something, the sooner it will be eaten. If extra time and effort is needed to procure food, then the nutritional reward (primarily in energy) has to be exceptional to justify it. Evolutionary pressures have made animals into excellent accountants. The human sweet tooth is a relic of humanity’s fruit-eating ape heritage, and the desire for fatty foods reflects an adaptation to prefer that energy-richest of foods. Fat (made of hydrocarbons) is the ultimate energy windfall of all foods.