Whatever the case may be, it appears clear that the population in Africa and Neanderthal population in Europe and the Middle East were isolated for tens of thousands of years, perhaps far more than 100,000 years, and humans used a toolkit like the Neanderthals’ until something happened between 70 and 50 kya. Just happened is a matter of great controversy, and in recent years, several disciplines have converged on the issue and are drawing a clearer picture today. Some key findings that shed light came from global DNA studies, linguistics partnering with evolutionary theory, and brain studies. In the past generation, as has been applied to many areas, a startling picture of the human journey has emerged. , probably for flexible power generation. For animals that reproduce sexually, the mother’s mitochondria are passed to her offspring, while virtually none comes from the father, if any. Geneticists can measure mutations in and approximate when two different animals shared a common ancestor, whether they belong to the same species or not. Similarly, regarding nuclear DNA, the produces a male mammal, and mutations in the Y chromosome can also be analyzed to estimate when two men shared the same ancestor. , but scientists have been aligning DNA results with fossil dates, which are considered more reliable, and have been resolving some limitations. But if the timing is suspect for such genetic analyses, far more confidence exists for descent relationships. Human DNA testing is a burgeoning business, used for everything from freeing to to examining the genetic heritage of the .
The (c. 5.3 to 2.6 mya) began warmer than , but was the prelude to today’s ice age, as temperatures steadily declined. An epoch of less than three million years reflects human interest in the recent past. Geologically and climatically, there was little noteworthy about the Pliocene (although the was created then), although two related events made for one of the most interesting evolutionary events yet studied. South America kept moving northward, and the currents that once in the Tethyan heyday were finally closed. The gap between North America and South America began to close about 3.5 mya, and by 2.7 mya the current land bridge had developed. Around three mya, the began, when fauna from each continent could raft or swim to the other side. South America had been isolated for 60 million years and only received the stray migrant, such as rodents and New World monkeys. North America, however, received repeated invasions from Asia and had exchanges with Europe and Greenland. North America also had much more diverse biomes than South America's, even though it had nothing like the Amazon rainforest. The ending of South America’s isolation provided the closest thing to a controlled experiment that paleobiologists would ever have. South America's fauna was devastated, far worse than European and African fauna were when Asia finally connected with them. More than 80% of all South American mammalian families and genera existing before the Oligocene were extinct by the Pleistocene. Proboscideans continued their spectacular success after leaving Africa, and species inhabited the warm, moist Amazonian biome, as well as the Andean mountainous terrain and pampas. The also invaded and thrived as a mixed feeder, grazing or browsing as conditions permitted. In came cats, dogs, camels (which became the ), horses, pigs, rabbits, raccoons, squirrels, deer, bears, tapirs, and others. They displaced virtually all species inhabiting the same niches on the South American side. All large South American predators were driven to extinction, as well as almost all browsers and grazers of the grasslands. The South American animals that migrated northward and survived in North America were almost always those that inhabited niches that no North American animal did, such as monkeys, (which survived because of their claws), and their small cousins (which survived because of their armor), , and (which survived because of their quills). The opossum was nearly eradicated by North American competition but survived and is the only marsupial that made it to North America and exists today. One large-hoofed herbivore survived: the . The (it weighed one metric ton!) survived for a million years after the interchange. , that , also survived and migrated to North America and lasted about a million years before dying out. In general, North American mammals were , which resulted from evolutionary pressures that South America had less of, in its isolation. They were able to outrun and outthink their South American competitors. South American animals made it past South America, but none of them drove any northern indigenous species of note to extinction.
Photosynthesis is the first stage of energy flow through an ecosystem. You and all other animals on earth rely on the energy that plants store for life. But animals aren’t the only organisms that burn energy. Plants burn energy as they grow, too. In both plants and animals, the process of — which releases stored energy for use — occurs in the mitochondria inside each cell.
The figure illustrates how closely photosynthesis and respiration are linked. As you can see, thanks to these two life-sustaining processes, plants and animals depend on each other to survive.
Most Americans, however, use solar energy in its secondhand form: fossil fuels. When sunlight strikes a plant, some of the energy is trapped through photosynthesis and is stored in chemical bonds as the plant grows. We can recover that energy months or years later by burning wood, which breaks the bonds and releases energy as heat and light. More often, though, we use the stored energy in the much more concentrated forms that result when organic matter, after millions of years of geological and chemical activity underground, turns into fossil fuels, such as coal, oil, or natural gas. Either way, we’re reclaiming the power of sunlight.
Human-caused began with , which in turn drove their predators to extinction and had , particularly those resulting from the loss of . The so-called that scientists have been studying have been all human-induced, as humans eliminated predators, who were their energetic competitors. Driving the world's large animals to extinction was probably the impetus behind the Domestication Revolution, and .
Once the energy is delivered into a gas tank or turbine boiler, the efficiency of turning that chemical energy into mechanical energy is what first dealt with. Mechanical energy is called , whether it comes from human muscles, the muscles of draft animals, or machines. Also, the concept of power is critical, which is the amount of work performed over a time period. How quickly energy can be released and used is the crux of how rockets work, for instance. To get more power, more sophisticated technology was required, from taller masts in sailing ships to stronger components for watermills to high-performance engines (which run hotter with greater pressures). Generating more power was always a technological feat but was dependent on how much energy could be delivered and utilized and how quickly.
Almost all traditional alternative energy sources and related technologies have low EROIs (direct solar 2-to-8, wind turbines 18, geothermal less than 5). Those alternative sources all have the same problems that wind and water power had before the Industrial Revolution and more, such as , not much energy is available to begin with, and they all create environmental impacts that, although not as great as fossil and nuclear fuels, are still considerable. Wind turbines not only kill vast numbers of birds each year, but they are noisy and create inland turbulence. In order to replace fossil fuels, there would need to be about four hundred times as many windmills on Earth as there already are, and I have driven through several windmill farms in the USA, which are spread across many miles of suitable terrain. In order to raise humanity to the American standard of living, there would need to be far more than a thousand times as many windmills. There may not be enough suitable land on Earth to host those windmills, and windmills are considered the most viable traditional alternative. Direct solar, including photovoltaics, makes the most sense in deserts. It does not deliver much energy, but it is considered the next most viable alternative, and there would have to be about four thousand times as many photovoltaic arrays as already exist to raise the world to the American standard of living. Again, finding the land to host them is a problem, and the materials need to be mined. There are maintenance issues and other problems. Rock is not a good conductor, so heat is rapidly depleted from the geothermal source and it quickly goes “dry,” and has to go “fallow” to recover.
For the first concept presented above, for conventional renewable energy sources, they are replenished by sunlight or radiation from Earth’s interior; one is fusion, and the other is fission. For so-called non-renewable energy sources, such as hydrocarbons and fissile materials, they are either renewed on timescales so vast that they are effectively non-renewable for humans (such as ), or are “renewed” by the (fissile materials), so could only be renewed with new planetary formation. In mainstream thought, the currently non-renewable energy resources are primarily hydrocarbons (petroleum, coal, and natural gas) and uranium. Much of the debate centers around the definition of oil. What has been called oil for the past 150 years is today called . It is the oil formed by the , and can be mined by drilling wells and extracting it with the conventional methods that have been used since the beginning, and new techniques are periodically invented to increase the rate and total extraction. For conventional oil, humanity has unearthed about 1.1 trillion barrels since 1859, and about as of 2014. Production of conventional oil peaked in 2006 at 25 billion barrels per year and has declined since then. At current production rates, conventional oil will be completely depleted in less than 50 years. About another five billion barrels per year are called unconventional oil, which is called heavy oil, extra heavy oil, and oil sands. Those unconventional oils comprise trillions more barrels, and total and arguably more. For fissile materials, primarily uranium, the peak may have already been reached by 2014, or it . For , in that the peak may have already been reached, or it is only a few decades into the future at most. For coal, may also be only a few decades into the future. Peak extraction usually occurs when about half of the recoverable energy resource has been mined. In summary, the energy resources that have powered the Industrial Revolution are all on their way to largely vanishing in this century. The only resources with seeming viability past this century are coal and unconventional oil, which brings us to the second concept: .
During the , when ecosystems had their energy supplies disrupted, they collapsed and mass extinctions resulted. The oceanic were probably caused by anoxia, temperature change, , and other physical events. An asteroid "winter" helped . When that asteroid-induced global "winter" blocked sunlight, the ecosystems had an energy shortage at their base, as photosynthesis was interrupted, both on land and in water. That dinosaur-destroying asteroid also incinerated land-based ecosystems. It is doubtful that any pre-human mass extinction was caused by a disruption at the food chains' tops, but near their bases. Human-caused extinctions had different dynamics.