The orientation of the Americas meant that few innovations traveled between continental civilizations. The only pack animals in the Americas, llamas and alpacas, never made it past South America before the European invasion. But there was a continual migration of innovations between China, Europe, and the Fertile Crescent. That is thought to be partly why Eurasian cultures became technologically advanced over those of sub-Saharan Africa, the Americas, and Australia.
Along with the disruptions that Europe caused to the world’s people, it was depressingly common how often the natives used the newcomers to conquer their neighbors. Although Spaniards inflicted onto the Western Hemisphere in the 1500s, they often had native assistance. The Aztecs were anything but benevolent rulers; their bloody altar constantly sacrificed prisoners (it was an ), and when that ultimately conquered the Aztecs, his native allies did most of the fighting. Any natives who helped the Spaniards helped depopulate their hemisphere. When the French allied with the Huron, the first thing that the Huron did was . That backfired on the Huron, as their tribe became extinct within 40 years. In Africa and North America, when European slavers came, the natives were often only too happy to sell their neighbors into slavery, and some American tribes made for Europeans before they themselves became extinct. With a , natives almost never realized what the coming of Europeans ultimately meant. With some notable exceptions, such as and , natives could not put aside their differences and try ridding their lands of the invaders, and when some tried, it was already too late. When the British began “settling” the South Pacific, the natives used European weapons to slaughter or or .
But the most impressive dual-use innovation in mollusks is what cephalopods invented. Their gill pumps are quite muscular and . That . Jet propulsion is not an energy-efficient means of transportation, but the cephalopod’s ability to pass oxygen-bearing water over its gills is unmatched. Cephalopods can live in waters too hypoxic for fish to survive. In the coming Ordovician Period, cephalopods would be apex predators of marine biomes and would hold that distinction for a long time. Cephalopods are today’s ; the and it has the largest brain-to-body-size ratio of all invertebrates. It is thought that the skills needed for predation stimulated cephalopodan intelligence. Today, the is the only survivor of that lineage of Ordovician apex predators.
But the branch of the that readers might find most interesting led to humans. Humans are in the phylum, and the last common ancestor that founded the Chordata phylum is still a mystery and understandably a source of controversy. Was our ancestor a ? A ? Peter Ward made the case, as have others for a long time, that it was the sea squirt, also called a tunicate, which in its larval stage resembles a fish. The nerve cord in most bilaterally symmetric animals runs below the belly, not above it, and a sea squirt that never grew up may have been our direct ancestor. Adult tunicates are also highly adapted to extracting oxygen from water, even too much so, with only about 10% of today’s available oxygen extracted in tunicate respiration. It may mean that tunicates adapted to low oxygen conditions early on. Ward’s respiration hypothesis, which makes the case that adapting to low oxygen conditions was an evolutionary spur for animals, will repeatedly reappear in this essay, as will . Ward’s hypothesis may be proven wrong or will not have the key influence that he attributes to it, but it also has plenty going for it. The idea that fluctuating oxygen levels impacted animal evolution has been gaining support in recent years, particularly in light of recent reconstructions of oxygen levels in the eon of complex life, called and , which have yielded broadly similar results, but their variances mean that much more work needs to be performed before on the can be done, if it ever can be. Ward’s basic hypotheses is that when oxygen levels are high, ecosystems are diverse and life is an easy proposition; when oxygen levels are low, animals adapted to high oxygen levels go extinct and the survivors are adapted to low oxygen with body plan changes, and their adaptations helped them dominate after the extinctions. The has a pretty wide range of potential error, particularly in the early years, and it also tracked atmospheric carbon dioxide levels. The challenges to the validity of a model based on data with such a wide range of error are understandable. But some broad trends are unmistakable, as it is with other models, some of which are generally declining carbon dioxide levels, some huge oxygen spikes, and the generally relationship between oxygen and carbon dioxide levels, which a geochemist would expect. The high carbon dioxide level during the Cambrian, of at least 4,000 PPM (the "RCO2" in the below graphic is a ratio of the calculated CO2 levels to today's levels), is what scientists think made the times so hot. (Permission: Peter Ward, June 2014)
An important evolutionary principle is organisms' developing a new feature for one purpose and then using that feature for other purposes as the opportunity arose. As complex life evolved in the newly oxygenated seafloors, several immediate survival needs had to be addressed. To revisit the , if an oxygen-dependent animal did not have access to oxygen, it meant immediate death. Obtaining oxygen would have been the salient requirement for early complex life that adopted aerobic respiration , which is how nearly all animals today respire. While animals in low-oxygen environments have adapted to other ways of respiring (or perhaps in the first place), they are all sluggish creatures and would have quickly lost in the coming arms race. , which is a critical connective tissue in animals, requires oxygen for its synthesis, and was one of numerous oxygen-dependencies that animals quickly adopted during the Cambrian Explosion.
The main reason for low-energy transportation lanes was so that energy supplies (primarily food and wood) could feed the cities, and that flow of energy was often reciprocated with the flow of manufactured goods. The standard pattern of early cities was energy supplies flowing to the cities and city-manufactured goods flowing outward, and cities thereby became hubs of exchange. The so-called “tyranny of distance,” which means how far goods could be effectively transported to cities, limited the size of their hinterland and thus limited a city’s size. More energy-intensive and energy-efficient transportation enlarged the exploitable hinterland, which allowed cities to grow. The introduction of the wheel could improve matters, but not always. In preindustrial Islamic cultures, the camel was often a more energy-efficient form of transportation than wheeled carts.
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 .
The explains plenty, and one reality is that women will always have a genetic investment in their offspring no matter who the fathers are. As civilizations rose and , they all had enhanced reproductive rights (many wives, harems, etc.), and many women found the situation tolerable and even attractive, although there could be coercion in the unions and there are many obvious disadvantages to being a "kept" woman. However, being a wife/concubine for an elite man usually meant a pretty good life and children being provided for. The biggest losers in such societies were non-dominant men, who had diminished procreation opportunities (and eunuchs guarded harems, for instance). With the rise of DNA testing, a repeating dynamic is seen: when one people at a higher economic level (energy use) encountered another, the women from the poorer culture bred with the men from the richer culture, and men from the poorer culture began vanishing from the gene pool. It is particularly noticeable among agriculturalist expansions into hunter-gatherer lands, such as the and from the Fertile Crescent into Europe and North Africa, and seems to be implicated in the spread of Mesoamerican farmers into the USA's Southwest. The general pattern during the Neolithic Expansion seems to have been farmers migrating to arable land and establishing agricultural communities that were surrounded by hunter-gatherers, and it seems more common that the farmer populations expanded and displaced (the men)/absorbed (the women) the hunter-gatherer population than hunter-gatherers learned agriculture. After a career of studying human migrations, Peter Bellwood had this to say about what motivated them:
Only when economic surpluses (primarily food) were redistributed, first by chiefs and then by early states, did men rise to dominance in those agricultural civilizations. Because the rise of civilization in the Fertile Crescent is the best studied and had the greatest influence on humanity, this chapter will tend to focus on it, although it will also survey similarities and differences with other regions where agriculture and civilization first appeared. Whenever agriculture appeared, cities nearly always eventually appeared, usually a few thousand years later. Agriculture’s chief virtue was that it extracted vast amounts of human-digestible energy from the land, and population densities hundreds of times greater than that of hunter-gatherers became feasible. The , but today it is widely thought that population pressures led to agriculture's appearance. The attractions of agricultural life over the hunter-gatherer lifestyle were not immediately evident, at least after the first easy phase, when intact forests and soils were there for the plundering. On the advancing front of agricultural expansion, life was easy, but as forests and soils were depleted, population pressures led to disease, "pests" learned to consume that human-raised food, and agricultural life became a life of drudgery compared to the hunter-gatherer or horticultural lifestyle. Sanitation issues, disease, and environmental decline plagued early settlements, and not long after they transitioned from hunter-gatherers to farmers, but the land could also support many times the people. Another aspect of biology that applies to human civilization is the idea of . Over history, the society with the higher carrying capacity prevailed, and the loser either adopted the winner’s practices or became enslaved, taxed, marginalized, or extinct. On the eve of the Domestication Revolution, Earth’s carrying capacity with the hunter-gatherer lifestyle was around 10 million people, and the actual population was somewhat less, maybe . On the eve of the Industrial Revolution in 1800, Earth’s population was , and again was considered to be about half of Earth's carrying capacity under that energy regime. No matter how talented a hunter-gatherer warrior was, he was no match for two hundred peasants armed with hoes.
The writers of the knew that deforestation led to droughts, and Gilgamesh’s war against the forest foreshadowed the fate of numerous Old World civilizations. The city-states of southern Mesopotamia made regular journeys to Lebanon’s cedar forest. The ruler of , not far from Uruk, had plans for aggrandizing his legacy and leveled cedar forests and rafted their logs downriver to Lagash to fulfill his grandiose schemes. The city-states of southern Mesopotamia deforested upstream river valleys and rafted logs to their downstream cities. Wars between the city-states, and wars of foreign conquest to secure forests and navigable rivers (particularly the Tigris, Euphrates, and of today’s Iran), were common then. Wood became such a coveted commodity that it could approach the value of precious metals and stones, and rulers placed names on mountains corresponding with what tree predominantly grew on each one.