Several lines of evidence have converged and more evidence is regularly amassed, which is telling a story of dramatic and rapid climate change spurring vegetation changes that initiated evolutionary adaptations in the cradle of humanity. Sediment cores off of East Africa in the Arabian Sea, land sediment records in East Africa, combined with studies of carbon-12/13 ratios of fossil teeth, are telling an interesting and familiar tale of human origins. Three mya, as Earth was moving toward an ice age and the climate dried, the familiar grasslands of the appeared for the first time. Cgrasses have higher proportions of carbon-13, and so will animals that eat them. The expanding Cgrasslands coincided with the disappearance of Lucy's species and the appearance of the robusts that (or perhaps eating animals that ate those plants), probably from those expanding grasslands.
There are three primary survival requirements for any species: obtain nutrients (always primarily energy), avoid becoming nutrients, and perform those first two tasks long enough to produce offspring. If those requirements are not met, the species will go extinct. The eating instinct outranks the sex drive, but avoiding becoming food is where the most energetic behaviors can usually be found. Primal survival instincts take over during the . In humans, that to enable the body to perform feats of physical survival. That is when pumps. All evolutionary adaptations studied by scientists always have those three primary requirements girding the explanatory framework.
Bonobos have an average party size of about 17, and party sizes are consistent. How can they have such large and stable foraging parties while no other chimps can? Because they eat gorilla food. Because gorillas no longer live south of the Congo, the young leaves and herb stems not available to chimps where gorillas live make for pleasant bonobo traveling snacks. Since the biomass concentration of gorillas and chimps is nearly the same where their ranges overlap, it meant that bonobos had twice the food supply that chimps did. Bonobos also evolved to better digest gorilla foods, and larger parties put females on a more equal footing with males. Bonobos, both males and females, did not tolerate the alpha male model of other chimp societies in which male gangs dominated.
The other colors represent ranges of other chimp species. Bonobos are separated from all other chimp species by the , which forms the north, east, and west borders of their range. When the , the current bonobo range began having droughts and the rainforest shrank. Gorillas are masters of the rainforest, and when the rainforest south of the Congo disappeared during one of the dry periods (and it seems to be about one mya, ), gorillas left and never returned. Humans are the only great apes that can swim, so the Congo was an impenetrable barrier for chimps and gorillas.
Bonobos are the only non-human African great ape exception to infanticide, and are also the only great ape species that does not sexually coerce females, humans included. The reason seems to be the social organization that arose from a plentiful food supply that allowed for larger groups in which females males actively reduced male violence. Many behaviors within and between bonobo bands are unknown with chimps. A male bonobo will remain with his mother for her entire life, and male bonobos do not vie for dominance. Instead, bonobos have a sexuality that no other animal on Earth has remotely approached. They settle nearly everything with sex. Female on female is common, particularly when bands meet, but , with the sole exception of mothers and sons, as the aversion to inbreeding is rooted deeply in animals and is also responsible for the human incest taboo. Bonobo societies are and seem to live by the slogan, “Make love, not war.” But it started with their economy, when their primary and dominant competitor moved away. In recent studies, the only bonobo sexual coercive acts observed are females abusing males, which is also rare. A likely influence on ending infanticide is that female bonobos, like humans, conceal their ovulation, so males are not cued to compete to be the father. Also, since virtually all bonobos have sex all the time, there is no way for bonobos to determine paternity.
But even though they were “regular” on the geologic time scale, driven by , there would have been nothing “regular” about them to evolving humans. When ice sheets advanced, global climate became cooler and dryer; rainforests shrank and deserts expanded. Human adaptations to those changes, which could even be discerned in one human lifetime, must have had profound impacts on the human journey. In short, humans had to readily adapt to rapidly changing conditions, and rapid adaptation would have had selective effects on burgeoning human intelligence and problem-solving ability; those that adapted, survived. Also, scientists think that the rapidly oscillating climate resulted in migrations and pockets of isolated members of species that then underwent rapid evolutionary adaptation, the kind that leads to speciation. This may have been partly responsible for the relatively rapid evolution of the human line, particularly in the past million years.
From the initial appearance of about 2.0-1.8 mya, Europe was periodically buried under the ice sheets that began growing and receding when the first stone tools were made, so tended to appear and disappear in Europe. The fact that humans evolved and spread during an ice age has led to competing hypotheses about many aspects of humanity’s rise. Although , and there have been 17 identified episodes of advancing and retreating ice sheets, particularly in North America and northern Eurasia, the early ones were not as severe, and they did not achieve , as the diagram below shows. (Source: Wikimedia Commons)
Humans took a different path 2.5 mya. There are generally two schools of thought regarding the appearance of among scientists: one is called the Multiregional Model, and the other is called the . In their essence, the Multiregional Model had those migrants eventually evolving into today’s races, and the “Out of Africa” Model had humans evolve in Africa and then spread across the world and replace/displace all other members of the genus. The rise of has largely resolved the issue in favor of the “Out of Africa” Model. There are also intermediate views and variations of each hypothesis, which generally relate to the invaders mating with the natives, even if they could be classified as separate species. For instance, Neanderthal DNA is part of the human genome, which reflects interbreeding. Since Neanderthals were largely confined to Europe and what became the Fertile Crescent, and the migration of the original was from Africa, sub-Saharan Africans . Africans also have the most genetic divergence, which reflects the idea that humans have lived longer in Africa than anywhere else. There is virtually no doubt that evolved in Africa.
Did the control of fire to , ? Or did merely use it to begin dominating the world? Was cooking the seminal event in the appearance of humans? Those questions may not be definitively answered in my lifetime, and led to the somewhat uncertain title of this chapter. Highly transformative developments coincided with the appearance and dispersal of , which was a radical break from all that came before – biologically, technically, and culturally – and strongly implies great cognitive enhancements. I believe that the control of fire and cooking would leave deep cultural and biological impacts on the human journey, and because barely changed during its nearly two-million year tenure on Earth, both in biology and in Acheulean artifacts, I favor Wrangham’s hypothesis, at least until the Next Big Finding. Just as Einstein said that and that his theories would one day become obsolete, but that their best parts would survive in the new theories, I suspect that significant aspects of Wrangham’s hypothesis will live on in successor hypotheses, and other scientists have been following Wrangham’s lead.
The energy from controlled fire allowed humans to , , and socially organize in new ways. Humans commandeered energy that otherwise and used it for immediate human benefit. It was also the first great human robbery. All heterotrophs “” energy from other life forms to live. The primary exception is the symbiosis that . But no animal had ever robbed energy from ecosystems on that scale before. By making fires, humans were liberating many times the energy that their biological processes used - energy that could have fed forest ecosystems. While humans were only using deadwood, it was the least destructive to forest ecosystems. But when humans began burning forests to flush out animals to kill and make biomes suitable for animals to hunt, they were destroying and altering ecosystems on a vast scale. A cord of wood provides about four years of the calories that fuel a human adult’s body, and one hectare can provide a sustainable annual harvest of about ten years of human calories. A family of four using a hectare for firewood on a sustainable basis would be using more than twice their caloric intake for burning wood. Very little of that released energy would benefit humans if they burned it over a campfire, as humans did for the entire epoch of the hunter-gatherer; that liberated energy largely went straight into the sky. The direct benefit to humans would be the energy that went into cooking food, what warmed human flesh, what was used to make tools, and the benefits of scaring off predators and providing light at night. More indirect benefits would have been ecosystem changes to provide human-digestible calories, such as American Indians burning the woodlands and plains to make environments conducive to animals that they could easily hunt. In , the earliest epochs are the most uncertain, but saying that hunter-gatherer humans used 2.5 times their dietary calories in their economy is probably, perhaps greatly, understating the case. That 5% efficiency number is also a rough estimate, and both numbers could be refined by a scientifically performed effort. Maybe somebody has already done it. The numbers in that table for subsequent epochs are more accurate, and the most accurate of all are those for , and I live in one. The increases in efficiency became more modest with each epoch as the limits of were approached.
Although are rather beautiful, anthropologists have lamented the “boring million years” that existed after Acheulean culture first appeared about 1.8-1.7 mya. It seems that not much was going on, anatomically or technologically, with the human line, from the first appearance of Acheulean culture to about a half-million years ago. There is evidence of Acheulean culture spreading in waves across Asia but in what became their refugia. Acheulean tools were even made by a likely . The Acheulean hand axe is the longest-lived technology in the human journey, other than the stick and maybe the campfire.