Humans
Of the various human species, the species homo sapiens has prevailed and displaced other species such as homo neandertalis.
Habitat and adaptation:
Of all primate species, humans have adapted most strongly to ground life, more so than baboons. While baboons can still climb trees well, the climbing abilities of humans have atrophied.
Just as gibbons have adapted to climbing with extra-long arms, humans have extra-long legs for their life on the ground. Unlike ground-dwelling baboons, humans move on two legs. Due to the special shape of their legs, hips and spine, they can run much more energy-efficiently, much faster and, above all, with greater endurance than all other primates.
They are hairless as an adaptation to living and running in savannahs with few trees. To protect them from the sun, their skin has a strong pigmentation, which has been lost again in some populations living in northern areas.
They are relatively unspecialized in their diet, which has enabled them to colonize a wide variety of habitats. They eat fruit, seeds, insects and meat. Leaves are rarely consumed.
In addition to their extra-long legs, humans have several other peculiarities that make them successful hunters. Humans have many powerful sweat glands on their skin that provide evaporative cooling. The shoulders have a shape unique among primates that allows for quick, accurate throwing. Unlike most savannah dwellers, humans can also hunt in extreme heat. By hunting in groups and throwing stones or sharpened wooden sticks, humans can hunt animals that are inferior to them in terms of strength and mass. Due to their good adaptation to hunting on the ground, humans spread across Europe and Asia in several waves. The last wave of emigration was the most far-reaching. It wiped out all humans from the previous waves of emigration (there is evidence of slight gene transmission during contact with the Neanderthals). The people of the last wave of expansion also colonized Australia and later America from Asia.
The big brain:
Humans have an unusual but very successful strategy for dealing with changing environmental conditions. While in other great apes sexual maturity usually occurs at the age of four to seven, in humans this does not happen until around the age of fifteen. This effect is triggered by an extremely large brain, which, however, contains no more instincts than the brains of other primates. The extremely long time until sexual maturity is reached is used to incorporate information from the environment into the brain. This means that adult humans – regardless of their habitat – have a better model of their habitat in their heads than any other primate.
The extremely large brain is of course a great hindrance. It complicates birth and consumes a lot of energy. The combination of a large head, narrow birth canal (upright gait) and limited resources during pregnancy means that newborn humans are much more helpless than other newborn primates. The care of these extremely helpless infants can only be managed in a close group. Typical human groups are larger than those of other primates (with the exception of some baboon groups), at around 120 people.
Because the environment of humans in such large groups consists mainly of other humans, a large part of the brain capacity is used to gain an advantageous position in the group. Humans have developed the ability to understand the feelings and plans of their conspecifics, which is unique in the animal kingdom.
Social structure:
Due to the very helpless offspring, humans have also developed an unusual social structure. In addition to the tendencies towards harem formation (as in chimpanzees) and promiscuity (as in bonobos), humans have an unusual monogamy within the group. Monogamy is only common in the animal kingdom where individuals live widely dispersed, such as in gibbons.
Because this monogamy is still a relatively new strategy that competes with other older strategies, it is still fragile and is used by the majority, but not all, individuals. Research reports many cases in which different strategies are used in parallel, depending on the occasion.
The social structure of people is flexible. Depending on the habitat and the available resources, the different strategies are successful in different ways and the social structures differ accordingly. There is a tendency for social structures to be more monogamous in harsh areas than in areas with good living conditions.
The social structure can be deduced from the external appearance of the individual: Since males no longer have pointed canines and males are only slightly larger than females, there is no harem culture associated with brutal fighting. The difference in size between males and females nevertheless suggests that there is no exclusive monogamy. The large testicles of the males indicate a certain importance of promiscuity. Large testicles develop evolutionarily when females have sex with several different males within a short period of time. Human females – unlike chimpanzee females – do not signal their fertile days. This non-signaling is advantageous for females in promiscuous and monagamous structures.
The large brain has greatly expanded the phenomenon of „culture“, which plays a certain role in other primates. Of all animals, humans are best able to transfer information from one generation to another (memes). The further development of spoken language as a communication tool has greatly promoted this. Cultural evolution, which progressed slowly in some other animal species – songbirds, whales and other apes – was rapid in speaking humans.
Symbionts:
Fruit-eating primates live in symbiosis with the plants whose fruits they eat. Similar to bees, they help the plants to reproduce by carrying the seeds out into the world and depositing them in nutrient-rich piles of excrement. When the trees gradually disappeared in East Africa and the predecessors of humans began to live on the ground, they no longer had these symbiotic partners. They made up for this by hunting successfully.
Recently – only ten thousand years ago – some human populations entered into new symbioses. The first new symbionts were grasses and yeasts. Humans began to plant grass seeds in the soil, protect the grass plants and eat the seeds of the grass plants. With the help of yeasts, the grass seeds were transformed into easily digestible and palatable food. These foods are called beer and bread. Such symbioses only exist in a few species, such as the leaf-cutter ants, which grow nutritious fungi on cut leaves in their burrow.
The symbiosis with some grass species was later joined by other symbioses with other plants. Some animals, especially herbivores, have also entered into a symbiosis with humans. Due to the different living and selection conditions within the symbiosis, these animals changed their appearance (e.g. the wolf became the handbag dog).
Through this symbiosis, the symbiotic humans (farmers) became more successful than the hunting humans. In the meantime, the symbiotic humans have almost completely replaced the hunting humans. These symbioses, together with other cultural evolution, have ensured that humans and their animal symbionts (pets) account for 98% of the biomass of all terrestrial vertebrates. (MacCreadyExplosion)
Animal and plant species that were not involved in the symbiosis and were a hindrance to humans were persecuted by humans and often exterminated.
The social structure of humans changed completely with the development of symbioses. The category of „possession“ has been added. There is hardly any „possession“ in other animal species. Territories are marked and defended. In some species, males conquer females and defend them. In humans, this principle has now also been applied to the symbionts cows and pigs. The substrate for the symbionts grass, the „fields“, also became „property“.
In many populations, „ownership“ with its concentration of resources once again enabled harem structures or monogamous relationships in which the woman became „property“. These female-possession structures could only be maintained with a „culture“ determined by the males. In some populations, however, it can already be observed how – despite continued „resource ownership“ – the culture dissolves the males‘ claim to ownership of the females. The tendency seems to be towards social structures in which monogamy and promiscuity exist in parallel. The desire of males to form harems is counteracted by promiscuity (with a certain competitive character).
More on this in: Sex is fun, but a lot of trouble; Chapter IV.