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Nitish Dec 04, 2020
How did Homo sapiens evolve much faster than other human and non-human primates?
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Evolution is a continuous process. Everything is evolving to become better than its earlier form. Humans, or Homo sapiens, are also the result of millions of years of evolution and brutal screening of natural selection. However, we were not the only species that evolved about ten to twelve million years ago, somewhere in Africa. Various fossil records emphasize the de-lineation of multiple Homo species (as well as other terrestrial, bipedal, cave-dwelling species) from a common arboreal, quadrupedal predecessor. Moreover, our genetic makeup is highly similar to other living non-human primates, and to the other human primates that went extinct. The unanswered questions, therefore, are:
- If the surrounding conditions were similar, why didn't the other non-human primates evolve the same way we did? What are the theories proposed to answer this?
- Did we have a role in the extinction of other Homo species?
- What experiments can we perform to answer these questions?
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"Stoned ape" hypothesis
This hypothesis was raised by an American ethnobotanist and author Terence McKenna. What it considers, in a nutshell, is that human evolution and the formation of human consciousness might have been catalyzed by psilocybin - a psychoactive compound found in mushrooms of the Psilocybe genus.
McKenna presented a scenario supported by consecutive arguments of how this may have happened. He emphasized the importance of diet in driving the evolutionary process, but in this case, it's not the cooked meat that gets in the spotlight. He argues that because of the significant change in geo-climatic conditions, namely the drying of the African continent, pre-human apes were forced out of their usual habitat - forest canopy into the expanding grasslands and urged to search for alternative food sources. Here's where Psilocybe mushrooms come in, particularly those that grow on animal dung (like Psilocybe cubensis), because apes would feed on the insects present in the dung and notice the visually outstanding mushrooms. That way by testing various possible food sources in the grasslands small amounts of those mushrooms were consumed in the process. In small amounts, psilocybin has been proven to increase visual acuity and McKenna argues that this then contributed to those primates who consumed it becoming better hunters and having better skills to avoid larger predators. Therefore natural selection was in favor of those individuals who consumed (more of) the mushrooms and they became dominant.
After some time mushroom eating might have become associated with better performance and survival rate and bigger quantities might have been consumed intentionally. In larger, but still moderate quantities, psilocybin is a CNS stimulant and might increase sexual drive. This once again goes in favor of survival and dominance for those who consume it. He further argues that increased sexual drive and sexual openness due to drug's effects resulted in polyamorous sexual style and in polyamorous social groups males can not consider certain females as belonging to them and also can't consider children to belong to them either, therefore this suppressed male dominance hierarchy, which exists in all primates. Males could only think in terms of "our women" and "our children" and this created social groups with a strong sense of group-bonding which may have contributed to the formation of exceptionally social behavior we now see in humans.
When even larger doses, those that can induce ecstatic experiences, began to be taken, then mushrooms were no longer consumed as survival chances increasing tool, but for the sake of the experience they give (there are many recorded cases of animals consuming natural psychoactive materials for the sake of their psychoactive effect). And thus consciousness-altering capacity of the drug influenced the transition from animal to human consciousness. The effects of the drug on the CNS of pre-human primates might have influenced the understanding of abstract concepts, language formation, ritual and symbolic activities, emergence of art, religion, etc. And drug-influenced epigenetic changes then were passed from generation to generation. The newly learned information/skills might have also been passed by simply teaching the offspring. Here's the hypothesis explained in detail by McKenna himself.
The theory has, of course, been criticized and due to lack of supporting evidence, it remains a hypothesis rather than a theory. However, having in mind that a rapid transition from animals to humans and the tripling of the brain size over a period of three million years have not been fully explained and proposed theories trying to explain "the missing link" don't sound very convincing, we might want to consider a more radical means and scenarios to serve as the missing link. Radical changes might need radical explanations.
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Neocortex and evolution of humans
An exciting question. Thanks for putting it into a session.
However, to begin with, the philosophy of the idea of evolution itself, would it be scientifically neutral and appropriate to posit that ‘humans evolved faster’? Is there a standard gauge to measure what is considered to be a ‘fast evolution’? How wrong would it be if I alternatively told you, by the rate of mutations and resultant evolutionary fitness, viruses (and other pathogens too) are the ones that are evolutionarily fastest?
On a lighter note, we can start our discussion by explicitly concentrating on the ‘cognitive’ evolution (and as far as I understand, this was what the session was aimed at). This is where we need to look at how our brain evolved: mostly similar in the overall biochemical mechanisms, anatomy and physiology, why are we capable of performing the ‘sentient’ and exclusively human-like activities that our primate relatives cannot? The answer might be found in the connection between developmental embryology and the evolution of the brain.
The neocortex, as the name suggests, is one of the most recently formed regions of the brain that was modified from the dorsal cortex of reptiles- gradually thicker and layered in the mammals. The neocortex is the most enlarged part of the mammalian brain, and it is the structure with enhanced information-processing and storing capacity, and it is considered to mediate consciousness in humans and possibly other mammals. Since the neural circuits in the neocortex are modifiable throughout life, it is the part of the brain that becomes tailored for relevant perceptual and behavioural abilities and enables individuals to acquire skills, personalities and memories. From a cognitive standpoint, this is very crucial. Hence, it is very likely that neocortex made all the difference in the pattern of cognitive evolution among the human vs other primates .
Evolutionarily, the brain features we share with close relatives like bonobos and chimpanzees are retained from the common ancestors. The fossil records, though not definitive on their own, suggest that the brain size relative to the individual’s body size is highly indicative of computing power and intelligence. We can look at the hominin clade that includes humans (emerged around 190 thousand years ago) and all extinct species more closely related to us than the chimpanzees, the closest living relatives. Given that all our other hominin relatives (Homo habilis, Homo erectus, for example) are extinct, we can try to see the fossil records to see if we can say anything about the last 5-8 million years of hominin brain evolution. Over the last 1.5 million years, the brain size has increased in size: modern human brains being three times as big than that of the great apes. There are also hints of proportional reduction of the primary visual cortex and reorganization of the cortical region, and also of the hemispheric asymmetries related to right-handedness and languages. It is guessed that cortical fields stabilized in size after a certain level of continual enlargement, and went through a reorganisation. This suggests that the neocortical region played a major role in making us humans what we are today. Indeed, there is extensive evidence from fMRI that the human brain houses more cortical areas (more than 150) than the brains of macaque monkeys.
Another reason to consider the importance of neocortex in setting us apart is that there is a unique trend of migration of bipolar neurons in humans that is unseen in other primates. The migratory cells (from ganglionic eminence to thalamus) rely on homotypic neutrophilic guidance, and they express DIX1 and DIX2 proteins that guide the migration. The process is anatomically linked to the association neocortex involved in higher cognitive functions like reasoning and language mastery .
There are other molecular underpinnings that point in the direction that neocortex is important for the human cognitive edge over its close relatives. One such phenomenon is the region-specific differential gene expression. There is a much greater transcriptional difference between neocortical areas in humans than there is in rodents . For instance, CNTNAP2 (contactin associated protein-like 2) is selectively and highly expressed in the orbital prefrontal cortex, the area involved in pro-social behaviour in humans. What is interesting about this is that the protein does not have any comparable analogue in rodents .
Hence, while there is a need for assessment of more fossil records and also more molecular data to be examined, one plausible answer to why humans evolved more (in terms of cognition) might be answered by looking at the evolution (and the developmental biology) of the human neocortex.
Also, joining with Shubhankar's earlier contribution, given the role of neocortex in the cognitive and meta-cognitive functions like socializing and forming relationships, the idea that Humans have something to do with the extinction of other Human-related species becomes even more plausible. As our forefathers grew more adept in performing tasks in groups- by the virtue of more complex neocortical prowess- we outdid other evolutionary relatives.
[1]Jon H. Kaas, Chapter 3 - The origin and evolution of neocortex: From early mammals to modern humans, Editor(s): Michel A. Hofman, Progress in Brain Research, Elsevier, Volume 250, 2019, Pages 61-81, ISSN 0079-6123, ISBN 9780444643179, https://doi.org/10.1016/bs.pbr.2019.03.017
[2]Letinic, K. & Rakic, P. Telencephalic origin of human thalamic GABAergic neurons. Nature Neurosci. 4, 931–936 (2001).
[3]Johnson, M. B. et al. Functional and evolutionary insights into human brain development through global transcriptome analysis. Neuron 62, 494–509 (2009).
[4]Mao, R. et al. Primary and secondary transcriptional effects in the developing human Down syndrome brain and heart. Genome Biol. 6, R107 (2005).
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Humans may have a role in the extinction of other Homo species
According to the book Sapiens, there may have been times when multiple Homo species existed on the planet. As more and more intelligent/ capable species evolved, they might have been a threat to the less evolved/ less intelligent/ less capable ones.
Harari says that Sapiens could dominate the world because it is the only animal that can cooperate/ adjust in large numbers. And, the ability of Sapiens to cooperate in large numbers arises from its unique capacity to believe in things existing purely in the imagination, such as Gods, nations, money, and human rights.
There are two theories that explain why the Homo sapiens prospered and spread across the planet, and the others did not.
- The interbreeding theory: Sapiens mated with the other species and gradually retained some of their DNA. Evidence suggests that modern European DNA contains between up to 4% Neanderthal DNA, and some of the other Homo species.
- The replacement theory: Sapiens competed with the other species for food and space and emerged victorious. Humans have caused a lot of large mammals to go extinct. As an example, about 50,000 years ago, Australia had huge ground sloths. Within a few thousand years of the human arrival to Australia, they became extinct. Sapiens were also responsible for the first large extinction during the Anthropocene. These pieces of evidence back the replacement theory.
Both theories are likely to be true and may have together contributed to the extinction of the other Homo species.
[1]Alexei, 2017. Medium. https://franticrock.medium.com/summarized-summaries-ep12-sapiens-yuval-noah-harari-883ecabd76a8
[2]Mark Looi, 2020. Medium. https://marklooi.medium.com/summary-of-yuval-hararis-sapiens-bb5430aa4fd9
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