Pyramidal cells make up approximately 70 percent of the cortical neurons. Among primates, they are more spinous in the frontal lobes. In humans, the pyramidal cells of the granular prefrontal cortex are more spinous than in the baboon and vervet monkey by a factor of three and ~70 percent more spinous in comparison to those in the macaque [57].
Evolution of Higher Bandwidth Synapses in Humans
Information processing and storage in the human brain is dependent on pyramidal cell firing, synaptic transmission, and the plasticity of neuronal circuitry. Compared to rodent brains, human pyramidal cell information transfer is greater by a factor of 4–9- fold, with frequencies measured in the beta and gamma range. Sensory synaptic features that process temporal data were also conveyed at wider bandwidth in humans compared to rodents. A human feature of “fast recovering synapses” augments the information transfer during spike trains. Human pyramidal cells also have the capability of encoding higher synaptic information quantities. Together, these and probably other features have enabled human cerebral microcircuits to transmit information at overall “wider band- widths” in comparison to the rodent microcircuitry [58].
Brief Genetic Insights into Human Evolution 81
melanin made in the skin in response to UV light, which controls how much vitamin D3 is made [62]. In agreement with Watson, that as no significant genetic differences have been noted in people discerned as different “races,” the evolutionary biological reasoning follows that societies might consider excluding such categorization altogether [59].
Despite these caveats, there have been impressive “genetic engineering” feats such as the CRISPR (clustered regularly interspaced short palindromic repeats) methodologies.
Termed a kind of gene editing tool, a customized RNA molecule is used that incorporates a bacterial nuclease such as Cas, which singles out a DNA sequence, usually a mutation, and restores it with a mending patch [63].
Epigenetic Mechanisms and Evolution of the Mind
In addition to genetic drivers, various epigenetic- based mechanisms shaped evolution and the human mind. Epigenetic “switches” affect gene regulation as well as behavior and are mostly reversible. This is nowadays generally regarded as the process linking nature and nurture in the evolution of biological systems. Epigenetics incurs a change in gene expression without altering DNA sequences. Furthermore, transgenerational epigenetics is a process that can extend beyond a person’s lifetime, such as may occur from a par- ent to their children. The neurobiological mechanisms are via DNA methylation, histone acetylation, and micro- RNA interference [64]. Epigenetics interject additional sources of variation among biological systems. The process of evolution can proceed through the epigenetic arena of hereditary mechanisms in the absence of genetic sequence alteration.
Epigenetic variation also occurs at a much higher rate and is more responsive to environ- mental factors, such as climate and nutritional drivers, that may induce several epigenetic variations simultaneously. Together the higher- frequency generation of epigenetic mech- anisms and that the change is more likely to be appropriate, in terms of adaptiveness, result in a more rapid adaptation by the organism.
Among the different epigenetic mechanisms are included memory of gene activity (one cell has genes turned on another has the genes turned off) and structural inheritance.
In the latter the architecture of the cell rather than gene activity mediates the effect – an example being prion proteins. These have an abnormal three- dimensional configur- ation causing fatal diseases such as Creutzfeldt–Jakob and mad cow disease. Chromatin- marking systems affect the relationship of DNA with its associated histone protein. DNA is entwined around the histone protein; methylation by adding methyl (CH3) groups and acetylation, for example, may slacken the chromatin structure rendering it more liable to be transcribed. Another mechanism is gene silencing. Small, interfering RNA molecules termed siRNAs may silence genes, with one mechanism being the Dicer enzyme that severs RNA into pieces. These can then be amplified, suppress messenger RNA (mRNA), and can migrate between cells within the body. The siRNA can also be associated with the gene from which it was derived and form enduring stable methylation or chromatin marks that can be transmitted to succeeding cell generations.
One of the roles that the RNAi system accomplishes is a type of cellular immune system. This is a type of intracellular defense mechanism for cells against viruses and so- called genomic parasites, also termed jumping genes or transposons, both of which generally generate double- stranded RNA. MicroRNAs (miRNA) are able to selectively inactivate genes by incorporating them into cells, and provide revolutionary tools for combating viral diseases – HIV and poliovirus being recent examples.
Behavioral inheritance may be transmitted to the offspring by substances impacting behavior, imitation- type learning, and social learning that is not imitative. An example of substances steering behavior include food preferences acquired through smell and taste characteristics of the mother while in the womb and later during breastfeeding. Imitative learning can be vocal or motor. Vocal imitative learning is common among birds, dol- phins, primates, and rodents. Motor imitative learning is much rarer in these species and more common among humans.
Symbolic behavior refers to the processing of information and transfer of informa- tion, which is likely unique to humans, prompting Ernst Cassirer to refer to humans as
“symbolic animals” [65]. He conceived of a neural system that, between receiving infor- mation and effecting action, has a third component that he referred to as the symbolic system. Jablonka and Lamb have proposed that genetic and even epigenetics are not the only drivers of evolution, but include the behavioral and symbolic systems and that evo- lution occurs in four dimensions [66].
Our Superior Information Processing and Working Memory Functions
The evolutionary history of our brain, literally honed by fire, ice, and environmental adversity, has endowed us with our 1350 cc brain, commonly referred to as the most intricate entity within our known universe. Our relatively large brain – with its expansive connectome measuring ~150 000 km of fibers – requires disproportionate amounts of energy. The downside is that an “Achilles heel” condition has made us prone to human- specific conditions such as degenerative (Alzheimer’s, frontotemporal dementias) and developmental (schizophrenia, autism) disease. With larger brains also come more syn- apses per neuron and disproportionately more fiber tracts, which become metabolically more extortionate. The majority of the energetic costs are incurred by synaptic signaling and maintaining neuronal electrochemical gradients.
Endeavors to correlate degrees of animal and human intelligence with absolute and relative brain size, corrected for body size, have yielded large inconsistencies. Often regarded as the most intelligent mammals, primates and humans do not have the lar- gest brains in absolute or relative terms. At the present time, perhaps the most accu- rate manner for deciphering the relation between degrees of intelligence and brain traits among mammals may be achieved by the combination of several factors. These include the overall number of cortical neurons, interneuronal distance, neuron packing density, and axon conduction velocity. These are traits that correlate with information- processing capacity (IPC), which may be seen as a marker or surrogate of general intelligence.
Although the pinnacle of known IPCs is generally considered to be the human IPC, with great apes and elephants next, the IPC of cetaceans and pinnipeds is probably even higher. Cetacean brain macro- and microanatomy components differ in several respects from humans. In a similar sense, corvid and psittacid birds also have differing anatomi- cal features to humans by having relatively small and densely packed pallial neurons and more numerous neurons, notwithstanding their particularly small brain volumes. These features may explain their superior intelligence. Referred to as an intelligence amplifier, language evolution, with its syntactical and grammatical elements in humans, may well have occurred in an analogous manner among songbirds and psittacids, termed “conver- gent evolution” [67].
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