The ARkStorm (for Atmospheric River
1,000) is a hypothetical megastorm, whose proposal is based on repeated
historical occurrences of atmospheric rivers and other major rain
events first developed and published by the Multi-Hazards Demonstration
Project (MHDP) of the United States Geological Survey (USGS) in 2010. An updated model was published as ARkStorm 2.0 in 2022.
ARkStorm 1.0 (2010 Study)
The ARkStorm 1.0 scenario describes an extreme storm that devastates much of California,
causing up to $725 billion in losses (mostly due to flooding and
erosion), and affecting a quarter of California's homes. The scenario
projects impacts of a storm that would be significantly less intense (25
days of rain) than the California storms that occurred between December 1861 and January 1862 (43 days). That event dumped nearly 10 feet (3.0 m) of rain in parts of California.
USGS sediment research in the San Francisco Bay Area, Santa Barbara Basin, Sacramento Valley, and the Klamath Mountain region found that "megastorms" have occurred in the years: 212, 440, 603, 1029, c. 1300, 1418, 1605, 1750, 1810, and, most recently, 1861–1862.
Based on the intervals of these known occurrences, ranging from 51 to
426 years, for a historic recurrence of, on average, every 100–200
years.
Geologic evidence indicates that several of the previous events
were more intense than the one in 1861–1862, particularly those in 440,
1418, 1605, and 1750, each of which deposited a layer of silt in the
Santa Barbara Basin more than one inch (2.5 cm) thick. The largest event
was the one in 1605,
which left a layer of silt two inches (5 cm) thick, indicating that
this flood was at least 50% more powerful than any of the others
recorded.
The conditions built into the scenario are "two super-strong
atmospheric rivers, just four days apart, one in Northern California and
one in Southern California, and one of them stalled for an extra day".
The ARkStorm 1.0 scenario would have the following effects:
The Central Valley would experience flooding 300 miles (480 km) long and at least 20 miles (30 km) wide.
Wind speeds in some places would reach 125 miles per hour (200 km/h).
Hundreds of landslides would damage roads, highways, and homes.
Property damage would exceed $300 billion, most from flooding.
Demand surge (an increase in labor rates and other repair costs
after major natural disasters) could increase property losses by 20
percent.
Agricultural losses and other costs to repair lifelines, drain
flooded islands, and repair damage from landslides, could bring the
total direct property loss to nearly $400 billion.
Power, water, sewer, and other lifelines would experience damage that could take weeks or months to restore.
Up to 1.5 million residents in the inland region and delta counties would need to evacuate due to flooding.
Business interruption costs could reach $325 billion, in addition to
the $400 billion required for property repair costs, meaning that an
ARkStorm scenario is projected to cost $750 billion (~$1 trillion in
2022 dollars), nearly three times the amount of damage predicted by the next "Big One", a hypothetical Southern California earthquake with roughly the same annual occurrence probability.
ARkStorm 2.0 (2022 update)
This update, with parts of the research on impacts still ongoing,
has examined how climate change is expected to increase the risk of
severe flooding from a hypothetical ARkStorm, with runoff 200% to 400%
above historical values for the Sierra Nevada
in part due to a decrease in the portion of precipitation that falls as
snow, as well as an increase in the amount of water that storms can
carry. The likelihood of the event outlined in the ARkStorm scenario is
now once every 25–50 years, with projected economic losses of over $1
trillion (or more than five times that of Hurricane Katrina).
Current
flood maps in the U.S. rarely take recent projections from projects
like ARkStorm into account, especially FEMA's maps, which many
decision-makers have relied on.
Land owners, flood insurers, governments and media outlets often use
maps like FEMA's that still fail to represent many significant risks due
to: 1) using only historical data (instead of incorporating climate
change models), 2) the omission of heavy rainfall events, and 3) lack of
modeling of flooding in urban areas. More robust and up-to-date models,
like the First Street Foundation's riskfactor.com, should better represent true flood risk though it is unclear if that model, for example, incorporates any ARkStorm science.
Government agencies may decide how much risk to accept, and how much risk to mitigate. The Netherlands' approach to flood control, for example, plans for 1 in 10,000 year events in heavily-populated areas and 1 in 4,000 year events in less well-populated areas.
https://en.wikipedia.org/wiki/Social_constructionism Social construction of technology (SCOT) is a theory within the field of science and technology studies. Advocates of SCOT—that is, social constructivists—argue
that technology does not determine human action, but that rather, human
action shapes technology. They also argue that the ways a technology is
used cannot be understood without understanding how that technology is
embedded in its social context. SCOT is a response to technological determinism and is sometimes known as technological constructivism.
SCOT draws on work done in the constructivist school of the sociology of scientific knowledge, and its subtopics include actor-network theory (a branch of the sociology of science and technology) and historical analysis of sociotechnical systems, such as the work of historian Thomas P. Hughes.
Its empirical methods are an adaptation of the Empirical Programme of
Relativism (EPOR), which outlines a method of analysis to demonstrate
the ways in which scientific findings are socially constructed (see strong program). Leading adherents of SCOT include Wiebe Bijker and Trevor Pinch.
SCOT holds that those who seek to understand the reasons for
acceptance or rejection of a technology should look to the social world.
It is not enough, according to SCOT, to explain a technology's success
by saying that it is "the best"—researchers must look at how the
criteria of being "the best" is defined and what groups and stakeholders
participate in defining it. In particular, they must ask who defines
the technical criteria success is measured by, why technical criteria
are defined this way, and who is included or excluded. Pinch and Bijker
argue that technological determinism is a myth that results when one
looks backwards and believes that the path taken to the present was the
only possible path.
SCOT is not only a theory, but also a methodology: it formalizes
the steps and principles to follow when one wants to analyze the causes
of technological failures or successes.
Legacy of the Strong Programme in the sociology of science
At the point of its conception, the SCOT approach was partly motivated by the ideas of the strong programme in the sociology of science (Bloor 1973). In their seminal article, Pinch and Bijker refer to the Principle of Symmetry
as the most influential tenet of the Sociology of Science, which should
be applied in historical and sociological investigations of technology
as well. It is strongly connected to Bloor's theory of social causation.
Symmetry
The Principle of Symmetry
holds that in explaining the origins of scientific beliefs, that is,
assessing the success and failure of models, theories, or experiments,
the historian/sociologist should deploy the same kind of
explanation in the cases of success as in cases of failure. When
investigating beliefs, researchers should be impartial to the (a posteriori
attributed) truth or falsehood of those beliefs, and the explanations
should be unbiased. The strong programme adopts a position of relativism
or neutralism regarding the arguments that social actors put forward
for the acceptance/rejection of any technology. All arguments (social,
cultural, political, economic, as well as technical) are to be treated
equally.
The symmetry principle addresses the problem that the historian
is tempted to explain the success of successful theories by referring to
their "objective truth", or inherent "technical superiority", whereas
s/he is more likely to put forward sociological explanations (citing
political influence or economic reasons) only in the case of failures.
For example, having experienced the obvious success of the chain-driven
bicycle for decades, it is tempting to attribute its success to its
"advanced technology" compared to the "primitiveness" of the Penny Farthing,
but if we look closely and symmetrically at their history (as Pinch and
Bijker do), we can see that at the beginning bicycles were valued
according to quite different standards than nowadays. The early adopters
(predominantly young, well-to-do gentlemen) valued the speed, the
thrill, and the spectacularity of the Penny Farthing – in contrast to
the security and stability of the chain-driven Safety Bicycle.
Many other social factors (e.g., the contemporary state of urbanism and
transport, women's clothing habits and feminism) have influenced and
changed the relative valuations of bicycle models.
A weak reading of the Principle of Symmetry points out
that there often are many competing theories or technologies, which all
have the potential to provide slightly different solutions to similar
problems. In these cases, sociological factors tip the balance between
them: that's why we should pay equal attention to them.
A strong, social constructivist reading would add that even the
emergence of the questions or problems to be solved are governed by
social determinations, so the Principle of Symmetry is applicable even
to the apparently purely technical issues.
Original Core concepts
The Empirical Programme of Relativism (EPOR) introduced the SCOT theory in two stage.
First Stage: Interpretative flexibility
The
first stage of the SCOT research methodology is to reconstruct the
alternative interpretations of the technology, analyze the problems and
conflicts these interpretations give rise to, and connect them to the
design features of the technological artifacts. The relations between
groups, problems, and designs can be visualized in diagrams.
Interpretative flexibility means that each technological
artifact has different meanings and interpretations for various groups.
Bijker and Pinch show that the air tire of the bicycle meant a more
convenient mode of transportation for some people, whereas it meant
technical nuisances, traction problems and ugly aesthetics to others. In racing air tires lent to greater speed.
These alternative interpretations generate different problems
to be solved. For the bicycle, it means how features such as
aesthetics, convenience, and speed should be prioritized. It also
considers tradeoffs, such as between traction and speed.
Relevant social groups
The most basic relevant groups are the users and the producers
of the technological artifact, but most often many subgroups can be
delineated – users with different socioeconomic status, competing
producers, etc. Sometimes there are relevant groups who are neither
users, nor producers of the technology, for example, journalists,
politicians, and civil organizations. Trevor Pinch has argued that the salespeople of technology should also be included in the study of technology. The groups can be distinguished based on their shared or diverging interpretations of the technology in question.
Design flexibility
Just
as technologies have different meanings in different social groups,
there are always multiple ways of constructing technologies. A
particular design is only a single point in the large field of technical
possibilities, reflecting the interpretations of certain relevant
groups.
Problems and conflicts
The
different interpretations often give rise to conflicts between criteria
that are hard to resolve technologically (e.g., in the case of the
bicycle, one such problem was how a woman could ride the bicycle in a
skirt while still adhering to standards of decency), or conflicts
between the relevant groups (the "Anti-cyclists" lobbied for the banning
of the bicycles). Different groups in different societies construct
different problems, leading to different designs.
Second Stage: Closure
The second stage of the SCOT methodology is to show how closure is achieved.
Over time, as technologies are developed, the interpretative and
design flexibility collapse through closure mechanisms. Two examples of
closure mechanisms:
Rhetorical closure: When social groups see the problem as being solved, the need for alternative designs diminishes. This is often the result of advertising.
Redefinition of the problem: A design standing in the focus
of conflicts can be stabilized by using it to solve a different, new
problem, which ends up being solved by this very design. As an example,
the aesthetic and technical problems of the air tire diminished, as the
technology advanced to the stage where air tire bikes started to win the
bike races. Tires were still considered cumbersome and ugly, but they
provided a solution to the "speed problem", and this overrode previous
concerns.
Closure is not permanent. New social groups may form and reintroduce
interpretative flexibility, causing a new round of debate or conflict
about a technology. (For instance, in the 1890s automobiles were seen as
the "green" alternative, a cleaner environmentally-friendly technology,
to horse-powered vehicles; by the 1960s, new social groups had
introduced new interpretations about the environmental effects of the
automobile, eliciting the opposite conclusion.)
Subsequent extension of the SCOT theory
Many other historians and sociologists of technology extended the original SCOT theory.
Technological Frame
Relating the content of the technological artifact to the wider sociopolitical milieu
This is often considered the third stage of the original theory.
For example, Paul N. Edwards shows in his book "The Closed World: Computers and the Politics of Discourse in Cold War America" the strong relations between the political discourse of the Cold War and the computer designs of this era.
Criticism
In 1993, Langdon Winner
published a critique of SCOT entitled "Upon Opening the Black Box and
Finding it Empty: Social Constructivism and the Philosophy of
Technology."
In it, he argues that social constructivism is an overly narrow
research program. He identifies the following specific limitations in
social constructivism:
It explains how technologies arise, but ignores the consequences
of the technologies after the fact. This results in a sociology that
says nothing about how such technologies matter in the broader context.
It examines social groups and interests that contribute to the
construction of technology, but ignores those who have no voice in the
process, yet are affected by it. Likewise, when documenting
technological contingencies and choices, it fails to account for those
options that never made it to the table. According to Winner, this
results in conservative and elitist sociology.
It is superficial in that it focuses on how the immediate needs,
interests, problems and solutions of chosen social groups influence
technological choice, but disregards any possible deeper cultural,
intellectual or economic origins of social choices concerning
technology.
It actively avoids taking any kind of moral stance or passing
judgment on the relative merits of the alternative interpretations of a
technology. This indifference makes it unhelpful in addressing important
debates about the place of technology in human affairs.
Other critics include Stewart Russell with his letter in the journal Social Studies of Science titled "The Social Construction of Artifacts: A Response to Pinch and Bijker".
Deborah Deliyannis, Hendrik Dey, and Paolo Squatriti criticize the concept of social construction of technology for being a false dichotomy with a technologically determinist straw man
that ignores third, fourth and more alternatives, as well as for
overlooking the process of how the technology is developed as something
that can work. For example, accounting for which groups would have
interests in a windmill
cannot explain how a windmill is practically constructed, nor does it
account for the difference between having the knowledge but for some
reason not using it and lacking the knowledge altogether. This
distinction between knowledge that have not yet been invented and
knowledge that is merely prevented from being used by commercial,
bureaucratic or other socially constructed factors, which it is argued
that SCOT overlooks, is argued to explain the archaeological evidence of
rich technological cultures in the aftermath of the collapse of
civilizations (such as early medieval technology in the aftermath of the
collapse of the Roman Empire, which was much richer than it is depicted
as by the "Dark Medieval" stereotype) as a result of technology being
remembered even when prevented from being used with the potential to
being put into use when the artificial repression is no longer in place
due to societal collapse.
Brain abscess in a person with a CSF shunt.
The abscess is the darker gray region in the lower left of the image
(corresponding to the right parietal lobe). The lateral ventricles are
visible in black in the center of the brain, adjacent to the abscess.
Brain abscess (or cerebral abscess) is an abscess within the brain tissue caused by inflammation and collection of infected material coming from local (ear infection, dental abscess, infection of paranasal sinuses, infection of the mastoid air cells of the temporal bone, epidural abscess) or remote (lung, heart, kidney etc.) infectious sources. The infection may also be introduced through a skull fracture following a head trauma or surgical procedures. Brain abscess is usually associated with congenital heart disease in young children. It may occur at any age but is most frequent in the third decade of life.
Signs and symptoms
Fever, headache, and neurological problems, while classic, only occur in 20% of people with brain abscess.
The famous triad of fever, headache and focal neurologic findings
are highly suggestive of brain abscess. These symptoms are caused by a
combination of increased intracranial pressure due to a space-occupying lesion (headache, vomiting, confusion, coma), infection (fever, fatigue etc.) and focal neurologic brain tissue damage (hemiparesis, aphasia etc.).
The most frequent presenting symptoms are headache, drowsiness, confusion, seizures,
hemiparesis or speech difficulties together with fever with a rapidly
progressive course. Headache is characteristically worse at night and in
the morning, as the intracranial pressure naturally increases when in
the supine position. This elevation similarly stimulates the medullary vomiting center and area postrema, leading to morning vomiting.
Other symptoms and findings depend largely on the specific
location of the abscess in the brain. An abscess in the cerebellum, for
instance, may cause additional complaints as a result of brain stem compression and hydrocephalus. Neurological examination may reveal a stiff neck in occasional cases (erroneously suggesting meningitis).
Pathophysiology
Bacterial
Brain abscess after metastasis treatment.
Anaerobic and microaerophilic cocci and gram-negative and gram-positive anaerobic bacilli are the predominant bacterial isolates. Many brain abscesses are polymicrobial. The predominant organisms include: Staphylococcus aureus, aerobic and anaerobic streptococci (especially Streptococcus intermedius), Bacteroides, Prevotella, and Fusobacterium species, Enterobacteriaceae, Pseudomonas species, and other anaerobes. Less common organisms include: Haemophillus influenzae, Streptococcus pneumoniae and Neisseria meningitidis.
Bacterial abscesses rarely (if ever) arise de novo within
the brain although establishing a cause can be difficult in many cases.
There is almost always a primary lesion elsewhere in the body that must
be sought assiduously because failure to treat the primary lesion will
result in relapse. In cases of trauma, for example in compound skull
fractures where fragments of bone are pushed into the substance of the
brain, the cause of the abscess is obvious. Similarly, bullets and
other foreign bodies may become sources of infection if left in place.
The location of the primary lesion may be suggested by the location of
the abscess: infections of the middle ear result in lesions in the middle and posterior cranial fossae; congenital heart disease with right-to-left shunts often result in abscesses in the distribution of the middle cerebral artery; and infection of the frontal and ethmoid sinuses usually results in collection in the subdural sinuses.
These organisms are associated with certain predisposing conditions:
Sinus and dental infections—Aerobic and anaerobic streptococci, anaerobic gram-negative bacilli (e.g. Prevotella, Porphyromonas, Bacteroides), Fusobacterium, S. aureus, and Enterobacteriaceae
Penetrating trauma—S. aureus, aerobic streptococci, Enterobacteriaceae, and Clostridium spp.
Pulmonary infections—Aerobic and anaerobic streptococci, anaerobic gram-negative bacilli (e.g. Prevotella, Porphyromonas, Bacteroides), Fusobacterium, Actinomyces, and Nocardia
Congenital heart disease—Aerobic and microaerophilic streptococci, and S. aureus
HIV infection—T. gondii, Mycobacterium, Nocardia, Cryptococcus, and Listeria monocytogenes
Transplantation—Aspergillus, Candida, Cryptococcus, Mucorales, Nocardia, and T. gondii
Neutropenia—Aerobic gram-negative bacilli, Aspergillus, Candida, and Mucorales
Diagnosis
MRI
(T1 with contrast) showing the ring-enhancing lesion. From a rare case
report of an abscess formed as a complication of the CSF shunt. Jamjoom
et al., 2009.
The diagnosis is established by a computed tomography (CT) (with contrast) examination. At the initial phase of the inflammation (which is referred to as cerebritis),
the immature lesion does not have a capsule and it may be difficult to
distinguish it from other space-occupying lesions or infarcts of the
brain. Within 4–5 days the inflammation and the concomitant dead brain
tissue are surrounded with a capsule, which gives the lesion the famous ring-enhancing lesion
appearance on CT examination with contrast (since intravenously applied
contrast material can not pass through the capsule, it is collected
around the lesion and looks as a ring surrounding the relatively dark
lesion). Lumbar puncture
procedure, which is performed in many infectious disorders of the
central nervous system is contraindicated in this condition (as it is in
all space-occupying lesions of the brain) because removing a certain
portion of the cerebrospinal fluid may alter the concrete intracranial pressure balances and causes the brain tissue to move across structures within the skull (brain herniation).
Ring enhancement may also be observed in cerebral hemorrhages (bleeding) and some brain tumors.
However, in the presence of the rapidly progressive course with fever,
focal neurologic findings (hemiparesis, aphasia etc.) and signs of
increased intracranial pressure, the most likely diagnosis should be the
brain abscess.
Treatment
The treatment includes lowering the increased intracranial pressure and starting intravenous antibiotics (and meanwhile identifying the causative organism mainly by blood culture studies).[citation needed]
Hyperbaric oxygen therapy (HBO2 or HBOT) is indicated as a primary and adjunct treatment which provides four primary functions.
Firstly, HBOT reduces intracranial pressure.
Secondly, high partial pressures of oxygen act as a bactericide and
thus inhibits the anaerobic and functionally anaerobic flora common in
brain abscess. Third, HBOT optimizes the immune function thus enhancing
the host defense mechanisms and fourth, HBOT has been found to be of
benefit when brain abscess is concomitant with cranial osteomyelitis.
Secondary functions of HBOT include increased stem cell
production and up-regulation of VEGF which aid in the healing and
recovery process.
Surgical drainage of the abscess remains part of the standard
management of bacterial brain abscesses. The location and treatment of
the primary lesion is also crucial, as is the removal of any foreign
material (bone, dirt, bullets, and so forth).
There are few exceptions to this rule: Haemophilus influenzae
meningitis is often associated with subdural effusions that are
mistaken for subdural empyemas. These effusions resolve with
antibiotics and require no surgical treatment. Tuberculosis
can produce brain abscesses that look identical to conventional
bacterial abscesses on CT imaging. Surgical drainage or aspiration is
often necessary to identify Mycobacterium tuberculosis, but once the diagnosis is made no further surgical intervention is necessary.
CT guided stereotactic aspiration is also indicated in the
treatment of brain abscess. The use of pre-operative imaging,
intervention with post-operative clinical and biochemical monitoring
used to manage brain abscesses today dates back to the Pennybacker
system pioneered by Somerset, Kentucky-born neurosurgeon Joseph Buford Pennybacker, director of the neurosurgery department of the Radcliffe Infirmary, Oxford from 1952 to 1971.
Prognosis
While death occurs in about 10% of cases, people do well about 70% of the time.
This is a large improvement from the 1960s due to improved ability to
image the head, more effective neurosurgery and more effective
antibiotics.
The evolution of the brain refers to the progressive development and complexity of neural structures
over millions of years, resulting in the diverse range of brain sizes
and functions observed across different species today, particularly in vertebrates.
The evolution of the brain has exhibited diverging adaptations
within taxonomic classes, such as Mammalia, and even more diverse
adaptations across other taxonomic classes. Brain-to-body size scales allometrically.
This means that as body size changes, so do other physiological,
anatomical, and biochemical connections between the brain and body.
Small-bodied mammals tend to have relatively large brains compared to
their bodies, while larger mammals (such as whales) have smaller
brain-to-body ratios. When brain weight is plotted against body weight
for primates, the regression line of the sample points can indicate the
brain power of a species. For example, lemurs fall below this line,
suggesting that for a primate of their size, a larger brain would be
expected. In contrast, humans lie well above this line, indicating they
are more encephalized than lemurs and, in fact, more encephalized than any other primate.
This suggests that human brains have undergone a larger evolutionary
increase in complexity relative to size. Some of these changes have been
linked to multiple genetic factors, including proteins and other
organelles.
One approach to understanding overall brain evolution is to use a paleoarchaeological
timeline to trace the necessity for ever increasing complexity in
structures that allow for chemical and electrical signaling. Because
brains and other soft tissues do not fossilize as readily as mineralized tissues,
scientists often look to other structures as evidence in the fossil
record to get an understanding of brain evolution. This, however, leads
to a dilemma as the emergence of organisms with more complex nervous
systems with protective bone or other protective tissues that can then
readily fossilize occur in the fossil record before evidence for
chemical and electrical signaling.
Evidence from 2008 showed that the ability to transmit electrical and
chemical signals existed even before more complex multicellular
lifeforms.
Fossilization of brain tissue, as well as other soft tissue, is
nonetheless possible, and scientists can infer that the first brain
structure appeared at least 521 million years ago, with fossil brain
tissue present in sites of exceptional preservation.
Another approach to understanding brain evolution is to look at
extant organisms that do not possess complex nervous systems, comparing
anatomical features that allow for chemical or electrical messaging. For
example, choanoflagellates are organisms that possess various membrane channels that are crucial to electrical signaling. The membrane channels of choanoflagellates' are homologous
to the ones found in animal cells, and this is supported by the
evolutionary connection between early choanoflagellates and the
ancestors of animals. Another example of extant organisms with the capacity to transmit electrical signals would be the glass sponge, a multicellular organism, which is capable of propagating electrical impulses without the presence of a nervous system.
Before the evolutionary development of the brain, nerve nets, the simplest form of a nervous system developed. These nerve nets were a sort of precursor for the more evolutionarily advanced brains. They were first observed in Cnidaria
and consist of a number of neurons spread apart that allow the organism
to respond to physical contact. They are able to rudimentarily detect
food and other chemicals, but these nerve nets do not allow them to
detect the source of the stimulus.
Ctenophores
also demonstrate this crude precursor to a brain or centralized nervous
system, however they phylogenetically diverged before the phylum Porifera
(the Sponges) and Cnidaria. There are two current theories on the
emergence of nerve nets. One theory is that nerve nets may have
developed independently in Ctenophores and Cnidarians. The other theory
states that a common ancestor may have developed nerve nets, but they
were lost in Porifera. While comparing the average neuron size and the
packing density the difference between primate and mammal brains is
shown.
A trend in brain evolution according to a study done with mice,
chickens, monkeys and apes concluded that more evolved species tend to
preserve the structures responsible for basic behaviors. A long term
human study comparing the human brain to the primitive brain found that
the modern human brain contains the primitive hindbrain region – what
most neuroscientists call the protoreptilian brain.
The purpose of this part of the brain is to sustain fundamental
homeostatic functions, which are self regulating processes organisms use
to help their bodies adapt. The pons and medulla
are major structures found there. A new region of the brain developed
in mammals about 250 million years after the appearance of the
hindbrain. This region is known as the paleomammalian brain, the major
parts of which are the hippocampi and amygdalas, often referred to as the limbic system.
The limbic system deals with more complex functions including
emotional, sexual and fighting behaviors. Of course, animals that are
not vertebrates also have brains, and their brains have undergone separate evolutionary histories.
The brainstem and limbic system are largely based on nuclei, which are essentially balled-up clusters of tightly packed neurons and the axon fibers that connect them to each other, as well as to neurons in other locations. The other two major brain areas (the cerebrum and cerebellum) are based on a cortical
architecture. At the outer periphery of the cortex, the neurons are
arranged into layers (the number of which vary according to species and
function) a few millimeters thick. There are axons that travel between
the layers, but the majority of axon mass is below the neurons
themselves. Since cortical neurons and most of their axon fiber tracts
do not have to compete for space, cortical structures can scale more
easily than nuclear ones. A key feature of cortex is that because it
scales with surface area, more of it can be fit inside a skull by
introducing convolutions, in much the same way that a dinner napkin can
be stuffed into a glass by wadding it up. The degree of convolution is
generally greater in species with more complex behavior, which benefits
from the increased surface area.
The cerebellum, or "little brain," is behind the brainstem and below the occipital lobe of the cerebrum
in humans. Its purposes include the coordination of fine sensorimotor
tasks, and it may be involved in some cognitive functions, such as
language and different motor skills that may involve hands and feet. The
cerebellum helps keep equilibrium. Damage to the cerebellum would
result in all physical roles in life to be affected. Human cerebellar
cortex is finely convoluted, much more so than cerebral cortex. Its
interior axon fiber tracts are called the arbor vitae, or Tree of Life.
The area of the brain with the greatest amount of recent evolutionary change is called the neocortex. In reptiles and fish, this area is called the pallium
and is smaller and simpler relative to body mass than what is found in
mammals. According to research, the cerebrum first developed about 200
million years ago. It is responsible for higher cognitive functions—for
example, language, thinking, and related forms of information
processing. It is also responsible for processing sensory input (together with the thalamus,
a part of the limbic system that acts as an information router). The
thalamus receives the different sensations before the information is
then passed onto the cerebral cortex. Most of its function is subconscious,
that is, not available for inspection or intervention by the conscious
mind. The neocortex is an elaboration, or outgrowth, of structures in
the limbic system, with which it is tightly integrated. The neocortex is
the main part controlling many brain functions as it covers half of the
whole brain in volume. The development of these recent evolutionary
changes in the neocortex likely occurred as a result of new neural
network formations and positive selections of certain genetic
components.
In addition to studying the fossil record, evolutionary history can be investigated via embryology. An embryo
is an unborn/unhatched animal and evolutionary history can be studied
by observing how processes in embryonic development are conserved (or
not conserved) across species. Similarities between different species
may indicate evolutionary connection. One way anthropologists study
evolutionary connection between species is by observing orthologs. An
ortholog is defined as two or more homologous genes between species that
are evolutionarily related by linear descent. By using embryology the
evolution of the brain can be tracked between various species.
Bone morphogenetic protein (BMP), a growth factor that plays a significant role in embryonic neural development, is highly conserved amongst vertebrates, as is sonic hedgehog
(SHH), a morphogen that inhibits BMP to allow neural crest development.
Tracking these growth factors with the use of embryology provides a
deeper understanding of what areas of the brain diverged in their
evolution. Varying levels of these growth factors lead to differing
embryonic neural development which then in turn affects the complexity
of future neural systems. Studying the brain's development at various
embryonic stages across differing species provides additional insight
into what evolutionary changes may have historically occurred. This then
allows scientists to look into what factors may have caused such
changes, such as links to neural network diversity, growth factor
production, protein- coding selections, and other genetic factors.
Randomizing access and increasing size
Some animal phyla have gone through major brain enlargement through evolution (e.g. vertebrates and cephalopods
both contain many lineages in which brains have grown through
evolution) but most animal groups are composed only of species with
extremely small brains. Some scientists
argue that this difference is due to vertebrate and cephalopod neurons
having evolved ways of communicating that overcome the scalability
problem of neural networks
while most animal groups have not. They argue that the reason why
traditional neural networks fail to improve their function when they
scale up is because filtering based on previously known probabilities
cause self-fulfilling prophecy-like
biases that create false statistical evidence giving a completely false
worldview and that randomized access can overcome this problem and
allow brains to be scaled up to more discriminating conditioned reflexes at larger brains that lead to new worldview forming abilities at certain thresholds.
This means when neurons scale in a non randomized fashion that their
functionality becomes more limited due to their neural networks being
unable to process more complex systems without the exposure to new
formations. This is explained by randomization allowing the entire brain
to eventually get access to all information over the course of many
shifts even though instant privileged access is physically impossible. They cite that vertebrate neurons transmit virus-like capsules containing RNA
that are sometimes read in the neuron to which it is transmitted and
sometimes passed further on unread which creates randomized access, and
that cephalopod neurons make different proteins from the same gene which
suggests another mechanism for randomization of concentrated
information in neurons, both making it evolutionarily worth scaling up
brains.
Brain re-organization
With the use of in vivo Magnetic resonance imaging (MRI)
and tissue sampling, different cortical samples from members of each
hominoid species were analyzed. In each species, specific areas were
either relatively enlarged or shrunken, which can detail neural
organizations. Different sizes in the cortical areas can show specific
adaptations, functional specializations and evolutionary events that
were changes in how the hominoid brain is organized. In early
prediction it was thought that the frontal lobe, a large part of the
brain that is generally devoted to behavior and social interaction,
predicted the differences in behavior between hominoid and humans.
Discrediting this theory was evidence supporting that damage to the
frontal lobe in both humans and hominoids show atypical social and
emotional behavior; thus, this similarity means that the frontal lobe
was not very likely to be selected for reorganization. Instead, it is
now believed that evolution occurred in other parts of the brain that
are strictly associated with certain behaviors. The reorganization that
took place is thought to have been more organizational than volumetric;
whereas the brain volumes were relatively the same but specific
landmark position of surface anatomical features, for example, the
lunate sulcus suggest that the brains had been through a neurological
reorganization.
There is also evidence that the early hominin lineage also underwent a
quiescent period, or a period of dormancy, which supports the idea of
neural reorganization.
Dental fossil records for early humans and hominins show that immature hominins, including australopithecines and members of Homo,
have a quiescent period (Bown et al. 1987). A quiescent period is a
period in which there are no dental eruptions of adult teeth; at this
time the child becomes more accustomed to social structure, and
development of culture. During this time the child is given an extra
advantage over other hominoids, devoting several years into developing
speech and learning to cooperate within a community.
This period is also discussed in relation to encephalization. It was
discovered that chimpanzees do not have this neutral dental period,
which suggests that a quiescent period occurred in very early hominin
evolution. Using the models for neurological reorganization it can be
suggested the cause for this period, dubbed middle childhood, is most
likely for enhanced foraging abilities in varying seasonal environments.
Genetic factors in recent evolution
Genes
involved in the neuro-development and in neuron physiology are
extremely conserved between mammalian species (94% of genes expressed in
common between humans and chimpanzees, 75% between humans and mice),
compared to other organs. Therefore, few genes account for species
differences in the human brain development and function.
Development of the human cerebral cortex
Main differences rely on the evolution of non-coding genomic regions,
involved in the regulation of gene expression. This leads to
differential expression of genes during the development of the human
brain compared to other species, including chimpanzees. Some of these
regions evolved fast in the human genome (human accelerated regions). The new genes expressed during human neurogenesis are notably associated with the NOTCH, WNT and mTOR pathways, but are also involved ZEB2, PDGFD and its receptor PDGFRβ. The human cerebral cortex is also characterized by a higher gradient of retinoic acid in the prefrontal cortex, leading to higher prefrontal cortex volume. All these differential gene expression lead to higher proliferation of the neural progenitors
leading to more neurons in the human cerebral cortex. Some genes are
lost in their expression during the development of the human cerebral
cortex like GADD45G and FLRT2/FLRT3.
Another source of molecular novelty rely on new genes in the human or hominid
genomes through segmental duplication. Around 30 new genes in the
hominid genomes are dynamically expressed during human corticogenesis.
Some were linked to higher proliferation of neural progenitors: NOTCH2NLA/B/C, ARHGAP11B, CROCCP2, TBC1D3, TMEM14B. Patients with deletions with NOTCH2NL genes display microcephaly, showing the necessity of such duplicated genes, acquired in the human genomes, in the proper corticogenesis.
MCPH1 and ASPM
Bruce Lahn, the senior author at the Howard Hughes Medical Center at
the University of Chicago and colleagues have suggested that there are
specific genes that control the size of the human brain. These genes
continue to play a role in brain evolution, implying that the brain is
continuing to evolve. The study began with the researchers assessing 214
genes that are involved in brain development. These genes were obtained
from humans, macaques, rats and mice. Lahn and the other researchers
noted points in the DNA sequences that caused protein alterations. These
DNA changes were then scaled to the evolutionary time that it took for
those changes to occur. The data showed the genes in the human brain
evolved much faster than those of the other species. Once this genomic
evidence was acquired, Lahn and his team decided to find the specific
gene or genes that allowed for or even controlled this rapid evolution.
Two genes were found to control the size of the human brain as it
develops. These genes are Microcephalin (MCPH1) and Abnormal Spindle-like Microcephaly (ASPM).
The researchers at the University of Chicago were able to determine
that under the pressures of selection, both of these genes showed
significant DNA sequence changes. Lahn's earlier studies displayed that
Microcephalin experienced rapid evolution along the primate lineage
which eventually led to the emergence of Homo sapiens. After the
emergence of humans, Microcephalin seems to have shown a slower
evolution rate. On the contrary, ASPM showed its most rapid evolution
in the later years of human evolution once the divergence between
chimpanzees and humans had already occurred.
Each of the gene sequences went through specific changes that led
to the evolution of humans from ancestral relatives. In order to
determine these alterations, Lahn and his colleagues used DNA sequences
from multiple primates then compared and contrasted the sequences with
those of humans. Following this step, the researchers statistically
analyzed the key differences between the primate and human DNA to come
to the conclusion, that the differences were due to natural selection.
The changes in DNA sequences of these genes accumulated to bring about a
competitive advantage and higher fitness that humans possess in
relation to other primates. This comparative advantage is coupled with a
larger brain size which ultimately allows the human mind to have a
higher cognitive awareness.
ZEB2 protein
ZEB2
ZEB2 is a protein- coding gene in the Homo sapien species. A 2021 study found that a delayed change in the shape of early brain cells causes the distinctly large human forebrain compared to other apes and identify ZEB2 as a genetic regulator of it, whose manipulation lead to acquisition of nonhuman ape cortical architecture in brain organoids.
NOVA1
In 2021, researchers reported that brain organoids created with stem cells into which they reintroduced the archaic gene variant NOVA1 present in Neanderthals and Denisovans via CRISPR-Cas9 shows that it has a major impact on neurodevelopment and that such genetic mutations during the evolution of the human brain underlie traits that separate modern humans from extinct Homo
species. They found that expression of the archaic NOVA1 in cortical
organoids leads to "modified synaptic protein interactions, affects glutamatergic signaling, underlies differences in neuronal connectivity, and promotes higher heterogeneity of neurons regarding their electrophysiological profiles".
This research suggests positive selection of the modern NOVA1 gene,
which may have promoted the randomization of neural scaling. A
subsequent study failed to replicate the differences in organoid
morphology between the modern human and the archaic NOVA1 variant, consistent with suspected unwanted side effects of CRISPR editing in the original study.
SRGAP2C and neuronal maturation
Less is known about neuronal maturation. Synaptic gene and protein expression are protracted, in line with the protracted
synaptic maturation of human cortical neurons so called neoteny. This
probably relies on the evolution of non-coding genomic regions. The
consequence of the neoteny could be an extension of the period of synaptic plasticity and therefore of learning. A human-specific duplicated gene, SRGAP2C accounts for this synaptic neoteny and acts by regulating molecular pathways linked to neurodevelopmental disorders. Other genes are deferentially expressed in human neurons during their development such as osteocrin or cerebelin-2.
LRRC37B and neuronal electrical properties
Even
less is known about molecular specificities linked to the physiology of
the human neurons. Human neurons are more divergent in the genes they
express compared to chimpanzees than chimpanzees to gorilla, which
suggests an acceleration of non-coding genomic regions associated with
genes involved in neuronal physiology, in particular linked to the
synapses. A hominid-specific duplicated gene, LRRC37B, codes for a transmembrane receptor that is selectively localized at the axon initial segment of human cortical pyramidal neurons. It inhibits their voltage-gated sodium channels that generate the action potentials leading to a lower neuronal excitability. Human cortical pyramidal neurons display a lower excitability compared to other mammalian species (including macaques and marmosets) which could lead to different circuit functions in the human species. Therefore, LRRC37B
whose expression has been acquired in the human lineage after the
separation from the chimpanzees could be a key gene in the function of
the human cerebral cortex. LRRC37B binds to secreted FGF13A and SCN1B and modulate indirectly the activity of SCN8A,
all involved in neural disorders such as epilepsy and autism.
Therefore, LRRC37B may contribute to human-specific sensitivities to
such disorders, both involved defects in neuronal excitability.
Genome repair
The genomicDNA of postmitotic neurons ordinarily does not replicate. Protection strategies have evolved to ensure the distinctive longevity of the neuronal genome. Human neurons are reliant on DNA repair
processes to maintain function during an individual's life-time. DNA
repair tends to occur preferentially at evolutionarily conserved sites
that are specifically involved with the regulation of expression of genes essential for neuronal identity and function.
Other factors
Many other genetics may also be involved in recent evolution of the brain.
For instance, scientists showed experimentally, with brain organoids grown from stem cells, how differences between humans and chimpanzees are also substantially caused by non-coding DNA (often discarded as relatively meaningless "junk DNA") – in particular via CRE-regulated expression of the ZNF558 gene for a transcription factor that regulates the SPATA18 gene.
SPATA18 gene encodes a protein and is able to influence lysosome-like
organelles that are found within mitochondria that eradicate oxidized
mitochondrial proteins. This helps monitor the quality of the
mitochondria as the disregulation of its quality control has been linked
to cancer and degenerative diseases. This example may contribute to illustrations of the complexity and scope of relatively recent evolution to Homo sapiens.
A change in gene TKTL1 could be a key factor of recent brain evolution and difference of modern humans to (other) apes and Neanderthals, related to neocortex-neurogenesis.However, the "archaic" allele attributed to Neanderthals is present in 0.03% of Homo sapiens, but no resultant phenotypic differences have been reported in these people.
Additionally, as Herai et al. contend, more is not always better. In
fact, enhanced neuron production "can lead to an abnormally enlarged
cortex and layer-specific imbalances in glia/neuron ratios and neuronal subpopulations during neurodevelopment."
Even the original study's authors agree that “any attempt to discuss
prefrontal cortex and cognitive advantage of modern humans over
Neandertals based on TKTL1 alone is problematic”.
Some of the prior study's authors reported a similar ARHGAP11B mutation in 2016.
Epigenetics also play a major role in the brain evolution in and to humans.
Recently evolved traits
Language
A genome-wide association studymeta-analysis reported genetic factors of, the so far uniquely human, language-related capacities, in particular factors of differences in skill-levels of five tested traits. It e.g. identified association with neuroanatomy of a language-related brain area via neuroimaging
correlation. The data contributes to identifying or understanding the
biological basis of this recently evolved characteristic capability.
One of the prominent ways of tracking the evolution of the human
brain is through direct evidence in the form of fossils. The
evolutionary history of the human brain shows primarily a gradually
bigger brain relative to body size during the evolutionary path from
early primates to hominids and finally to Homo sapiens. Because
fossilized brain tissue is rare, a more reliable approach is to observe
anatomical characteristics of the skull that offer insight into brain
characteristics. One such method is to observe the endocranial cast
(also referred to as endocasts).
Endocasts occur when, during the fossilization process, the brain
deteriorates away, leaving a space that is filled by surrounding
sedimentary material over time. These casts, give an imprint of the
lining of the brain cavity, which allows a visualization of what was
there.
This approach, however, is limited in regard to what information can be
gathered. Information gleaned from endocasts is primarily limited to
the size of the brain (cranial capacity or endocranial volume), prominent sulci and gyri, and size of dominant lobes or regions of the brain.
While endocasts are extremely helpful in revealing superficial brain
anatomy, they cannot reveal brain structure, particularly of deeper
brain areas. By determining scaling metrics of cranial capacity as it
relates to total number of neurons present in primates, it is also
possible to estimate the number of neurons through fossil evidence.
Facial reconstruction of a Homo georgicus from over 1.5 Mya
Despite the limitations to endocasts, they can and do provide a basis
for understanding human brain evolution, which shows primarily a
gradually bigger brain. The evolutionary history of the human brain
shows primarily a gradually bigger brain relative to body size during
the evolutionary path from early primates to hominins and finally to Homo sapiens.
This trend that has led to the present day human brain size indicates
that there has been a 2-3 factor increase in size over the past 3
million years. This can be visualized with current data on hominin evolution, starting with Australopithecus—a group of hominins from which humans are likely descended.
After all of the data, all observations concluded that the main
development that occurred during evolution was the increase of brain
size.
However, recent research has called into question the hypothesis of a threefold increase in brain size when comparing Homo sapiens with Australopithecus
and chimpanzees. For example, in an article published in 2022 compiled a
large data set of contemporary humans and found that the smallest human
brains are less than twice that of large brained chimpanzees. As the
authors write '...the upper limit of chimpanzee brain size is 500g/ml
yet numerous modern humans have brain size below 900 g/ml.' (Note that in this quote, the unit g/ml is to be understood not in the usual way as gram per millilitre but rather as gram or
millilitre. This is consistent because brain density is close to 1
g/ml.) Consequently, the authors argue that the notion of an increase in
brain size being related to advances in cognition needs to be
re-thought in light of global variation in brain size, as the brains of
many modern humans with normal cognitive capacities are only 400g/ml
larger than chimpanzees. Additionally, much of the increase in brain
size - which occurs to a much greater degree in specific modern
populations - can be explained by increases in correlated body size
related to diet and climatic factors.
Australopiths
lived from 3.85 to 2.95 million years ago with the general cranial
capacity somewhere near that of the extant chimpanzee—around 300–500 cm3. Considering that the volume of the modern human brain is around 1,352 cm3 on average this represents a substantial amount of brain mass evolved. Australopiths are estimated to have a total neuron count of ~30-35 billion.
Progressing along the human ancestral timeline, brain size continues to steadily increase (see Homininae) when moving into the era of Homo. For example, Homo habilis, living 2.4 million to 1.4 million years ago and argued to be the first Homo species based on a host of characteristics, had a cranial capacity of around 600 cm3. Homo habilis is estimated to have had ~40 billion neurons.
A little closer to present day, Homo heidelbergensis lived from around 700,000 to 200,000 years ago and had a cranial capacity of around 1290 cm3 and having around 76 billion neurons.
Homo neaderthalensis, living 400,000 to 40,000 years ago, had a cranial capacity comparable to that of modern humans at around 1500–1600 cm3on average, with some specimens of Neanderthal having even greater cranial capacity. Neanderthals are estimated to have had around 85 billion neurons. The increase in brain size topped with Neanderthals, possibly due to their larger visual systems.
It is also important to note that the measure of brain mass or volume, seen as cranial capacity, or even relative brain size,
which is brain mass that is expressed as a percentage of body mass, are
not a measure of intelligence, use, or function of regions of the
brain.
Total neurons, however, also do not indicate a higher ranking in
cognitive abilities. Elephants have a higher number of total neurons
(257 billion) compared to humans (100 billion).
Relative brain size, overall mass, and total number of neurons are only
a few metrics that help scientists follow the evolutionary trend of
increased brain to body ratio through the hominin phylogeny.
In 2021, scientists suggested that the brains of early Homo from Africa and Dmanisi, Georgia, Western Asia "retained a great ape-like structure of the frontal lobe" for far longer than previously thought – until about 1.5 million years ago. Their findings imply that Homofirst dispersed out of Africa
before human brains evolved to roughly their modern anatomical
structure in terms of the location and organization of individual brain
regions. It also suggests that this evolution occurred – not during –
but only long after the Homo lineage evolved ~2.5 million years ago and
after they – Homo erectus in particular – evolved to walk upright. What is the least controversial is that the brain expansion started about 2.6 Ma (about the same as the start of the Pleistocene), and ended around 0.2 Ma.
In
addition to just the size of the brain, scientists have observed
changes in the folding of the brain, as well as in the thickness of the cortex.
The more convoluted the surface of the brain is, the greater the
surface area of the cortex which allows for an expansion of cortex. It
is the most evolutionarily advanced part of the brain.
Greater surface area of the brain is linked to higher intelligence as
is the thicker cortex but there is an inverse relationship—the thicker
the cortex, the more difficult it is for it to fold. In adult humans,
thicker cerebral cortex has been linked to higher intelligence.
The neocortex is the most advanced and most evolutionarily young
part of the human brain. It is six layers thick and is only present in
mammals. It is especially prominent in humans and is the location of
most higher level functioning and cognitive ability. The six-layered neocortex found in mammals is evolutionarily derived from a three-layer cortex present in all modern reptiles.
This three-layer cortex is still conserved in some parts of the human
brain such as the hippocampus and is believed to have evolved in mammals
to the neocortex during the transition between the Triassic and
Jurassic periods. After looking at history, the mammals had little neocortex compared to the primates as they had more cortex. The three layers of this reptilian cortex correlate strongly to the first, fifth and sixth layers of the mammalian neocortex.
Across species of mammals, primates have greater neuronal density
compared to rodents of similar brain mass and this may account for
increased intelligence.
Theories of human brain evolution
Explanations
of the rapid evolution and exceptional size of the human brain can be
classified into five groups: instrumental, social, environmental,
dietary, and anatomo-physiological. The instrumental hypotheses are based on the logic that evolutionary selection for larger brains is beneficial for species survival, dominance, and spread, because larger brains facilitate food-finding and mating success. The social hypotheses suggest that social behavior stimulates evolutionary expansion of brain size. Similarly, the environmental hypotheses suppose that encephalization is promoted by environmental factors such as stress, variability, and consistency.
The dietary theories maintain that food quality and certain
nutritional components directly contributed to the brain growth in the Homo genus. The anatomo-physiologic concepts, such as cranio-cerebral vascular hypertension due to head-down posture of the anthropoid fetus during pregnancy, are primarily focused on anatomic-functional changes that predispose to brain enlargement.
No single theory can completely account for human brain
evolution. Multiple selective pressures in combination seems to have
been involved. Synthetic theories have been proposed,
but have not clearly explained reasons for the uniqueness of the human
brain. Puzzlingly, brain enlargement has been found to have occurred
independently in different primate lineages, but only human lineage ended up with an exceptional brain capacity. Fetal head-down posture may be an explanation of this conundrum because Homo sapiens is the only primate obligatory biped with upright posture.
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