Neuroscience

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Neuroscience

Drawing by Santiago Ramón y Cajal (1899) of neurons in the pigeon cerebellum

Neuroscience is the scientific study of the nervous system (the brain, spinal cord, and peripheral nervous system), its functions, and its disorders. It is a multidisciplinary science that combines physiology, anatomy, molecular biology, developmental biology, cytology, psychology, physics, computer science, chemistry, medicine, statistics, and mathematical modeling to understand the fundamental and emergent properties of neurons, glia, and neural circuits. The understanding of the biological basis of learning, memory, behavior, perception, and consciousness has been described by Eric Kandel as the "epic challenge" of the biological sciences.

The scope of neuroscience has broadened over time to include different approaches used to study the nervous system at different scales. The techniques used by neuroscientists have expanded enormously, from molecular and cellular studies of individual neurons to imaging of sensory, motor, and cognitive tasks in the brain.

History

Main article: History of neuroscience

Illustration from Gray's Anatomy (1918) of a lateral view of the human brain, featuring the hippocampus among other neuroanatomical features

The earliest study of the nervous system dates to ancient Egypt. Trepanation, the surgical practice of either drilling or scraping a hole into the skull for the purpose of curing head injuries or mental disorders, or relieving cranial pressure, was first recorded during the Neolithic period. Manuscripts dating to 1700 BC indicate that the Egyptians had some knowledge about symptoms of brain damage.

Early views on the function of the brain regarded it to be a "cranial stuffing" of sorts. In Egypt, from the late Middle Kingdom onwards, the brain was regularly removed in preparation for mummification. It was believed at the time that the heart was the seat of intelligence. According to Herodotus, the first step of mummification was to "take a crooked piece of iron, and with it draw out the brain through the nostrils, thus getting rid of a portion, while the skull is cleared of the rest by rinsing with drugs."

The view that the heart was the source of consciousness was not challenged until the time of the Greek physician Hippocrates. He believed that the brain was not only involved with sensation—since most specialized organs (e.g., eyes, ears, tongue) are located in the head near the brain—but was also the seat of intelligence. Plato also speculated that the brain was the seat of the rational part of the soul. Aristotle, however, believed the heart was the center of intelligence and that the brain regulated the amount of heat from the heart. This view was generally accepted until the Roman physician Galen, a follower of Hippocrates and physician to Roman gladiators, observed that his patients lost their mental faculties when they had sustained damage to their brains.

Abulcasis, Averroes, Avicenna, Avenzoar, and Maimonides, active in the Medieval Muslim world, described a number of medical problems related to the brain. In Renaissance Europe, Vesalius (1514–1564), René Descartes (1596–1650), Thomas Willis (1621–1675) and Jan Swammerdam (1637–1680) also made several contributions to neuroscience.

The Golgi stain first allowed for the visualization of individual neurons.

Luigi Galvani's pioneering work in the late 1700s set the stage for studying the electrical excitability of muscles and neurons. In 1843 Emil du Bois-Reymond demonstrated the electrical nature of the nerve signal, whose speed Hermann von Helmholtz proceeded to measure, and in 1875 Richard Caton found electrical phenomena in the cerebral hemispheres of rabbits and monkeys. Adolf Beck published in 1890 similar observations of spontaneous electrical activity of the brain of rabbits and dogs. Studies of the brain became more sophisticated after the invention of the microscope and the development of a staining procedure by Camillo Golgi during the late 1890s. The procedure used a silver chromate salt to reveal the intricate structures of individual neurons. His technique was used by Santiago Ramón y Cajal and led to the formation of the neuron doctrine, the hypothesis that the functional unit of the brain is the neuron. Golgi and Ramón y Cajal shared the Nobel Prize in Physiology or Medicine in 1906 for their extensive observations, descriptions, and categorizations of neurons throughout the brain.

In parallel with this research, in 1815 Jean Pierre Flourens induced localized lesions of the brain in living animals to observe their effects on motricity, sensibility and behavior. Work with brain-damaged patients by Marc Dax in 1836 and Paul Broca in 1865 suggested that certain regions of the brain were responsible for certain functions. At the time, these findings were seen as a confirmation of Franz Joseph Gall's theory that language was localized and that certain psychological functions were localized in specific areas of the cerebral cortex.^[22][23] The localization of function hypothesis was supported by observations of epileptic patients conducted by John Hughlings Jackson, who correctly inferred the organization of the motor cortex by watching the progression of seizures through the body. Carl Wernicke further developed the theory of the specialization of specific brain structures in language comprehension and production. In 1894, neurologist and psychiatrist Edward Flatau published a human brain atlas “Atlas of the Human Brain and the Course of the Nerve-Fibres” which consisted of long-exposure photographs of fresh brain sections. In 1897, Charles Scott Sherrington introduced the name "synapse" for the connection between neurons.

Brodmann's diagram of the cerebral cortex with the areas he identified

In 1909, German anatomist Korbinian Brodmann published his original research on brain mapping, defining 52 distinct regions of the cerebral cortex, known as Brodmann areas. Modern research through neuroimaging techniques, still uses the Brodmann cerebral cytoarchitectonic map (referring to the study of cell structure) anatomical definitions from this era in continuing to show that distinct areas of the cortex are activated in the execution of specific tasks.

During the 20th century, neuroscience began to be recognized as a distinct academic discipline in its own right, rather than as studies of the nervous system within other disciplines. Eric Kandel and collaborators have cited David Rioch, Francis O. Schmitt, and Stephen Kuffler as having played critical roles in establishing the field. Rioch originated the integration of basic anatomical and physiological research with clinical psychiatry at the Walter Reed Army Institute of Research, starting in the 1950s. During the same period, Schmitt established a neuroscience research program within the Biology Department at the Massachusetts Institute of Technology, bringing together biology, chemistry, physics, and mathematics. The first freestanding neuroscience department (then called Psychobiology) was founded in 1964 at the University of California, Irvine by James L. McGaugh. This was followed by the Department of Neurobiology at Harvard Medical School, which was founded in 1966 by Stephen Kuffler.

3-D sensory and motor homunculus models at the Natural History Museum, London

In the process of treating epilepsy, Wilder Penfield produced maps of the location of various functions (motor, sensory, memory, vision) in the brain. He summarized his findings in a 1950 book called The Cerebral Cortex of Man. Wilder Penfield and his co-investigators Edwin Boldrey and Theodore Rasmussen are considered to be the originators of the cortical homunculus.

The understanding of neurons and of nervous system function became increasingly precise and molecular during the 20th century. For example, in 1952, Alan Lloyd Hodgkin and Andrew Huxley presented a mathematical model for the transmission of electrical signals in neurons of the giant axon of a squid, which they called "action potentials", and how they are initiated and propagated, known as the Hodgkin–Huxley model. In 1961–1962, Richard FitzHugh and J. Nagumo simplified Hodgkin–Huxley, in what is called the FitzHugh–Nagumo model. In 1962, Bernard Katz modeled neurotransmission across the space between neurons known as synapses. Beginning in 1966, Eric Kandel and collaborators examined biochemical changes in neurons associated with learning and memory storage in Aplysia. In 1981 Catherine Morris and Harold Lecar combined these models in the Morris–Lecar model. Such increasingly quantitative work gave rise to numerous biological neuron models and models of neural computation.

As a result of the increasing interest about the nervous system, several prominent neuroscience organizations have been formed to provide a forum to all neuroscientists during the 20th century. For example, the International Brain Research Organization was founded in 1961, the International Society for Neurochemistry in 1963, the European Brain and Behaviour Society in 1968, and the Society for Neuroscience in 1969. Recently, the application of neuroscience research results has also given rise to applied disciplines as neuroeconomics, neuroeducation, neuroethics, and neurolaw.

Over time, brain research has gone through philosophical, experimental, and theoretical phases, with work on neural implants and brain simulation predicted to be important in the future.

Modern neuroscience

Main article: Outline of neuroscience

Human nervous system

The scientific study of the nervous system increased significantly during the second half of the twentieth century, principally due to advances in molecular biology, electrophysiology, and computational neuroscience. This has allowed neuroscientists to study the nervous system in all its aspects: how it is structured, how it works, how it develops, how it malfunctions, and how it can be changed.

For example, it has become possible to understand, in much detail, the complex processes occurring within a single neuron. Neurons are cells specialized for communication. They are able to communicate with neurons and other cell types through specialized junctions called synapses, at which electrical or electrochemical signals can be transmitted from one cell to another. Many neurons extrude a long thin filament of axoplasm called an axon, which may extend to distant parts of the body and are capable of rapidly carrying electrical signals, influencing the activity of other neurons, muscles, or glands at their termination points. A nervous system emerges from the assemblage of neurons that are connected to each other in neural circuits, and networks.

The vertebrate nervous system can be split into two parts: the central nervous system (defined as the brain and spinal cord), and the peripheral nervous system. In many species—including all vertebrates—the nervous system is the most complex organ system in the body, with most of the complexity residing in the brain. The human brain alone contains around one hundred billion neurons and one hundred trillion synapses; it consists of thousands of distinguishable substructures, connected to each other in synaptic networks whose intricacies have only begun to be unraveled. At least one out of three of the approximately 20,000 genes belonging to the human genome is expressed mainly in the brain.

Due to the high degree of plasticity of the human brain, the structure of its synapses and their resulting functions change throughout life.

Making sense of the nervous system's dynamic complexity is a formidable research challenge. Ultimately, neuroscientists would like to understand every aspect of the nervous system, including how it works, how it develops, how it malfunctions, and how it can be altered or repaired. Analysis of the nervous system is therefore performed at multiple levels, ranging from the molecular and cellular levels to the systems and cognitive levels. The specific topics that form the main focus of research change over time, driven by an ever-expanding base of knowledge and the availability of increasingly sophisticated technical methods. Improvements in technology have been the primary drivers of progress. Developments in electron microscopy, computer science, electronics, functional neuroimaging, and genetics and genomics have all been major drivers of progress.

Advances in the classification of brain cells have been enabled by electrophysiological recording, single-cell genetic sequencing, and high-quality microscopy, which have combined into a single method pipeline called patch-sequencing in which all three methods are simultaneously applied using miniature tools. The efficiency of this method and the large amounts of data that is generated has allowed researchers to make some general conclusions about cell types; for example that the human and mouse brain have different versions of fundamentally the same cell types.

Molecular and cellular neuroscience

Main articles: Molecular neuroscience and Cellular neuroscience

Photograph of a stained neuron in a chicken embryo

Basic questions addressed in molecular neuroscience include the mechanisms by which neurons express and respond to molecular signals and how axons form complex connectivity patterns. At this level, tools from molecular biology and genetics are used to understand how neurons develop and how genetic changes affect biological functions. The morphology, molecular identity, and physiological characteristics of neurons and how they relate to different types of behavior are also of considerable interest.

Questions addressed in cellular neuroscience include the mechanisms of how neurons process signals physiologically and electrochemically. These questions include how signals are processed by neurites and somas and how neurotransmitters and electrical signals are used to process information in a neuron. Neurites are thin extensions from a neuronal cell body, consisting of dendrites (specialized to receive synaptic inputs from other neurons) and axons (specialized to conduct nerve impulses called action potentials). Somas are the cell bodies of the neurons and contain the nucleus.

Another major area of cellular neuroscience is the investigation of the development of the nervous system. Questions include the patterning and regionalization of the nervous system, axonal and dendritic development, trophic interactions, synapse formation and the implication of fractones in neural stem cells, differentiation of neurons and glia (neurogenesis and gliogenesis), and neuronal migration.

Computational neurogenetic modeling is concerned with the development of dynamic neuronal models for modeling brain functions with respect to genes and dynamic interactions between genes, on the cellular level (Computational Neurogenetic Modeling (CNGM) can also be used to model neural systems).

Neural circuits and systems

Main articles: Neural circuit and Systems neuroscience

Proposed organization of motor-semantic neural circuits for action language comprehension. Adapted from Shebani et al. (2013).

Systems neuroscience research centers on the structural and functional architecture of the developing human brain, and the functions of large-scale brain networks, or functionally-connected systems within the brain. Alongside brain development, systems neuroscience also focuses on how the structure and function of the brain enables or restricts the processing of sensory information, using learned mental models of the world, to motivate behavior.

Questions in systems neuroscience include how neural circuits are formed and used anatomically and physiologically to produce functions such as reflexes, multisensory integration, motor coordination, circadian rhythms, emotional responses, learning, and memory. In other words, this area of research studies how connections are made and morphed in the brain, and the effect it has on human sensation, movement, attention, inhibitory control, decision-making, reasoning, memory formation, reward, and emotion regulation.

Specific areas of interest for the field include observations of how the structure of neural circuits effect skill acquisition, how specialized regions of the brain develop and change (neuroplasticity), and the development of brain atlases, or wiring diagrams of individual developing brains.

The related fields of neuroethology and neuropsychology address the question of how neural substrates underlie specific animal and human behaviors. Neuroendocrinology and psychoneuroimmunology examine interactions between the nervous system and the endocrine and immune systems, respectively. Despite many advancements, the way that networks of neurons perform complex cognitive processes and behaviors is still poorly understood.

Cognitive and behavioral neuroscience

Main articles: Behavioral neuroscience and Cognitive neuroscience

Cognitive neuroscience addresses the questions of how psychological functions are produced by neural circuitry. The emergence of powerful new measurement techniques such as neuroimaging (e.g., fMRI, PET, SPECT), EEG, MEG, electrophysiology, optogenetics and human genetic analysis combined with sophisticated experimental techniques from cognitive psychology allows neuroscientists and psychologists to address abstract questions such as how cognition and emotion are mapped to specific neural substrates. Although many studies hold a reductionist stance looking for the neurobiological basis of cognitive phenomena, recent research shows that there is an interplay between neuroscientific findings and conceptual research, soliciting and integrating both perspectives. For example, neuroscience research on empathy solicited an interdisciplinary debate involving philosophy, psychology and psychopathology. Moreover, the neuroscientific identification of multiple memory systems related to different brain areas has challenged the idea of memory as a literal reproduction of the past, supporting a view of memory as a generative, constructive and dynamic process.

Neuroscience is also allied with the social and behavioral sciences, as well as with nascent interdisciplinary fields. Examples of such alliances include neuroeconomics, decision theory, social neuroscience, and neuromarketing to address complex questions about interactions of the brain with its environment. A study into consumer responses for example uses EEG to investigate neural correlates associated with narrative transportation into stories about energy efficiency.

Computational neuroscience

Main article: Computational neuroscience

Questions in computational neuroscience can span a wide range of levels of traditional analysis, such as development, structure, and cognitive functions of the brain. Research in this field utilizes mathematical models, theoretical analysis, and computer simulation to describe and verify biologically plausible neurons and nervous systems. For example, biological neuron models are mathematical descriptions of spiking neurons which can be used to describe both the behavior of single neurons as well as the dynamics of neural networks. Computational neuroscience is often referred to as theoretical neuroscience.

Neuroscience and medicine

Clinical neuroscience

Main article: Neurotherapy

Further information: Clinical neuroscience

Neurology, psychiatry, neurosurgery, psychosurgery, anesthesiology and pain medicine, neuropathology, neuroradiology, ophthalmology, otolaryngology, clinical neurophysiology, addiction medicine, and sleep medicine are some medical specialties that specifically address the diseases of the nervous system. These terms also refer to clinical disciplines involving diagnosis and treatment of these diseases.

Neurology works with diseases of the central and peripheral nervous systems, such as amyotrophic lateral sclerosis (ALS) and stroke, and their medical treatment. Psychiatry focuses on affective, behavioral, cognitive, and perceptual disorders. Anesthesiology focuses on perception of pain, and pharmacologic alteration of consciousness. Neuropathology focuses upon the classification and underlying pathogenic mechanisms of central and peripheral nervous system and muscle diseases, with an emphasis on morphologic, microscopic, and chemically observable alterations. Neurosurgery and psychosurgery work primarily with surgical treatment of diseases of the central and peripheral nervous systems.

Neuroscience underlies the development of various neurotherapy methods to treat diseases of the nervous system.

Translational research

Further information: Translational research and Translational neuroscience

An MRI of a human head showing benign familial macrocephaly (head circumference > 60 cm)

Recently, the boundaries between various specialties have blurred, as they are all influenced by basic research in neuroscience. For example, brain imaging enables objective biological insight into mental illnesses, which can lead to faster diagnosis, more accurate prognosis, and improved monitoring of patient progress over time.

Integrative neuroscience describes the effort to combine models and information from multiple levels of research to develop a coherent model of the nervous system. For example, brain imaging coupled with physiological numerical models and theories of fundamental mechanisms may shed light on psychiatric disorders.

Another important area of translational research is brain–computer interfaces (BCIs), or machines that are able to communicate and influence the brain. They are currently being researched for their potential to repair neural systems and restore certain cognitive functions. However, some ethical considerations have to be dealt with before they are accepted.

Major branches

Modern neuroscience education and research activities can be very roughly categorized into the following major branches, based on the subject and scale of the system in examination as well as distinct experimental or curricular approaches. Individual neuroscientists, however, often work on questions that span several distinct subfields.

List of the major branches of neuroscience

Branch	Description
Affective neuroscience	Affective neuroscience is the study of the neural mechanisms involved in emotion, typically through experimentation on animal models.
Behavioral neuroscience	Behavioral neuroscience (also known as biological psychology, physiological psychology, biopsychology, or psychobiology) is the application of the principles of biology to the study of genetic, physiological, and developmental mechanisms of behavior in humans and non-human animals.
Cellular neuroscience	Cellular neuroscience is the study of neurons at a cellular level including morphology and physiological properties.
Clinical neuroscience	The scientific study of the biological mechanisms that underlie the disorders and diseases of the nervous system.
Cognitive neuroscience	Cognitive neuroscience is the study of the biological mechanisms underlying cognition.
Computational neuroscience	Computational neuroscience is the theoretical study of the nervous system.
Cultural neuroscience	Cultural neuroscience is the study of how cultural values, practices and beliefs shape and are shaped by the mind, brain and genes across multiple timescales.
Developmental neuroscience	Developmental neuroscience studies the processes that generate, shape, and reshape the nervous system and seeks to describe the cellular basis of neural development to address underlying mechanisms.
Evolutionary neuroscience	Evolutionary neuroscience studies the evolution of nervous systems.
Molecular neuroscience	Molecular neuroscience studies the nervous system with molecular biology, molecular genetics, protein chemistry, and related methodologies.
Nanoneuroscience	An interdisciplinary field that integrates nanotechnology and neuroscience.
Neural engineering	Neural engineering uses engineering techniques to interact with, understand, repair, replace, or enhance neural systems.
Neuroanatomy	Neuroanatomy is the study of the anatomy of nervous systems.
Neurochemistry	Neurochemistry is the study of how neurochemicals interact and influence the function of neurons.
Neuroethology	Neuroethology is the study of the neural basis of non-human animals behavior.
Neurogastronomy	Neurogastronomy is the study of flavor and how it affects sensation, cognition, and memory.
Neurogenetics	Neurogenetics is the study of the genetical basis of the development and function of the nervous system.
Neuroimaging	Neuroimaging includes the use of various techniques to either directly or indirectly image the structure and function of the brain.
Neuroimmunology	Neuroimmunology is concerned with the interactions between the nervous and the immune system.
Neuroinformatics	Neuroinformatics is a discipline within bioinformatics that conducts the organization of neuroscience data and application of computational models and analytical tools.
Neurolinguistics	Neurolinguistics is the study of the neural mechanisms in the human brain that control the comprehension, production, and acquisition of language.
Neuro-ophthalmology	Neuro-ophthalmology is an academically oriented subspecialty that merges the fields of neurology and ophthalmology, often dealing with complex systemic diseases that have manifestations in the visual system.
Neurophysics	Neurophysics is the branch of biophysics dealing with the development and use of physical methods to gain information about the nervous system.
Neurophysiology	Neurophysiology is the study of the structure and function of the nervous system, generally using physiological techniques that include measurement and stimulation with electrodes or optically with ion- or voltage-sensitive dyes or light-sensitive channels.
Neuropsychology	Neuropsychology is a discipline that resides under the umbrellas of both psychology and neuroscience, and is involved in activities in the arenas of both basic science and applied science. In psychology, it is most closely associated with biopsychology, clinical psychology, cognitive psychology, and developmental psychology. In neuroscience, it is most closely associated with the cognitive, behavioral, social, and affective neuroscience areas. In the applied and medical domain, it is related to neurology and psychiatry.
Neuropsychopharmacology	Neuropsychopharmacology, an interdisciplinary science related to psychopharmacology and fundamental neuroscience, is the study of the neural mechanisms that drugs act upon to influence behavior.
Optogenetics	Optogenetics is a biological technique to control the activity of neurons or other cell types with light.
Paleoneurobiology	Paleoneurobiology is a field that combines techniques used in paleontology and archeology to study brain evolution, especially that of the human brain.
Social neuroscience	Social neuroscience is an interdisciplinary field devoted to understanding how biological systems implement social processes and behavior, and to using biological concepts and methods to inform and refine theories of social processes and behavior.
Systems neuroscience	Systems neuroscience is the study of the function of neural circuits and systems.

Careers in neuroscience

The career options for neuroscience graduates vary widely depending on the level of education. At the bachelor’s level, graduates often enter laboratory research, healthcare support, biotechnology, or science communication, though some pursue broader fields such as policy or nonprofit work. With a master’s degree, training may prepare individuals for applied health professions (e.g., occupational therapy, medicine -neurology, psychiatry, neuroimaging-, genetic counseling), research management, or public health. An advanced degree (PhD or equivalent) is usually required for independent research or university teaching. Source:

Bachelor's Level

Pharmaceutical Sales	Residential Counselor
Laboratory Technician	Regulatory Affairs Specialist
Psychometrist*	Medical Technician*
Science Writer	Clinical Research Assistant
Science Advocacy	Special Education Assistant
Nonprofit Work	Patient Care Assistant*
Health Educator	Orthotic and Prosthetic Technician*
EEG Technologist*	Lab Animal Care Technician
Medical and Healthcare Manager	Sales Engineer
Forensic Science Technician	Law Enforcement
Pharmacy Technician*	Natural Sciences Manager
Public Policy	Advertising/Marketing

Master's Level

Nurse Practitioner	Neuroimaging Technician
Physician's Assistant	Teacher
Genetic Counselor	Epidemiology
Occupational Therapist	Biostatistician
Orthotist/Prosthetist	Speech-Language Pathologist
Neural Engineer	Public Health

Advanced Degree

Medicine (MD, DO)	Food Scientist
Research Scientist	Pharmacist
Dentist	Veterinarian
Physical Therapist	Audiologist
Optometrist	Lawyer
Clinical Psychologist	Professor
Neuropsychologist	Chiropractor

Neuroscience organizations

See also: Category:Neuroscience organizations

The largest professional neuroscience organization is the Society for Neuroscience (SFN), which is based in the United States but includes many members from other countries. Since its founding in 1969 the SFN has grown steadily: as of 2010 it recorded 40,290 members from 83 countries. Annual meetings, held each year in a different American city, draw attendance from researchers, postdoctoral fellows, graduate students, and undergraduates, as well as educational institutions, funding agencies, publishers, and hundreds of businesses that supply products used in research.

Other major organizations devoted to neuroscience include the International Brain Research Organization (IBRO), which holds its meetings in a country from a different part of the world each year, and the Federation of European Neuroscience Societies (FENS), which holds a meeting in a different European city every two years. FENS comprises a set of 32 national-level organizations, including the British Neuroscience Association, the German Neuroscience Society (Neurowissenschaftliche Gesellschaft), and the French Société des Neurosciences. The first National Honor Society in Neuroscience, Nu Rho Psi, was founded in 2006. Numerous youth neuroscience societies which support undergraduates, graduates and early career researchers also exist, such as Simply Neuroscience and Project Encephalon.

In 2013, the BRAIN Initiative was announced in the US. The International Brain Initiative was created in 2017, currently integrated by more than seven national-level brain research initiatives (US, Europe, Allen Institute, Japan, China, Australia, Canada, Korea, and Israel) spanning four continents.

Public education and outreach

In addition to conducting traditional research in laboratory settings, neuroscientists have also been involved in the promotion of awareness and knowledge about the nervous system among the general public and government officials. Such promotions have been done by both individual neuroscientists and large organizations. For example, individual neuroscientists have promoted neuroscience education among young students by organizing the International Brain Bee, which is an academic competition for high school or secondary school students worldwide. In the United States, large organizations such as the Society for Neuroscience have promoted neuroscience education by developing a primer called Brain Facts, collaborating with public school teachers to develop Neuroscience Core Concepts for K-12 teachers and students, and cosponsoring a campaign with the Dana Foundation called Brain Awareness Week to increase public awareness about the progress and benefits of brain research. In Canada, the Canadian Institutes of Health Research's (CIHR) Canadian National Brain Bee is held annually at McMaster University.

Neuroscience educators formed a Faculty for Undergraduate Neuroscience (FUN) in 1992 to share best practices and provide travel awards for undergraduates presenting at Society for Neuroscience meetings.

Neuroscientists have also collaborated with other education experts to study and refine educational techniques to optimize learning among students, an emerging field called educational neuroscience. Federal agencies in the United States, such as the National Institute of Health (NIH) and National Science Foundation (NSF), have also funded research that pertains to best practices in teaching and learning of neuroscience concepts.

Engineering applications of neuroscience

Neuromorphic computer chips

Neuromorphic engineering is a branch of neuroscience that deals with creating functional physical models of neurons for the purposes of useful computation. The emergent computational properties of neuromorphic computers are fundamentally different from conventional computers in the sense that they are complex systems, and that the computational components are interrelated with no central processor.

One example of such a computer is the SpiNNaker supercomputer.

Sensors can also be made smart with neuromorphic technology. An example of this is the Event Camera's BrainScaleS (brain-inspired Multiscale Computation in Neuromorphic Hybrid Systems), a hybrid analog neuromorphic supercomputer located at Heidelberg University in Germany. It was developed as part of the Human Brain Project's neuromorphic computing platform and is the complement to the SpiNNaker supercomputer, which is based on digital technology. The architecture used in BrainScaleS mimics biological neurons and their connections on a physical level; additionally, since the components are made of silicon, these model neurons operate on average 864 times (24 hours of real time is 100 seconds in the machine simulation) that of their biological counterparts.

Recent advances in neuromorphic microchip technology have led a group of scientists to create an artificial neuron that can replace real neurons in diseases.

Snowball Earth

From Wikipedia, the free encyclopedia

https://en.wikipedia.org/wiki/Snowball_Earth

Artist's rendition of a fully-frozen Snowball Earth with no remaining liquid surface water

Proterozoic snowball periods
−750 — – −725 — – −700 — – −675 — – −650 — – −625 — – −600 — – −575 — – −550 — – −525 —	Tonian Cryogenian Ediacaran Cambrian Sturtian Marinoan Gaskiers Baykonurian
Neoproterozoic era   Snowball Earth
Estimate of Proterozoic glacial periods.Vertical axis: millions of years ago

The Snowball Earth is a geohistorical hypothesis that proposes that during one or more of Earth's icehouse climates, the planet's surface became nearly entirely frozen with no liquid oceanic or surface water exposed to the atmosphere. The most academically mentioned period of such a global ice age is believed to have occurred some time before 650 mya during the Cryogenian period, which included at least two large glacial periods, the Sturtian and Marinoan glaciations.

Proponents of the hypothesis argue that it best explains sedimentary deposits that are generally believed to be of glacial origin at tropical palaeolatitudes and other enigmatic features in the geological record. Opponents of the hypothesis contest the geological evidence for global glaciation and the geophysical feasibility of an ice- or slush-covered ocean, and they emphasize the difficulty of escaping an all-frozen condition. Several unanswered questions remain, including whether Earth was a full "snowball" or a "slushball" with a thin equatorial band of open (or seasonally open) water. The Snowball Earth episodes are proposed to have occurred before the sudden radiations of multicellular bioforms known as the Avalon and Cambrian explosions; the most recent Snowball episode may have triggered the evolution of multicellularity.

History

First evidence for ancient glaciation

Long before the idea of a global glaciation was first proposed, a series of discoveries occurred that accumulated evidence for ancient Precambrian glaciations. The first of these discoveries was published in 1871 by J. Thomson, who found ancient glacier-reworked material (tillite) in Islay, Scotland. Similar findings followed in Australia (1884) and India (1887). A fourth and very illustrative finding, which came to be known as "Reusch's Moraine," was reported by Hans Reusch in northern Norway in 1891. Many other findings followed, but their understanding was hampered by the rejection (at the time) of continental drift.

Global glaciation proposed

Douglas Mawson, an Australian geologist and Antarctic explorer, spent much of his career studying the stratigraphy of the Neoproterozoic in South Australia, where he identified thick and extensive glacial sediments. As a result, late in his career, he speculated about the possibility of global glaciation.

Mawson's ideas of global glaciation, however, were based on the mistaken assumption that the geographic position of Australia, and those of other continents where low-latitude glacial deposits are found, have remained constant through time. With the advancement of the continental drift hypothesis, and eventually plate tectonic theory, came an easier explanation for the glaciogenic sediments—they were deposited at a time when the continents were at higher latitudes.

In 1964, the idea of global-scale glaciation reemerged when W. Brian Harland published a paper in which he presented palaeomagnetic data showing that glacial tillites in Svalbard and Greenland were deposited at tropical latitudes. From this data and the sedimentological evidence that the glacial sediments interrupt successions of rocks commonly associated with tropical to temperate latitudes, he argued that an ice age occurred that was so extreme that it resulted in marine glacial rocks being deposited in the tropics.

In the 1960s, Mikhail Budyko, a Soviet climatologist, developed a simple energy-balance climate model to investigate the effect of ice cover on global climate. Using this model, Budyko found that if ice sheets advanced far enough out of the polar regions, a feedback loop ensued where the increased reflectiveness (albedo) of the ice led to further cooling and the formation of more ice, until the entire Earth was covered in ice and stabilized in a new ice-covered equilibrium. While Budyko's model showed that this ice-albedo stability could happen, he concluded that it had, in fact, never happened, as his model offered no way to escape from such a feedback loop.

In 1971, Aron Faegre, an American physicist, showed that a similar energy-balance model predicted three stable global climates, one of which was snowball Earth. This model introduced Edward Norton Lorenz's concept of intransitivity, indicating that there could be a major jump from one climate to another, including to snowball Earth.

The term "snowball Earth" was coined by Joseph Kirschvink in a short paper published in 1992 within a lengthy volume concerning the biology of the Proterozoic eon. The major contributions from this work were: (1) the recognition that the presence of banded iron formations is consistent with such a global glacial episode, and (2) the introduction of a mechanism by which to escape from a completely ice-covered Earth—specifically, the accumulation of CO₂ from volcanic outgassing leading to an ultra-greenhouse effect.

Franklyn Van Houten's discovery of a consistent geological pattern in which lake levels rose and fell is now known as the "Van Houten cycle". His studies of phosphorus deposits and banded iron formations in sedimentary rocks made him an early adherent of the snowball Earth hypothesis postulating that the planet's surface froze more than 650 Ma.

Interest in the notion of a snowball Earth increased dramatically after Paul F. Hoffman and his co-workers applied Kirschvink's ideas to a succession of Neoproterozoic sedimentary rocks in Namibia and elaborated upon the hypothesis in the journal Science in 1998 by incorporating such observations as the occurrence of cap carbonates.

In 2010, Francis A. Macdonald, assistant professor at Harvard in the Department of Earth and Planetary Sciences, and others, reported evidence that Rodinia was at equatorial latitude during the Cryogenian period with glacial ice at or below sea level, and that the associated Sturtian glaciation was global.

Evidence

The snowball Earth hypothesis was originally devised to explain geological evidence for the apparent presence of glaciers at tropical latitudes. According to modelling, an ice–albedo feedback would result in glacial ice rapidly advancing to the equator once the glaciers spread to within 25° to 30° of the equator. Therefore, the presence of glacial deposits within the tropics suggests global ice cover.

Critical to an assessment of the validity of the theory, therefore, is an understanding of the reliability and significance of the evidence that led to the belief that ice ever reached the tropics. This evidence must prove three things:

1. That a bed contains sedimentary structures that could have been formed only by glacial activity;
2. That the bed lay within the tropics when it was deposited.
3. That glaciers were active at different global locations at the same time, and that no other deposits of the same age are in existence.

This last point is very difficult to prove. Before the Ediacaran, the biostratigraphic markers usually used to correlate rocks are absent; therefore there is no way to prove that rocks in different places across the globe were deposited at the same time. The best that can be done is to estimate the age of the rocks using radiometric dating, which are rarely accurate to better than a million years or so.

The first two points are often the source of contention on a case-to-case basis. Many glacial features can also be formed by non-glacial means, and estimating the approximate latitudes of landmasses even as recently as 200 Ma can be riddled with difficulties.

Palaeomagnetism

The snowball Earth hypothesis was first posited to explain what were then considered to be glacial deposits near the equator. Since tectonic plates move slowly over time, ascertaining their position at a given point in Earth's long history is not easy. In addition to considerations of how the recognizable landmasses could have fit together, the latitude at which a rock was deposited can be constrained by palaeomagnetism.

When sedimentary rocks form, magnetic minerals within them tend to align with Earth's magnetic field. Through the precise measurement of this palaeomagnetism, it is possible to estimate the latitude (but not the longitude) where the rock matrix was formed. Palaeomagnetic measurements have indicated that some sediments of glacial origin in the Neoproterozoic rock record were deposited within 10 degrees of the equator, although the accuracy of this reconstruction is in question. This palaeomagnetic location of apparently glacial sediments (such as dropstones) has been taken to suggest that glaciers extended from land to sea level in tropical latitudes at the time the sediments were deposited. It is not clear whether this implies a global glaciation or the existence of localized, possibly land-locked, glacial regimes. Others have even suggested that most data do not constrain any glacial deposits to within 25° of the equator.

Skeptics suggest that the palaeomagnetic data could be corrupted if Earth's ancient magnetic field was substantially different from today's. Depending on the rate of cooling of Earth's core, it is possible that during the Proterozoic, the magnetic field did not approximate a simple dipolar distribution, with north and south magnetic poles roughly aligning with the planet's axis as they do today. Instead, a hotter core may have circulated more vigorously and given rise to 4, 8 or more poles. Palaeomagnetic data would then have to be re-interpreted, as the sedimentary minerals could have aligned pointing to a "west pole" rather than the north magnetic pole. Alternatively, Earth's dipolar field could have been oriented such that the poles were close to the equator. This hypothesis has been posited to explain the extraordinarily rapid motion of the magnetic poles implied by the Ediacaran palaeomagnetic record; the alleged motion of the north magnetic pole would occur around the same time as the Gaskiers glaciation.

Another weakness of reliance on palaeomagnetic data is the difficulty in determining whether the magnetic signal recorded is original, or whether it has been reset by later activity. For example, a mountain-building orogeny releases hot water as a by-product of metamorphic reactions; this water can circulate to rocks thousands of kilometers away and reset their magnetic signature. This makes the authenticity of rocks older than a few million years difficult to determine without painstaking mineralogical observations. Moreover, further evidence is accumulating that large-scale remagnetization events have taken place which may necessitate revision of the estimated positions of the palaeomagnetic poles.

There is currently only one deposit, the Elatina deposit of Australia, that was indubitably deposited at low latitudes; its depositional date is well-constrained, and the signal is demonstrably original.

Low-latitude glacial deposits

Diamictite of the Neoproterozoic Pocatello Formation, a "snowball Earth"-type deposit

Elatina Fm diamictite below Ediacaran GSSP site in the Flinders Ranges NP, South Australia. A$1 coin is for scale.

Sedimentary rocks that are deposited by glaciers have distinctive features that enable their identification. Long before the advent of the snowball Earth hypothesis, many Neoproterozoic sediments had been interpreted as having a glacial origin, including some apparently at tropical latitudes at the time of their deposition. However, many sedimentary features traditionally associated with glaciers can also be formed by other means. Thus the glacial origin of many of the key occurrences for snowball Earth has been contested. As of 2007, there was only one "very reliable"—still challenged—datum point identifying tropical tillites, which makes statements of equatorial ice cover somewhat presumptuous. However, evidence of sea-level glaciation in the tropics during the Sturtian glaciation is accumulating. Evidence of possible glacial origin of sediment includes:

• Dropstones (stones dropped into marine sediments), which can be deposited by glaciers or other phenomena.
• Varves (annual sediment layers in periglacial lakes), which can form at higher temperatures.
• Glacial striations (formed by embedded rocks scraped against bedrock): similar striations are from time to time formed by mudflows or tectonic movements.
• Diamictites (poorly sorted conglomerates). Originally described as glacial till, most were in fact formed by debris flows.

Open-water deposits

It appears that some deposits formed during the snowball period could only have formed in the presence of an active hydrological cycle. Bands of glacial deposits up to 5,500 meters thick, separated by small (meters) bands of non-glacial sediments, demonstrate that glaciers melted and re-formed repeatedly for tens of millions of years; solid oceans would not permit this scale of deposition. It is considered^{[by whom?]} possible that ice streams such as seen in Antarctica today could have caused these sequences. Further, sedimentary features that could only form in open water (for example: wave-formed ripples, far-traveled ice-rafted debris and indicators of photosynthetic activity) can be found throughout sediments dating from the snowball-Earth periods. While these may represent "oases" of meltwater on a completely frozen Earth, computer modelling suggests that large areas of the ocean must have remained ice-free, arguing that a "hard" snowball is not plausible in terms of energy balance and general circulation models.

Carbon isotope ratios

There are two stable isotopes of carbon in sea water: carbon-12 (¹²C) and the rare carbon-13 (¹³C), which makes up about 1.109 percent of carbon atoms. Biochemical processes, of which photosynthesis is one, tend to preferentially incorporate the lighter ¹²C isotope. Thus ocean-dwelling photosynthesizers, both protists and algae, tend to be very slightly depleted in ¹³C, relative to the abundance found in the primary volcanic sources of Earth's carbon. Therefore, an ocean with photosynthetic life will have a lower ¹³C/¹²C ratio within organic remains and a higher ratio in corresponding ocean water. The organic component of the lithified sediments will remain very slightly, but measurably, depleted in ¹³C.

Silicate weathering, an inorganic process by which carbon dioxide is drawn out of the atmosphere and deposited in rock, also fractionates carbon. The emplacement of several large igneous provinces shortly before the Cryogenian and the subsequent chemical weathering of the enormous continental flood basalts produced by them, aided by the breakup of Rodinia that exposed many of these flood basalts to warmer, moister conditions closer to the coast and accelerated chemical weathering, is also believed to have caused a major positive shift in carbon isotopic ratios and contributed to the beginning of the Sturtian glaciation.

During the proposed episode of snowball Earth, there are rapid and extreme negative excursions in the ratio of ¹³C to ¹²C. Close analysis of the timing of ¹³C 'spikes' in deposits across the globe allows the recognition of four, possibly five, glacial events in the late Neoproterozoic.

Banded iron formations

2.1 billion-year-old rock with black-band ironstone

Banded iron formations (BIF) are sedimentary rocks of layered iron oxide and iron-poor chert. In the presence of oxygen, iron naturally rusts and becomes insoluble in water. The banded iron formations are commonly very old and their deposition is often related to the oxidation of Earth's atmosphere during the Palaeoproterozoic era, when dissolved iron in the ocean came in contact with photosynthetically produced oxygen and precipitated out as iron oxide.

The bands were produced at the tipping point between an anoxic and an oxygenated ocean. Since today's atmosphere is oxygen-rich (nearly 21% by volume) and in contact with the oceans, it is not possible to accumulate enough iron oxide to deposit a banded formation. The only extensive iron formations that were deposited after the Palaeoproterozoic (after 1.8 billion years ago) are associated with Cryogenian glacial deposits.

For such iron-rich rocks to be deposited there would have to be anoxia in the ocean, so that much dissolved iron (as ferrous oxide) could accumulate before it met an oxidant that would precipitate it as ferric oxide. For the ocean to become anoxic it must have limited gas exchange with the oxygenated atmosphere. Proponents of the hypothesis argue that the reappearance of BIF in the sedimentary record is a result of limited oxygen levels in an ocean sealed by sea-ice. Near the end of a glaciation period, a reestablishment of gas exchange between the ocean and atmosphere oxidised seawater rich in ferrous iron would occur. A positive shift in δ⁵⁶Fe_IRMM-014 from the lower to upper layers of Cryogenian BIFs may reflect an increase in ocean acidification, as the upper layers were deposited as more and more oceanic ice cover melted away and more carbon dioxide was dissolved by the ocean.

Opponents of the hypothesis suggest that the rarity of the BIF deposits may indicate that they formed in inland seas. Being isolated from the oceans, such lakes could have been stagnant and anoxic at depth, much like today's Black Sea; a sufficient input of iron could provide the necessary conditions for BIF formation. A further difficulty in suggesting that BIFs marked the end of the glaciation is that they are found interbedded with glacial sediments; such interbedding has been suggested to be an artefact of Milankovitch cycles, which would have periodically warmed the seas enough to allow gas exchange between the atmosphere and ocean and precipitate BIFs.

Cap carbonate rocks

A present-day glacier

Around the top of Neoproterozoic glacial deposits there is commonly a sharp transition into a chemically precipitated sedimentary limestone or dolomite metres to tens of metres thick. These cap carbonates sometimes occur in sedimentary successions that have no other carbonate rocks, suggesting that their deposition is result of a profound aberration in ocean chemistry.

Volcanoes may have had a role in replenishing CO₂, possibly ending the global ice age of the Cryogenian Period.

These cap carbonates have unusual chemical composition as well as strange sedimentary structures that are often interpreted as large ripples. The formation of such sedimentary rocks could be caused by a large influx of positively charged ions, as would be produced by rapid weathering during the extreme greenhouse following a snowball Earth event. The δ¹³C isotopic signature of the cap carbonates is near −5 ‰, consistent with the value of the mantle—such a low value could be taken to signify an absence of life, since photosynthesis usually acts to raise the value; alternatively the release of methane deposits could have lowered it from a higher value and counterbalance the effects of photosynthesis.

The mechanism involved in the formation of cap carbonates is not clear, but the most cited explanation suggests that at the melting of a snowball Earth, water would dissolve the abundant CO₂ from the atmosphere to form carbonic acid, which would fall as acid rain. This would weather exposed silicate and carbonate rock (including readily attacked glacial debris), releasing large amounts of calcium, which when washed into the ocean would form distinctively textured layers of carbonate sedimentary rock. Such an abiotic "cap carbonate" sediment can be found on top of the glacial till that gave rise to the snowball Earth hypothesis.

However, there are some problems with the designation of a glacial origin to cap carbonates. The high carbon dioxide concentration in the atmosphere would cause the oceans to become acidic and dissolve any carbonates contained within—starkly at odds with the deposition of cap carbonates. The thickness of some cap carbonates is far above what could reasonably be produced in the relatively quick deglaciations. The cause is further weakened by the lack of cap carbonates above many sequences of clear glacial origin at a similar time and the occurrence of similar carbonates within the sequences of proposed glacial origin. An alternative mechanism, which may have produced the Doushantuo cap carbonate at least, is the rapid, widespread release of methane. This accounts for incredibly low—as low as −48 ‰—δ¹³C values—as well as unusual sedimentary features which appear to have been formed by the flow of gas through the sediments.

Changing acidity

Isotopes of boron suggest that the pH of the oceans dropped dramatically before and after the Marinoan glaciation. This may indicate a buildup of carbon dioxide in the atmosphere, some of which would dissolve into the oceans to form carbonic acid. Although the boron variations may be evidence of extreme climate change, they need not imply a global glaciation.

Space dust

Earth's surface is very depleted in iridium, which primarily resides in Earth's core. The only significant source of the element at the surface is cosmic particles that reach Earth. During a snowball Earth, iridium would accumulate on the ice sheets, and when the ice melted the resulting layer of sediment would be rich in iridium. An iridium anomaly has been discovered at the base of the cap carbonate formations and has been used to suggest that the glacial episode lasted for at least 3 million years, but this does not necessarily imply a global extent to the glaciation; indeed, a similar anomaly could be explained by the impact of a large meteorite.

Cyclic climate fluctuations

Using the ratio of mobile cations to those that remain in soils during chemical weathering (the chemical index of alteration), it has been shown that chemical weathering varied in a cyclic fashion within a glacial succession, increasing during interglacial periods and decreasing during cold and arid glacial periods. This pattern, if a true reflection of events, suggests that the "snowball Earths" bore a stronger resemblance to Pleistocene ice age cycles than to a completely frozen Earth.

In addition, glacial sediments of the Port Askaig Tillite Formation in Scotland clearly show interbedded cycles of glacial and shallow marine sediments. The significance of these deposits is highly reliant upon their dating. Glacial sediments are difficult to date, and the closest dated bed to the Port Askaig group is 8 km stratigraphically above the beds of interest. Its dating to 600 Ma means the beds can be tentatively correlated to the Sturtian glaciation, but they may represent the advance or retreat of a snowball Earth.

Mechanisms

One computer simulation of conditions during a snowball Earth period

The initiation of a snowball Earth event would involve some initial cooling mechanism, which would result in an increase in Earth's coverage of snow and ice. The increase in Earth's coverage of snow and ice would in turn increase Earth's albedo, which would result in positive feedback for cooling. If enough snow and ice accumulates, run-away cooling would result. This positive feedback is facilitated by an equatorial continental distribution, which would allow ice to accumulate in the regions closer to the equator, where solar radiation is most direct.

Many possible triggering mechanisms could account for the beginning of a snowball Earth, such as a reduction in the atmospheric concentration of greenhouse gases including methane and/or carbon dioxide, the eruption of a supervolcano, changes in Solar energy output, or perturbations of Earth's orbit. The start of the Cryogenian period is known to correspond with a rapid rise in atmospheric oxygen known as the Neoproterozoic Oxygenation Event (NOE). This rise in atmospheric oxygen resulted in a reduction in atmospheric methane, a potent greenhouse gas. Regardless of the trigger, initial cooling results in an increase in the area of Earth's surface covered by ice and snow, and the additional ice and snow reflects more solar energy back to space, further cooling Earth and further increasing the area of Earth's surface covered by ice and snow. This positive feedback loop could eventually produce a frozen equator as cold as modern Antarctica.

Global warming associated with large accumulations of carbon dioxide in the atmosphere over millions of years, emitted primarily by volcanic activity, is the proposed trigger for melting a snowball Earth. Due to positive feedback for melting, the eventual melting of the snow and ice covering most of Earth's surface would require as little as a millennium.

Initiation of glaciation

A tropical distribution of the continents is, perhaps counter-intuitively, necessary to allow the initiation of a snowball Earth. Tropical continents are more reflective than open ocean and so absorb less of the Sun's heat: most absorption of solar energy on Earth today occurs in tropical oceans. Further, tropical continents are subject to more rainfall, which leads to increased river discharge and erosion. When exposed to air, silicate rocks undergo weathering reactions which remove carbon dioxide from the atmosphere. These reactions proceed in the general form

Rock-forming mineral + CO₂ + H₂O → cations + bicarbonate + SiO₂

An example of such a reaction is the weathering of wollastonite:

CaSiO₃ + 2 CO₂ + H₂O → Ca²⁺ + SiO₂ + 2 HCO⁻
₃

The released calcium cations react with the dissolved bicarbonate in the ocean to form calcium carbonate as a chemically precipitated sedimentary rock. This transfers carbon dioxide, a greenhouse gas, from the air into the geosphere, and, in steady-state on geologic time scales, offsets the carbon dioxide emitted from volcanoes into the atmosphere.

As of 2003, a precise continental distribution during the Neoproterozoic was difficult to establish because there were too few suitable sediments for analysis.^[56] Some reconstructions point towards polar continents—which have been a feature of all other major glaciations, providing a point upon which ice can nucleate. Changes in ocean circulation patterns may then have provided the trigger of snowball Earth.

Additional factors that may have contributed to the onset of the Neoproterozoic snowball include the introduction of atmospheric free oxygen, which may have reached sufficient quantities to react with methane in the atmosphere, oxidizing it to carbon dioxide, a much weaker greenhouse gas, and a younger—thus fainter—Sun, which would have emitted 6 percent less radiation in the Neoproterozoic.

Normally, as Earth gets colder due to natural climatic fluctuations and changes in incoming solar radiation, the cooling slows these weathering reactions. As a result, less carbon dioxide is removed from the atmosphere and Earth warms as this greenhouse gas accumulates—this 'negative feedback' process limits the magnitude of cooling. During the Cryogenian, however, Earth's continents were all at tropical latitudes, which made this moderating process less effective, as high weathering rates continued on land even as Earth cooled. This caused ice to advance beyond the polar regions. Once ice advanced to within 30° of the equator, a positive feedback could ensue such that the increased reflectiveness (albedo) of the ice led to further cooling and the formation of more ice, until the whole Earth is ice-covered.

Polar continents, because of low rates of evaporation, are too dry to allow substantial carbon deposition—restricting the amount of atmospheric carbon dioxide that can be removed from the carbon cycle. A gradual rise of the proportion of the isotope ¹³C relative to ¹²C in sediments pre-dating "global" glaciation indicates that CO₂ draw-down before snowball Earths was a slow and continuous process. The start of snowball Earths are marked by a sharp downturn in the δ¹³C value of sediments, a hallmark that may be attributed to a crash in biological productivity as a result of the cold temperatures and ice-covered oceans.

In January 2016, Gernon et al. proposed a "shallow-ridge hypothesis" involving the breakup of Rodinia, linking the eruption and rapid alteration of hyaloclastites along shallow ridges to massive increases in alkalinity in an ocean with thick ice cover. Gernon et al. demonstrated that the increase in alkalinity over the course of glaciation is sufficient to explain the thickness of cap carbonates formed in the aftermath of Snowball Earth events.

Dating of the Sturtian glaciation's onset has found it to be coeval with the emplacement of a large igneous province in the tropics. Weathering of this equatorial large igneous province is believed to have sucked enough carbon dioxide out of the air to enable the development of major glaciation.

During the frozen period

Global ice sheets may have formed the bottleneck required for the evolution of multicellular life.

Global temperature fell so low that the equator was as cold as modern-day Antarctica. This low temperature was maintained by the high albedo of the ice sheets, which reflected most incoming solar energy into space. A lack of heat-retaining clouds, caused by water vapor freezing out of the atmosphere, amplified this effect. Degassing of carbon dioxide has been speculated to have been unusually low during the Cryogenian, enabling the persistence of global glaciation.

Breaking out of global glaciation

The carbon dioxide levels necessary to thaw Earth have been estimated as being 350 times what they are today, about 13% of the atmosphere. Since Earth was almost completely covered with ice, carbon dioxide could not be withdrawn from the atmosphere by release of alkaline metal ions weathering out of siliceous rocks. Over 4 to 30 million years, enough CO₂ and methane, mainly emitted by volcanoes but also produced by microbes converting organic carbon trapped under the ice into the gas, would accumulate to finally cause enough greenhouse effect to make surface ice melt in the tropics until a band of permanently ice-free land and water developed; this would be darker than the ice and thus absorb more energy from the Sun—initiating a "positive feedback".

The first areas to become free of permanent ice cover may have been in the mid-latitudes rather than in the tropics, because a rapid hydrological cycle would have inhibited the melting of ice at low latitudes. As these mid-latitude regions became ice free, dust from them blew over onto ice sheets elsewhere, decreasing their albedo and accelerating the process of deglaciation. Destabilization of substantial deposits of methane hydrates locked up in low-latitude permafrost may also have acted as a trigger and/or strong positive feedback for deglaciation and warming.

Methanogens were an important contributor to the deglaciation of the Marinoan Snowball Earth. The return of high primary productivity in surficial waters fueled extensive microbial sulphur reduction, causing deeper waters to become highly euxinic. Euxinia caused the formation of large amounts of methyl sulphides, which in turn was converted into methane by methanogens. A major negative nickel isotope excursion confirms high methanogenic activity during this period of deglaciation and global warming.

On the continents, the melting of glaciers would release massive amounts of glacial deposit, which would erode and weather. The resulting sediments supplied to the ocean would be high in nutrients such as phosphorus, which combined with the abundance of CO₂ would trigger a cyanobacteria population explosion, which would cause a relatively rapid reoxygenation of the atmosphere and may have contributed to the rise of the Ediacaran biota and the subsequent Cambrian explosion—a higher oxygen concentration allowing large multicellular lifeforms to develop. Although the positive feedback loop would melt the ice in geological short order, perhaps less than 1,000 years, replenishment of atmospheric oxygen and depletion of the CO₂ levels would take further millennia.

It is possible that carbon dioxide levels fell enough for Earth to freeze again; this cycle may have repeated until the continents had drifted to more polar latitudes. More recent evidence suggests that with colder oceanic temperatures, the resulting higher ability of the oceans to dissolve gases led to the carbon content of sea water being more quickly oxidized to carbon dioxide. This leads directly to an increase of atmospheric carbon dioxide, enhanced greenhouse warming of Earth's surface, and the prevention of a total snowball state.

During millions of years, cryoconite would have accumulated on and inside the ice. Psychrophilic microorganisms, volcanic ash and dust from ice-free locations would settle on ice covering several million square kilometers. Once the ice started to melt, these layers would become visible and darken the icy surfaces, helping to accelerate the process. Also, ultraviolet light from the Sun produced hydrogen peroxide (H₂O₂) when it hit water molecules. Normally H₂O₂ breaks down in sunlight, but some would have been trapped inside the ice. When the glaciers started to melt, it would have been released in both the ocean and the atmosphere, where it was split into water and oxygen molecules, increasing atmospheric oxygen.

Slushball Earth hypothesis

While the presence of glaciers is not disputed, the idea that the entire planet was covered in ice is more contentious, leading some scientists to posit a "slushball Earth", in which a band of ice-free, or ice-thin, waters remains around the equator, allowing for a continued hydrologic cycle. This hypothesis appeals to scientists who observe certain features of the sedimentary record that can only be formed under open water or rapidly moving ice (which would require somewhere ice-free to move to). Recent research observed geochemical cyclicity in clastic rocks, showing that the snowball periods were punctuated by warm spells, similar to ice age cycles in recent Earth history. Attempts to construct computer models of a snowball Earth have struggled to accommodate global ice cover without fundamental changes in the laws and constants which govern the planet.

A less extreme snowball Earth hypothesis involves continually evolving continental configurations and changes in ocean circulation. Synthesised evidence has produced slushball Earth models where the stratigraphic record does not permit postulating complete global glaciations. Kirschvink's original hypothesis had recognised that warm tropical puddles would be expected to exist in a snowball Earth.

A more extreme hypothesis, the Waterbelt Earth hypothesis, suggests that ice-free areas of ocean continued to exist even as tropical continents were glaciated.

Scientific dispute

The argument against the hypothesis is evidence of fluctuation in ice cover and melting during "snowball Earth" deposits. Evidence for such melting comes from evidence of glacial dropstones, geochemical evidence of climate cyclicity, and interbedded glacial and shallow marine sediments. A longer record from Oman, constrained to 13°N, covers the period from 712 to 545 million years ago—a time span containing the Sturtian and Marinoan glaciations—and shows both glacial and ice-free deposition. The snowball Earth hypothesis does not explain the alternation of glacial and interglacial events, nor the oscillation of glacial sheet margins.

There have been difficulties in recreating a snowball Earth with global climate models. Simple GCMs with mixed-layer oceans can be made to freeze to the equator; a more sophisticated model with a full dynamic ocean (though only a primitive sea ice model) failed to form sea ice to the equator. In addition, the levels of CO₂ necessary to melt a global ice cover have been calculated to be 130,000 ppm, which is considered by some to be unreasonably large.

Strontium isotopic data have been found to be at odds with proposed snowball Earth models of silicate weathering shutdown during glaciation and rapid rates immediately post-glaciation. Therefore, methane release from permafrost during marine transgression was proposed to be the source of the large measured carbon excursion in the time immediately after glaciation.

"Zipper rift" hypothesis

Nick Eyles suggests that the Neoproterozoic Snowball Earth was in fact no different from any other glaciation in Earth's history, and that efforts to find a single cause are likely to end in failure. The "zipper rift" hypothesis proposes two pulses of continental "unzipping"—first, the breakup of Rodinia, forming the proto-Pacific Ocean; then the splitting of the continent Baltica from Laurentia, forming the proto-Atlantic—coincided with the glaciated periods. The associated tectonic uplift would form high plateaus, just as the East African Rift is responsible for high topography; this high ground could then host glaciers.

Banded iron formations have been taken as unavoidable evidence for global ice cover, since they require dissolved iron ions and anoxic waters to form; however, the limited extent of the Neoproterozoic banded iron deposits means that they may have formed in inland seas rather than in frozen oceans. Such seas can experience a wide range of chemistries; high rates of evaporation could concentrate iron ions, and a periodic lack of circulation could allow anoxic bottom water to form. Continental rifting, with associated subsidence, tends to produce such landlocked water bodies. This rifting, and associated subsidence, would produce the space for the fast deposition of sediments, negating the need for an immense and rapid melting to raise the global sea levels.

High-obliquity hypothesis

A competing hypothesis to explain the presence of ice on the equatorial continents was that Earth's axial tilt was quite high, in the vicinity of 60°, which would place Earth's land in high "latitudes", although supporting evidence is scarce. A less extreme possibility would be that it was merely Earth's magnetic pole that wandered to this inclination, as the magnetic readings which suggested ice-filled continents depend on the magnetic and rotational poles being relatively similar. In either of these two situations, the freeze would be limited to relatively small areas, as is the case today; severe changes to Earth's climate are not necessary.

Inertial interchange true polar wander

The evidence for low-latitude glacial deposits during the supposed snowball Earth episodes has been reinterpreted via the concept of inertial interchange true polar wander. This hypothesis, created to explain palaeomagnetic data, suggests that Earth's orientation relative to its axis of rotation shifted one or more times during the general time-frame attributed to snowball Earth. This could feasibly produce the same distribution of glacial deposits without requiring any of them to have been deposited at equatorial latitude. While the physics behind the proposition is sound, the removal of one flawed data point from the original study rendered the application of the concept in these circumstances unwarranted.

Survival of life through frozen periods

A black smoker, a type of hydrothermal vent

A tremendous glaciation would curtail photosynthetic life on Earth, thus depleting atmospheric oxygen, and thereby allowing non-oxidized iron-rich rocks to form. Detractors argue that this kind of glaciation would have made life extinct entirely. However, microfossils such as stromatolites and oncolites prove that, in shallow marine environments at least, life did not suffer any perturbation. Instead life developed a trophic complexity and survived the cold period unscathed. Proponents counter that it may have been possible for life to survive in these ways:

• In reservoirs of anaerobic and low-oxygen life powered by chemicals in deep oceanic hydrothermal vents surviving in Earth's deep oceans and crust; but photosynthesis would not have been possible there.
• Under the ice layer, in chemolithotrophic (mineral-metabolizing) ecosystems theoretically resembling those in existence in modern glacier beds, high-alpine and Arctic talus permafrost, and basal glacial ice. This is especially plausible in areas of volcanism or geothermal activity.
• In pockets of liquid water within and under the ice caps, similar to Lake Vostok in Antarctica. In theory, this system may resemble microbial communities living in the perennially frozen lakes of the Antarctic dry valleys. Photosynthesis can occur under ice up to 20 m thick, and at the temperatures predicted by models, equatorial sublimation would prevent equatorial ice thickness from exceeding 10 m.
• As eggs and dormant cells and spores deep-frozen into ice during the most severe phases of the frozen period.
• In small regions of open sea water: polynya. These natural ice holes can occur from the action of winds, currents or a local heat source (e.g. geothermal), even if the surrounding sea is completely frozen over. They could preserve enclaves of photosynthesizers with access to light and CO₂ to generate trace amounts of oxygen, enough to sustain some oxygen-dependent organisms. It is not necessary that a hole form in the ice, merely that some parts of the ice become thin enough to admit light. These small regions may have occurred in deep ocean, far from Rodinia or its remnants as it broke apart and drifted on the tectonic plates.
• In layers of "dirty ice" on top of the ice sheet covering shallow seas below. Animals and mud from the sea would be frozen into the base of the ice and gradually concentrate on the top as the ice above evaporates. Small ponds of water would teem with life thanks to the flow of nutrients through the ice. Such environments may have covered approximately 12 per cent of the global surface area.
• In small oases of liquid water, as would be found near geothermal hotspots resembling Iceland today.^[93]
• In nunatak areas in the tropics, where daytime tropical sun or volcanic heat heated bare rock sheltered from cold wind and made small temporary melt pools, which would freeze at sunset.^{[citation needed]}
• Oxygenated subglacial meltwater, along with iron-rich sediments dissolved in the glacial water, generated a meltwater oxygen pump when it entered the ocean, where it provided eukaryotes with some oxygen, and both photosynthetic and chemosynthetic organisms with sufficient nutrients to support an ecosystem. The freshwater would also mix with the hypersaline seawater, which formed areas less hostile to eukaryotic life than elsewhere in the ocean.

However, organisms and ecosystems, as far as it can be determined by the fossil record, do not appear to have undergone the significant change that would be expected by a mass extinction. With the advent of more precise dating, a phytoplankton extinction event which had been associated with snowball Earth was shown to precede glaciations by 16 million years. Even if life were to cling on in all the ecological refuges listed above, a whole-Earth glaciation would result in a biota with a noticeably different diversity and composition. This change in diversity and composition has not yet been observed—in fact, the organisms which should be most susceptible to climatic variation emerge unscathed from the snowball Earth. One rebuttal to this is the fact that in many of these places where an argument is made against a mass extinction caused by snowball Earth, the Cryogenian fossil record is impoverished. Sponges, a basal group of animals managed to survive the glaciations after having emerged in the Tonian period. The diversity of animals would later grow dramatically during the Ediacaran.

Implications

A snowball Earth has profound implications in the history of life on Earth. While many refugia have been postulated, global ice cover would certainly have ravaged ecosystems dependent on sunlight. Geochemical evidence from rocks associated with low-latitude glacial deposits have been interpreted to show a crash in oceanic life during the glacials. High magnitude glacial retreats favoured the survival of macroalgae.

Because about half of the oceans' water was frozen solid as ice, the remaining water would be twice as salty as it is today, lowering its freezing point. When the ice sheet melted under a hot atmosphere rich in carbon dioxide, it would cover the oceans with a layer of warm (50°C) freshwater up to 2 kilometres thick. Only after the warm surface water mixed with the colder and deeper saltwater did the sea return to a warmer and less salty state. The melting of the ice may have presented many new opportunities for diversification, and may indeed have driven the rapid evolution which took place at the end of the Cryogenian period. Global ice cover, if it existed, may—in concert with geothermal heating—have led to a lively, well mixed ocean with great vertical convective circulation.

Effect on early evolution

Dickinsonia costata, an Ediacaran organism of unknown affinity, with a quilted appearance

The Neoproterozoic was a time of remarkable diversification of multicellular organisms, including animals. Organism size and complexity increased considerably after the end of the snowball glaciations. This rapid development of multicellular organisms may have been the result of increased evolutionary pressures resulting from multiple icehouse-hothouse cycles; in this sense, snowball Earth episodes may have "pumped" evolution, much as glaciations during the Pleistocene are known to have acted as a diversity pump in Antarctica. Alternatively, fluctuating copper levels and rising oxygen may have played a part. Many Sturtian diamictites unconformably overlie copper-mineralised strata in Greenland, North America, Australia, and Africa; the glacial breakup and erosion of rocks heavily enriched in copper during the Sturtian glaciation, combined with the chemical weathering of the Franklin Large Igneous Province, greatly elevated copper concentrations in the ocean. Because copper is an essential component of many proteins involved in mitigating oxygen toxicity, synthesising adenosine triphosphate, and producing elastin and collagen, among other biological functions, this spike in copper concentrations was essential to the explosive evolution of multicellular life throughout the latter portion of the Neoproterozoic. Elevated copper concentrations persisted into the Cambrian explosion at the beginning of the Phanerozoic and likely influenced its course too.

One hypothesis which has been gaining currency in recent years: that early snowball Earths did not so much affect the evolution of life on Earth as result from it. In fact the two hypotheses are not mutually exclusive. The idea is that Earth's life forms affect the global carbon cycle and so major evolutionary events alter the carbon cycle, redistributing carbon within various reservoirs within the biosphere system and in the process temporarily lowering the atmospheric (greenhouse) carbon reservoir until the revised biosphere system settled into a new state. The cool period of the Huronian glaciation is speculated to be linked to the decline in the atmospheric content of greenhouse gases during the Great Oxygenation Event. Similarly, the possible snowball Earth of the Precambrian's Cryogenian between 580 and 850 million years ago (and which itself had a number of distinct episodes) could be related to the rise of more advanced multicellular animal life and life's colonisation of the land. However, a 2022 study, based on findings of previous studies, suggested land plant evolution was driven by the Cryogenian glaciations, which they also theorized to be the reason why the Zygnematophyceae (sister group of land plants) became unicellular and cryophilic, lost their flagella and evolved sexual conjugation.

Occurrence and timing

Palaeoproterozoic

Main article: Huronian glaciation

The Snowball Earth hypothesis has been invoked to explain glacial deposits in the Huronian Supergroup of Canada, though the palaeomagnetic evidence that suggests ice sheets at low latitudes is contested, and stratigraphic evidence clearly shows only three distinct depositions of glacial material (the Ramsay, Bruce and Gowganda Formations) separated by significant periods without. The glacial sediments of the Makganyene formation of South Africa are slightly younger than the Huronian glacial deposits (~2.25 billion years old) and were possibly deposited at tropical latitudes. It has been proposed that rise of free oxygen that occurred during the Great Oxygenation Event removed atmospheric methane through oxidation. As the solar irradiance was notably weaker at the time, Earth's climate may have relied on methane, a powerful greenhouse gas, to maintain surface temperatures above freezing. In the absence of this methane greenhousing, temperatures plunged and a global glaciation could have occurred between 2.5 and 2.2 Gya, during the Siderian and Rhyacian periods of the Paleoproterozoic era.

Neoproterozoic

Main articles: Sturtian glaciation, Marinoan glaciation, and Baykonurian glaciation

There were three or four significant ice ages during the late Neoproterozoic. Of these, the Marinoan was the most significant, and the Sturtian glaciations were also widespread. Even the leading snowball proponent Hoffman agrees that the 350 thousand-year-long Gaskiers glaciation did not lead to global glaciation, although it was probably as intense as the late Ordovician glaciation. The status of the Kaigas "glaciation" or "cooling event" is currently unclear; some scientists do not recognise it as a glacial, others suspect that it may reflect poorly dated strata of Sturtian association, and others believe it may indeed be a third ice age. It was certainly less significant than the Sturtian or Marinoan glaciations, and probably not global in extent. Emerging evidence suggests that Earth underwent a number of glaciations during the Neoproterozoic, which would stand strongly at odds with the snowball hypothesis.