Germ theory of disease

 From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Germ_theory_of_disease

Scanning electron microscope image of Vibrio cholerae, the bacterium that causes cholera

The germ theory of disease is the currently accepted scientific theory explaining the cause of infectious diseases. It states that microorganisms known as pathogens or "germs" can cause disease. These small organisms, which are too small to be seen without magnification, invade animals, plants, and even bacteria. Their growth and reproduction within their hosts can cause disease. "Germ" refers not just to bacteria but to any type of microorganism, such as protists or fungi, or other pathogens, including parasites, viruses, prions, or viroids. Even when a pathogen is the principal cause of a disease, environmental and hereditary factors often influence the severity of the disease, and whether a potential host individual becomes infected when exposed to the pathogen. Pathogens are disease-causing agents that can pass from one individual to another, across multiple domains of life.

Basic forms of germ theory were proposed by Girolamo Fracastoro in 1546, and expanded upon by Marcus von Plenciz in 1762. However, such views were held in disdain in Europe, where Galen's miasma theory remained dominant among scientists and doctors.

By the early 19th century, the first vaccine, smallpox vaccination, was commonplace in Europe, though doctors were unaware of how it worked or how to extend the principle to other diseases. A transitional period began in the late 1850s with the work of Louis Pasteur. This work was later extended by Robert Koch in the 1880s. By the end of that decade, the miasma theory was struggling to compete with the germ theory of disease. Viruses were initially discovered in the 1890s. Eventually, a "golden era" of bacteriology ensued, during which the germ theory quickly led to the identification of the actual organisms that cause many diseases.

Miasma theory

Main article: Miasma theory

A representation by Robert Seymour of the cholera epidemic depicts the spread of the disease in the form of poisonous air.

The miasma theory was the predominant theory of disease transmission before the germ theory took hold towards the end of the 19th century; it is no longer accepted as a correct explanation for disease by the scientific community. It held that diseases such as cholera, chlamydia infection, or the Black Death were caused by a miasma (μίασμα, Ancient Greek: "pollution"), a noxious form of "bad air" emanating from rotting organic matter. Miasma was considered to be a poisonous vapor or mist filled with particles from decomposed matter (miasmata) that was identifiable by its foul smell. The theory posited that diseases were the product of environmental factors such as contaminated water, foul air, and poor hygienic conditions. Such infections, according to the theory, were not passed between individuals but would affect those within a locale that gave rise to such vapors.

Development of germ theory

Greece and Rome

In Antiquity, the Greek historian Thucydides (c. 460 – c. 400 BC) was the first known person to write, in his account of the plague of Athens, that diseases could spread from an infected person to others.

One theory of the spread of contagious diseases that were not spread by direct contact was that they were spread by spore-like "seeds" (Latin: semina) that were present in and dispersible through the air. In his poem, De rerum natura (On the Nature of Things, c. 56 BC), the Roman poet Lucretius (c. 99 BC – c. 55 BC) stated that the world contained various "seeds", some of which could sicken a person if they were inhaled or ingested.

The Roman statesman Marcus Terentius Varro (116–27 BC) wrote, in his Rerum rusticarum libri III (Three Books on Agriculture, 36 BC): "Precautions must also be taken in the neighborhood of swamps... because there are bred certain minute creatures which cannot be seen by the eyes, which float in the air and enter the body through the mouth and nose and there cause serious diseases."

The Greek physician Galen (AD 129 – c. 200/216) speculated in his On Initial Causes (c. 175 AD) that some patients might have "seeds of fever". In his On the Different Types of Fever (c. 175 AD), Galen speculated that plagues were spread by "certain seeds of plague", which were present in the air. And in his Epidemics (c. 176–178 AD), Galen explained that patients might relapse during recovery from fever because some "seed of the disease" lurked in their bodies, which would cause a recurrence of the disease if the patients did not follow a physician's therapeutic regimen.

The Middle Ages

A hybrid form of miasma and contagion theory was proposed by Persian physician Ibn Sina (known as Avicenna in Europe) in The Canon of Medicine (1025). He mentioned that people can transmit disease to others by breath, noted contagion with tuberculosis, and discussed the transmission of disease through water and dirt.

During the early Middle Ages, Isidore of Seville (c. 560–636) mentioned "plague-bearing seeds" (pestifera semina) in his On the Nature of Things (c. AD 613). Later, in 1345, Tommaso del Garbo (c. 1305–1370) of Bologna, Italy mentioned Galen's "seeds of plague" in his work Commentaria non-parum utilia in libros Galeni (Helpful commentaries on the books of Galen).

The 16th century Reformer Martin Luther appears to have had some idea of the contagion theory, commenting, "I have survived three plagues and visited several people who had two plague spots which I touched. But it did not hurt me, thank God. Afterwards when I returned home, I took up Margaret," (born 1534), "who was then a baby, and put my unwashed hands on her face, because I had forgotten; otherwise I should not have done it, which would have been tempting God." In 1546, Italian physician Girolamo Fracastoro published De Contagione et Contagiosis Morbis (On Contagion and Contagious Diseases), a set of three books covering the nature of contagious diseases, categorization of major pathogens, and theories on preventing and treating these conditions. Fracastoro blamed "seeds of disease" that propagate through direct contact with an infected host, indirect contact with fomites, or through particles in the air.

The Early Modern Period

In 1668, Italian physician Francesco Redi published experimental evidence rejecting spontaneous generation, the theory that living creatures arise from nonliving matter. He observed that maggots only arose from rotting meat that was uncovered. When meat was left in jars covered by gauze, the maggots would instead appear on the gauze's surface, later understood as rotting meat's smell passing through the mesh to attract flies that laid eggs.

Microorganisms are said to have been first directly observed in the 1670s by Anton van Leeuwenhoek, an early pioneer in microbiology, considered "the Father of Microbiology". Leeuwenhoek is said to be the first to see and describe bacteria in 1674, yeast cells, the teeming life in a drop of water (such as algae), and the circulation of blood corpuscles in capillaries. The word "bacteria" did not exist yet, so he called these microscopic living organisms "animalcules", meaning "little animals". Those "very little animalcules" he was able to isolate from different sources, such as rainwater, pond and well water, and the human mouth and intestine.

Yet German Jesuit priest and scholar Athanasius Kircher (historically often misspelled as "Kirchner") may have observed such microorganisms prior to this. One of his books written in 1646 contains a chapter in Latin, which reads in translation: "Concerning the wonderful structure of things in nature, investigated by microscope...who would believe that vinegar and milk abound with an innumerable multitude of worms." Kircher defined the invisible organisms found in decaying bodies, meat, milk, and secretions as "worms." His studies with the microscope led him to the belief, which he was possibly the first to hold, that disease and putrefaction, or decay were caused by the presence of invisible living bodies, writing that "a number of things might be discovered in the blood of fever patients." When Rome was struck by the bubonic plague in 1656, Kircher investigated the blood of plague victims under the microscope. He noted the presence of "little worms" or "animalcules" in the blood and concluded that the disease was caused by microorganisms.

Kircher was the first to attribute infectious disease to a microscopic pathogen, inventing the germ theory of disease, which he outlined in his Scrutinium Physico-Medicum, published in Rome in 1658. Kircher's conclusion that disease was caused by microorganisms was correct, although it is likely that what he saw under the microscope were in fact red or white blood cells and not the plague agent itself. Kircher also proposed hygienic measures to prevent the spread of disease, such as isolation, quarantine, burning clothes worn by the infected, and wearing facemasks to prevent the inhalation of germs. It was Kircher who first proposed that living beings enter and exist in the blood.

In the 18th century, more proposals were made, but struggled to catch on. In 1700, physician Nicolas Andry argued that microorganisms he called "worms" were responsible for smallpox and other diseases. In 1720, Richard Bradley theorised that the plague and "all pestilential distempers" were caused by "poisonous insects", living creatures viewable only with the help of microscopes.

In 1762, the Austrian physician Marcus Antonius von Plenciz (1705–1786) published a book titled Opera medico-physica. It outlined a theory of contagion stating that specific animalcules in the soil and the air were responsible for causing specific diseases. Von Plenciz noted the distinction between diseases which are both epidemic and contagious (like measles and dysentery), and diseases which are contagious but not epidemic (like rabies and leprosy). The book cites Anton van Leeuwenhoek to show how ubiquitous such animalcules are and was unique for describing the presence of germs in ulcerating wounds. Ultimately, the theory espoused by von Plenciz was not accepted by the scientific community.

19th and 20th centuries

Main article: Germ theory's key 19th century figures

Agostino Bassi, Italy

During the early 19th century, driven by economic concerns over collapsing silk production, Italian entomologist Agostino Bassi researched a silkworm disease known as "muscardine" in French and "calcinaccio" or "mal del segno" in Italian, causing white fungal spots along the caterpillar. From 1835 to 1836, Bassi published his findings that fungal spores transmitted the disease between individuals. In recommending the rapid removal of diseased caterpillars and disinfection of their surfaces, Bassi outlined methods used in modern preventative healthcare. Italian naturalist Giuseppe Gabriel Balsamo-Crivelli named the causative fungal species after Bassi, currently classified as Beauveria bassiana.

Louis-Daniel Beauperthuy, France

In 1838 French specialist in tropical medicine Louis-Daniel Beauperthuy pioneered using microscopy in relation to diseases and independently developed a theory that all infectious diseases were due to parasitic infection with "animalcules" (microorganisms). With the help of his friend M. Adele de Rosseville, he presented his theory in a formal presentation before the French Academy of Sciences in Paris. By 1853, he was convinced that malaria and yellow fever were spread by mosquitos. He even identified the particular group of mosquitos that transmit yellow fever as the "domestic species" of "striped-legged mosquito", which can be recognised as Aedes aegypti, the actual vector. He published his theory in 1854 in the Gaceta Oficial de Cumana ("Official Gazette of Cumana"). His reports were assessed by an official commission, which discarded his mosquito theory.

Ignaz Semmelweis, Austria

Ignaz Semmelweis, a Hungarian obstetrician working at the Vienna General Hospital (Allgemeines Krankenhaus) in 1847, noticed the dramatically high maternal mortality from puerperal fever following births assisted by doctors and medical students. However, those attended by midwives were relatively safe. Investigating further, Semmelweis made the connection between puerperal fever and examinations of delivering women by doctors, and further realized that these physicians had usually come directly from autopsies. Asserting that puerperal fever was a contagious disease and that matter from autopsies was implicated in its spread, Semmelweis made doctors wash their hands with chlorinated lime water before examining pregnant women. He then documented a sudden reduction in the mortality rate from 18% to 2.2% over a period of a year. Despite this evidence, he and his theories were rejected by most of the contemporary medical establishment.

Gideon Mantell, UK

Gideon Mantell, the Sussex doctor more famous for discovering dinosaur fossils, spent time with his microscope, and speculated in his Thoughts on Animalcules (1850) that perhaps "many of the most serious maladies which afflict humanity, are produced by peculiar states of invisible animalcular life".

John Snow, UK

Main article: 1854 Broad Street cholera outbreak

British physician John Snow is credited as a founder of modern epidemiology for studying the 1854 Broad Street cholera outbreak. Snow criticized the Italian anatomist Giovanni Maria Lancisi for his early 18th century writings that claimed swamp miasma spread malaria, rebutting that bad air from decomposing organisms was not present in all cases. In his 1849 pamphlet On the Mode of Communication of Cholera, Snow proposed that cholera spread through the fecal–oral route, replicating in human lower intestines.

In the book's second edition, published in 1855, Snow theorized that cholera was caused by cells smaller than human epithelial cells, leading to Robert Koch's 1884 confirmation of the bacterial species Vibrio cholerae as the causative agent. In recognizing a biological origin, Snow recommended boiling and filtering water, setting the precedent for modern boil-water advisory directives.

Through a statistical analysis tying cholera cases to specific water pumps associated with the Southwark and Vauxhall Waterworks Company, which supplied sewage-polluted water from the River Thames, Snow showed that areas supplied by this company experienced fourteen times as many deaths as residents using Lambeth Waterworks Company pumps that obtained water from the upriver, cleaner Seething Wells. While Snow received praise for convincing the Board of Guardians of St James's Parish to remove the handles of contaminated pumps, he noted that the outbreak's cases were already declining as scared residents fled the region.

Louis Pasteur, France

Louis Pasteur's spontaneous generation experiment illustrates that liquid nutrients are spoiled by particles in the air rather than the air itself. These results of these experiments supported the germ theory of disease.

During the mid-19th century, French microbiologist Louis Pasteur showed that treating the female genital tract with boric acid killed the microorganisms causing postpartum infections while avoiding damage to mucous membranes.

Building on Redi's work, Pasteur disproved spontaneous generation by constructing swan neck flasks containing nutrient broth. Since the flask contents were only fermented when in direct contact with the external environment's air by removing the curved tubing, Pasteur demonstrated that bacteria must travel between sites of infection to colonize environments.

Similar to Bassi, Pasteur extended his research on germ theory by studying pébrine, a disease that causes brown spots on silkworms. While Swiss botanist Carl Nägeli discovered the fungal species Nosema bombycis in 1857, Pasteur applied the findings to recommend improved ventilation and screening of silkworm eggs, an early form of disease surveillance.

Robert Koch, Germany

In 1884, German bacteriologist Robert Koch published four criteria for establishing causality between specific microorganisms and diseases, now known as Koch's postulates:

1. The microorganism must be found in abundance in all organisms with the disease, but should not be found in healthy organisms.
2. The microorganism must be isolated from a diseased organism and grown in pure culture.
3. The cultured microorganism should cause disease when introduced into a healthy organism.
4. The microorganism must be re-isolated from the inoculated, diseased experimental host and identified as being identical to the original specific causative agent.

During his lifetime, Koch recognized that the postulates were not universally applicable, such as asymptomatic carriers of cholera violating the first postulate. For this same reason, the third postulate specifies "should", rather than "must", because not all host organisms exposed to an infectious agent will acquire the infection, potentially due to differences in prior exposure to the pathogen. Limiting the second postulate, it was later discovered that viruses cannot be grown in pure cultures because they are obligate intracellular parasites, making it impossible to fulfill the second postulate. Similarly, pathogenic misfolded proteins, known as prions, only spread by transmitting their structure to other proteins, rather than self-replicating.

While Koch's postulates retain historical importance for emphasizing that correlation does not imply causation, many pathogens are accepted as causative agents of specific diseases without fulfilling all of the criteria. In 1988, American microbiologist Stanley Falkow published a molecular version of Koch's postulates to establish correlation between microbial genes and virulence factors.

Joseph Lister, UK

After reading Pasteur's papers on bacterial fermentation, British surgeon Joseph Lister recognized that compound fractures, involving bones breaking through the skin, were more likely to become infected due to exposure to environmental microorganisms. He recognized that carbolic acid could be applied to the site of injury as an effective antiseptic.

Universal Darwinism

https://en.wikipedia.org/wiki/Universal_Darwinism
From Wikipedia, the free encyclopedia

 Universal Darwinism is a variety of approaches that extend the theory of Darwinism beyond its original domain of biological evolution on Earth. Universal Darwinism aims to formulate a generalized version of the mechanisms of variation, selection and heredity proposed by Charles Darwin, so that they can apply to explain evolution in a wide variety of other domains, including psychology, linguistics, economics, culture, medicine, computer science, and physics.

Examples of patterns that have been postulated to undergo variation and selection, and thus adaptation, are genes, ideas (memes), theories, technologies, neurons and their connections, words, computer programs, firms, antibodies, institutions, law and judicial systems, quantum states and even whole universes.

History and development

Conceptually, "evolutionary theorizing about cultural, social, and economic phenomena" preceded Darwin, but was still lacking the concept of natural selection.

Starting in the 1950s, Donald T. Campbell was one of the first and most influential authors to revive the tradition, and to formulate a generalized Darwinian algorithm directly applicable to phenomena outside of biology. In this, he was inspired by William Ross Ashby's view of self-organization and intelligence as fundamental processes of selection. His aim was to explain the development of science and other forms of knowledge by focusing on the variation and selection of ideas and theories, thus laying the basis for the domain of evolutionary epistemology. In the 1990s, Campbell's formulation of the mechanism of "blind-variation-and-selective-retention" (BVSR) was further developed and extended to other domains under the labels of "universal selection theory" or "universal selectionism" by his disciples Gary Cziko, Mark Bickhard, and Francis Heylighen.

Richard Dawkins may have first coined the term "universal Darwinism" in 1983 to describe his conjecture that any possible life forms existing outside the Solar System would evolve by natural selection just as they do on Earth. This conjecture was also presented in 1983 in a paper entitled “The Darwinian Dynamic” that dealt with the evolution of order in living systems and certain nonliving physical systems. It was suggested “that ‘life’, wherever it might exist in the universe, evolves according to the same dynamical law” termed the Darwinian dynamic. Henry Plotkin in his 1997 book on Darwin machines makes the link between universal Darwinism and Campbell's evolutionary epistemology.

The philosopher of mind Daniel Dennett, in his 1995 book Darwin's Dangerous Idea, developed the idea of a Darwinian process, involving variation, selection and retention, as a generic algorithm that is substrate-neutral and could be applied to many fields of knowledge outside of biology. He described the idea of natural selection as a "universal acid" that cannot be contained in any vessel, as it seeps through the walls and spreads ever further, touching and transforming ever more domains. He notes in particular the field of memetics in the social sciences.

In agreement with Dennett's prediction, over the past decades the Darwinian perspective has spread ever more widely, in particular across the social sciences as the foundation for numerous schools of study including memetics, evolutionary economics, evolutionary psychology, evolutionary anthropology, neural Darwinism, and evolutionary linguistics. Researchers have postulated Darwinian processes as operating at the foundations of physics, cosmology and chemistry via the theories of quantum Darwinism, observation selection effects and cosmological natural selection.

Author D. B. Kelley has formulated one of the most all-encompassing approaches to universal Darwinism. In his 2013 book The Origin of Everything, he holds that natural selection involves not the preservation of favored races in the struggle for life, as shown by Darwin, but the preservation of favored systems in contention for existence. The fundamental mechanism behind all such stability and evolution is therefore what Kelley calls "survival of the fittest systems." Because all systems are cyclical, the Darwinian processes of iteration, variation and selection are operative not only among species but among all natural phenomena both large-scale and small. Kelley thus maintains that, since the Big Bang especially, the universe has evolved from a highly chaotic state to one that is now highly ordered with many stable phenomena, naturally selected.

Gene-based Darwinian extensions

• Evolutionary psychology assumes that our emotions, preferences and cognitive mechanisms are the product of natural selection
  • Evolutionary educational psychology applies evolutionary psychology to education
  • Evolutionary developmental psychology applies evolutionary psychology to cognitive development
  • Darwinian Happiness applies evolutionary psychology to understand the optimal conditions for human well-being
  • Darwinian literary studies tries to understand the characters and plots of narrative on the basis of evolutionary psychology
  • Evolutionary aesthetics applies evolutionary psychology to explain our sense of beauty, especially for landscapes and human bodies
  • Evolutionary musicology applies evolutionary aesthetics to music
• Evolutionary anthropology studies the evolution of human beings
• Sociobiology proposes that social systems in animals and humans are the product of Darwinian biological evolution
• Human behavioral ecology investigates how human behavior has become adapted to its environment via variation and selection
• Evolutionary medicine investigates the origin of diseases by looking at the evolution both of the human body and of its parasites
• Paleolithic diet proposes that the most healthy nutrition is the one to which our hunter-gatherer ancestors have adapted over millions of years
• Paleolithic lifestyle generalizes the Paleolithic diet to include exercise, behavior and exposure to the environment
• Molecular evolution studies evolution at the level of DNA, RNA and proteins
• Biosocial criminology studies crime using several different approaches that include genetics and evolutionary psychology
• Evolutionary linguistics studies the evolution of language, biologically as well as culturally

Other Darwinian extensions

• Quantum Darwinism sees the emergence of classical states in physics as a natural selection of the most stable quantum properties
• Cosmological natural selection hypothesizes that universes reproduce and are selected for having fundamental constants that maximize "fitness"
• Complex adaptive systems models the dynamics of complex systems in part on the basis of the variation and selection of its components
• Evolutionary archaeology is a Darwinian approach to the cultural evolution of tools
• Evolutionary computation is a Darwinian approach to the generation of adapted computer programs
• Genetic algorithms, a subset of evolutionary computation, models variation by "genetic" operators (mutation and recombination)
• Evolutionary robotics applies Darwinian algorithms to the design of autonomous robots
• Artificial life uses Darwinian algorithms to let organism-like computer agents evolve in a software simulation
• Evolutionary art uses variation and selection to produce works of art
• Evolutionary music does the same for works of music
• Clonal selection theory sees the creation of adapted antibodies in the immune system as a process of variation and selection
• Neural Darwinism proposes that neurons and their synapses are selectively pruned during brain development
• Evolutionary epistemology of theories assumes that scientific theories develop through variation and selection
• Memetics is a theory of the variation, transmission, and selection of cultural items, such as ideas, fashions, and traditions
• Dual inheritance theory a framework for cultural evolution developed largely independently of memetics
• Cultural selection theory is a theory of cultural evolution related to memetics
• Cultural materialism is an anthropological approach that contends that the physical world impacts and sets constraints on human behavior.
• Environmental determinism is a social science theory that proposes that it is the environment that ultimately determines human culture.
• Evolutionary economics studies the variation and selection of economic phenomena, such as commodities, technologies, institutions and organizations.
• Evolutionary ethics investigates the origin of morality, and uses Darwinian foundations to formulate ethical values
• Big History is the science-based narrative integrating the history of the universe, earth, life, and humanity. Scholars consider Universal Darwinism to be a possible unifying theme for the discipline.

Ancient philosophy and cosmology

Some philosophers produced conceptions opposed to the dominant teleological and essentialist thesis (Plato) and resembling Darwin's but applied to every being and not only living ones. Therefore, they can be considered as ontological, cosmological, general and universal darwinisms. Darwin was aware of it : in On the Origin of Species, he cites Empedocles (cited by Aristotle).

Dark skin

From Wikipedia, the free encyclopedia
https://en.wikipedia.org/wiki/Dark_skin

A woman with dark skin

Dark skin is a type of human skin color that is rich in melanin pigments. People with dark skin are often referred to as black people, although this usage can be ambiguous in some countries where it is also used to specifically refer to different ethnic groups or populations.

The evolution of dark skin is believed to have begun around 1.2 million years ago, in light-skinned early hominid species after they moved from the equatorial rainforest to the sunny savannas. In the heat of the savannas, better cooling mechanisms were required, which were achieved through the loss of body hair and development of more efficient perspiration. The loss of body hair led to the development of dark skin pigmentation, which acted as a mechanism of natural selection against folate (vitamin B9) depletion, and to a lesser extent, DNA damage. The primary factor contributing to the evolution of dark skin pigmentation was the breakdown of folate in reaction to ultraviolet radiation; the relationship between folate breakdown induced by ultraviolet radiation and reduced fitness as a failure of normal embryogenesis and spermatogenesis led to the selection of dark skin pigmentation. By the time modern Homo sapiens evolved, all humans were dark-skinned.

Humans with dark skin pigmentation have skin naturally rich in melanin, especially eumelanin, and have more melanosomes which provide superior protection against the deleterious effects of ultraviolet radiation. This helps the body to retain its folate reserves and protects against damage to DNA.

Dark-skinned people who live in high latitudes with mild sunlight are at an increased risk—especially in the winter—of vitamin D deficiency. As a consequence of vitamin D deficiency, they are at a higher risk of developing rickets, numerous types of cancers, and possibly cardiovascular disease and low immune system activity. However, some recent studies have questioned if the thresholds indicating vitamin D deficiency in light-skinned individuals are relevant for dark-skinned individuals, as they found that, on average, dark-skinned individuals have higher bone density and lower risk of fractures than lighter-skinned individuals with the same levels of vitamin D. This is possibly attributed to lower presence of vitamin D binding agents (and thus its higher bioavailability) in dark-skinned individuals.

The global distribution of generally dark-skinned populations is strongly correlated with the high ultraviolet radiation levels of the regions inhabited by them. These populations, with the exception of indigenous Tasmanians, almost exclusively live near the equator, in tropical areas with intense sunlight: Africa, Australia, Melanesia, South Asia, Southeast Asia, West Asia, and the Americas. Studies into non-African populations indicates dark skin is not necessarily a retention of the pre-existing high UVR-adapted state of modern humans before the out of Africa migration, but may in fact be a later evolutionary adaptation to tropical rainforest regions. Due to mass migration and increased mobility of people between geographical regions in the recent past, dark-skinned populations today are found all over the world.

Evolution

Due to natural selection, people who lived in areas of intense sunlight developed dark skin colouration to protect against ultraviolet (UV) light, mainly to protect their body from folate depletion. Evolutionary pigmentation of the skin was caused by ultraviolet radiation of the sun. As hominids gradually lost their fur between 1.2 and 4 million years ago, to allow for better cooling through sweating, their naked and lightly pigmented skin was exposed to sunlight. In the tropics, natural selection favoured dark-skinned human populations as high levels of skin pigmentation protected against the harmful effects of sunlight. Indigenous populations' skin reflectance (the amount of sunlight the skin reflects) and the actual UV radiation in a particular geographic area is highly correlated, which supports this idea. Genetic evidence also supports this notion, demonstrating that around 1.2 million years ago there was a strong evolutionary pressure which acted on the development of dark skin pigmentation in early members of the genus Homo. The effect of sunlight on folic acid levels has been crucial in the development of dark skin.

Savannas in Africa are where most of the hominid evolution of dark skin may have taken place

The earliest primate ancestors of modern humans most likely had pale skin, like our closest modern relative—the chimpanzee. About 7 million years ago human and chimpanzee lineages diverged, and between 4.5 and 2 million years ago early humans moved out of rainforests to the savannas of sub-Saharan Africa. They not only had to cope with more intense sunlight but had to develop a better cooling system. It was harder to get food in the hot savannas and as mammalian brains are prone to overheating—5 or 6 °C rise in temperature can lead to heatstroke—there was a need for the development of better heat regulation. The solution was sweating and loss of body hair.

Sweating dissipated heat through evaporation. Early humans, like chimpanzees now, had few sweat glands, and most of them were located in the palms of the hand and the soles of the feet. At times, individuals with more sweat glands were born. These humans could search for food and hunt for longer periods before being forced back to the shades. The more they could forage, the more and healthier offspring they could produce, and the higher the chance they had to pass on their genes for abundant sweat glands. With less hair, sweat could evaporate more easily and cool the bodies of humans faster. A few million years of evolution later, early humans had sparse body hair and more than 2 million sweat glands in their body.

Hairless skin, however, is particularly vulnerable to be damaged by ultraviolet light and this proved to be a problem for humans living in areas of intense UV radiation, and the evolutionary result was the development of dark-coloured skin as a protection. Scientists have long assumed that humans evolved melanin in order to absorb or scatter harmful sun radiation. Some researchers assumed that melanin protects against skin cancer. While high UV radiation can cause skin cancer, the development of cancer usually occurs after child-bearing age. As natural selection favours individuals with traits of reproductive success, skin cancer had little effect on the evolution of dark skin. Previous hypotheses suggested that sunburned nipples impeded breastfeeding, but a slight tan is enough to protect mothers against this issue.

A 1978 study examined the effect of sunlight on folate—a vitamin B complex—levels. The study found that even short periods of intense sunlight are able to halve folate levels if someone has light skin. Low folate levels are correlated with neural tube defects, such as anencephaly and spina bifida. UV rays can strip away folate, which is important to the development of healthy foetuses. In these abnormalities children are born with an incomplete brain or spinal cord. Nina Jablonski, a professor of anthropology and expert on evolution of human skin colouration, found several cases in which mothers' visits to tanning studios were connected to neural tube defects in early pregnancy. She also found that folate was crucial to sperm development; some male contraception drugs are based on folate inhibition. It has been found that folate may have been the driving force behind the evolution of dark skin.

As humans dispersed from equatorial Africa to low UVR areas and higher latitudes sometime between 120,000 and 65,000 years ago, dark skin posed a disadvantage. Populations with light skin pigmentation evolved in climates of little sunlight. Light skin pigmentation protects against vitamin D deficiency. It is known that dark-skinned people who have moved to climates of limited sunlight can develop vitamin D-related conditions such as rickets, and different forms of cancer.

A 2022 study revealed that traits such as dark skin show strong signals for Convergent evolution and selective pressure (positive Selection).

Other hypotheses

The main other hypotheses that have been put forward through history to explain the evolution of dark skin colouration relate to increased mortality due to skin cancers, enhanced fitness as a result of protection against sunburns, and increasing benefits due to antibacterial properties of eumelanin.

Darkly pigmented, eumelanin-rich skin protects against DNA damage caused by the sunlight. This is associated with lower skin cancer rates among dark-skinned populations. The presence of pheomelanin in light skin increases the oxidative stress in melanocytes, and this combined with the limited ability of pheomelanin to absorb UVR contributes to higher skin cancer rates among light-skinned individuals. The damaging effect of UVR on DNA structure and the entailing elevated skin cancer risk is widely recognized.

However, these cancer types usually affect people at the end or after their reproductive career and could have not been the evolutionary reason behind the development of dark skin pigmentation. Of all the major skin cancer types, only melanoma has a major effect in a person's reproductive age. However, because the mortality rates of melanoma have been very low before the mid-20th century (less than 5 per 100,000), it has therefore been argued that the low melanoma mortality rates during reproductive age cannot be the principal reason behind the development of dark skin pigmentation.

Studies have found that even serious sunburns could not affect sweat gland function and thermoregulation. There are no data or studies that support that sunburn can cause damage so seriously it can affect reproductive success.

Another group of hypotheses contended that dark skin pigmentation developed as antibacterial protection against tropical infectious diseases and parasites. Although it is true that eumelanin has antibacterial properties, its importance is secondary to 'physical adsorption' (physisorption) to protect against UVR-induced damage. This hypothesis is not consistent with the evidence that most of the hominid evolution took place in savanna environments and not in tropical rainforests. Humans living in hot and sunny environments have darker skin than humans who live in wet and cloudy environments. The antimicrobial hypothesis also does not explain why some populations (like the Inuit or Tibetans) who live far from the tropics and are exposed to high UVR have darker skin pigmentation than their surrounding populations.

Biochemistry and genetics

The layers in darkly pigmented skin

Dark-skinned humans have high amounts of melanin found in their skin. Melanin is derivative of the amino acid tyrosine. Eumelanin is the dominant form of melanin found in human skin. Eumelanin protects tissues and DNA from the radiation damage of UV light. Melanin is produced in specialized cells called melanocytes, which are found at the lowest level of the epidermis.

Melanin is produced inside small membrane-bound packages called melanosomes. People with naturally occurring dark skin have melanosomes which are clumped, large and full of eumelanin. A four-fold difference in naturally occurring dark skin gives seven- to eight-fold protection against DNA damage, but even the darkest skin colour cannot protect against all damage to DNA.

Dark skin offers great protection against UVR because of its eumelanin content, the UVR-absorbing capabilities of large melanosomes, and because eumelanin can be mobilized faster and brought to the surface of the skin from the depths of the epidermis. For the same body region, light- and dark-skinned individuals have similar numbers of melanocytes (there is considerable variation between different body regions), but pigment-containing organelles, called melanosomes, are larger and more numerous in dark-skinned individuals.

Keratocytes from dark skin cocultured with melanocytes give rise to a melanosome distribution pattern characteristic of dark skin. Melanosomes are not in aggregated state in darkly pigmented skin compared to lightly pigmented skin. Due to the heavily melanised melanosomes in darkly pigmented skin, it can absorb more energy from UVR and thus offers better protection against sunburns and by absorption and dispersion UV rays.

Darkly pigmented skin protects against direct and indirect DNA damage. Photodegration occurs when melanin absorbs photons. Recent research suggest that the photoprotective effect of dark skin is increased by the fact that melanin can capture free radicals, such as hydrogen peroxide, which are created by the interaction of UVR and layers of the skin. Heavily pigmented melanocytes have greater capacity to divide after ultraviolet irradiation, which suggests that they receive less damage to their DNA. Despite this, medium-wave ultraviolet radiation (UVB) damages the immune system even in darker skinned individuals due to its effect on Langerhans cells. The stratum corneum of people with dark or heavily tanned skin is more condensed and contains more cornified cell layers than in lightly pigmented humans. These qualities of dark skin enhance the barrier protection function of the skin.

Although darkly pigmented skin absorbs about 30 to 40% more sunlight than lightly pigmented skin, dark skin does not increase the body's internal heat intake in conditions of intense solar radiation. Solar radiation heats up the body's surface and not the interior. Furthermore, this amount of heat is negligible compared to the heat produced when muscles are actively used during exercise. Regardless of skin colour, humans have excellent capabilities to dissipate heat through sweating. Half of the solar radiation reaching the Earth's surface is in the form of infrared light and is absorbed similarly regardless of skin colouration.

In people with naturally occurring dark skin, the tanning occurs with the dramatic mobilization of melanin upward in the epidermis and continues with the increased production of melanin. This accounts for the fact that dark-skinned people get visibly darker after one or two weeks of sun exposure, and then lose their colour after months when they stay out of the sun. Darkly pigmented people tend to exhibit fewer signs of aging in their skin than the lightly pigmented because their dark skin protects them from most photoaging.

Skin colour is a polygenic trait, which means that several different genes are involved in determining a specific phenotype. Many genes work together in complex, additive, and non-additive combinations to determine the skin colour of an individual. The skin colour variations are normally distributed from light to dark, as it is usual for polygenic traits.

Data collected from studies on MC1R gene has shown that there is a lack of diversity in dark-skinned African samples in the allele of the gene compared to non-African populations. This is remarkable given that the number of polymorphisms for almost all genes in the human gene pool is greater in African samples than in any other geographic region. So, while the MC1Rf gene does not significantly contribute to variation in skin colour around the world, the allele found in high levels in African populations probably protects against UV radiation and was probably important in the evolution of dark skin.

Skin colour seems to vary mostly due to variations in a number of genes of large effect as well as several other genes of small effect (TYR, TYRP1, OCA2, SLC45A2, SLC24A5, MC1R, KITLG and SLC24A4). This does not take into account the effects of epistasis, which would probably increase the number of related genes. Variations in the SLC24A5 gene account for 20–25% of the variation between dark- and light-skinned populations of Africa, and appear to have arisen as recently as within the last 10,000 years. The Ala111Thr or rs1426654 polymorphism in the coding region of the SLC24A5 gene reaches fixation in Europe, and is also common among populations in North Africa, the Horn of Africa, West Asia, Central Asia and South Asia.

Health implications

Skin pigmentation is an evolutionary adaptation to various UVR levels around the world. As a consequence there are many health implications that are the product of population movements of humans of certain skin pigmentation to new environments with different levels of UVR. Modern humans are often ignorant of their evolutionary history at their peril. Cultural practices that increase problems of conditions among dark-skinned populations are traditional clothing and vitamin D-poor diet.

Advantages in high sunlight

Dark-pigmented people living in high sunlight environments are at an advantage due to the high amounts of melanin produced in their skin. The dark pigmentation protects from DNA damage and absorbs the right amounts of UV radiation needed by the body, as well as protects against folate depletion. Folate is a water-soluble vitamin B complex which naturally occurs in green, leafy vegetables, whole grains, and citrus fruits. Women need folate to maintain healthy eggs, for proper implantation of eggs, and for the normal development of placenta after fertilization. Folate is needed for normal sperm production in men. Folate is essential for fetal growth, organ development, and neural tube development. Folate breaks down in high intensity UVR.

Dark-skinned women suffer the lowest level of neural tube defects. Folate plays an important role in DNA production and gene expression. It is essential for maintaining proper levels of amino acids which make up proteins. Folate is used in the formation of myelin, the sheath that covers nerve cells and makes it possible to send electrical signals quickly. Folate also plays an important role in the development of many neurotransmitters, e.g. serotonin which regulates appetite, sleep, and mood. Serum folate is broken down by UV radiation or alcohol consumption. Because the skin is protected by the melanin, dark-pigmented people have a lower chance of developing skin cancer and conditions related to folate deficiency, such as neural tube defects.

Disadvantages in low sunlight

Main article: Vitamin D deficiency

Rickets is a condition associated with dark skin.

Dark-skinned people living in low sunlight environments have been recorded to be very susceptible to vitamin D deficiency due to reduced vitamin D synthesis. A dark-skinned person requires about six times as much UVB than lightly pigmented persons. This is not a problem near the equator; however, it can be a problem at higher latitudes. For humans with dark skin in climates of low UVR, it can take about two hours to produce the same amount of vitamin D as humans with light skin produce in 15 minutes. Dark-skinned people having a high body-mass index and not taking vitamin D supplements were associated with vitamin D deficiency.

Vitamin D plays an important role in regulating the human immune system. Chronic deficiencies in vitamin D can make humans susceptible to specific types of cancers and many kinds of infectious diseases. Vitamin D deficiency increases the risk of developing tuberculosis five-fold and also contributes to the development of breast, prostate, and colourectal cancer.

The most prevalent disease to follow vitamin D deficiency is rickets, the softening of bones in children potentially leading to fractures and deformity. Rickets is caused by reduced vitamin D synthesis that causes an absence of vitamin D, which then causes the dietary calcium to not be properly absorbed. This disease in the past was commonly found among dark-skinned Americans of the southern part of the United States who migrated north into low sunlight environments. The popularity of sugary drinks and decreased time spent outside have contributed to significant rise of developing rickets. Deformities of the female pelvis related to severe rickets impair normal childbirth, which leads to higher mortality of the infant, mother, or both.

Vitamin D deficiency is most common in regions with low sunlight, especially in the winter. Chronic deficiencies in vitamin D may also be linked with breast, prostate, colon, ovarian, and possibly other types of cancers. The relationship between cardiovascular disease and vitamin D deficiency also suggest a link between health of cardiac and smooth muscle. Low vitamin D levels have also been linked to impaired immune system and brain functions. In addition, recent studies have linked vitamin D deficiency to autoimmune diseases, hypertension, multiple sclerosis, diabetes and incidence of memory loss.

Outside the tropics UVR has to penetrate through a thicker layer of atmosphere, which results in most of the intermediate wavelength UVB reflected or destroyed en route; because of this there is less potential for vitamin D biosynthesis in regions far from the equator. Higher amount of vitamin D intake for dark-skinned people living in regions with low levels of sunlight are advised by doctors to follow a vitamin D-rich diet or take vitamin D supplements, although there is recent evidence that dark-skinned individuals are able to process vitamin D more efficiently than lighter-skinned individuals so may have a lower threshold of sufficiency.

Geographic distribution

There is a correlation between the geographic distribution of UV radiation (UVR) and the distribution of skin pigmentation around the world. Areas that have higher amounts of UVR have darker-skinned populations, generally located nearer the equator. Areas that are further away from the equator and generally closer to the poles have a lower concentration of UVR and contain lighter-skinned populations. This is the result of human evolution which contributed to variable melanin content in the skin to adapt to certain environments.

A larger percentage of dark-skinned people are found in the Southern Hemisphere because latitudinal land mass distribution is disproportionate. The present distribution of skin colour variation does not completely reflect the correlation of intense UVR and dark skin pigmentation due to mass migration and movement of peoples across continents in the recent past. Dark-skinned populations inhabiting Africa, Australia, Melanesia, South Asia, Southeast Asia, West Asia and South America live in some of the areas with the highest UV radiation in the world, and have evolved very dark skin pigmentations as protection from the sun's harmful rays.

Evolution has restricted humans with darker skin in tropical latitudes, especially in non-forested regions, where ultraviolet radiation from the sun is usually the most intense. Different dark-skinned populations are not necessarily closely related genetically. Before the modern mass migration, it has been argued that the majority of dark-pigmented people lived within 20° of the equator.

Ati woman, Philippines – the Negritos are an indigenous people of Southeast Asia.

Natives of Buka and Bougainville at the northern Solomon Islands in Melanesia and the Chopi people of Mozambique in the southeast coast of Africa have darker skin than other surrounding populations. The native people of the Autonomous Region of Bougainville have some of the darkest skin pigmentation in the world. Although these people are widely separated they share similar physical environments. In both regions, they experience very high UVR exposure from cloudless skies near the equator which is reflected from water or sand. Water reflects, depending on colour, about 10 to 30% of UVR that falls on it.

People in these populations spend long hours fishing on the sea. Because it is impractical to wear extensive clothing in a watery environment, culture and technology does little to buffer UVR exposure. The skin takes a very large amount of ultraviolet radiation. These populations are probably near or at the maximum darkness that human skin can achieve.

More recent research has found that human populations over the past 50,000 years have changed from dark-skinned to light-skinned and vice versa. Only 100–200 generations ago, the ancestors of most people living today likely also resided in a different place and had a different skin colour. According to Nina Jablonski, darkly pigmented modern populations in South India and Sri Lanka are an example of this, having re-darkened after their ancestors migrated down from areas much farther north. Scientists originally believed that such shifts in pigmentation occurred relatively slowly. However, researchers have since observed that changes in skin colouration can happen in as little as 100 generations (~2,500 years), with no intermarriage required. The speed of change is also affected by clothing, which tends to slow it down.

Australia

An Aboriginal Australian man with dark skin

Indigenous Australians, as with all other populations outside of Africa, are descendants of the Out-of-Africa wave. Genetic evidence has pointed out that the Indigenous peoples of Australia are genetically dissimilar to the dark-skinned populations of Africa and that they are more closely related to other non-African populations.

The term black initially has been applied as a reference to the skin pigmentation of Indigenous Australians; today it has been embraced by Aboriginal activists as a term for shared culture and identity, regardless of skin colour.

Melanesia

A dark-skinned blond boy from Vanuatu, Melanesia

Melanesia, a subregion of Oceania, whose name means "black islands", have several islands that are inhabited by people with dark skin pigmentation. The islands of Melanesia are located immediately north and northeast of Australia as well as east coast of Papua New Guinea. The western end of Melanesia from New Guinea through the Solomon Islands were first colonized by humans about 40,000 to 29,000 years ago.

In the world, blond hair is exceptionally rare outside Europe, North Africa and West Asia, especially among dark-skinned populations. However, Melanesians are one of the dark-skinned human populations known to have naturally occurring blond hair.

New Guinea

Buka boys from Bougainville Island, Papua New Guinea. People from Bougainville have some of the darkest skin tones among humans.

The indigenous Papuan people of New Guinea have dark skin pigmentation and have inhabited the island for at least 40,000 years. Due to their similar phenotype and the location of New Guinea being in the migration route taken by Indigenous Australians, it was generally believed that Papuans and Aboriginal Australians shared a common origin. However, a 1999 study failed to find clear indications of a single shared genetic origin between the two populations, suggesting multiple waves of migration into Sahul with distinct ancestries.

Sub-Saharan Africa

People from Congo

Sub-Saharan Africa is the region in Africa situated south of the Sahara where a large number of dark-skinned populations live. Dark-skinned groups on the continent have the same receptor protein as Homo ergaster and Homo erectus had. According to scientific studies, populations in Africa also have the highest skin colour diversity. High levels of skin colour variation exists between different populations in Sub-Saharan Africa. These differences depend in part on general distance from the equator, illustrating the complex interactions of evolutionary forces which have contributed to the geographic distribution of skin colour at any point of time.

Due to frequently differing ancestry among dark-skinned populations, the presence of dark skin in general is not a reliable genetic marker, including among groups in Africa. For example, Wilson et al. (2001) found that most of their Ethiopian samples showed closer genetic affinities with lighter-skinned Armenians than with darker-skinned Bantu populations. Mohamoud (2006) likewise observed that their Somali samples were genetically more similar to Arab populations than to other African populations.

South Asia

Fisherman from Chennai, Tamil Nadu in southern India.

South Asia has some of the greatest skin colour diversity outside of Africa. Skin colour among South Indians is on average darker than North Indians. This is mainly because of the weather conditions in South Asia—higher UV indices are in the south. Several genetic surveys of South Asian populations in different regions have found a weak negative correlation between social status and skin darkness, represented by the melanin index.

Tamils and Dravidian people descended from the original inhabitants of India. They have a dark skin tone because those original inhabitants were a dark skinned people who had migrated from out of Africa into India and other surrounding areas about 65,000 years ago.

A study of caste populations in the Gangetic Plain found an association between the proportion of dark skin and ranking in the caste hierarchy. Dalits had, on average, the darkest skin. A pan-India study of Telugu and North Indian castes found a similar correlation between skin colour and caste association, linked to the absence of the rs1426654-A variant of the SLC245-A gene, but are also linked to mutations overriding these variants.

Americas

A dark-skinned Wayuu couple from Colombia. Many other Indigenous peoples of tropical or subtropical areas of the Americas have dark skin.

Relatively dark skin remains among the Inuit and other Arctic populations. A combination of protein-heavy diets and summer snow reflection have been speculated as favouring the retention of pigmented skin.

Earliest European colonial descriptions of North American populations include terms such as "brown", "tawny" or "olive", though some populations were also described as "light-skinned". Most North American indigenous populations rank similar to African and Oceanian populations in regards to the presence of the allele Ala111.

Native South Americans and Mesoamericans are also typically considered dark-skinned. High ultraviolet radiation levels occur throughout the Andes region of Peru, Bolivia, Chile, and Argentina.

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  An Indigenous mother and daughter from the Urus islands.
   
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  An indigenous boy from Peru.
   
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  An indigenous fisherman from Lake Titicaca.
   
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  A man from Bolivia.

Culture

Main article: Discrimination based on skin color

The preference or disfavour for darker skin has varied depending on geographical area and time.