Recent African origin of modern humans

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This article is about modern humans. For migrations of early humans, see Early expansions of hominins out of Africa.

 

Successive dispersals of
  Homo erectus greatest extent (yellow),
  Homo neanderthalensis greatest extent (ochre) and
  Homo sapiens (red).

 

Expansion of early modern humans from Africa through the Near East

 

Map of the migration of modern humans out of Africa, based on mitochondrial DNA. Colored rings indicate thousand years before present.

In paleoanthropology, the recent African origin of modern humans, also called the "Out of Africa" theory (OOA), recent single-origin hypothesis (RSOH), replacement hypothesis, or recent African origin model (RAO), is the dominant model of the geographic origin and early migration of anatomically modern humans (Homo sapiens). It follows the early expansions of hominins out of Africa, accomplished by Homo erectus and then Homo neanderthalensis.

The model proposes a "single origin" of Homo sapiens in the taxonomic sense, precluding parallel evolution of traits considered anatomically modern in other regions, but not precluding multiple admixture between H. sapiens and archaic humans in Europe and Asia. H. sapiens most likely developed in the Horn of Africa between 300,000 and 200,000 years ago. The "recent African origin" model proposes that all modern non-African populations are substantially descended from populations of H. sapiens that left Africa after that time.

There were at least several "out-of-Africa" dispersals of modern humans, possibly beginning as early as 270,000 years ago, including 215,000 years ago to at least Greece, and certainly via northern Africa about 130,000 to 115,000 years ago. These early waves appear to have mostly died out or retreated by 80,000 years ago.

The most significant "recent" wave took place about 70,000–50,000 years ago, via the so-called "Southern Route", spreading rapidly along the coast of Asia and reaching Australia by around 65,000–50,000 years ago, (though some researchers question the earlier Australian dates and place the arrival of humans there at 50,000 years ago at earliest, while others have suggested that these first settlers of Australia may represent an older wave before the more significant out of Africa migration and thus not necessarily be ancestral to the region's later inhabitants) while Europe was populated by an early offshoot which settled the Near East and Europe less than 55,000 years ago.

In the 2010s, studies in population genetics uncovered evidence of interbreeding that occurred between H. sapiens and archaic humans in Eurasia, Oceania and Africa, indicating that modern population groups, while mostly derived from early H. sapiens, are to a lesser extent also descended from regional variants of archaic humans.

Proposed waves

Layer sequence at Ksar Akil in the Levantine corridor, and discovery of two fossils of Homo sapiens, dated to 40,800 to 39,200 years BP for "Egbert", and 42,400–41,700 BP for "Ethelruda".

"Recent African origin," or Out of Africa II, refers to the migration of anatomically modern humans (Homo sapiens) out of Africa after their emergence at c. 300,000 to 200,000 years ago, in contrast to "Out of Africa I", which refers to the migration of archaic humans from Africa to Eurasia roughly 1.8 to 0.5 million years ago. Omo-Kibish I (Omo I) from southern Ethiopia is the oldest anatomically modern Homo sapiens skeleton currently known (196 ± 5 ka).

Since the beginning of the 21st century, the picture of "recent single-origin" migrations has become significantly more complex, not only due to the discovery of modern-archaic admixture but also due to the increasing evidence that the "recent out-of-Africa" migration took place in a number of waves spread over a long time period. As of 2010, there were two main accepted dispersal routes for the out-of-Africa migration of early anatomically modern humans: via the "Northern Route" (via Nile Valley and Sinai) and the "Southern Route" via the Bab al Mandab strait.

• Posth et al. (2017) suggest that early Homo sapiens, or "another species in Africa closely related to us," might have first migrated out of Africa around 270,000 years ago.
• Finds at Misliya cave, which include a partial jawbone with eight teeth, have been dated to around 185,000 years ago. Layers dating from between 250,000 and 140,000 years ago in the same cave contained tools of the Levallois type which could put the date of the first migration even earlier if the tools can be associated with the modern human jawbone finds.
• An Eastward Dispersal from Northeast Africa to Arabia 150,000–130,000  years ago based on the finds at Jebel Faya dated to 127,000 years ago (discovered in 2011). Possibly related to this wave are the finds from Zhirendong cave, Southern China, dated to more than 100,000 years ago. Other evidence of modern human presence in China has been dated to 80,000 years ago.
• The most significant dispersal took place around 50–70,000 years ago via the so-called Southern Route, either before or after the Toba event, which happened between 69,000 and 77,000 years ago. This dispersal followed the southern coastline of Asia, and reached Australia around 65,000-50,000 years ago, or according to some research, by 50,000 years ago at earliest. Western Asia was "re-occupied" by a different derivation from this wave around 50,000 years ago, and Europe was populated from Western Asia beginning around 43,000 years ago.
• Wells (2003) describes an additional wave of migration after the southern coastal route, namely a northern migration into Europe at circa 45,000 years ago. However, this possibility is ruled out by Macaulay et al. (2005) and Posth et al. (2016), who argue for a single coastal dispersal, with an early offshoot into Europe.

Northern Route dispersal

Anatomically Modern Humans known archaeological remains in Europe and Africa, directly dated, calibrated carbon dates as of 2013.

Beginning 135,000 years ago, tropical Africa experienced megadroughts which drove humans from the land and towards the sea shores, and forced them to cross over to other continents.

Modern humans crossed the Straits of Bab el Mandeb in the southern Red Sea, and moved along the green coastlines around Arabia, and thence to the rest of Eurasia. Fossils of early Homo sapiens were found in Qafzeh cave in Israel and have been dated 80,000 to 100,000 years ago. These humans seem to have either become extinct or retreated back to Africa 70,000 to 80,000 years ago, possibly replaced by southbound Neanderthals escaping the colder regions of ice-age Europe. Hua Liu et al. analyzed autosomal microsatellite markers dating to about 56,000 years ago. They interpret the paleontological fossil as an isolated early offshoot that retracted back to Africa.

The discovery of stone tools in the United Arab Emirates in 2011 at the Faya-1 site in Mleiha, Sharjah, indicated the presence of modern humans at least 125,000 years ago, leading to a resurgence of the "long-neglected" North African route.

In Oman, a site was discovered by Bien Joven in 2011 containing more than 100 surface scatters of stone tools belonging to the late Nubian Complex, known previously only from archaeological excavations in the Sudan. Two optically stimulated luminescence age estimates placed the Arabian Nubian Complex at approximately 106,000 years old. This provides evidence for a distinct Stone Age technocomplex in southern Arabia, around the earlier part of the Marine Isotope Stage 5.

According to Kuhlwilm and his co-authors, Neanderthals contributed genetically to modern humans then living outside of Africa around 100,000 years ago: humans which had already split off from other modern humans around 200,000 years ago, and this early wave of modern humans outside Africa also contributed genetically to the Altai Neanderthals. They found that "the ancestors of Neanderthals from the Altai Mountains and early modern humans met and interbred, possibly in the Near East, many thousands of years earlier than previously thought". According to co-author Ilan Gronau, "This actually complements archaeological evidence of the presence of early modern humans out of Africa around and before 100,000 years ago by providing the first genetic evidence of such populations." Similar genetic admixture events have been noted in other regions as well.

In China, the Liujiang man (Chinese: 柳江人) is among the earliest modern humans found in East Asia. The date most commonly attributed to the remains is 67,000 years ago. High rates of variability yielded by various dating techniques carried out by different researchers place the most widely accepted range of dates with 67,000 BP as a minimum, but do not rule out dates as old as 159,000 BP.  Liu, Martinón-Torres et al. (2015) claim that modern human teeth have been found in China dating to at least 80,000 years ago.

Southern Route dispersal

Coastal route

Red Sea crossing

By some 50-70,000 years ago, a subset of the bearers of mitochondrial haplogroup L3 migrated from East Africa into the Near East. It has been estimated that from a population of 2,000 to 5,000 individuals in Africa, only a small group, possibly as few as 150 to 1,000 people, crossed the Red Sea. The group that crossed the Red Sea travelled along the coastal route around Arabia and the Persian Plateau to India, which appears to have been the first major settling point. Wells (2003) argued for the route along the southern coastline of Asia, across about 250 kilometres (155 mi), reaching Australia by around 50,000 years ago.

Today at the Bab-el-Mandeb straits, the Red Sea is about 20 kilometres (12 mi) wide, but 50,000 years ago sea levels were 70 m (230 ft) lower (owing to glaciation) and the water was much narrower. Though the straits were never completely closed, they were narrow enough to have enabled crossing using simple rafts, and there may have been islands in between. Shell middens 125,000 years old have been found in Eritrea, indicating the diet of early humans included seafood obtained by beachcombing.

The dating of the Southern Dispersal is a matter of dispute. It may have happened either pre- or post-Toba, a catastrophic volcanic eruption that took place between 69,000 and 77,000 years ago at the site of present-day Lake Toba. Stone tools discovered below the layers of ash disposed in India may point to a pre-Toba dispersal but the source of the tools is disputed. An indication for post-Toba is haplo-group L3, that originated before the dispersal of humans out of Africa and can be dated to 60,000–70,000 years ago, "suggesting that humanity left Africa a few thousand years after Toba". Some research showing slower than expected genetic mutations in human DNA was published in 2012, indicating a revised dating for the migration to between 90,000 and 130,000 years ago. Some more recent research suggests a migration out-of-Africa of around 50,000-65,000 years ago of the ancestors of modern non-African populations, similar to most previous estimates.

Western Asia

A fossil of a modern human dated to 54,700 years ago was found in Manot Cave in Israel, named Manot 1, though the dating was questioned by Groucutt et al. (2015).

South Asia and Australia

It is thought that Australia was inhabited around 65,000–50,000 years ago. As of 2017, the earliest evidence of humans in Australia is at least 65,000 years old, while McChesney stated that

...genetic evidence suggests that a small band with the marker M168 migrated out of Africa along the coasts of the Arabian Peninsula and India, through Indonesia, and reached Australia very early, between 60,000 and 50,000 years ago. This very early migration into Australia is also supported by Rasmussen et al. (2011).

Fossils from Lake Mungo, Australia, have been dated to about 42,000 years ago. Other fossils from a site called Madjedbebe have been dated to at least 65,000 years ago, though some researchers doubt this early estimate and date the Madjedbebe fossils at about 50,000 years ago at the oldest.

East Asia

Tianyuan man from China has a probable date range between 38,000 and 42,000 years ago, while Liujiang man from the same region has a probable date range between 67,000 and 159,000 years ago. According to 2013 DNA tests, Tianyuan man is related "to many present-day Asians and Native Americans". Tianyuan is similar in morphology to Minatogawa Man, modern humans dated between 17,000 and 19,000 years ago and found on Okinawa Island, Japan.

Europe

According to Macaulay et al. (2005), an early offshoot from the southern dispersal with haplogroup N followed the Nile from East Africa, heading northwards and crossing into Asia through the Sinai. This group then branched, some moving into Europe and others heading east into Asia. This hypothesis is supported by the relatively late date of the arrival of modern humans in Europe as well as by archaeological and DNA evidence. Based on an analysis of 55 human mitochondrial genomes (mtDNAs) of hunter-gatherers, Posth et al. (2016) argue for a "rapid single dispersal of all non-Africans less than 55,000 years ago."

Genetic reconstruction

Mitochondrial haplogroups

Within Africa

Map of early diversification of modern humans according to mitochondrial population genetics (see: Haplogroup L).

The first lineage to branch off from Mitochondrial Eve was L0. This haplogroup is found in high proportions among the San of Southern Africa and the Sandawe of East Africa. It is also found among the Mbuti people.  These groups branched off early in human history and have remained relatively genetically isolated since then. Haplogroups L1, L2 and L3 are descendants of L1–L6, and are largely confined to Africa. The macro haplogroups M and N, which are the lineages of the rest of the world outside Africa, descend from L3. L3 is about 70,000 years old, while haplogroups M and N are about 65-55,000 years old. The relationship between such gene trees and demographic history is still debated when applied to dispersals.

Of all the lineages present in Africa, only the female descendants of one lineage, mtDNA haplogroup L3, are found outside Africa. If there had been several migrations, one would expect descendants of more than one lineage to be found. L3's female descendants, the M and N haplogroup lineages, are found in very low frequencies in Africa (although haplogroup M1 populations are very ancient and diversified in North and North-east Africa) and appear to be more recent arrivals. A possible explanation is that these mutations occurred in East Africa shortly before the exodus and became the dominant haplogroups thereafter by means of the founder effect. Alternatively, the mutations may have arisen shortly afterwards.

Southern Route and haplogroups M and N

Results from mtDNA collected from aboriginal Malaysians called Orang Asli indicate that the hapologroups M and N share characteristics with original African groups from approximately 85,000 years ago, and share characteristics with sub-haplogroups found in coastal south-east Asian regions, such as Australasia, the Indian subcontinent and throughout continental Asia, which had dispersed and separated from their African progenitor approximately 65,000 years ago. This southern coastal dispersal would have occurred before the dispersal through the Levant approximately 45,000 years ago. This hypothesis attempts to explain why haplogroup N is predominant in Europe and why haplogroup M is absent in Europe. Evidence of the coastal migration is thought to have been destroyed by the rise in sea levels during the Holocene epoch. Alternatively, a small European founder population that had expressed haplogroup M and N at first, could have lost haplogroup M through random genetic drift resulting from a bottleneck (i.e. a founder effect).

The group that crossed the Red Sea travelled along the coastal route around Arabia and Persia until reaching India. Haplogroup M is found in high frequencies along the southern coastal regions of Pakistan and India and it has the greatest diversity in India, indicating that it is here where the mutation may have occurred. Sixty percent of the Indian population belong to Haplogroup M. The indigenous people of the Andaman Islands also belong to the M lineage. The Andamanese are thought to be offshoots of some of the earliest inhabitants in Asia because of their long isolation from the mainland. They are evidence of the coastal route of early settlers that extends from India to Thailand and Indonesia all the way to eastern New Guinea. Since M is found in high frequencies in highlanders from New Guinea and the Andamanese and New Guineans have dark skin and Afro-textured hair, some scientists think they are all part of the same wave of migrants who departed across the Red Sea ~60,000 years ago in the Great Coastal Migration. The proportion of haplogroup M increases eastwards from Arabia to India; in eastern India, M outnumbers N by a ratio of 3:1. Crossing into Southeast Asia, haplogroup N (mostly in the form of derivatives of its R subclade) reappears as the predominant lineage. M is predominant in East Asia, but amongst Indigenous Australians, N is the more common lineage. This haphazard distribution of Haplogroup N from Europe to Australia can be explained by founder effects and population bottlenecks.

Autosomal DNA

A 2002 study of African, European and Asian populations, found greater genetic diversity among Africans than among Eurasians, and that genetic diversity among Eurasians is largely a subset of that among Africans, supporting the out of Africa model. A large study by Coop et al. (2009) found evidence for natural selection in autosomal DNA outside of Africa. The study distinguishes non-African sweeps (notably KITLG variants associated with skin color), West-Eurasian sweeps (SLC24A5) and East-Asian sweeps (MC1R, relevant to skin color). Based on this evidence, the study concluded that human populations encountered novel selective pressures as they expanded out of Africa. MC1R and its relation to skin color had already been discussed by Liu, Harding et al. (2000), p. 135. According to this study, Papua New Guineans continued to be exposed to selection for dark skin color so that, although these groups are distinct from Africans in other places, the allele for dark skin color shared by contemporary Africans, Andamanese and New Guineans is an archaism. Endicott et al. (2003) suggest convergent evolution. A 2014 study by Gurdasani et al. indicates that the higher genetic diversity in Africa was further increased in some regions by relatively recent Eurasian migrations affecting parts of Africa.

Pathogen DNA

Another promising route towards reconstructing human genetic genealogy is via the JC virus (JCV), a type of human polyomavirus which is carried by 70–90 percent of humans and which is usually transmitted vertically, from parents to offspring, suggesting codivergence with human populations. For this reason, JCV has been used as a genetic marker for human evolution and migration. This method does not appear to be reliable for the migration out of Africa, in contrast to human genetics, JCV strains associated with African populations are not basal. From this Shackelton et al. (2006) conclude that either a basal African strain of JCV has become extinct or that the original infection with JCV post-dates the migration from Africa.

Admixture of archaic and modern humans

Evidence for archaic human species (descended from Homo heidelbergensis) having interbred with modern humans outside of Africa, was discovered in the 2010s. This concerns primarily Neanderthal admixture in all modern populations except for Sub-Saharan Africans but evidence has also been presented for Denisova hominin admixture in Australasia (i.e. in Melanesians, Aboriginal Australians and some Negritos).

The rate of admixture of Neanderthal admixture to European and Asian populations as of 2017 has been estimated at between about 2–3%.

Archaic admixture in some Sub-Saharan African populations hunter-gatherer groups (Biaka Pygmies and San), derived from archaic hominins that broke away from the modern human lineage around 700,000 years, was discovered in 2011. The rate of admixture was estimated at around 2%. Admixture from archaic hominins of still earlier divergence times, estimated at 1.2 to 1.3 million years ago, was found in Pygmies, Hadza and five Sandawe in 2012. From an analysis of Mucin 7, a highly divergent haplotype that has an estimated coalescence time with other variants around 4.5 million years BP and is specific to African populations is inferred to have been derived from interbreeding between African modern and archaic humans.

Stone tools

In addition to genetic analysis, Petraglia et al. also examines the small stone tools (microlithic materials) from the Indian subcontinent and explains the expansion of population based on the reconstruction of paleoenvironment. He proposed that the stone tools could be dated to 35 ka in South Asia, and the new technology might be influenced by environmental change and population pressure.

History of the theory

Classical paleoanthropology

The frontispiece to Huxley's Evidence as to Man's Place in Nature (1863): the image compares the skeleton of a human to other apes.

The cladistic relationship of humans with the African apes was suggested by Charles Darwin after studying the behaviour of African apes, one of which was displayed at the London Zoo. The anatomist Thomas Huxley had also supported the hypothesis and suggested that African apes have a close evolutionary relationship with humans. These views were opposed by the German biologist Ernst Haeckel, who was a proponent of the Out of Asia theory. Haeckel argued that humans were more closely related to the primates of South-east Asia and rejected Darwin's African hypothesis. 

In the Descent of Man, Darwin speculated that humans had descended from apes, which still had small brains but walked upright, freeing their hands for uses which favoured intelligence; he thought such apes were African:

In each great region of the world the living mammals are closely related to the extinct species of the same region. It is, therefore, probable that Africa was formerly inhabited by extinct apes closely allied to the gorilla and chimpanzee; and as these two species are now man's nearest allies, it is somewhat more probable that our early progenitors lived on the African continent than elsewhere. But it is useless to speculate on this subject, for an ape nearly as large as a man, namely the Dryopithecus of Lartet, which was closely allied to the anthropomorphous Hylobates, existed in Europe during the Upper Miocene period; and since so remote a period the earth has certainly undergone many great revolutions, and there has been ample time for migration on the largest scale.

— Charles Darwin, Descent of Man

In 1871, there were hardly any human fossils of ancient hominins available. Almost fifty years later, Darwin's speculation was supported when anthropologists began finding fossils of ancient small-brained hominins in several areas of Africa (list of hominina fossils). The hypothesis of recent (as opposed to archaic) African origin developed in the 20th century. The "Recent African origin" of modern humans means "single origin" (monogenism) and has been used in various contexts as an antonym to polygenism. The debate in anthropology had swung in favour of monogenism by the mid-20th century. Isolated proponents of polygenism held forth in the mid-20th century, such as Carleton Coon, who thought as late as 1962 that H. sapiens arose five times from H. erectus in five places.

Multiregional origin hypothesis

The historical alternative to the recent origin model is the multiregional origin of modern humans, initially proposed by Milford Wolpoff in the 1980s. This view proposes that the derivation of anatomically modern human populations from H. erectus at the beginning of the Pleistocene 1.8 million years BP, has taken place within a continuous world population. The hypothesis necessarily rejects the assumption of an infertility barrier between ancient Eurasian and African populations of Homo. The hypothesis was controversially debated during the late 1980s and the 1990s. The now-current terminology of "recent-origin" and "Out of Africa" became current in the context of this debate in the 1990s. Originally seen as an antithetical alternative to the recent origin model, the multiregional hypothesis in its original "strong" form is obsolete, while its various modified weaker variants have become variants of a view of "recent origin" combined with archaic admixture. Stringer (2014) distinguishes the original or "classic" Multiregional model as having existed from 1984 (its formulation) until 2003, to a "weak" post-2003 variant that has "shifted close to that of the Assimilation Model".

Mitochondrial analyses

In the 1980s, Allan Wilson together with Rebecca L. Cann and Mark Stoneking worked on genetic dating of the matrilineal most recent common ancestor of modern human populations (dubbed "Mitochondrial Eve"). To identify informative genetic markers for tracking human evolutionary history, Wilson concentrated on mitochondrial DNA (mtDNA), passed from mother to child. This DNA material mutates quickly, making it easy to plot changes over relatively short times. With his discovery that human mtDNA is genetically much less diverse than chimpanzee mtDNA, Wilson concluded that modern human populations had diverged recently from a single population while older human species such as Neanderthals and Homo erectus had become extinct. With the advent of archaeogenetics in the 1990s, the dating of mitochondrial and Y-chromosomal haplogroups became possible with some confidence. By 1999, estimates ranged around 150,000 years for the mt-MRCA and 60,000 to 70,000 years for the migration out of Africa.

From 2000–2003, there was controversy about the mitochondrial DNA of "Mungo Man 3" (LM3) and its possible bearing on the multiregional hypothesis. LM3 was found to have more than the expected number of sequence differences when compared to modern human DNA (CRS). Comparison of the mitochondrial DNA with that of ancient and modern aborigines, led to the conclusion that Mungo Man fell outside the range of genetic variation seen in Aboriginal Australians and was used to support the multiregional origin hypothesis. A reanalysis on LM3 and other ancient specimens from the area published in 2016, showed it to be akin to modern Aboriginal Australian sequences, inconsistent with the results of the earlier study.

Y-Chromosome analyses

As current estimates on The male most recent common ancestor ("Y-chromosomal Adam" or Y-MRCA) converge with estimates for the age of anatomically modern humans and well predate the Out of Africa migration, geographical origin hypotheses continue to be limited to the African continent.

The most basal lineages have been detected in West, Northwest and Central Africa, suggesting plausibility for the Y-MRCA living in the general region of "Central-Northwest Africa".

another study finds a plausible placement in "the north-western quadrant of the African continent" for the emergence of the A1b haplogroup. The 2013 report of haplogroup A00 found among the Mbo people of western present-day Cameroon is also compatible with this picture.

The revision of Y-chromosomal phylogeny since 2011 has affected estimates for the likely geographical origin of Y-MRCA as well as estimates on time depth. By the same reasoning, future discovery of presently-unknown archaic haplogroups in living people would again lead to such revisions. In particular, the possible presence of between 1% and 4% Neanderthal-derived DNA in Eurasian genomes implies that the (unlikely) event of a discovery of a single living Eurasian male exhibiting a Neanderthal patrilineal line would immediately push back T-MRCA ("time to MRCA") to at least twice its current estimate. However, the discovery of a Neanderthal Y-chromosome by Mendez et al. A 2016 study suggests the extinction of Neanderthal patrilineages, as the lineage inferred from the Neanderthal sequence is outside of the range of contemporary human genetic variation. Questions of geographical origin would become part of the debate on Neanderthal evolution from Homo erectus.

Complex adaptive system

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A complex adaptive system is a system in which a perfect understanding of the individual parts does not automatically convey a perfect understanding of the whole system's behavior. In complex adaptive systems, the whole is more complex than its parts, and more complicated and meaningful than the aggregate of its parts. The study of complex adaptive systems, a subset of nonlinear dynamical systems, is highly interdisciplinary and blends insights from the natural and social sciences to develop system-level models and insights that allow for heterogeneous agents, phase transition, and emergent behavior.

They are complex in that they are dynamic networks of interactions, and their relationships are not aggregations of the individual static entities, i.e., the behavior of the ensemble is not predicted by the behavior of the components. They are adaptive in that the individual and collective behavior mutate and self-organize corresponding to the change-initiating micro-event or collection of events. They are a "complex macroscopic collection" of relatively "similar and partially connected micro-structures" formed in order to adapt to the changing environment and increase their survivability as a macro-structure. The Complex Adaptive Systems approach builds on replicator dynamics.

The term complex adaptive system was coined in 1968 by sociologist Walter F. Buckley who proposed a model of cultural evolution which regards psychological and socio-cultural systems as analogous with biological species. In the modern context, complex adaptive system is either directly linked to memetics, or proposed as a more viable reformulation to replace the memetic concept.

Overview

The term complex adaptive systems, or complexity science, is often used to describe the loosely organized academic field that has grown up around the study of such systems. Complexity science is not a single theory—it encompasses more than one theoretical framework and is highly interdisciplinary, seeking the answers to some fundamental questions about living, adaptable, changeable systems. Complex adaptive systems may adopt hard or softer approaches. Hard theories use formal language that is precise, tend to see agents as having tangible properties, and usually see objects in a behavioral system that can be manipulated in some way. Softer theories use natural language and narratives that may be imprecise, and agents are subjects having both tangible and intangible properties. Examples of hard complexity theories include Complex Adaptive Systems (CAS) and Viability Theory, and a class of softer theory is Viable System Theory. Many of the propositional consideration made in hard theory are also of relevance to softer theory. From here on, interest will now center on CAS.

The study of CAS focuses on complex, emergent and macroscopic properties of the system. John H. Holland said that CAS "are systems that have a large numbers of components, often called agents, that interact and adapt or learn".

Typical examples of complex adaptive systems include: climate; cities; firms; markets; governments; industries; ecosystems; social networks; power grids; animal swarms; traffic flows; social insect (e.g. ant) colonies; the brain and the immune system; and the cell and the developing embryo. Human social group-based endeavors, such as political parties, communities, geopolitical organizations, war, and terrorist networks are also considered CAS. The internet and cyberspace—composed, collaborated, and managed by a complex mix of human–computer interactions, is also regarded as a complex adaptive system. CAS can be hierarchical, but more often exhibit aspects of "self-organization".

General properties

What distinguishes a CAS from a pure multi-agent system (MAS) is the focus on top-level properties and features like self-similarity, complexity, emergence and self-organization. A MAS is defined as a system composed of multiple interacting agents; whereas in CAS, the agents as well as the system are adaptive and the system is self-similar. A CAS is a complex, self-similar collectivity of interacting, adaptive agents. Complex Adaptive Systems are characterized by a high degree of adaptive capacity, giving them resilience in the face of perturbation.

Other important properties are adaptation (or homeostasis), communication, cooperation, specialization, spatial and temporal organization, and reproduction. They can be found on all levels: cells specialize, adapt and reproduce themselves just like larger organisms do. Communication and cooperation take place on all levels, from the agent to the system level. The forces driving co-operation between agents in such a system, in some cases, can be analyzed with game theory.

Characteristics

Some of the most important characteristics of complex systems are:

• The number of elements is sufficiently large that conventional descriptions (e.g. a system of differential equations) are not only impractical, but cease to assist in understanding the system. Moreover, the elements interact dynamically, and the interactions can be physical or involve the exchange of information
• Such interactions are rich, i.e. any element or sub-system in the system is affected by and affects several other elements or sub-systems
• The interactions are non-linear: small changes in inputs, physical interactions or stimuli can cause large effects or very significant changes in outputs
• Interactions are primarily but not exclusively with immediate neighbours and the nature of the influence is modulated
• Any interaction can feed back onto itself directly or after a number of intervening stages. Such feedback can vary in quality. This is known as recurrency
• The overall behavior of the system of elements is not predicted by the behavior of the individual elements
• Such systems may be open and it may be difficult or impossible to define system boundaries
• Complex systems operate under far from equilibrium conditions. There has to be a constant flow of energy to maintain the organization of the system
• Complex systems have a history. They evolve and their past is co-responsible for their present behaviour
• Elements in the system may be ignorant of the behaviour of the system as a whole, responding only to the information or physical stimuli available to them locally

Robert Axelrod & Michael D. Cohen identify a series of key terms from a modeling perspective:

• Strategy, a conditional action pattern that indicates what to do in which circumstances
• Artifact, a material resource that has definite location and can respond to the action of agents
• Agent, a collection of properties, strategies & capabilities for interacting with artifacts & other agents
• Population, a collection of agents, or, in some situations, collections of strategies
• System, a larger collection, including one or more populations of agents and possibly also artifacts
• Type, all the agents (or strategies) in a population that have some characteristic in common
• Variety, the diversity of types within a population or system
• Interaction pattern, the recurring regularities of contact among types within a system
• Space (physical), location in geographical space & time of agents and artifacts
• Space (conceptual), "location" in a set of categories structured so that "nearby" agents will tend to interact
• Selection, processes that lead to an increase or decrease in the frequency of various types of agent or strategies
• Success criteria or performance measures, a "score" used by an agent or designer in attributing credit in the selection of relatively successful (or unsuccessful) strategies or agents

Turner and Baker synthesized the characteristics of complex adaptive systems from the literature and tested these characteristics in the context of creativity and innovation. Each of these eight characteristics had been shown to be present in the creativity and innovative processes:

• Path dependent: Systems tend to be sensitive to their initial conditions. The same force might affect systems differently.
• Systems have a history: The future behavior of a system depends on its initial starting point and subsequent history.
• Non-linearity: React disproportionately to environmental perturbations. Outcomes differ from those of simple systems.
• Emergence: Each system's internal dynamics affect its ability to change in a manner that might be quite different from other systems.
• Irreducible: Irreversible process transformations cannot be reduced back to its original state.
• Adaptive/Adaptability: Systems that are simultaneously ordered and disordered are more adaptable and resilient.
• Operates between order and chaos: Adaptive tension emerges from the energy differential between the system and its environment.
• Self-organizing: Systems are composed of interdependency, interactions of its parts, and diversity in the system. 

Modeling and simulation

CAS are occasionally modeled by means of agent-based models and complex network-based models. Agent-based models are developed by means of various methods and tools primarily by means of first identifying the different agents inside the model. Another method of developing models for CAS involves developing complex network models by means of using interaction data of various CAS components.

In 2013 SpringerOpen/BioMed Central has launched an online open-access journal on the topic of complex adaptive systems modeling (CASM).

Evolution of complexity

Passive versus active trends in the evolution of complexity. CAS at the beginning of the processes are colored red. Changes in the number of systems are shown by the height of the bars, with each set of graphs moving up in a time series.

Living organisms are complex adaptive systems. Although complexity is hard to quantify in biology, evolution has produced some remarkably complex organisms. This observation has led to the common misconception of evolution being progressive and leading towards what are viewed as "higher organisms".

If this were generally true, evolution would possess an active trend towards complexity. As shown below, in this type of process the value of the most common amount of complexity would increase over time. Indeed, some artificial life simulations have suggested that the generation of CAS is an inescapable feature of evolution.

However, the idea of a general trend towards complexity in evolution can also be explained through a passive process. This involves an increase in variance but the most common value, the mode, does not change. Thus, the maximum level of complexity increases over time, but only as an indirect product of there being more organisms in total. This type of random process is also called a bounded random walk.

In this hypothesis, the apparent trend towards more complex organisms is an illusion resulting from concentrating on the small number of large, very complex organisms that inhabit the right-hand tail of the complexity distribution and ignoring simpler and much more common organisms. This passive model emphasizes that the overwhelming majority of species are microscopic prokaryotes, which comprise about half the world's biomass and constitute the vast majority of Earth's biodiversity.

 Therefore, simple life remains dominant on Earth, and complex life appears more diverse only because of sampling bias.

If there is a lack of an overall trend towards complexity in biology, this would not preclude the existence of forces driving systems towards complexity in a subset of cases. These minor trends would be balanced by other evolutionary pressures that drive systems towards less complex states.

Individual and political action on climate change

From Wikipedia, the free encyclopedia

Global warming has intensified since the 1980s, as reflected in global average temperature datasets from NASA, NOAA, Berkeley Earth, and meteorological offices of the U.K. and Japan.

Individual and political action on climate change can take many forms. Many actions aim to build social and political support to limit and reduce the concentration of greenhouse gases (GHGs) in the atmosphere, with the goal of mitigating climate change. Other actions seek to address the ethical and moral aspects of climate justice, especially with regard to the anticipated unequal impacts of climate change adaptation.

Political action

Although the world average per person emissions is about 7 tonnes per year, so needs to be reduced to meet the global carbon budget, parts of an individual's emissions are outside their control (for example in many countries it is not possible for an individual to switch to a green energy supplier). Political action can change laws and regulations that relate to climate change (for example by abolishing fossil fuel subsidies and taxing air pollution so that green energy suppliers can compete fairly).

Carbon pricing methods, such as a carbon tax or an emissions trading system, are favored by many economists as the most efficient and effective means to reduce GHG emissions, and are increasingly being deployed around the world. In the U.S., groups such as the bipartisan legislative Climate Solutions Caucus and the Citizens' Climate Lobby work to build support for carbon pricing. The first bipartisan climate policy in 10 years was introduced in the United States House of Representatives in 2018 as the Energy Innovation and Carbon Dividends Act.

Pictured above is a protest in the streets of Paris against climate change.

Regulations can strengthen GHG emission standards from particular sectors of the economy, such as the EPA's proposed Clean Power Plan for United States power plants, or vehicle standards in Europe and the United States.

Political action can also gain media and public attention to climate change. Political action from the community, however, is often challenged by interests within the fossil-fuel industry, which have been charged with promoting climate change denial views in order to hold off a "carbon bubble" valuation crash.

There are many forms of political action on climate change including letter writing, direct lobbying, and public shaming of politicians and media organizations. Political action campaigns require building a base of support at local level.

There are many facts that point to the existence of climate change. The plentiful hurricanes, droughts, and fires are all in part due to the ever changing atmosphere caused by climate change. One of the ways in which the human race can get in front of this change is through political action. Legislative action is one strategy that politicians are using to combat carbon emissions. The way these laws will come about is via political action. Carbon emissions are a significant causing factor for climate change and by putting federal regulations such as a carbon tax there will overall be a decrease in carbon emissions. 

This allows the private sector to decide how to do so cost effectively which in turn benefits the environment. The private sector is one deciding factor in how governments enact their policies. In the case of climate change an action that needs to be taken is one that influences them rather than the other way around.

Activist movements

Political figures have a vested interest in remaining on the good side of the public. This is because in democratic countries the public are the ones electing these government officials. Thus keeping up with protests is a way they can ensure they have the public's wants in mind. Climate change is a prevalent issue in many societies. Some believe that some of the long-term negative effects of climate change can be ameliorated through individual and community actions to reduce resource consumption. Thus, many environmental advocacy organizations associated with the climate movement (such as the Earth Day Network) focus on encouraging such individual conservation and grassroots organizing around environmental issues.

Placard "Change the administration, not the climate", at the People's Climate March (2017).

Many environmental, economic, and social issues find common ground in mitigation of global warming.

To raise awareness of climate issues, activists organized a series of international labor and school strikes in late September 2019, with estimates of total participants ranging between 6 and 7.3 million.

A number of groups from around the world have come together to work on the issue of global warming. Non-governmental organizations (NGOs) from diverse fields of work have united on this issue. A coalition of 50 NGOs called Stop Climate Chaos launched in Britain in 2005 to highlight the issue of climate change.

The Campaign against Climate Change was created to focus purely on the issue of climate change and to pressure governments into action by building a protest movement of sufficient magnitude to effect political change.

Critical Mass is an event typically held on the last Friday of every month in various cities around the world wherein bicyclists and, less frequently, unicyclists, skateboarders, inline skaters, roller skaters and other self-propelled commuters take to the streets en masse. While the ride was founded in San Francisco with the idea of drawing attention to how unfriendly the city was to bicyclists, the leaderless structure of Critical Mass makes it impossible to assign it any one specific goal. In fact, the purpose of Critical Mass is not formalized beyond the direct action of meeting at a set location and time and traveling as a group through city or town streets.

One of the elements of the Occupy movement is global warming action.

Following environmentalist Bill McKibben's mantra that "if it's wrong to wreck the climate, it's wrong to profit from that wreckage," fossil fuel divestment campaigns attempt to get public institutions, such as universities and churches, to remove investment assets from fossil fuel companies. By December 2016, a total of 688 institutions and over 58,000 individuals representing $5.5 trillion in assets worldwide had been divested from fossil fuels.

Groups such as NextGen America and Climate Hawks Vote are working in the United States to elect officials who will make action on climate change a high priority.

On 20 July 2020, Swedish climate activist Greta Thunberg, who was awarded a Portuguese rights award, pledged to donate the Gulbenkian Prize money of 1 million euros to organizations focused on the environment and climate change.

Climate disobedience

Climate disobedience is a form of civil disobedience, deliberate action intended to critique government climate policy. In 2008, American climate activist Tim DeChristopher posed as a bidder at an auction of US Bureau of Land Management oil and gas leases of public land in Utah, won the auction, reneged on payment, and was imprisoned for 21 months. In September 2015, five climate activists known as the Delta 5 obstructed an oil train in Everett, Washington. At trial, the Delta 5 were allowed the necessity defense, that is, breaking a law in the service of preventing a greater harm. After testimony, the judge determined the grounds for the necessity defense were not met and instructed the jury to disregard testimony admitted under the necessity defense. The Delta 5 were fined for trespassing but were acquitted of more serious charges.

The first example of a judge accepting the climate necessity defense was on March 27, 2018 when Judge Mary Ann Driscoll acquitted all 13 defendants of civil charges from a protest held in 2016 in Boston, Massachusetts.

International political frameworks

Paris Agreement

Map of countries participating in revised Kyoto Protocol and their commitments.

The Paris Agreement is an agreement within the United Nations Framework Convention on Climate Change (UNFCCC) dealing with greenhouse gases emissions mitigation, adaptation and finance starting in the year 2020. The language of the agreement was negotiated by representatives of 195 countries at the 21st Conference of the Parties of the UNFCCC (COP21) in Paris and adopted by consensus on 12 December 2015.

The head of the Paris Conference, France's foreign minister Laurent Fabius, called the plan "ambitious and balanced" and an "historic turning point" in the goal of reducing global warming. Critics note that the agreement is not sufficient to achieve the 2 °C warming target, and the lack of any binding enforcement mechanism. Subsequent Conference of the Parties meetings are expected to address shortcomings in the Paris Agreement.

Amid fierce opposition from scientists and other leaders around the world, U.S. President Donald Trump has pledged to pull out of the Paris Agreement.

The Paris Agreement, which was brought to discussion on November 12, 2016, was made with a goal in mind to unite all countries against the threat of climate change. This argument stipulates that all countries involved contribute financially and regularly report on emissions and the status of national progress. At this point, over half the countries in Convention who were responsible for over half of greenhouse emissions, had ratified.

The European Union

Pictured above are US Secretary Kerry and the UN Secretary-General at the COP21 meeting of the UNFCCC in Paris.

Under the Kyoto Protocol, countries with targets could elect to reach these targets in co-operation with other countries. The European Union has decided to work as a unit to meet its emissions targets. The European climate change program attempts to do this by utilising an emissions trading scheme known as the European Union Greenhouse Gas Emission Trading Scheme. The principle of this scheme is quite simple: to make their legally binding commitments under Kyoto, countries can either make these savings within their own country, or they can buy these emissions reductions from other countries. These other countries would still need to meet their Kyoto targets, but the use of a free market system ensures the reductions are made for the least possible costs. Most reductions are made where these reductions are cheapest, and the excess reductions can be sold on to other countries where such cuts would be less economically viable. The EU ETS is arguably the global template for emissions trading schemes that are being implemented globally (China, South Korea, Tokyo and others).

The commitment of the European Union to cut back on its excessive green house gas emissions came from the implementation of the Kyoto Protocol. Since its application, the Doha amendments, which were made to ratify the Kyoto Protocol, have come to regulate this protocol and update the Union's commitment to reducing greenhouse gas emissions. This second round of the Kyoto Protocol created a legally binding agreement for participating EU countries until 2020. Under this plan, by 2030, there are hopes of the EU cutting their emissions by 40%.

States that become more involving or are and concerned about ability are more than likely to have more consistency; compared to the least become more refrained with agreements that comply decrease of legalization. “Empirical analyses of ratification of the FCCC and the Kyoto Protocol yield strong support for these propositions” .

Contraction and Convergence

The concept of Cap, Contraction and Convergence was proposed as a replacement to the Kyoto agreement. The idea here is that the limits to carbon emissions need to be capped at 350-450 parts per million, currently considered to produce a raise in world temperatures above pre-industrial levels of between 1 and 2 degrees Celsius. It is currently believed that further increases would bring about major positive feedbacks (the burning of forests and the loss of carbon from soils and oceans) which currently limit greenhouse gas emissions, and would lead to a run-away global warming similar to the Eocene period, during which there was no ice at the poles.

To sustain this figure, it has been proposed that on equity grounds, all people should be allocated an equal carbon footprint (currently about 2 tonnes per person, which by 2050 could fall to 1.5 tonnes per person through population increase). World per capita carbon emissions, currently in excess of 4 tonnes per person needs to contract to those levels, if these targets are to be met. As a result, in the name of global and inter-generational equity, policies needing to be instituted need to converge, over a fixed period towards this figure for every country. A trading regime, whereby which countries in excess of these figures (from example the US at 20 tonnes per capita), purchase carbon credits from a country using less than its allocation (e.g. Kenya at 1.3 tonnes per capita), is considered by many as the best way of solving this problem.

For example, the Contract and Converge strategy was adopted by India, China and many African countries as the basis for future negotiations. The UK Royal Commission on Environmental Pollution said in 2000 "the UK should be prepared to accept the contraction and convergence principle is the basis for international agreement on greenhouse gas emissions".

In 2015, the United Nations Framework Convention on Climate Change met in Paris to negotiate a new agreement. This meeting was the 21st Conference of Parties (COP21). In this agreement, the goal was to keep temperatures under 1.5 degrees Celsius. While this goal was seen as progress in the fight against climate change, some more liable countries saw this as unattainable. They also thought some important steps were not taken during this meeting; however, everyone seemed optimistic for the future.

Sub-national level action

Some states, regions, and cities in the world are taking the lead on developing emissions reduction methods in the absence of national policy, and may provide models for future national efforts. Their efforts are achieving real measurable emissions reductions and by pursuing policies and programs that have climate benefits, they have promoted state economic development, improved air quality and trimmed their vulnerability to energy price spikes. In the long run, addressing climate change will require comprehensive national policy and international agreements. However, in the US, due to absence of federal policy, states and regions are taking the lead on developing policies that may provide models for future national efforts.

Climate budget by the State of Odisha

While presenting the fiscal year 2020-2021 state budget for the Indian state of Odisha, Finance Minister of the state Niranjan Pujari introduced the Climate Budget. Climate budget aims to keep track of the expenses made by the government for climate change or to support mitigation and adaption actions to address climate change. As per the document, It will help the government to decide whether to redesign or safeguard the existing projects by seeing their impact on the climate change. Odisha has become the first state in India to introduce climate budget.

The Columbia River Basin

Large efforts are being made across many continents and many nations such as the United States. The Columbia River, which runs through the United States and Canada, has an abundance of naturally rich soil and wildlife; thus making it a natural resource to the North American continent. The Columbia River Basin uses the flow of the current of the river to create hydropower which makes it an economic resource as well. A study on the hydrology of the river was done in order to see the effects of climate in the future and how they can be managed.

The Danish Council on Climate Change

In response to the Climate Change Act, the Danish Council on Climate Change was formed in order to improve the quality of life through lessening the amount of carbon emitted in the atmosphere. This includes promoting cleaner future structures being built, clean renewable energy, and transportation. A group of experts is working with the council in order to ensure that accurate data is being attained and more action is being made to see improvements.

Regional Greenhouse Gas Initiative (United States)

The Regional Greenhouse Gas Initiative (RGGI, pronounced "Reggie") is the first mandatory market based program in the United States to reduce greenhouse gas emissions. RGGI is a cooperative effort among the states of Connecticut, Delaware, Maine, Maryland, Massachusetts, New Hampshire, New York, Rhode Island, and Vermont to cap and reduce CO
2 emissions from the power sector. In effect since January 1, 2009, the program is now in its third 3-year compliance period (2015-2017). As of 2016, RGGI has cut power plant carbon emissions in the region by 37%, while saving customers over $395 million on their bills.

Ghent, Belgium

The city promotes a meat-free day on Thursdays called Veggiedag, with vegetarian-only food in public canteens for civil servants and elected councillors, soon in all schools, and promotion of vegetarian eating options in town (through the distribution of "veggie street maps"). This campaign is linked to the recognition of the detrimental environmental effects of meat production, which the United Nations' Food and Agriculture Organization has established to represent nearly one-fifth of global greenhouse gas emissions.

United Kingdom

The town of Totnes in Devon through its "Transition Town Totnes" Project has adopted an Energy Descent Plan, as a response in answer to the twin problems of greenhouse gas emissions and peak oil. As a result of a series of large, well attended public gatherings with key experts from around the world, and the organisation of a number of special interest groups, the community has come together with lecturers and trainers shared with Schumacher College, through a process of participative strategic planning, to hone their skills in project development. As a result of the initiatives in Totnes, a large number of other communities have started "Transition Town" projects, and there are now more than 400 around the world, ranging from small communities to whole cities (e.g. Berlin).

The concepts of including food miles or carbon neutral labels on packaging has been gaining interest in the UK.

Individual action

Studies published in peer-reviewed scientific journals have posited that the most significant way for individuals to reduce their own ecological footprint is to have fewer children,(however this is disputed) followed by traveling without a vehicle, forgoing air travel and adopting a largely plant-based diet. According to another study if the wealthiest 10% of people in the world (net wealth over about US$100 thousand) cut their emissions to the level of the average EU citizen that would cut total emissions by a third.

Energy Usage

Around 66% of worldwide greenhouse gas emissions are connected to consuming fossil fuels for energy to utilize for warming, power, transport and industry. In Europe, as well, energy processes are the biggest producer of greenhouse gases, 78% of total EU emissions in 2015. In some countries individual consumers can switch energy supplier to consume only sustainable energy.

Vegetables on sale

Transport

Individuals concerned about climate change should not buy SUV's powered by gasoline or diesel, and should not fly frequently.

Diet

The 2019 World Scientists’ Warning of a Climate Emergency, endorsed by over 11,000 scientists from more than 150 countries, stated that "eating mostly plant-based foods while reducing the global consumption of animal products, especially ruminant livestock, can improve human health and significantly lower GHG emissions."

Carbon Conversations

Carbon Conversations is a "psychosocial project that addresses the practicalities of carbon reduction while taking account of the complex emotions and social pressures that make this difficult". The project touches on five main topics: i) home energy; ii) food; iii) travel; iv) consumption and waste; and v) talking with family and friends. The project understands that individuals often fail to adopt low-carbon lifestyles not because of practical barriers to change (e.g.: there is no renewable energy available), but because of aspects related to their values, emotions, and identity. The project offers a supportive group experience that helps people reduce their personal carbon dioxide emissions by 1 tonne CO
2 on average and aim at halving it in the long term. They deal with the difficulties of change by connecting to values, emotions and identity. The groups are based on a psychosocial understanding of how people change. Groups of 6-8 members meet six or twelve times with trained facilitators in homes, community centres, workplaces or other venues. The meetings create a non-judgmental atmosphere where people are encouraged to make serious lifestyle changes.

Carbon Conversations was cited in The Guardian newspaper as one of the 20 best ideas to tackle climate change.