The
Toba supereruption was a
supervolcanicA supervolcano is a volcano capable of producing a volcanic eruption with an ejecta volume greater than 1,000 cubic kilometers . This is thousands of times larger than most historic volcanic eruptions. Supervolcanoes can occur when magma in the Earth rises into the crust from a hotspot but is...
eruptionDuring a volcanic eruption, lava, tephra , and various gases are expelled from a volcanic vent or fissure. Several types of volcanic eruptions have been distinguished by volcanologists. These are often named after famous volcanoes where that type of behavior has been observed...
that occurred some time between 69,000 and 77,000 years ago at
Lake TobaLake Toba is a lake and supervolcano. The lake is 100 kilometres long and 30 kilometres wide, and 505 metres at its deepest point. Located in the middle of the northern part of the Indonesian island of Sumatra with a surface elevation of about , the lake stretches from to...
(
SumatraSumatra is an island in western Indonesia, westernmost of the Sunda Islands. It is the largest island entirely in Indonesia , and the sixth largest island in the world at 473,481 km2 with a population of 50,365,538...
,
IndonesiaIndonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...
). It is recognized as one of the Earth's largest known eruptions. The related
catastrophe theory holds that this event plunged the planet into a 6-to-10-year
volcanic winterA volcanic winter is the reduction in temperature caused by volcanic ash and droplets of sulfuric acid obscuring the sun and raising Earth's albedo after a large particularly explosive type of volcanic eruption...
and possibly an additional 1,000-year cooling episode. This change in temperature resulted in the world's human population being reduced to 10,000 or even a mere 1,000 breeding pairs, creating a
bottleneckA population bottleneck is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing....
in
human evolutionHuman evolution refers to the evolutionary history of the genus Homo, including the emergence of Homo sapiens as a distinct species and as a unique category of hominids and mammals...
.
The Toba event is the most closely studied supereruption. In 1993, science journalist Ann Gibbons first suggested a link between the eruption and a bottleneck in human evolution. Michael R. Rampino of
New York UniversityNew York University is a private, nonsectarian research university based in New York City. NYU's main campus is situated in the Greenwich Village section of Manhattan...
and Stephen Self of the
University of Hawaii at ManoaThe University of Hawaii at Mānoa is a public, co-educational university and is the flagship campus of the greater University of Hawaii system...
quickly lent their support to the idea. The theory was further developed in 1998 by Stanley H. Ambrose of the
University of Illinois at Urbana-ChampaignThe University of Illinois at Urbana–Champaign is a large public research-intensive university in the state of Illinois, United States. It is the flagship campus of the University of Illinois system...
.
Supereruption
The
Toba eruption or
Toba event occurred at what is now Lake Toba about or ago. The Toba eruption was the latest of the three major eruptions which occurred at Toba in the last 1 million years. The last eruption had an estimated
Volcanic Explosivity IndexThe Volcanic Explosivity Index was devised by Chris Newhall of the U.S. Geological Survey and Stephen Self at the University of Hawaii in 1982 to provide a relative measure of the explosiveness of volcanic eruptions....
of 8 (described as "mega-colossal"), or magnitude ≥ M8; it thus made a sizeable contribution to the
calderaA caldera is a cauldron-like volcanic feature usually formed by the collapse of land following a volcanic eruption, such as the one at Yellowstone National Park in the US. They are sometimes confused with volcanic craters...
complex.
Dense-rock equivalentDense-rock equivalent is a volcanologic calculation used to estimate volcanic eruption volume. One of the widely accepted measures of the size of a historic or prehistoric eruption is the volume of magma ejected as pumice and volcanic ash, known as tephra during an explosive phase of the eruption,...
estimates of eruptive volume for the eruption vary between and , but the most frequently quoted DRE is (about ) of erupted magma, of which was deposited as ash fall. It was two orders of magnitude greater in erupted mass than the largest volcanic eruption in historic times, in 1815 at
Mount TamboraMount Tambora is an active stratovolcano, also known as a composite volcano, on the island of Sumbawa, Indonesia. Sumbawa is flanked both to the north and south by oceanic crust, and Tambora was formed by the active subduction zone beneath it. This raised Mount Tambora as high as , making it...
in Indonesia, which caused the 1816 "
Year Without a SummerThe Year Without a Summer was 1816, in which severe summer climate abnormalities caused average global temperatures to decrease by about 0.4–0.7 °C , resulting in major food shortages across the Northern Hemisphere...
" in the northern hemisphere.
Although the Toba eruption took place in Indonesia, it deposited an ash layer approximately 15 centimetres thick over the entirety of
South AsiaSouth Asia, also known as Southern Asia, is the southern region of the Asian continent, which comprises the sub-Himalayan countries and, for some authorities , also includes the adjoining countries to the west and the east...
. A blanket of volcanic ash was also deposited over the
Indian OceanThe Indian Ocean is the third largest of the world's oceanic divisions, covering approximately 20% of the water on the Earth's surface. It is bounded on the north by the Indian Subcontinent and Arabian Peninsula ; on the west by eastern Africa; on the east by Indochina, the Sunda Islands, and...
, and the
ArabianThe Arabian Sea is a region of the Indian Ocean bounded on the east by India, on the north by Pakistan and Iran, on the west by the Arabian Peninsula, on the south, approximately, by a line between Cape Guardafui in northeastern Somalia and Kanyakumari in India...
and
South China SeaThe South China Sea is a marginal sea that is part of the Pacific Ocean, encompassing an area from the Singapore and Malacca Straits to the Strait of Taiwan of around...
. Deep-sea cores retrieved from the South China Sea extended the known distribution of the eruption and suggest that the calculation of the eruption magnitude is a minimum value or even an underestimate.
Volcanic winter and cooling
The apparent coincidence of the eruption with the onset of the last glacial period attracted the scientists' interest. Michael L. Rampino and Stephen Self argued that the eruption caused a "brief, dramatic cooling or 'volcanic winter'", which resulted in a global mean surface temperature drop of 3–5°C and accelerated the glacial transition from warm to cold temperatures of the last glacial cycle. Zielinski showed
GreenlandGreenland is an autonomous country within the Kingdom of Denmark, located between the Arctic and Atlantic Oceans, east of the Canadian Arctic Archipelago. Though physiographically a part of the continent of North America, Greenland has been politically and culturally associated with Europe for...
ice coreAn ice core is a core sample that is typically removed from an ice sheet, most commonly from the polar ice caps of Antarctica, Greenland or from high mountain glaciers elsewhere. As the ice forms from the incremental build up of annual layers of snow, lower layers are older than upper, and an ice...
evidence for a 1,000-year cool period with low
δ18O and increased dust deposition immediately following the eruption. He further suggested that this 1,000-year cool period (stadial) could have been caused by the eruption, and that the longevity of the Toba stratospheric loading may account at least for the first two centuries of the cooling episode. Rampino and Self believe that global cooling was already underway at the time of the eruption, but the procedure was extremely slow; YTT "may have provided the extra 'kick' that caused the climate system to switch from warm to cold states." Oppenheimer discounts the arguments that the eruption triggered the last glaciation, but he accepts that it may have been responsible for a millennium of cool climate prior to the
Dansgaard-Oeschger eventDansgaard–Oeschger events are rapid climate fluctuations that occurred 25 times during the last glacial period. Some scientists claim that the events occur quasi-periodically with a recurrence time being a multiple of 1,470 years, but this is debated...
.
According to Alan Robock, the Toba incident did not initiate an
ice ageAn ice age or, more precisely, glacial age, is a generic geological period of long-term reduction in the temperature of the Earth's surface and atmosphere, resulting in the presence or expansion of continental ice sheets, polar ice sheets and alpine glaciers...
. Using an emission of 6,000 million tons of sulphur dioxide, his simulations demonstrated a maximum global cooling of around 15°C, approximately three years after the eruption. As the saturated adiabatic lapse rate is 4.9°C/1,000 m for temperatures above freezing, this means that the tree line and the
snow lineThe climatic snow line is the point above which snow and ice cover the ground throughout the year. The actual snow line may seasonally be significantly lower....
were around 3,000 m (9,900 ft) lower at this time. Nevertheless, the climate recovered over a few decades. Robock found no evidence that the 1,000-year cold period seen in Greenland ice core records was directly generated by the Toba eruption. Nevertheless, he argues that the volcanic winter would have been colder and longer-lasting than Ambrose assumed, which strengthens his argument for a genetic bottleneck. Contrary to Robock, Oppenheimer believes that estimates of a surface temperature drop of 3–5°C after the eruption are probably too high; a figure closer to 1°C appears more realistic. Robock criticized Oppenheimer's analysis, arguing that it is based on simplistic
T-forcingIn statistics, a standard score indicates how many standard deviations an observation or datum is above or below the mean. It is a dimensionless quantity derived by subtracting the population mean from an individual raw score and then dividing the difference by the population standard deviation...
relationships.
Despite the different approaches and estimates, scientists agree that a supereruption like the one at Lake Toba must have led to very extensive ash-fall layers and injection of noxious gases into the atmosphere, having severe worldwide effects on climate and weather. Additionally, the Greenland ice core data display an abrupt climate change around this time, but there is no consensus that the eruption directly generated the 1,000-year cold period seen in Greenland or triggered the last glaciation.
Genetic bottleneck theory
Ann Gibbons first suggested, in an article in the October 1993 edition of
ScienceScience is the academic journal of the American Association for the Advancement of Science and is one of the world's top scientific journals....
, that a bottleneck in human evolution about 50,000 years ago could be linked to the Toba eruption. Rampino and Self backed up this idea in a letter to the journal later that year. The bottleneck theory was then further developed by Ambrose in 1998 and Rampino & Ambrose in 2000, who invoked the Toba eruption to explain a severe culling of the human population.
According to the supporters of the genetic bottleneck theory, between 50,000 and 100,000 years ago, human population suffered a severe population decrease—only 3,000 to 10,000 individuals survived—followed eventually by rapid population increase, innovation, progress and migration. Several geneticists, including Lynn Jorde and
Henry HarpendingHenry C. Harpending is an anthropologist and population geneticist at the University of Utah, where he is a distinguished professor...
have proposed that the human race was reduced to approximately five to ten thousand people. Genetic evidence suggests that all humans alive today, despite apparent variety, are descended from a very small population, perhaps between 1,000 to 10,000 breeding pairs about 70,000 years ago. Note that this is an estimate of ancestors, not of total human population. Isolated pockets of humans who eventually died out without descendants may have also existed in numbers that cannot be reliably estimated by geneticists.
Ambrose and Rampino proposed in the late 1990s that a genetic bottleneck could have been caused by the climate effects of the Toba eruption. The supporters of the Toba catastrophe theory suggest that the eruption resulted in a global ecological disaster with extreme phenomena, such as worldwide vegetation destruction, and severe drought in the tropical rainforest belt and in monsoonal regions. Τhis massive environmental change created population bottlenecks in species that existed at the time, including hominids; this in turn accelerated differentiation of the reduced human population. Therefore, Toba may have caused modern races to differentiate abruptly only 70,000 years ago, rather than gradually over one million years. Robock believes that, indeed, a 10-year volcanic winter triggered by YTT could have largely destroyed the food supplies of humans and therefore caused a significant reduction in population sizes.
Gene analysis of some genes shows divergence anywhere from 60,000 to 2 million years ago. This does not contradict the Toba theory, however, because Toba is not conjectured to be an extreme bottleneck event. The complete picture of gene lineages, including present-day levels of human
genetic variationGenetic variation, variation in alleles of genes, occurs both within and among populations. Genetic variation is important because it provides the “raw material” for natural selection. Genetic variation is brought about by mutation, a change in a chemical structure of a gene. Polyploidy is an...
, allows the theory of a Toba-induced human population bottleneck.
However, research by archaeologist Michael Petraglia's team cast doubt on Ambrose's theory. Petraglia and his team found stone tools in southern India, above and below a thick layer of ash from the Toba eruption. The tools from each layer were remarkably similar, and Petraglia says that this shows that the huge dust clouds from the eruption did not wipe out the local population of people:
A 2009 study by Martin A. J. Williams's team challenges Petraglia's findings. Williams analysed pollen from a marine core in the
Bay of BengalThe Bay of Bengal , the largest bay in the world, forms the northeastern part of the Indian Ocean. It resembles a triangle in shape, and is bordered mostly by the Eastern Coast of India, southern coast of Bangladesh and Sri Lanka to the west and Burma and the Andaman and Nicobar Islands to the...
with stratified Toba ash, and argued that the eruption caused prolonged deforestation in South Asia. Ambrose, who is a co-author of the study, calls the evidence "unambiguous", and further argues that YTT may have forced our ancestors to adopt new survival strategies, which permitted them to replace
NeanderthalThe Neanderthal is an extinct member of the Homo genus known from Pleistocene specimens found in Europe and parts of western and central Asia...
s and "other archaic human species". However, both Neanderthals in Europe and the small-brained
Homo floresiensisHomo floresiensis is a possible species, now extinct, in the genus Homo. The remains were discovered in 2003 on the island of Flores in Indonesia. Partial skeletons of nine individuals have been recovered, including one complete cranium...
in Southeastern Asia survived YTT by 50,000 and 60,000 years respectively.
Oppenheimer accepts that the arguments proposed by Rampino and Ambrose are plausible, but they are not yet compelling for two reasons: it is difficult to estimate the global and regional climatic impacts of the eruption, and, at the same time, we cannot conclude with any confidence that the eruption actually preceded the bottleneck. Furthermore, a 2010 geneticists' study seems to question the foundations of the Toba bottleneck theory: analysis of
Alu sequenceAn Alu element is a short stretch of DNA originally characterized by the action of the Alu restriction endonuclease. Alu elements of different kinds occur in large numbers in primate genomes. In fact, Alu elements are the most abundant mobile elements in the human genome. They are derived from the...
s across the entire
human genomeThe human genome is the genome of Homo sapiens, which is stored on 23 chromosome pairs plus the small mitochondrial DNA. 22 of the 23 chromosomes are autosomal chromosome pairs, while the remaining pair is sex-determining...
has shown that the effective human population was already less than 26,000 as far back as 1.2 million years ago, suggesting that no Toba bottleneck was necessary. Possible explanations for the low population size of human ancestors may include repeated population bottlenecks or periodic replacement events from competing
HomoHomo may refer to:*the Greek prefix ὅμο-, meaning "the same"*the Latin for man, human being*Homo, the taxonomical genus including modern humans...
subspecies.
Genetic bottlenecks related to the human population
Evolutionary biologist
Richard DawkinsClinton Richard Dawkins, FRS, FRSL , known as Richard Dawkins, is a British ethologist, evolutionary biologist and author...
has postulated that human
mitochondrial DNAMitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...
(inherited only from one's mother) and
Y chromosomeThe Y chromosome is one of the two sex-determining chromosomes in most mammals, including humans. In mammals, it contains the gene SRY, which triggers testis development if present. The human Y chromosome is composed of about 60 million base pairs...
DNA (from one's father) show coalescence at around 140,000 and 60,000 years ago respectively. In other words, all living humans'
female line ancestryMatrilineality is a system in which descent is traced through the mother and maternal ancestors. Matrilineality is also a societal system in which one belongs to one's matriline or mother's lineage, which can involve the inheritance of property and/or titles.A matriline is a line of descent from a...
trace back to a single female (
Mitochondrial EveIn the field of human genetics, Mitochondrial Eve refers to the matrilineal "MRCA" . In other words, she was the woman from whom all living humans today descend, on their mother's side, and through the mothers of those mothers and so on, back until all lines converge on one person...
) at around 140,000 years ago. Via the
male linePatrilineality is a system in which one belongs to one's father's lineage. It generally involves the inheritance of property, names or titles through the male line as well....
, all humans can trace their ancestry back to a single male (
Y-chromosomal AdamIn human genetics, Y-chromosomal Adam is the theoretical most recent common ancestor from whom all living people are descended patrilineally . Many studies report that Y-chromosomal Adam lived as early as around 142,000 years ago and possibly as recently as 60,000 years ago...
) at 60,000 to 90,000 years ago.
This is consistent with the Toba catastrophe theory which suggests that a
bottleneck of the human populationA population bottleneck is an evolutionary event in which a significant percentage of a population or species is killed or otherwise prevented from reproducing....
occurred c. 70,000 years ago, proposing that the human population was reduced to c. 15,000 individuals when the
TobaLake Toba is a lake and supervolcano. The lake is 100 kilometres long and 30 kilometres wide, and 505 metres at its deepest point. Located in the middle of the northern part of the Indonesian island of Sumatra with a surface elevation of about , the lake stretches from to...
supervolcanoA supervolcano is a volcano capable of producing a volcanic eruption with an ejecta volume greater than 1,000 cubic kilometers . This is thousands of times larger than most historic volcanic eruptions. Supervolcanoes can occur when magma in the Earth rises into the crust from a hotspot but is...
in
IndonesiaIndonesia , officially the Republic of Indonesia , is a country in Southeast Asia and Oceania. Indonesia is an archipelago comprising approximately 13,000 islands. It has 33 provinces with over 238 million people, and is the world's fourth most populous country. Indonesia is a republic, with an...
erupted and triggered a major
environmentThe natural environment encompasses all living and non-living things occurring naturally on Earth or some region thereof. It is an environment that encompasses the interaction of all living species....
al change, including a
volcanic winterA volcanic winter is the reduction in temperature caused by volcanic ash and droplets of sulfuric acid obscuring the sun and raising Earth's albedo after a large particularly explosive type of volcanic eruption...
. The theory is based on geological evidences of sudden climate change at that time, and on coalescence evidences of some genes (including mitochondrial DNA, Y-chromosome and some nuclear genes) and the relatively low level of genetic variation among present-day humans.
However, such coalescence is genetically expected and does not, in itself, indicate a population bottleneck, because mitochondrial DNA and Y-chromosome DNA are only a small part of the entire
genomeIn modern molecular biology and genetics, the genome is the entirety of an organism's hereditary information. It is encoded either in DNA or, for many types of virus, in RNA. The genome includes both the genes and the non-coding sequences of the DNA/RNA....
, and are atypical in that they are inherited exclusively through the mother or through the father, respectively. Most genes in the genome are inherited randomly from either father or mother, thus can be traced back in time via either matrilineal or patrilineal ancestry. Research on many (but not necessarily most) genes find various coalescence points from 2 million years ago to 60,000 years ago, according to the genes considered, thus disproving the existence of more recent extreme bottlenecks (i.e. a
single breeding pair).
On the other hand, in 2000, a
Molecular Biology and Evolution paper suggested a transplanting model or a 'long bottleneck' to account for the limited genetic variation, rather than a catastrophic environmental change. This would be consistent with suggestions that in sub-Saharan Africa human populations could have dropped at times as low as 2,000, for perhaps as long as 100,000 years, before numbers began to expand again in the
Late Stone AgeThe Upper Paleolithic is the third and last subdivision of the Paleolithic or Old Stone Age as it is understood in Europe, Africa and Asia. Very broadly it dates to between 40,000 and 10,000 years ago, roughly coinciding with the appearance of behavioral modernity and before the advent of...
One early oversight of many early studies is that the fixation of
alleleAn allele is one of two or more forms of a gene or a genetic locus . "Allel" is an abbreviation of allelomorph. Sometimes, different alleles can result in different observable phenotypic traits, such as different pigmentation...
s (the object of coalescent theory study) is not a discrete mathematical function, but a
probabilistic functionProbability theory is the branch of mathematics concerned with analysis of random phenomena. The central objects of probability theory are random variables, stochastic processes, and events: mathematical abstractions of non-deterministic events or measured quantities that may either be single...
, and it is highly dependent on the
ploidyPloidy is the number of sets of chromosomes in a biological cell.Human sex cells have one complete set of chromosomes from the male or female parent. Sex cells, also called gametes, combine to produce somatic cells. Somatic cells, therefore, have twice as many chromosomes. The haploid number is...
being studied.
Takahata (1999) was the first molecular anthropologist to point out that conclusions drawn from single locus studies suffer from the large randomness of the fixation process. Schaffner (2004) has cleared up this issue by demonstrating the three sets of fixation ranges, haploid, X-linked and diploid where TMCRAs for different loci are expected to fall. Takahata (1993) estimated the effective human population size at 11,000 individuals, and Schaffner working on an improved set of X-linked markers from low recombination regions of the X-chromosome identified an effective size of approximately 12,000 individuals. PDHA1 falls on the edge of fixation times for X-linked chromosome. For autosomes, the MX1 locus and the HLA loci appear to preserve past diversity in the human population. With few exceptions, however, X-linked and autosomes appear to coalesce under a common population size.
Just as mitochondria are inherited matrilineally, Y-chromosomes are inherited patrilineally. Y chromosomal TMRCA, the time of the
Y-chromosomal AdamIn human genetics, Y-chromosomal Adam is the theoretical most recent common ancestor from whom all living people are descended patrilineally . Many studies report that Y-chromosomal Adam lived as early as around 142,000 years ago and possibly as recently as 60,000 years ago...
, lie in the 42 to 110ky range, which is a little less than half the TMRCA of mtDNA. Importantly, the genetic evidence suggests that the most recent patriarch of all humanity is much more recent than the most recent matriarch, suggesting that 'Adam' and 'Eve' were not alive at the same time. While 'Eve' is believed to have lived more than 140,000 years ago, 'Adam' appears to have lived less than 110,000 years ago. According to Wilder et al. (2004), the lower TMRCA of Y is due to an effective population size of males 1/2 that of females over most of human evolution.
Even with a reduced effective population size there are problems with this explanation . Recently, with more mitogenomic sequences from Africa, evidence has grown for an early population size expansion. This expansion probably started prior to 100,000 years ago and greatly increasing after 100,000 years ago. The effective size of the human population should have well exceeded 10
4 individuals between 80,000 to 120,000 years ago. Given this expansion, implicit male populations sizes would have improbably coalesced to
Y-Adam within that time frame. However, the greatest age for Y TMRCA is more recent than the evidence for expansion. In addition, despite evidence of a bottleneck, the human mtDNA TMRCA range remains consistent with population sizes estimates from X-linked and autosomal loci. However, Y-chromosomes TMRCA is not consistent with mtDNA or either of these sets (see figure:
TMRCAs of loci).
This inconsistency maybe explained by some form of Y chromosome selection (cultural, or genetic). A Y-chromosomal lineage might have swept the male population. However, if true the place of greatest Y chromosomal diversity could be anywhere that humans inhabited Africa. However, Y diversity is greatest in Southern Africa, close to the earliest female population split predicted by Behar et al. (2009) suggesting the earliest branch in Y should be between 125,000 and 150,000 Ka in age. This suggests a SNP rate inaccuracy in the Y-chromosomal and/or mtDNA molecular clock. A recent study of X-chromosome suggests that different rates of male sperm production between humans and chimps has altered the molecular clock in sex chromosomes. This shift in the molecular clock would not affect the mtDNA SNP rate and would affect the Y-chromosomal rate more than X-linked and autosomes, since these Y-chromosomal lineages spend the most time in male testes.
The term
bottleneck has been used to describe the population structure that created mtDNA Eve. The appearance of a bottleneck was a consequence of the appearance of a 'big bang' of HVR branching about the time humans first left Africa. From that point back to the TMRCA was less than 100,000 years and the population size estimate was below 5000 effective females. Looking backwards in time this is what might be called a
retrograde bottleneck, however it is an artifact of coalescence process, since the coalescence of mitogenomes on the sequence of the MRCA (the event which initiated with mtDNA Eve and extended to the extant population) conceals the population size from all points earlier than that mutation (see figure
Retrograde look at bottlenecks). Therefore the population size could have been of equal size going back 100,000s of years, to the time in which Neanderthals' ancestors and Modern humans' ancestors were part of a single population.
Evidence against a population bottleneck
The work done on Neanderthal sequencing (Green 2007) has identified little evidence of Neanderthal contribution to humans, moreover it describes an effective size of the population when humans and Neanderthals split was about 3000 individuals. Taken in the light of Schaffner's and Takahata's effective populations sizes, 3000 < N
e, female < 6000 and 2000 < N
e, male < 4000 does not appear to represent a magnitude shift downward from the average size. Taking a null hypothesis, prior to and after the mtDNA MRCA population sizes appear to reflect long-term small population structure up until 70,000~150,000 years ago, not a brief constricting bottleneck, but a long period of constrained size followed by an expansion.
Evidence for a population bottleneck
Confidence intervals of population size do not require an alternative,
population bottleneck, hypothesis. However, a bottleneck may have existed. If the population size were at 12,000 individuals as suggested by X-chromosomal studies, the N
e for mtDNA and Y in particular, is below the expected median TMRCAs (See image Above and on the left). Y chromosome and mtDNA may be more representative of population structure immediately prior to expansion. However, meshing mtDNA TMRCA and Y TMRCA is problematic. If these two loci could be treated together, they would likely fall significantly below the X-linked and autosome-derived size estimates for any given T
CHLCA.
show that prior to 150,000 years ago the population could have been as low as 1000 effective females (~1500 total, 4500 census) and as high as 11,000 effective females with a lower population size between 150,000 to 200,000 years ago. Whereas X-chromosome and autosomes warrant larger population size minima, thousands of females, these loci of larger ploidy are capable of sensing population structure of much longer periods. Such periods may include recent and ancient population structures and size oscillations. Most population structure models for Africa have assumed much of the growth occurred very recently, however Atkinson et al. (2009) shows that by 100,000 years ago the minimum female population size exceed the estimated population size for females. The flat population/recent growth model is troubled in considering an ancient population core in Tanzania (Gonder. et al. (2007) early East African/Khoisan split (Behar et al. 2008), and spread of L2 in parts of Africa where L0 and L1 are found in low abundance. Simply, the evidence of lineage growth appears to correlate with growth in geographic regions in which humans live. Retrospectively, this suggests that population size was growing as new lineages appears to expand territory. Comparing these observations with populations sizes suggested by X-chromosome (~7000 females) one might expect a low stand of the human population size of 1/3 to 1/2 this size between 150,000 to 250,000 years ago. This indicates that earlier periods had a reciprocal, or larger size (>7000 females) between 200,000 and 500,000 years ago.
Other authors such as think that bottlenecks in the human prehistory were such a common feature that they interfere with TMRCA determinations and imply the possible effect of the
OIS-6Marine isotope stages , marine oxygen-isotope stages, or oxygen isotope stages , are alternating warm and cool periods in the Earth's paleoclimate, deduced from oxygen isotope data reflecting changes in temperature derived from data from deep sea core samples...
on population size reduction with a TMRCA around the time of late pliestocene climate optimum, approximately 120,000 years ago.
Human parasite: analysis of louse genes
Alan RogersAlan Rogers may refer to:*Alan Rogers , XXth century Bishop of Mauritius, Bishop of Fulham and Bishop of Edmonton*Alan Rogers , camping enthusiast and publisher...
, a co-author of this study and professor of
anthropologyAnthropology is the study of humanity. It has origins in the humanities, the natural sciences, and the social sciences. The term "anthropology" is from the Greek anthrōpos , "man", understood to mean mankind or humanity, and -logia , "discourse" or "study", and was first used in 1501 by German...
at the
University of UtahThe University of Utah, also known as the U or the U of U, is a public, coeducational research university in Salt Lake City, Utah, United States. The university was established in 1850 as the University of Deseret by the General Assembly of the provisional State of Deseret, making it Utah's oldest...
, says: “The record of our past is written in our
parasitesParasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that needed more than one host . These are now called macroparasites...
.” Rogers and others have proposed the bottleneck may have occurred because of a mass die-off of early humans due to a globally catastrophic volcanic eruption. The analysis of
louseLice is the common name for over 3,000 species of wingless insects of the order Phthiraptera; three of which are classified as human disease agents...
genes confirmed that the population of
Homo sapiens mushroomed after a small band of early humans left Africa sometime between 150,000 and 50,000 years ago.
Human pathogen: analysis of Helicobacter pylori genes
Recent research states that genetic diversity in the
pathogenic bacteriumPathogenic bacteria are bacteria that cause bacterial infection. This article deals with human pathogenic bacteria.Although the vast majority of bacteria are harmless or beneficial, quite a few bacteria are pathogenic...
Helicobacter pyloriHelicobacter pylori , previously named Campylobacter pyloridis, is a Gram-negative, microaerophilic bacterium found in the stomach. It was identified in 1982 by Barry Marshall and Robin Warren, who found that it was present in patients with chronic gastritis and gastric ulcers, conditions that were...
decreases with geographic distance from East Africa, the birthplace of modern humans. Using the genetic diversity data, the researchers have created simulations that indicate the bacteria seem to have spread from East Africa around 58,000 years ago. Their results indicate modern humans were already infected by
H. pylori before their migrations out of Africa, and
H. pylori remained associated with human hosts since that time.
Genetic bottlenecks of other mammals
The population of the Eastern African
chimpanzeeChimpanzee, sometimes colloquially chimp, is the common name for the two extant species of ape in the genus Pan. The Congo River forms the boundary between the native habitat of the two species:...
, Bornean
orangutanOrangutans are the only exclusively Asian genus of extant great ape. The largest living arboreal animals, they have proportionally longer arms than the other, more terrestrial, great apes. They are among the most intelligent primates and use a variety of sophisticated tools, also making sleeping...
, central Indian
macaqueThe Rhesus macaque , also called the Rhesus monkey, is one of the best-known species of Old World monkeys. It is listed as Least Concern in the IUCN Red List of Threatened Species in view of its wide distribution, presumed large population, and its tolerance of a broad range of habitats...
, the
cheetahThe cheetah is a large-sized feline inhabiting most of Africa and parts of the Middle East. The cheetah is the only extant member of the genus Acinonyx, most notable for modifications in the species' paws...
, the
tigerThe tiger is the largest cat species, reaching a total body length of up to and weighing up to . Their most recognizable feature is a pattern of dark vertical stripes on reddish-orange fur with lighter underparts...
, and the separation of the nuclear gene pools of eastern and western lowland
gorillaGorillas are the largest extant species of primates. They are ground-dwelling, predominantly herbivorous apes that inhabit the forests of central Africa. Gorillas are divided into two species and either four or five subspecies...
s, all recovered from very low numbers around 70,000–55,000 years ago.
Migration after Toba
It is currently not known where human populations were living at the time of the eruption. The most plausible scenario is that all the survivors were populations living in
AfricaAfrica is the world's second largest and second most populous continent, after Asia. At about 30.2 million km² including adjacent islands, it covers 6% of the Earth's total surface area and 20.4% of the total land area...
, whose descendants would go on to populate the world. However, recent archeological finds, mentioned above, have suggested that a human population may have survived in
JwalapuramJwalapuram is an archaeological site in South India which shows hominid habitation before and after the Toba event. It is unclear what species of humans settled Jwalapuram as no fossil remains appear to have been found....
,
Southern IndiaSouth India is the area encompassing India's states of Andhra Pradesh, Karnataka, Kerala and Tamil Nadu as well as the union territories of Lakshadweep and Pondicherry, occupying 19.31% of India's area...
.
Recent analyses of
mitochondrial DNAMitochondrial DNA is the DNA located in organelles called mitochondria, structures within eukaryotic cells that convert the chemical energy from food into a form that cells can use, adenosine triphosphate...
have set the estimate for the major migration from Africa from 60,000–70,000 years ago, around 10–20,000 years earlier than previously thought, and in line with dating of the Toba eruption to around 66,000–76,000 years ago. During the subsequent tens of thousands of years, the descendants of these migrants populated
AustraliaAustralia , officially the Commonwealth of Australia, is a country in the Southern Hemisphere comprising the mainland of the Australian continent, the island of Tasmania, and numerous smaller islands in the Indian and Pacific Oceans. It is the world's sixth-largest country by total area...
,
East AsiaEast Asia or Eastern Asia is a subregion of Asia that can be defined in either geographical or cultural terms...
,
EuropeEurope is, by convention, one of the world's seven continents. Comprising the westernmost peninsula of Eurasia, Europe is generally 'divided' from Asia to its east by the watershed divides of the Ural and Caucasus Mountains, the Ural River, the Caspian and Black Seas, and the waterways connecting...
, and finally the
AmericasThe Americas, or America , are lands in the Western hemisphere, also known as the New World. In English, the plural form the Americas is often used to refer to the landmasses of North America and South America with their associated islands and regions, while the singular form America is primarily...
.
It has been suggested that nearby hominid populations, such as
Homo erectus soloensisHomo erectus soloensis, known as Solo Man and formerly classified as Homo sapiens soloensis, is generally now regarded as a subspecies of the extinct hominin, Homo erectus. The only known specimens of this anomalous hominid were retrieved from sites along the Bengawan Solo River, on the Indonesian...
on
JavaJava is an island of Indonesia. With a population of 135 million , it is the world's most populous island, and one of the most densely populated regions in the world. It is home to 60% of Indonesia's population. The Indonesian capital city, Jakarta, is in west Java...
, and
Homo floresiensisHomo floresiensis is a possible species, now extinct, in the genus Homo. The remains were discovered in 2003 on the island of Flores in Indonesia. Partial skeletons of nine individuals have been recovered, including one complete cranium...
on
FloresFlores is one of the Lesser Sunda Islands, an island arc with an estimated area of 14,300 km² extending east from the Java island of Indonesia. The population was 1.831.000 in the 2010 census and the largest town is Maumere. Flores is Portuguese for "flowers".Flores is located east of Sumbawa...
, survived because they were
upwindWindward is the direction upwind from the point of reference. Leeward is the direction downwind from the point of reference. The side of a ship that is towards the leeward is its lee side. If the vessel is heeling under the pressure of the wind, this will be the "lower side"...
of Toba.
See also
- Wallace line
The Wallace Line separates the ecozones of Asia and Wallacea, a transitional zone between Asia and Australia. West of the line are found organisms related to Asiatic species; to the east, a mixture of species of Asian and Australian origin is present...
- Recent African origin of modern humans
In paleoanthropology, the recent African origin of modern humans is the most widely accepted model describing the origin and early dispersal of anatomically modern humans...
- Most recent common ancestor
In genetics, the most recent common ancestor of any set of organisms is the most recent individual from which all organisms in the group are directly descended...
- Timetable of major worldwide volcanic eruptions
This article is a list of volcanic eruptions of approximately at least magnitude 6 on the Volcanic Explosivity Index or equivalent sulfur dioxide emission around the Quaternary period. Some cooled the global climate; the extent of this effect depends on the amount of sulfur dioxide emitted...
- Early human migrations
Early human migrations began when Homo erectus first migrated out of Africa over the Levantine corridor and Horn of Africa to Eurasia about 1.8 million years ago, a migration probably sparked by the development of language Early human migrations began when Homo erectus first migrated out of Africa...
- Quaternary extinction event
The Quaternary period saw the extinctions of numerous predominantly larger, especially megafaunal, species, many of which occurred during the transition from the Pleistocene to the Holocene epoch. However, the extinction wave did not stop at the end of the Pleistocene, but continued especially on...
External links
- Population Bottlenecks and Volcanic Winter
- 1998 article based on news release regarding Ambrose's paper
- Homepage of Professor Stanley H. Ambrose
- Toba Volcano, by George Weber
- "The proper study of mankind"—Article in The Economist
- Journey of Mankind by The Bradshaw Foundation—includes discussion on Toba eruption, DNA and human migrations]
- Geography Predicts Human Genetic Diversity ScienceDaily (Mar. 17, 2005)—By analyzing the relationship between the geographic location of current human populations in relation to East Africa and the genetic variability within these populations, researchers have found new evidence for an African origin of modern humans.
- Out Of Africa – Bacteria, As Well: Homo Sapiens And H. Pylori Jointly Spread Across The Globe ScienceDaily (Feb. 16, 2007)—When man made his way out of Africa some 60,000 years ago to populate the world, he was not alone: He was accompanied by the bacterium Helicobacter pylori
Helicobacter pylori , previously named Campylobacter pyloridis, is a Gram-negative, microaerophilic bacterium found in the stomach. It was identified in 1982 by Barry Marshall and Robin Warren, who found that it was present in patients with chronic gastritis and gastric ulcers, conditions that were...
...; illus. migration map.