The savannah hypothesis (or savanna hypothesis) is a hypothesis that human bipedalism evolved as a direct result of human ancestors' transition from an arboreal lifestyle to one on the savannas. According to this hypothesis, millions of years ago hominins left the woodlands that had previously been their natural habitat, and adapted to their new habitat by walking upright.
The idea that a climate-driven retraction of tropical forests
forced early hominini into bipedalism has been around for a long time,
often implicitly. Some early authors saw savannahs as open grasslands,
while others saw a mosaic of environments from woodlands to grasslands.
The hypothesis has seen rising criticism since at least the late 1960s.
The open grasslands version is mostly dismissed, while the mosaic
version still has relatively wide support, although the transition from
forest to savanna probably was more gradual than previously thought.
History
The fundamental ideas behind it date back to Lamarck, Darwin and Wallace. Also Gustav Steinmann saw reducing rain forest due to climate change as important driver for bipedalism. Osborn thought man probably originated from the forests and flood-plains of southern Asia. Hilzheimer stated it was open landscapes that stimulated development.
The hypothesis first came to prominence however with the discovery of Australopithecus africanus by Raymond Dart in 1924. In an article on the discovery, published in the journal Nature, Dart wrote:
"For the production of man a
different apprenticeship was needed to sharpen the wits and quicken the
higher manifestations of intellect – a more open veldt country where
competition was keener between swiftness and stealth, and where
adroitness of thinking and movement played a preponderating role in the
preservation of the species. Darwin has said, "no country in the world
abounds in a greater degree with dangerous beasts than Southern Africa."
and, in my opinion, Southern Africa, by providing a vast open country
with occasional wooded belts and a relative scarcity of water, together
with a fierce and bitter mammalian competition, furnished a laboratory
such as was essential to this penultimate phase of human evolution."
— Raymond Dart, Australopithecus africanus: The Man-Ape of South Africa
Weinert
stated apes are very reluctant to leave the safety of the trees, and
the ancestors of modern man did not leave the trees, but the trees left
them. Grabau echoed this by saying Instead of the apes leaving the trees, the trees left the apes.
Not everyone agreed with this hypothesis, such as Weidenreich, but he did conclude it was a widely spread belief.
The work of Robert Ardrey helped popularize the ideas that Dart had developed with a wide audience.
In the decades following Dart's discovery, more hominid fossils
were found in Eastern and Southern Africa, leading researches to
conclude that these were savanna dwellers as well. Much of the academic
discussion at the time took for granted that the transition to the
savannas was responsible for the emergence of bipedalism, and focused
instead on determining particular mechanisms by which this happened.
One of the proposed mechanisms was the knuckle-walking
hypothesis; the claim that early human ancestors walked on all fours
when they first emerged into the savannas. This was based on
observations of morphological characteristics found in Australopithecus anamensis and Australopithecus afarensis, and posited that knuckle-walking was an example of convergent evolution in chimpanzees and gorillas, which was then lost by the genus Homo.
Paleoanthropologists also posited that the upright posture would have
been advantageous to savanna-dwelling hominids, as it allowed them to
peer over tall grasses for predators, or in search of prey.
P. E. Wheeler suggested that another advantage lay in reducing the
amount of skin exposed to the sun, which helped regulate body
temperatures. The turnover-pulse hypothesis, first described by Elizabeth Vrba seemed to support the savanna hypothesis by suggesting that climate change events resulting in the shrinking of forested areas forced animals out into the open grasslands.
Robinson investigated adaptive radiation for Australopithecus
and saw grass savanna and other more arid environments expanding at
this time, thus providing increased opportunity for animals capable of
adapting to such conditions. Monod
investigated the role in human evolution of the Sahara during wet
periods as a place that was covered with steppes, savannas, and lakes.
He saw advantages for the process of hominization in a wooded savanna.
In analogy with geladaJolly proposed that [i]n
the basal hominid, therefore, the 'gelada' specialisations would be
superimposed upon a behavioural repertoire and post-cranial structure
already attuned to some degree of truncal erectness. The transition to bipedality would have been instigated by seed-eating and would probably took place in a dambo-like environment, later shifting to wider floodplains.
An early critic of the savanna hypothesis was Lovejoy in 1981. He stated [i]t
is more likely that hominids venturing into open habitats were already
bipedal and that their regular occupation of savannahs was not possible
until intensified social behavior was well developed.
In the latter parts of the 20th century,
new fossil evidence began to emerge which called the savanna hypothesis
into question. These newly-discovered remains showed indications that
they were still well adapted to climbing trees, even after they had
begun to walk upright. Both humans and chimpanzees tend to walk upright when moving along long branches of trees, increasing their reach.
In 1993, 4.4 million year old fossil teeth were found in Aramis, Ethiopia, by a group led by Tim D. White attributed to a new species, Australopithecus ramidus, later called Ardipithecus ramidus. The age was thus half a million years older than previously known A. afarensis and had a more monkey-like appearance. After extensive research, in 2009 in a series of eleven articles in Science, more was published about Ardi. It concluded that Ar. ramidus preferred more wooded areas instead of open grassland, which would not support the climate-driven savannah hypothesis.
A year later, these conclusions were questioned: In contrast,
we find the environmental context of Ar. ramidus at Aramis to be
represented by what is commonly referred to as "tree or bush savanna"
with 25% or less woody canopy cover. The habitats involved probably
ranged from riparian forest to grassland.
For Phillip Tobias, the 1994 find of Little Foot, the collection of Australopithecus africanus
foot bones demonstrating features consistent with tree-climbing as well
as an upright gait, contributed to calling the savannah hypothesis
obsolete, stating Open the window and throw out the savannah hypothesis; it's dead and we need a new paradigm.
In 2000 Brigitte Senut and Martin Pickford found the 6 million year old Orrorin tugenensis
in Kenya. The skeleton seems to indicate both bipedalism and good
climbing skills. The latter indicates a wooded environment, as does the
discovery of black-and-white colobuses. The discovery of impalas points more towards a more open landscape. It later led Senut to the conclusion that the savannah hypothesis was no longer tenable. If these fossils are indeed early ancestors of modern man, then the environment of the later Australopithecus is less relevant.
In 2001 the 7 million year old Sahelanthropus tchadensis was discovered in Chad. Based on animal finds in the vicinity, this suggests a mosaic of environments from gallery forest at the edge of a lake area to a dominance of large savannah and grassland, although more research was needed to determine this precisely. The 5.6 million year old Ardipithecus kadabba discovered in 1997 was found in a similar terrain.
Definition of savannah
Not
everyone was willing to write off the savannah hypothesis. A poor
definition of what a savannah actually is, contributed to this. Critics
of the hypothesis often saw the savannah as open grasslands with
sporadic tree growth. However, savannas can have a high tree density and
can also be humid. The big difference between savannas and forests is
the lack of grasses in the latter. Thure E. Cerling
developed a method to determine the forest cover of ancient landscapes,
thus no longer requiring a definition of what a savannah is. By
distinguishing between the C3 plants of the tropical forests and the mix of trees and C4 grasses of the savannah, they investigated the stable carbon isotope of paleosols from some sites in East Africa. They described landscapes varying from forest, woodland/bushland/shrubland, wooded grasslands to grasslands. They concluded that the early hominini lived in a more open environment than Australopithecus, rendering the savannah hypothesis still a plausible possibility.
Following on from Cerling, Manuel Domínguez-Rodrigo
stated that the usual division of landscapes into grassy, wooded and
wooded is of little use, because it tells nothing about the evolutionary pressure
on mammals. For example, the selection pressure of grass fields in
tropical forests is incomparable to the grasslands of savannas. Tropical
forests also have many different species of trees, while savannas only
have a few species, which hardly carry any fruit. Another factor is that
of scale. Paleontologists often only investigate the site itself, an
area of several hundred to thousands of meters. These habitats are
referred to as biomes,
but the latter include many hundreds of kilometres. According to
Domínguez-Rodrigo, the savannah hypothesis can still give a good
explanation, although the transition of environment has probably been
less abrupt than some earlier authors thought.
The control of fire by early humans was a turning point in the technological evolution of human beings. Fire provided a source of warmth and lighting, protection from predators (especially at night), a way to create more advanced hunting tools, and a method for cooking
food. These cultural advances allowed human geographic dispersal,
cultural innovations, and changes to diet and behavior. Additionally, creating fire allowed human activity to continue into the dark and colder hours of the evening.
Claims for the earliest definitive evidence of control of fire by a member of Homo range from 1.7 to 2.0 million years ago (Mya). Evidence for the "microscopic traces of wood ash" as controlled use of fire by Homo erectus, beginning some 1,000,000 years ago, has wide scholarly support. Flint blades burned in fires roughly 300,000 years ago were found near fossils of early but not entirely modern Homo sapiens in Morocco. Fire was used regularly and systematically by early modern humans to heat treat silcrete stone to increase its flake-ability for the purpose of toolmaking approximately 164,000 years ago at the South African site of Pinnacle Point. Evidence of widespread control of fire by anatomically modern humans dates to approximately 125,000 years ago.
Control of fire
Use
and control of fire was a gradual process, proceeding through more than
one stage. One was a change in habitat, from dense forest, where
wildfires were common, to savanna (mixed grass/woodland) where wildfires were of higher intensity. Such a change may have occurred about 3 Mya, when the savanna expanded in East Africa due to cooler and drier climate.
The next stage involved interaction with burned landscapes and foraging in the wake of wildfires, as observed in various wild animals. In the African savanna, animals that preferentially forage in recently burned areas include savanna chimpanzees (a variety of Pan troglodytes verus), vervet monkeys (Cercopithecus aethiops) and a variety of birds, some of which also hunt insects and small vertebrates in the wake of grass fires.
The next step would be to make some use of residual hot spots
that occur in the wake of wildfires. For example, foods found in the
wake of wildfires tend to be either burned or undercooked. This might
have provided incentives to place undercooked foods on a hotspot or to
pull food out of the fire if it were in danger of getting burned. This
would require familiarity with fire and its behavior.
An early step in the control of fire would have been transporting
it from burned to unburned areas and lighting them on fire, providing
advantages in food acquisition.
Maintaining a fire over an extended period of time, as for a season
(such as the dry season), may have led to the development of base
campsites. Building a hearth or other fire enclosure such as a circle of stones would have been a later development. The ability to make fire, generally with a friction device with hardwood rubbing against softwood (as in a bow drill), was a later development.
Each of these stages could occur at different intensities, ranging from occasional or "opportunistic" to "habitual" to "obligate" (unable to survive without it).
Lower Paleolithic evidence
Most of the evidence of controlled use of fire during the Lower Paleolithic is uncertain and has limited scholarly support. Some of the evidence is inconclusive because other plausible explanations exist, such as natural processes, for the findings. Recent findings support that the earliest known controlled use of fire took place in Wonderwerk Cave, South Africa, 1.0 Mya.
Africa
Findings from the Wonderwerk Cave site, in the Northern Cape
province of South Africa, provide the earliest evidence for controlled
use of fire. Intact sediments were analyzed using micromorphological
analysis and Fourier Transform Infrared Microspectroscopy (mFTIR) and
yielded evidence, in the form of burned bones and ashed plant remains,
that burning took place at the site 1.0 Mya.
East African sites, such as Chesowanja near Lake Baringo, Koobi Fora, and Olorgesailie in Kenya, show some possible evidence that fire was controlled by early humans.
In Chesowanja, archaeologists found red clay clasts
dated to 1.4 Mya. These clasts must have been heated to 400 °C (750 °F)
to harden. However, tree stumps burned in bush fires in East Africa
produce clasts, which when broken by erosion, are like those described
at Chesownja. Controlled use of fire at Chesowanja is unproven.
In Koobi Fora, sites show evidence of control of fire by Homo erectus at 1.5 Mya with findings of reddened sediment that could come from heating at 200–400 °C (400–750 °F).
Evidence of possible human control of fire, found at Swartkrans, South Africa, includes several burned bones, including ones with hominin-inflicted cut marks, along with Acheulean and bone tools. This site also shows some of the earliest evidence of carnivorous behavior in H. erectus.
A "hearth-like depression" that could have been used to burn
bones was found at a site in Olorgesailie, Kenya. However, it did not
contain any charcoal
and no signs of fire have been observed. Some microscopic charcoal was
found, but it could have resulted from a natural brush fire.
In Gadeb, Ethiopia, fragments of welded tuff
that appeared to have been burned were found in Locality 8E but
refiring of the rocks might have occurred due to local volcanic
activity.
In the Middle Awash
River Valley, cone-shaped depressions of reddish clay were found that
could have been formed by temperatures of 200 °C (400 °F). These
features, thought to have been created by burning tree stumps, were
hypothesized to have been produced by early hominids lighting tree
stumps so they could have fire away from their habitation site. This
view is not widely accepted, though.
Burned stones are also found in Awash Valley, but volcanic welded tuff
is also found in the area, which could explain the burned stones.
Burned flints discovered near Jebel Irhoud, Morocco, dated by thermoluminescence to around 300,000 years old, were discovered in the same sedimentary layer as skulls of early Homo sapiens. Paleoanthropologist Jean-Jacques Hublin believes the flints were used as spear tips and left in fires used by the early humans for cooking food.
Asia
In Xihoudu in Shanxi Province, China,
the black, blue, and grayish-green discoloration of mammalian bones
found at the site illustrates the evidence of burning by early hominids.
In 1985, a parallel site in China, Yuanmou in the Yunnan Province, archaeologists found blackened mammal bones that date back to 1.7 Mya.
At Trinil, Java, burned wood has been found in layers that carried H. erectus (Java Man) fossils dating from 830,000 to 500,000 BP. The burned wood has been claimed to indicate the use of fire by early hominids.
Middle Paleolithic evidence
Africa
The Cave of Hearths
in South Africa has burn deposits, which date from 700,000 to
200,000 BP, as do various other sites such as Montagu Cave (200,000 to
58,000 BP) and the Klasies River Mouth (130,000 to 120,000 BP).
Strong evidence comes from Kalambo Falls in Zambia,
where several artifacts related to the use of fire by humans have been
recovered, including charred logs, charcoal, carbonized grass stems and
plants, and wooden implements, which may have been hardened by fire. The
site has been dated through radiocarbon dating to be between 110,000 BP and 61,000 BP through amino-acid racemization.
Fire was used for heat treatment of silcrete stones to increase their workability before they were knapped into tools by Stillbay culture in South Africa.
These Stillbay sites date back from 164,000 to 72,000 years ago, with
the heat treatment of stone beginning by about 164,000 years ago.
Evidence at Zhoukoudian cave in China suggests control of fire as early as 460,000 to 230,000 BP.
Fire in Zhoukoudian is suggested by the presence of burned bones,
burned chipped-stone artifacts, charcoal, ash, and hearths alongside H. erectus fossils in Layer 10, the earliest archaeological horizon at the site. This evidence comes from Locality 1, also known as the Peking Man
site, where several bones were found to be uniformly black to grey. The
extracts from the bones were determined to be characteristic of burned
bone rather than manganese staining. These residues also showed IR spectra
for oxides, and a bone that was turquoise was reproduced in the
laboratory by heating some of the other bones found in Layer 10. At the
site, the same effect might have been due to natural heating, as the
effect was produced on white, yellow, and black bones.
Layer 10 itself is described as ash with biologically produced
silicon, aluminum, iron, and potassium, but wood ash remnants such as siliceous
aggregates are missing. Among these are possible hearths "represented
by finely laminated silt and clay interbedded with reddish-brown and
yellow brown fragments of organic matter, locally mixed with limestone
fragments and dark brown finely laminated silt, clay, and organic
matter." The site itself does not show that fires were made in Zhoukoudian, but the association of blackened bones with quartzite artifacts at least shows that humans did control fire at the time of the habitation of the Zhoukoudian cave.
Middle East
At the Amudian site of Qesem Cave, near the city of Kfar Qasim, evidence exists of the regular use of fire from before 382,000 BP to around 200,000 BP at the end of Lower Pleistocene.
Large quantities of burned bone and moderately heated soil lumps were
found, and the cut marks found on the bones suggest that butchering and
prey-defleshing took place near fireplaces. In addition, hominins living in Qesem cave managed to heat their flint to varying temperatures before knapping it into different tools.
Europe
Multiple sites in Europe, such as Torralba and Ambrona, Spain, and St. Esteve-Janson, France, have also shown evidence of use of fire by later versions of H. erectus. The oldest has been found in England at the site of Beeches Pit, Suffolk; uranium series dating and thermoluminescence dating place the use of fire at 415,000 BP. At Vértesszőlős, Hungary, while no charcoal has been found, burned bones have been discovered dating from c. 350,000 years ago. At Torralba and Ambrona, Spain, objects such as Acheulean stone tools, remains of large mammals such as extinct elephants, charcoal, and wood were discovered. At Saint-Estève-Janson in France, there is evidence of five hearths and reddened earth in the Escale Cave. These hearths have been dated to 200,000 BP. Evidence for fire making dates to at least the Middle Paleolithic, with dozens of Neanderthalhand axes from France exhibiting use-wear traces suggesting these tools were struck with the mineral pyrite to produce sparks around 50,000 years ago.
Impact on human evolution
Cultural innovation
Uses of fire by early humans
The discovery of fire came to provide a wide variety of uses for early hominids.
It acted as a source of warmth, making getting through low nighttime
temperatures possible and allowing survival in colder environments,
through which geographic expansion from tropical and subtropical climates to areas of temperate
climates containing colder winters began to occur. The use of fire
continued to aid hominids at night by also acting as a means by which to
ward off predatory animals.
Fire also played a major role in changing how food was obtained and consumed, primarily by the practice of cooking. This caused a significant increase in meat consumption and calorie intake.
In addition to cooking, it was soon discovered that meat could be dried
through the use of fire, allowing it to be preserved for times in which
harsh environmental conditions made hunting difficult. Fire was even used in manufacturing tools to be used for hunting and cutting meat. Hominids found that large fires had their uses, as well. By starting wildfires, they were able to increase land fertility and clear large amounts of brush and trees to make hunting easier.
As early they began to understand how to use fire, such a useful skill
may have led to specialized social roles through the separation of
cooking from hunting.
Protection and hunting
The
early discovery of fire had numerous benefits for early humans. They
were able to protect themselves from the weather, and were also able to
devise an entirely new way of hunting. Evidence of fire has been found in caves, suggesting it was used to keep warm. This is significant, because it allowed them to migrate to cooler climates and thrive. This evidence also suggests that fire was used to clear out caves prior to living in them. Use of shelter was a major advancement in protection from the weather and from other species.
In addition to protection from the weather, the discovery of fire
allowed for innovations in hunting. Initially, it was used to set grass fires to hunt and control the population of pests in the surrounding areas. Evidence shows that early hominids were able to corral and trap animals by means of fire prior to consumption.
Tool and weapon making
In
addition to the many other benefits that fire provided to early humans,
it also had a major impact on the innovation of tool and weapon
manufacture. The use of fire by early humans as an engineering tool to
modify the effectiveness of their weaponry was a major technological
advancement. In an archeological dig that dates to around 400,000 years
ago, researchers excavating in an area known as the 'Spear Horizon' in Schöningen, in the district of Helmstedt, Germany, unearthed eight wooden spears among a trove of preserved artifacts. The spears were found along with stone tools and horse remains, one of which still had a spear through its pelvis. At another dig site located in Lehringen, Germany, a fire-hardened lance was found thrust into the rib cage of a 'straight-tusked elephant'.
These archeological digs provide evidence that suggests the spears were
deliberately fire-hardened, which allowed early humans the ability to
modify their hunting tactics and use the spears as thrusting rather than
throwing weapons. Researchers further uncovered environmental evidence
that indicated early humans may have been waiting in nearby vegetation
that provided enough concealment for them to ambush their prey.
Fire-hardened spear circa 380,000 to 400,000 years old.
Early evidence for the extensive heat treatment of silcrete in the Howiesons Poort at Klipdrift Shelter (Layer PBD, 65 ka), South Africa.
More recent evidence dating to roughly 164,000 years ago found that early humans living in South Africa
in the Middle Stone Age used fire as an engineering tool to alter the
mechanical properties of the materials they used to make tools and
improve their lives. Researchers found evidence that suggests early
humans applied a method of heat treatment to a fine-grained, local rock
called silcrete.
Once treated, the heated rocks were modified and tempered into
crescent-shaped blades or arrowheads. The evidence suggests that early
humans probably used the modified tools for hunting or cutting meat from
killed animals. Researchers postulate that this may have been the
first time that a bow and arrows
were used for hunting, an advancement that had a significant impact on
how early humans may have lived, hunted, and existed as community
groups.
Art and ceremonial uses
Fire
was also used in the creation of art. Scientists have discovered
several small, 1- to 10-inch statues in Europe referred to as the Venus figurines. These statues date back to the Paleolithic period. Several of these figures were created from stone and ivory, while some were created with clay and then fired. These are some of the earliest examples of ceramics. Fire was also commonly used to create pottery.
Although the advent of pottery was first thought to have begun with the
use of agriculture around 10,000 years ago, scientists in China discovered pottery fragments in the Xianrendong Cave that were about 20,000 years old. During the Neolithic Age,
which began about 10,000 years ago, though, the creation and use of
pottery became far more widespread. These items were often carved and
painted with simple linear designs and geometric shapes.
Developments and expansion in early hominid societies
Fire was an important factor in expanding and developing societies of early hominids. One impact fire might have had was social stratification.
Those who could make and wield fire had more power than those who could
not, and may have, therefore, had a higher position in society.
The presence of fire also led to an increase in length of "daytime",
and allowed more activity to occur in the night that was not previously
possible.
Evidence of large hearths indicate that the majority of this nighttime
activity was spent around the fire, contributing to social interactions
among individuals.
This increased amount of social interaction is speculated to be
important in the development of language, as it fostered more
communication among individuals.
Another effect that the presence of fire had on hominid societies
is that it required larger and larger groups to work together to
maintain and sustain the fire. Individuals had to work together to find
fuel for the fire, maintain the fire, and complete other necessary
tasks. These larger groups might have included older individuals,
grandparents, to help care for children. Ultimately, fire had a
significant influence on the size and social interactions of early
hominid communities.
Environment and nighttime activity
The
control of fire enabled important changes in human behavior, health,
energy expenditure, and geographic expansion. Humans were able to modify
their environments to their own benefit.
This ability to manipulate their environments allowed them to move into
much colder regions that would have previously been uninhabitable after
the loss of body hair. Evidence of more complex management to change biomes
can be found as far back as 200,000 to 100,000 years ago at a minimum.
Furthermore, activity was no longer restricted to daylight hours due to
the use of fire. Exposure to artificial light during later hours of the
day changed humans' circadian rhythms, contributing to a longer waking day. The modern human's waking day is 16 hours, while most mammals are only awake for half as many hours.
Additionally, humans are most awake during the early evening hours,
while other primates' days begin at dawn and end at sundown. Many of
these behavioral changes can be attributed to the control of fire and
its impact on daylight extension.
The supporting evidence of the cooking hypothesis argues that
compared to the nutrients in the raw food, nutrients in cooked food are
much easier to digest for hominids as shown in the research of protein
ingestion from raw vs. cooked egg.
Such a feature is essential for brain evolution; through studying the
metabolic activities between primate species, scientists found that a
limitation of energy harvesting through food sources exists due to
shorter days without fire.
Besides the brain, other organs in the human body also demand a high level of metabolism.
At the same time, the body-mass portion of different organs was
changing throughout the process of evolution as a means for brain
expansion. Genus Homo
was able to break through the limit by cooking food to lower their
feeding times and be able to absorb more nutrients to accommodate the
increasing need for energy. In addition, scientists argue that the Homo species was also able to obtain nutrients like docosahexaenoic acid
from algae that were especially beneficial and critical for brain
evolution, and as mentioned in the previous sections, the detoxification
of the cooking process enabled early humans to access these resources.
Changes to diet
Before the advent of fire, the hominid diet was limited to mostly plant parts composed of simple sugars and carbohydrates
such as seeds, flowers, and fleshy fruits. Parts of the plant such as
stems, mature leaves, enlarged roots, and tubers would have been
inaccessible as a food source due to the indigestibility of raw cellulose and starch.
Cooking, however, made starchy and fibrous foods edible and greatly
increased the diversity of other foods available to early humans.
Toxin-containing foods including seeds and similar carbohydrate sources,
such as cyanogenic glycosides found in linseed and cassava, were incorporated into their diets as cooking rendered them nontoxic.
Cooking could also kill parasites,
reduce the amount of energy required for chewing and digestion, and
release more nutrients from plants and meat. Due to the difficulty of
chewing raw meat and digesting tough proteins (e.g. collagen)
and carbohydrates, the development of cooking served as an effective
mechanism to efficiently process meat and allow for its consumption in
larger quantities. With its high caloric density and store of important
nutrients, meat thus became a staple in the diet of early humans. By increasing digestibility, cooking allowed hominids
to maximize the energy gained from consuming foods. Studies show that
caloric intake from cooking starches improves 12-35% and 45-78% for
protein. As a result of the increases in net energy gain from food
consumption, survival and reproductive rates in hominids increased.
Through lowering food toxicity and increasing nutritive yield, cooking
allows for an earlier weaning age, permitting females to have more
children. In this way, too, it facilitates population growth.
It has been proposed that the use of fire for cooking caused environmental toxins to accumulate in the placenta, which led to a species-wide taboo on human placentophagy around the time of the mastery of fire. Placentophagy is common in other primates.
Biological changes
Before their use of fire, the hominid species had large premolars,
which were used to chew harder foods, such as large seeds. In addition,
due to the shape of the molar cusps, the diet is inferred to be more
leaf- or fruit-based. In response to consuming cooked foods, the molar
teeth of H. erectus had gradually shrunk, suggesting that their
diet had changed from tougher foods such as crisp root vegetables to
softer cooked foods such as meat.
Cooked foods further selected for the differentiation of their teeth
and eventually led to a decreased jaw volume with a variety of smaller
teeth in hominids. Today, a smaller jaw volume and teeth size of humans
is seen in comparison to other primates.
Due to the increased digestibility of many cooked foods, less
digestion was needed to procure the necessary nutrients. As a result,
the gastrointestinal tract and organs in the digestive system
decreased in size. This is in contrast to other primates, where a
larger digestive tract is needed for fermentation of long carbohydrate
chains. Thus, humans evolved from the large colons and tracts that are
seen in other primates to smaller ones.
According to Wrangham, control of fire allowed hominids to sleep
on the ground and in caves instead of trees and led to more time being
spent on the ground. This may have contributed to the evolution of bipedalism, as such an ability became increasingly necessary for human activity.
Criticism
Critics of the hypothesis argue that while a linear increase in brain volume of the genus Homo is seen over time, adding fire control and cooking does not add anything meaningful to the data. Species such as H. ergaster
existed with large brain volumes during time periods with little to no
evidence of fire for cooking. Little variation exists in the brain sizes
of H. erectus dated from periods of weak and strong evidence for cooking.
An experiment involving mice fed raw versus cooked meat, found that
cooking meat did not increase the amount of calories taken up by mice,
leading to the study's conclusion that the energetic gain is the same,
if not greater, in raw meat diets than cooked meats.
Studies such as this and others led criticisms of the hypothesis to
state that the increases in human brain-size occurred well before the
advent of cooking due to a shift away from the consumption of nuts and
berries to the consumption of meat.
Other anthropologists argue that the evidence suggests that cooking
fires began in earnest only 250,000 BP, when ancient hearths, earth
ovens, burned animal bones, and flint appear across Europe and the
Middle East.
The
fire maps show the locations of actively burning fires around the world
on a monthly basis, based on observations from the Moderate Resolution
Imaging Spectroradiometer (MODIS) on NASA's Terra satellite.
The colors are based on a count of the number (not size) of fires
observed within a 1,000-square-kilometer area. White pixels show the
high end of the count—as many as 100 fires in a 1,000-square-kilometer
area per day. Yellow pixels show as many as 10 fires, orange shows as
many as five fires, and red areas as few as one fire per day.
Fire is the rapid oxidation of a material in the exothermic chemical process of combustion, releasing heat, light, and various reaction products.
Fire is hot because the conversion of the weak double bond in molecular oxygen, O2, to the stronger bonds in the combustion products carbon dioxide and water releases energy (418 kJ per 32 g of O2); the bond energies of the fuel play only a minor role here. At a certain point in the combustion reaction, called the ignition point, flames are produced. The flame
is the visible portion of the fire. Flames consist primarily of carbon
dioxide, water vapor, oxygen and nitrogen. If hot enough, the gases may
become ionized to produce plasma. Depending on the substances alight, and any impurities outside, the color of the flame and the fire's intensity will be different.
Fire in its most common form can result in conflagration, which has the potential to cause physical damage through burning.
Fire is an important process that affects ecological systems around
the globe. The positive effects of fire include stimulating growth and
maintaining various ecological systems.
Its negative effects include hazard to life and property, atmospheric
pollution, and water contamination. If fire removes protective vegetation, heavy rainfall may lead to an increase in soil erosion by water. Also, when vegetation is burned, the nitrogen it contains is released into the atmosphere, unlike elements such as potassium and phosphorus which remain in the ash
and are quickly recycled into the soil. This loss of nitrogen caused by
a fire produces a long-term reduction in the fertility of the soil, but
this fecundity can potentially be recovered as molecular nitrogen in
the atmosphere is "fixed" and converted to ammonia by natural phenomena such as lightning and by leguminous plants that are "nitrogen-fixing" such as clover, peas, and green beans.
Fire has been used by humans in rituals, in agriculture for clearing land, for cooking, generating heat and light, for signaling, propulsion purposes, smelting, forging, incineration of waste, cremation, and as a weapon or mode of destruction.
Fires start when a flammable or a combustible material, in combination with a sufficient quantity of an oxidizer
such as oxygen gas or another oxygen-rich compound (though non-oxygen
oxidizers exist), is exposed to a source of heat or ambient temperature above the flash point for the fuel/oxidizer mix, and is able to sustain a rate of rapid oxidation that produces a chain reaction. This is commonly called the fire tetrahedron.
Fire cannot exist without all of these elements in place and in the
right proportions. For example, a flammable liquid will start burning
only if the fuel and oxygen are in the right proportions. Some
fuel-oxygen mixes may require a catalyst, a substance that is not consumed, when added, in any chemical reaction during combustion, but which enables the reactants to combust more readily.
Once ignited, a chain reaction must take place whereby fires can
sustain their own heat by the further release of heat energy in the
process of combustion and may propagate, provided there is a continuous
supply of an oxidizer and fuel.
If the oxidizer is oxygen from the surrounding air, the presence of a force of gravity, or of some similar force caused by acceleration, is necessary to produce convection,
which removes combustion products and brings a supply of oxygen to the
fire. Without gravity, a fire rapidly surrounds itself with its own
combustion products and non-oxidizing gases from the air, which exclude
oxygen and extinguish the fire. Because of this, the risk of fire in a spacecraft is small when it is coasting in inertial flight. This does not apply if oxygen is supplied to the fire by some process other than thermal convection.
Fire can be extinguished
by removing any one of the elements of the fire tetrahedron. Consider a
natural gas flame, such as from a stove-top burner. The fire can be
extinguished by any of the following:
turning off the gas supply, which removes the fuel source;
covering the flame completely, which smothers the flame as the
combustion both uses the available oxidizer (the oxygen in the air) and
displaces it from the area around the flame with CO2;
application of water, which removes heat from the fire faster than
the fire can produce it (similarly, blowing hard on a flame will
displace the heat of the currently burning gas from its fuel source, to
the same end), or
application of a retardant chemical such as Halon
to the flame, which retards the chemical reaction itself until the rate
of combustion is too slow to maintain the chain reaction.
In contrast, fire is intensified by increasing the overall rate of
combustion. Methods to do this include balancing the input of fuel and
oxidizer to stoichiometric
proportions, increasing fuel and oxidizer input in this balanced mix,
increasing the ambient temperature so the fire's own heat is better able
to sustain combustion, or providing a catalyst, a non-reactant medium
in which the fuel and oxidizer can more readily react.
Photo of a fire taken with a 1/4000th of a second exposure
Fire is affected by gravity. Left: Flame on Earth; Right: Flame on the ISS
A flame is a mixture of reacting gases and solids emitting visible, infrared, and sometimes ultraviolet light, the frequency spectrum
of which depends on the chemical composition of the burning material
and intermediate reaction products. In many cases, such as the burning
of organic matter, for example wood, or the incomplete combustion of gas, incandescent solid particles called soot produce the familiar red-orange glow of "fire". This light has a continuous spectrum.
Complete combustion of gas has a dim blue color due to the emission of
single-wavelength radiation from various electron transitions in the
excited molecules formed in the flame. Usually oxygen is involved, but hydrogen burning in chlorine also produces a flame, producing hydrogen chloride (HCl). Other possible combinations producing flames, amongst many, are fluorine and hydrogen, and hydrazine and nitrogen tetroxide. Hydrogen and hydrazine/UDMH flames are similarly pale blue, while burning boron and its compounds, evaluated in mid-20th century as a high energy fuel for jet and rocket engines, emits intense green flame, leading to its informal nickname of "Green Dragon".
The glow of a flame is complex. Black-body radiation
is emitted from soot, gas, and fuel particles, though the soot
particles are too small to behave like perfect blackbodies. There is
also photon emission by de-excited atoms and molecules
in the gases. Much of the radiation is emitted in the visible and
infrared bands. The color depends on temperature for the black-body
radiation, and on chemical makeup for the emission spectra.
The dominant color in a flame changes with temperature. The photo of
the forest fire in Canada is an excellent example of this variation.
Near the ground, where most burning is occurring, the fire is white, the
hottest color possible for organic material in general, or yellow.
Above the yellow region, the color changes to orange, which is cooler,
then red, which is cooler still. Above the red region, combustion no
longer occurs, and the uncombusted carbon particles are visible as black
smoke.
The common distribution of a flame under normal gravity conditions depends on convection, as soot tends to rise to the top of a general flame, as in a candle in normal gravity conditions, making it yellow. In micro gravity or zero gravity, such as an environment in outer space,
convection no longer occurs, and the flame becomes spherical, with a
tendency to become more blue and more efficient (although it may go out
if not moved steadily, as the CO2 from combustion does not
disperse as readily in micro gravity, and tends to smother the flame).
There are several possible explanations for this difference, of which
the most likely is that the temperature is sufficiently evenly
distributed that soot is not formed and complete combustion occurs. Experiments by NASA reveal that diffusion flames
in micro gravity allow more soot to be completely oxidized after they
are produced than diffusion flames on Earth, because of a series of
mechanisms that behave differently in micro gravity when compared to
normal gravity conditions. These discoveries have potential applications in applied science and industry, especially concerning fuel efficiency.
In combustion engines,
various steps are taken to eliminate a flame. The method depends mainly
on whether the fuel is oil, wood, or a high-energy fuel such as jet fuel.
Typical adiabatic temperatures
The adiabatic flame temperature of a given fuel and oxidizer pair is that at which the gases achieve stable combustion.
Every natural ecosystem has its own fire regime, and the organisms in those ecosystems are adapted to or dependent upon that fire regime. Fire creates a mosaic of different habitat patches, each at a different stage of succession.
Different species of plants, animals, and microbes specialize in
exploiting a particular stage, and by creating these different types of
patches, fire allows a greater number of species to exist within a
landscape.
The fossil record of fire first appears with the establishment of a land-based flora in the Middle Ordovician period, 470 million years ago, permitting the accumulation of oxygen
in the atmosphere as never before, as the new hordes of land plants
pumped it out as a waste product. When this concentration rose above
13%, it permitted the possibility of wildfire. Wildfire is first recorded in the Late Silurian fossil record, 420 million years ago, by fossils of charcoalified plants. Apart from a controversial gap in the Late Devonian, charcoal is present ever since.
The level of atmospheric oxygen is closely related to the prevalence of
charcoal: clearly oxygen is the key factor in the abundance of
wildfire. Fire also became more abundant when grasses radiated and became the dominant component of many ecosystems, around 6 to 7million years ago; this kindling provided tinder which allowed for the more rapid spread of fire. These widespread fires may have initiated a positive feedback process, whereby they produced a warmer, drier climate more conducive to fire.
The ability to control fire was a dramatic change in the habits of early humans. Making fire to generate heat and light made it possible for people to cook
food, simultaneously increasing the variety and availability of
nutrients and reducing disease by killing organisms in the food.
The heat produced would also help people stay warm in cold weather,
enabling them to live in cooler climates. Fire also kept nocturnal
predators at bay. Evidence of cooked food is found from 1 million years ago, although fire was probably not used in a controlled fashion until 400,000 years ago. There is some evidence that fire may have been used in a controlled fashion about 1 million years ago.
Evidence becomes widespread around 50 to 100 thousand years ago,
suggesting regular use from this time; interestingly, resistance to air pollution started to evolve in human populations at a similar point in time.
The use of fire became progressively more sophisticated, with it being
used to create charcoal and to control wildlife from 'tens of
thousands' of years ago.
Fire has also been used for centuries as a method of torture and execution, as evidenced by death by burning as well as torture devices such as the iron boot, which could be filled with water, oil, or even lead and then heated over an open fire to the agony of the wearer.
By the Neolithic Revolution, during the introduction of grain-based agriculture, people all over the world used fire as a tool in landscape management. These fires were typically controlled burns or "cool fires",
as opposed to uncontrolled "hot fires", which damage the soil. Hot
fires destroy plants and animals, and endanger communities. This is
especially a problem in the forests of today where traditional burning
is prevented in order to encourage the growth of timber crops. Cool
fires are generally conducted in the spring and autumn. They clear
undergrowth, burning up biomass
that could trigger a hot fire should it get too dense. They provide a
greater variety of environments, which encourages game and plant
diversity. For humans, they make dense, impassable forests traversable.
Another human use for fire in regards to landscape management is its use
to clear land for agriculture. Slash-and-burn agriculture is still
common across much of tropical Africa, Asia and South America. "For
small farmers, it is a convenient way to clear overgrown areas and
release nutrients from standing vegetation back into the soil", said
Miguel Pinedo-Vasquez, an ecologist at the Earth Institute’s Center for Environmental Research and Conservation.
However this useful strategy is also problematic. Growing population,
fragmentation of forests and warming climate are making the earth's
surface more prone to ever-larger escaped fires. These harm ecosystems
and human infrastructure, cause health problems, and send up spirals of
carbon and soot that may encourage even more warming of the atmosphere –
and thus feed back into more fires. Globally today, as much as 5
million square kilometres – an area more than half the size of the
United States – burns in a given year.
There are numerous modern applications of fire. In its broadest
sense, fire is used by nearly every human being on earth in a controlled
setting every day. Users of internal combustion vehicles employ fire every time they drive. Thermal power stations provide electricity for a large percentage of humanity.
Hamburg after four fire-bombing raids in July 1943, which killed an estimated 50,000 people
The use of fire in warfare has a long history. Fire was the basis of all early thermal weapons. Homer detailed the use of fire by Greek soldiers who hid in a wooden horse to burn Troy during the Trojan war. Later the Byzantine fleet used Greek fire to attack ships and men. In the First World War, the first modern flamethrowers were used by infantry, and were successfully mounted on armoured vehicles in the Second World War. In the latter war, incendiary bombs were used by Axis and Allies alike, notably on Tokyo, Rotterdam, London, Hamburg and, notoriously, at Dresden; in the latter two cases firestorms were deliberately caused in which a ring of fire surrounding each city
was drawn inward by an updraft caused by a central cluster of fires.
The United States Army Air Force also extensively used incendiaries
against Japanese targets in the latter months of the war, devastating
entire cities constructed primarily of wood and paper houses. The use of
napalm was employed in July 1944, towards the end of the Second World War; although its use did not gain public attention until the Vietnam War. Molotov cocktails were also used.
The unburnable solid remains of a combustible material left after a fire is called clinker if its melting point is below the flame temperature, so that it fuses and then solidifies as it cools, and ash if its melting point is above the flame temperature.
Protection and prevention
Wildfire prevention programs around the world may employ techniques such as wildland fire use and prescribed or controlled burns. Wildland fire use refers to any fire of natural causes that is monitored but allowed to burn. Controlled burns are fires ignited by government agencies under less dangerous weather conditions.
Fire fighting services are provided in most developed areas to extinguish or contain uncontrolled fires. Trained firefighters use fire apparatus, water supply resources such as water mains and fire hydrants or they might use A and B class foam depending on what is feeding the fire.
Fire prevention is intended to reduce sources of ignition. Fire
prevention also includes education to teach people how to avoid causing
fires. Buildings, especially schools and tall buildings, often conduct fire drills to inform and prepare citizens on how to react to a building fire. Purposely starting destructive fires constitutes arson and is a crime in most jurisdictions.
This
visualization shows fires detected in the United States from July 2002
through July 2011. Look for fires that reliably burn each year in
western states and across the Southeast.
Different restoration methods and measures are used depending on the
type of fire damage that occurred. Restoration after fire damage can be
performed by property management
teams, building maintenance personnel, or by the homeowners themselves;
however, contacting a certified professional fire damage restoration
specialist is often regarded as the safest way to restore fire damaged
property due to their training and extensive experience.
Most are usually listed under "Fire and Water Restoration" and they can
help speed repairs, whether for individual homeowners or for the
largest of institutions.
Fire and Water Restoration companies are regulated by the
appropriate state's Department of Consumer Affairs – usually the state
contractors license board. In California, all Fire and Water Restoration
companies must register with the California Contractors State License
Board.
Presently, the California Contractors State License Board has no
specific classification for "water and fire damage restoration." Hence,
the Contractor's State License Board requires both an asbestos
certification (ASB) as well as a demolition classification (C-21) in
order to perform Fire and Water Restoration work.
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