question: If there are so many natural fires in forests per year that are then contained by humans, how wasn´t earth burned before humans appeared?
comment: When humans prevent a fire, it allows more undergrowth and deadfall to accumulate, which makes the next fire even worse. When the fires occur naturally, without that accumulation, they are smaller and burn themselves out more quickly. Trees don't burn easily, it takes very special conditions to create a forest fire. Even when there is a major fire, it only destroys a tiny percentage of the forest.
passages: 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 rare and potentially catastrophic, to savanna (mixed grass/woodland) where wildfires were very rare and of lower intensity . Such a change may have occurred about three million years ago, when the savanna expanded in East Africa due to cooler and drier climate.
   Fires among the low, scrubby, wetland plants of the Silurian can only have been limited in scope. Not until the forests of the Middle Devonian could large-scale wildfires really gain a foothold. Fires really took off in the high-oxygen, high-biomass period of the Carboniferous, where the coal-forming forests frequently burned; the coal that is the fossilised remains of those trees may contain as much as 10-20% charcoal by volume. These represent fires which may have had approximately a 100-year repeat cycle.
   Section::::Prehistoric fires.
  
  Sustained wildfire can only exist once oxygen levels and fuel sources were present in sufficient quantities. Between 400 and 450 million year ago, fire became a feature of the landscape. The presence of fusain (fossil charcoal), beginning in the early Carboniferous attests to this fire history and forms an important element of the Cretaceous–Paleogene boundary.
  
  Section::::Tree-ring data.
   Fossil charcoal indicates that wildfires began soon after the appearance of terrestrial plants 420 million years ago. Wildfire's occurrence throughout the history of terrestrial life invites conjecture that fire must have had pronounced evolutionary effects on most ecosystems' flora and fauna. Earth is an intrinsically flammable planet owing to its cover of carbon-rich vegetation, seasonally dry climates, atmospheric oxygen, and widespread lightning and volcanic ignitions.
   In the Welsh Borders, the first evidence of wildfire is rhyniophytoid plant fossils preserved as charcoal, dating to the Silurian period (about ). Smoldering surface fires started to occur sometime before the Early Devonian period . Low atmospheric oxygen during the Middle and Late Devonian was accompanied by a decrease in charcoal abundance. Additional charcoal evidence suggests that fires continued through the Carboniferous period. Later, the overall increase of atmospheric oxygen from 13% in the Late Devonian to 30-31% by the Late Permian was accompanied by a more widespread distribution of wildfires. Later, a decrease in wildfire-related charcoal deposits from the late Permian to the Triassic periods is explained by a decrease in oxygen levels.
   Both destructive and at times constructive, anthropogenic fire is the most immediately visible human-mediated disturbance, and without it, many landscapes would become denatured. Humans have practiced controlled burns of forests globally for thousands of years, shaping landscapes in order to better fit their needs. They burned vegetation and forests to create space for crops, sometimes resulting in higher levels of species diversity. Today, in the absence of indigenous populations who once practiced controlled burns (most notably in North America and Australia), naturally ignited wildfires have increased. In addition, there has been destabilization of "ecosystem after ecosystem, and there is good documentation to suggest fire exclusion by Europeans has led to floral and faunal extinctions."
   At the end of the Permian, oxygen levels plummeted, and fires became less common. In the early Triassic, after the 'mother of all extinctions' at the end of the Permian, there is an enigmatic coal gap, suggesting a very low biomass; this is accompanied by a paucity of charcoal throughout the entire Triassic period.
   Fire was used to keep large areas of forest and mountains free of undergrowth for hunting or travel, or to create berry patches.
  
  Section::::Grasslands and savannas.
  
  When first encountered by Europeans, many ecosystems were the result of repeated fires every one to three years, resulting in the replacement of forests with grassland or savanna, or opening up the forest by removing undergrowth. Terra preta soils, created by slow burning, are found mainly in the Amazon basin, where estimates of the area covered range from 0.1 to 0.3%, or 6,300 to 18,900 km² of low forested Amazonia to 1.0% or more.
   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.
   A snowball earth, from around 850-630 mya, is believed to have been caused by early photosynthetic organisms, which reduced the concentration of carbon dioxide and increased the amount of oxygen in the atmosphere. The establishment of a land-based flora increased the rate of accumulation of oxygen in the atmosphere, as the land plants produced oxygen as a waste product. When this concentration rose above 13%, wildfires became possible, evident from charcoal in the fossil record. Apart from a controversial gap in the Late Devonian, charcoal is present ever since.
   Land clearing practices which use fire, often starting forest fires, may be the cause of coal seam fires in Indonesia. In 1982–83 one of the largest forest fires in this century raged for several months through an estimated 5 million hectares of Borneo's tropical rainforests. Goldammer and Seibert however concluded that there are indications that coal seam fires already occurred between 13,200 and 15,000 BP.
   Section::::Middle Paleolithic evidence.:Europe.
   The ability to control fire probably began in "Homo erectus" (or "Homo ergaster"), probably at least 790,000 years ago but perhaps as early as 1.5 Ma. The use and discovery of controlled fire may even predate "Homo erectus". Fire was possibly used by the early Lower Paleolithic (Oldowan) hominid "Homo habilis" or strong australopithecines such as "Paranthropus."
   The most significant type of environmental change brought about by Precolumbian human activity was the modification of vegetation. [...] Vegetation was primarily altered by the clearing of forest and by intentional burning. Natural fires certainly occurred but varied in frequency and strength in different habitats. Anthropogenic fires, for which there is ample documentation, tended to be more frequent but weaker, with a different seasonality than natural fires, and thus had a different type of influence on vegetation. The result of clearing and burning was, in many regions, the conversion of forest to grassland, savanna, scrub, open woodland, and forest with grassy openings. (William M. Denevan)
   Scorched layers of trees which survived fires can also provide evidence of fire frequency - especially as they can be related to the annual growth rings of the affected tree. These are useful for relatively recent times, but there are only putative reports of this phenomenon in pre-Tertiary strata.
  
  Section::::Geochemical evidence.
  
  The amount of oxygen in the atmosphere is the major control on the abundance of fire; this can be approximated by a number of proxies.
  
  Section::::Development through time.
   The human use of fire for agricultural and hunting purposes during the Paleolithic and Mesolithic ages altered the preexisting landscapes and fire regimes. Woodlands were gradually replaced by smaller vegetation that facilitated travel, hunting, seed-gathering and planting. In recorded human history, minor allusions to wildfires were mentioned in the Bible and by classical writers such as Homer. However, while ancient Hebrew, Greek, and Roman writers were aware of fires, they were not very interested in the uncultivated lands where wildfires occurred. Wildfires were used in battles throughout human history as early thermal weapons. From the Middle ages, accounts were written of occupational burning as well as customs and laws that governed the use of fire. In Germany, regular burning was documented in 1290 in the Odenwald and in 1344 in the Black Forest. In the 14th century Sardinia, firebreaks were used for wildfire protection. In Spain during the 1550s, sheep husbandry was discouraged in certain provinces by Philip II due to the harmful effects of fires used in transhumance. As early as the 17th century, Native Americans were observed using fire for many purposes including cultivation, signaling, and warfare. Scottish botanist David Douglas noted the native use of fire for tobacco cultivation, to encourage deer into smaller areas for hunting purposes, and to improve foraging for honey and grasshoppers. Charcoal found in sedimentary deposits off the Pacific coast of Central America suggests that more burning occurred in the 50 years before the Spanish colonization of the Americas than after the colonization. In the post-World War II Baltic region, socio-economic changes led more stringent air quality standards and bans on fires that eliminated traditional burning practices. In the mid-19th century, explorers from observed Australian Aborigines using fire for ground clearing, hunting, and regeneration of plant food in a method later named fire-stick farming. Such careful use of fire has been employed for centuries in the lands protected by Kakadu National Park to encourage biodiversity.
   First to advance were woody pioneer species that were quick to spread, such as birch and pine. They were followed by heat-loving species such as oak and elm. Finally they were followed by slower migrating woody species that evolved into a climax community (see cyclic succession). With the end of the interglacial period and the onset of cooler climates, these species then retreated to their refuges again or simply died out.
  
  Section::::Most recent post-glacial period.
   Section::::Impact on human evolution.:Cultural innovations.:Protection and hunting.
   Small scale deforestation was practiced by some societies for tens of thousands of years before the beginnings of civilization. The first evidence of deforestation appears in the Mesolithic period. It was probably used to convert closed forests into more open ecosystems favourable to game animals. With the advent of agriculture, larger areas began to be deforested, and fire became the prime tool to clear land for crops. In Europe there is little solid evidence before 7000 BC. Mesolithic foragers used fire to create openings for red deer and wild boar. In Great Britain, shade-tolerant species such as oak and ash are replaced in the pollen record by hazels, brambles, grasses and nettles. Removal of the forests led to decreased transpiration, resulting in the formation of upland peat bogs. Widespread decrease in elm pollen across Europe between 8400–8300 BC and 7200–7000 BC, starting in southern Europe and gradually moving north to Great Britain, may represent land clearing by fire at the onset of Neolithic agriculture.
   A new analysis of burned antelope bones from caves in Swartkrans, South Africa, confirms that "Australopithecus robustus" and "Homo erectus" built campfires roughly 1.6 million years ago. Nearby evidence within Wonderwerk Cave, at the edge of the Kalahari Desert, has been called the oldest known controlled fire. Microscopic analysis of plant ash and charred bone fragments suggests that materials in the cave were not heated above about . This is consistent with preliminary findings that the fires burned grasses, brush, and leaves. Such fuel would not produce hotter flames. The data suggests humans were cooking prey by campfire as far back as the first appearance of "Homo erectus" 1.9 million years ago.
   Historical and contemporary human activity has profoundly influenced the composition of forests in the densely populated region of Central Europe. Remaining forest in Central Europe today is not generally considered natural forest, but rather a cultural landscape created over thousands of years which consists almost exclusively of replacement communities. The oldest evidence of human and forest interaction in Central Europe is about 500 000 years ago, whenfrom some hand axes have been found. The degree of hemeroby (human influence) and the extent of the original natural state from so long ago are difficult to estimate. It is believed that during glacial times during the ongoing ice age Central Europe was largely deforested and, in the period of "natural" re-emergence of the forest, since the end of the last glacial period, the Würm glaciation (about 11 700 years BP), people began to play a part transforming the potential natural vegetation. Sedentary, Neolithic farmers of the Linear Pottery Culture, about 7 500 years ago, began to change the forested landscape massively.
   Perhaps the earliest fuel employed by humans is wood. Evidence shows controlled fire was used up to 1.5 million years ago at Swartkrans, South Africa. It is unknown which hominid species first used fire, as both "Australopithecus" and an early species of "Homo" were present at the sites. As a fuel, wood has remained in use up until the present day, although it has been superseded for many purposes by other sources. Wood has an energy density of 10–20 MJ/kg.
   Savannas are subject to regular wildfires and the ecosystem appears to be the result of human use of fire. For example, Native Americans created the Pre-Columbian savannas of North America by periodically burning where fire-resistant plants were the dominant species. Pine barrens in scattered locations from New Jersey to coastal New England are remnants of these savannas. Aboriginal burning appears to have been responsible for the widespread occurrence of savanna in tropical Australia and New Guinea, and savannas in India are a result of human fire use. The maquis shrub savannas of the Mediterranean region were likewise created and maintained by anthropogenic fire.
   Today's forest communities in central Europe are influenced by the usefulness of the individual tree species. Apart from a few remnants of "near-natural forest" the vast majority of today's Central European forests are either artificial forests or whose present composition has arisen as a result of active or passive human intervention. Far and away the most common are commercial timber forests, which may be more or less near-natural, with beech and oak, spruce and pine. "Ancient forests" in Central Europe refer to the few remaining stands that are neither currently used for forestry nor were exploited in historical times.
   Section::::Impact on human evolution.:Cultural innovations.:Developments and expansion in early hominid societies.
  
presuppositions: Because there have been so many forest fires in the past that only stopped because humans stopped them, the earth should have burned before humans existed because there was no one to stop the fires.
corrections: Natural fires are usually much smaller and usually subside before causing major damage, they usually destroy a small percentage of forest when they do occur. 
raw_presuppositions: Because there have been so many forest fires in the past that only stopped because humans stopped them, the earth should have burned before humans existed because there was no one to stop the fires. Natural fires in forests require humans to contain them in order to prevent the earth burning completely.
raw_corrections: Natural fires are usually much smaller and usually subside before causing major damage, they usually destroy a small percentage of forest when they do occur.  Naturally occuring fires which are not contained by humans are smaller and burn themselves out more quickly, and even a major fire only destroys a tiny percentage of the forest.