Last Glacial Maximum refugia were places (refugia) in which humans and other species survived during the
Last Glacial Period, around 25,000 to 18,000 years ago.[1]Glacial refugia are areas that climate changes were not as severe, and where species could recolonize after deglaciation .[2]
Globally, the temperatures during the
Last Glacial Maximum (LGM) were 4.0 ± 0.8 °C cooler than present day.[3] The colder climate contributed to ice sheet growth in North America, Europe, and Antarctica. At this time there were major climate shifts around the world. Some areas became too dry to support life, and others housed more vegetation and mammals.
The
northern hemisphere was heavily impacted by
ice sheets during the LGM. Some recent archaeological evidence suggests the possibility that
human arrival in the Americas may have occurred prior to the Last Glacial Maximum more than 30,000 years ago. This evidence was found adjacent to ice sheets,[4] but research is still in an early stage. The best attested shelters are therefore mainly those in
Eurasia. Aside from human habitation in the north, other animals and vegetation thrived in refugium south of ice sheets.
The
southern hemisphere saw much less extensive glaciation, and outside Antarctica lacked continental ice sheet growth. Vast areas of
Australia and
Africa were too dry for human habitation of any sort,[5] even by the most specialized and well-adapted
foragers. However,
Sub-Saharan Africa was a refuge for many humans.[6]South America was not inhabited by humans during the LGM, but many other animals existed and thrived there.
Northern Hemisphere
North America
The
Laurentide and
Cordilleran ice sheets overtook the majority of Canada and parts of the United States during the last glaciation. South of the glaciers, the major biomes on the continent were tropical semi-desert, subalpine parkland, temperate
steppe grassland, and main
taiga.[7] In the present day climate, the biomes in
North America are tundra, boreal forest, temperate forest, grassland, desert and several more.[8] As the
ice sheets retreated, biomes moved northward.
During the LGM,
beech and
maple trees were found in temperate deciduous forests in the southeast United States. These areas were a refuge to many species.[9] As the glacier retreated, trees and other vegetation would have migrated north to follow the climatic conditions they required.[9]
The first humans were thought to have arrived in North America around 30,000 years ago from
Beringia.
Homo sapiens were discovered in the high latitude northern hemisphere 30,000 years ago, however they did not migrate south until almost 15,000 years ago.[10]
Europe
The majority of the regions north of 40° N were overtaken by glaciers during the LGM. In these areas the climate was 10-25 °C lower than the current temperatures.[11] South of the glaciers housed a steppe-tundra climate, along with small sections of forest steppe and open boreal woodlands.[7] In these lowland areas the temperature was more mild, 2-5 °C less than the present day.[11] In the present,
Europe has several biomes such as the
Mediterranean, temperate forests, boreal forests and several steppes.
Due to the harsh conditions in Europe during the LGM, humans remained in certain regions. West of the Alps housed the
Solutrean and Proto Solutrean cultures. The Solutrean people lived in between a forest steppe and a steppe tundra. This area had the highest probability of containing human refugia during the LGM.[12] Parts of Italy and extending into Romania contained the
Epi-Gravettian cultures.[13]
Most trees in Europe lived in select refugium. Several requirements of these refugium include: soil moisture, relatively warm temperatures, shelter from wind, and no permafrost. There are two proposed
hypotheses for tree refugia during the LGM in Europe. The first pertains to trees thriving in high-altitude locations in southern Europe. The second hypothesis suggests that forest trees existed only in small scattered refugia where there was adequate moisture.[11]
Asia
In
Asia the main biomes during the LGM were forest steppes, semi-arid temperate woodlands, scrub woodlands, and
tropical woodlands. There were some
deserts in the region as well; 75% of the area consisted of dry steppes, semi-deserts, and deserts.[7][14] The remaining areas in Asia were covered with
tropical rainforests.
There were several factors that controlled where plants and animals thrived in Asia during the LGM. In the south,
precipitation was the major factor that contributed to species refugia. Contrastingly, the higher latitude's main limiting factor was
temperature.[15] Climate conditions in central Asia were too harsh to allow human habitation. However, several
hominin sites have been discovered in Uzbekistan. This implies that during unfavorable glacial conditions, hominin did not move.[16]
Northern Africa
The climate was drastically different between
Northern Africa and
Southern Africa. In the north,
Africa was mainly a tropical extreme desert, but also housed small sections of tropical semi-desert, tropical grassland and tropical woodland.[7]
The
Sahara desert falls in Northern Africa, and during the LGM it moved slightly south due to the ice sheets in the north. When that occurred, the northern tip of Africa became a haven for humid and moist
mixed forests. These climatic conditions were similar to
southern Europe, now called the
Mediterranean.[17] During the LGM, winter global temperatures were 10-20 °C cooler. Even with the cold, the Mediterranean housed a mosaic of suitable micro climates for temperate and
thermophilic animals.[18] Many animals that moved to this area would have died from the cold
tundra during the LGM.[17]
Southern Hemisphere
Southern Africa
The
Congo tropical rainforest resides in the southern region of
Africa. During the Last Glacial Maximum, the rainforest was not as large as it is today. This was because the Earth was drier. With less humidity, the Congo shrank by 54%.[19] In addition, the LGM allowed for
savannas, tropical
grasslands, tropical thorn scrub, tropical
semi-desert, tropical extreme desert, and semi-arid temperate woodland.[20] Today,
Southern Africa consists mainly of savannas, deciduous woodlands, tropical rainforests, and deserts.
Modern
humans evolved from Africa, with their first known appearance being ~195,000 years ago.[21] The earliest dated fossils outside of Africa are between 90,000 to 120,000 years ago.[21] During the LGM, 27-66% of Africa was a refugium for humans. Humans thrived here because of the abundance of humidity, accessibility of woodlands, and lastly precipitation/access to water. The
Zambezi,
Omo river, and the
Great Rift Valley lakes were major sources of water in southern Africa.[22]
South America
South America contained several biomes during the LGM. First, the
Andes mountains held glaciers which created a
tundra in surrounding areas. Secondly, the
Amazon rainforest contained tropical rainforests, grasslands, deserts, and savannas.[20] The
ITCZ,
trade winds, and
insolation created an environment that allowed high precipitation rates in the Amazon.[23] South of the Amazon rainforest was significantly drier. For example, precipitation east of the Andes was ~20-30% lower than present day values. Additionally, the southern tip of the continent had ~40-50% lower precipitation.[24]
Humans arrived in South America approximately 15,000 years ago.[25] Due to the timing, humans arrived after the LGM.
The South American deer, Hippocamelus, was known to live in high altitude locations and cold valleys. Historically, they lived anywhere between 36.5° S and 54° S. Presently, they live between 40° S and 51° S. The habitat of Hippocamelus shrank as the LGM came to an end. The change in location is theorized to be caused by a lack of cold tundra areas, as well as higher
sea level.[26]
Australia
During the Last Glacial Maximum,
Australia was cooled and became arid. The annual temperatures decreased approximately 10 °C, and rainfall decreased 60%.[27] Australian biomes during the LGM include tropical extreme desert, tropical semi-desert, tropical thorn scrub and scrub woodland, tropical grassland, and tropical woodland.[20] The largest biome was the tropical extreme desert. This region was considered a "barrier", meaning devoid of human activity. Australia housed refugia such as the
Gulf Plans/
Einasleigh Uplands,
Brigalow Belt South,
Murray Darling Depression,
Tasmanian Central Highlands, and many others.[27]
The majority of refugia existed along the coast where woodlands and grasslands were found. The Australian
Wet Tropics are a region in the northeast that contain rainforests. The rain forest expanded and contracted throughout the glacial cycles. During the LGM, it withdrew to a refugia near the coast. A majority of the wet tropics were replaced with woodlands and grasslands after the LGM.[28]