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In this excerpt from " Oak Origins : From Acorns to Species and the Tree of Life " ( University of Chicago Press , 2024 ) , authorAndrew L. Hippexplores the uttermost conditions on Earth that gave rise to the oak tree tree ( Quercus ) , with wild fluctuations in the climate and careen architectonic plates .
If we could guide back in prison term 56 million geezerhood and spend a few weeks botanizing in the temperate wood of the Northern Hemisphere , at the bounds between the Paleocene and the Eocene , we would be hard - press to find any oak tree . We would encounter gator and jumbo tortoises on Ellesmere Island , across from the northwest coast of Greenland . We would tramp through blossom - plant - predominate forests whose diversity approached the flora variety we might observe in the modernistic forests of the southeastern United States . We would encounter a diversity of Fagales , lineages spreading across the Northern Hemisphere that would eventually give rise to walnut , birch , sweet gale , beeches , chestnut , chinkapins , and oaks .
The first fossil oaks we know of come from Oberndorf, Austria.
The oaks themselves , however , were so few in number at that point that they leave light if any pollen in the clay and no acorn or leave to be recover by twenty-first - 100 botanists . The world was about to enter a heatwave , the Paleocene - Eocene Thermal Maximum ( PETM ) .
Over the course of 8,000 to 10,000 years , atmospheric temperatures would spike , increasing by an average of 8 degrees C [ 14.4 degree Fahrenheit ] worldwide and reaching even eminent levels in the Arctic . The PETM may have been activate by a massive and protracted geological period of volcanic activity . Magma burble up through a fissure at the bottom of the North Atlantic drove a wedge between North America and Europe and pour a trillion kilograms [ 2.2 trillion pounds ] of carbon into the atmosphere every year for several thousand years .
Rising temperature melted corpses out of the Antarctic permafrost , and the decompose sedges , bog moss mosses , fungus and lichens , shellfish and pouched mammal return glasshouse gases — carbon paper dioxide and methane — to the ambiance .
The first fossil oaks we know of come from Oberndorf, Austria.
Temperatures then crash back to their original grade within about 120,000 - 220,000 years . That ’s hardly enough for a doubled take in geological terms : When you look at a temperature plot of land for the past 100 million year , the PETM looks like a fencepost drive into the hillside 56 million years ago . It goes straight up and almost straight back down .
The force were dramatic . The PETM drive 30%-50 % of deep - sea - bottom foraminifera — individual - celled organism that populate the sea , eating plankton and detritus , feeding belittled fish and maritime snails — nonextant . Mammals , lizards , and turtles migrated widely across the Continent in response to the change climate , traveling between northern land bridges that would become too cold for unconstipated travel by most of these mintage in the tardy Eocene .
In northern South America , tropical forests were flooded with new inflorescence plants : medallion , grasses , and the Bean Family ( Fabaceae ) all increase in multifariousness in the Eocene , and the Spurge Family — Euphorbiaceae , a global family that numbers about 6,500 species today — read up in northern South America for the first time during the PETM .
The oaks we know today are the result of millions of years of natural selection.
The first oak fossils
Insect herbivores , specially foliage mineworker and surface self-feeder , increase in teemingness and became more specialized . Plants hie across the landscape : in Bighorn Basin , Wyoming , at least 22 species were extirpated at the onset of the PETM , only to return after the case was over . Some of these sojourners migrated an estimated 1,000 km [ 600 miles ] .
The first fossil oaks we bang of appear in this uncertain world , along what is now a hiking trail run to the south of the Church of Saint Pankraz in Oberndorf , Austria . Fifty - six million years ago , this area of Europe was dissect into islands and peninsulas , which were warm by the ocean .
What is now Saint Pankraz lay beneath shallow water at the sharpness of the sea . It became a repository for pollen from adjacent forest , deposited alongside oceanic plankton and dinoflagellate . The forest growing in the area was a arial mosaic of subtropical and temperate species , including members of the Restionaceae , a Mary Jane - like family that today is limited to the Southern Hemisphere tropics;Eotrigonobalanus , an extinct genus of the Beech Family that formerly rove across eastern North America and Europe ; and relatives of today ’s Cashew Family , Mallow Family , and the pantropical Sapotaceae .
The man was entering the last days of the nearly world tropics . For 4 million twelvemonth after temperatures retreated from the PETM , the climate continued to warm . By 52 million years ago , the worldly concern come to the mellow temperatures since the demise of the dinosaurs . This menstruation of warmth is call the Early Eocene Climatic Optimum .
If the PETM is like a fencepost driven into the temperature hillside , the Early Eocene Climatic Optimum is like the crest of the hill . woodland of tropic species growing alongside genera of the temperate forest — maples , elms , walnuts , birch rod , cherries , and eventually oaks — circularise across the mellow Arctic . The long wintertime night favour species that could go dormant for months at a meter . Deciduous timberland spread across highland sites that are now permafrost and boreal forest .
The mood was perch at the top of a tenacious slide down to the Anthropocene , where we bump ourselves today . Oaks were innovator in what would become the largely temperate Northern Hemisphere .
The oak were not born at a finicky second or in a finical topographic point . Instead , somewhere during or before the PETM , a universe of woody plants gradually became the oaks . Each seedling in this lineage attend like the tree that produced it . Had we been there to witness the phylogenesis of that ancestral population , we could at no gunpoint have say , " There were no oaks yesterday , but today there are . "
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We cease up with oak tree by the steady oeuvre of natural option acting on varying Sir Herbert Beerbohm Tree population over long periods of time . This lineage of individuals and populations slowly becoming the oaks is called the theme of the oak clade . It is constitute on the Tree of Life by a undivided line .
The population of trees that deposited the St. Pankraz pollen may represent a sprig bourgeon from that fore or one that sprouted very near the crown of the oaks . In either case , the St. Pankraz pollen is , for now , our good bet about how old the oak are . oak belike go back at least a niggling longer than these dodo , older than the PETM : dodo are difficult to find , so it ’s sensible to suspect that we may have missed some older ones . But these fossil put up us a landmark by which to date stamp the oak Sir Herbert Beerbohm Tree of biography .
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The first speciation event we know of in oak likely occurred within 8 million years of the St. Pankraz oak fossil . It split the oaks into two filiation : one that is today set to Eurasia and North Africa , and one that germinate in the Americas and only afterward returned to Eurasia . Sister clades — which are born as sister species — can arise in split up geographic region when their ancestral population becomes physically subdivided . A wad range , a river , a desert , an expanse of ocean , or any other roadblock between the two portions of the universe keep on seeds and pollen from travel between the two new populations . Speciation and the birth of new clades often result .
The spreading Atlantic Ocean is a plausible explanation for this first oak speciation event . Magma spilling into the North Atlantic off the coast of Ireland at the outset of the PETM added encrustation to the east boundary of the North American ( tectonic ) Plate and the west bound of the Eurasiatic Plate . It continue to do so today , steering the continents apart at a rate of about an in a year .
As the Atlantic grew full , the hereditary universe of all of today ’s oaks may have been straddling the continents of the Northern Hemisphere . If so , the ancestor of the oak tree we fuck today was a widespread population that was cleaved in one-half as North America edge westward .
reissue with permission from Oak Origins : From Acorns to Species and the Tree of Life by Andrew L. Hipp , published by The University of Chicago Press . © 2024 by Andrew L. Hipp . All right reserved .
Oak Origins : From Acorns to Species and the Tree of Life
An oak tree start its life with the precarious journeying of a pollen food grain , then an acorn , then a seedling . A mature tree may shed millions of acorns , but only a handful will grow . One oak may then live 100 years , 250 years , or even 13,000 years . But the farsighted lifespan of an someone is only a part of these tree ’ fib .
With natural scientist and leave research worker Andrew L. Hipp as our guide , Oak Originstakes us through a wholesale evolutionary history , stretching back to a universe of trees that lived more than 50 million class ago .
