In this excerpt from “Oak Origins: From Acorns to Species and the Tree of Life” (University of Chicago Press, 2024), author Andrew L. Hipp explores the extreme conditions on Earth that gave rise to the oak tree (Quercus), with wild fluctuations in the climate and shifting tectonic plates.
If we could head back in time 56 million years and spend a few weeks botanizing in the temperate forests of the Northern Hemisphere, at the boundary between the Paleocene and the Eocene, we would be hard-pressed to find any oaks. We would find alligators and giant tortoises on Ellesmere Island, across from the northwest coast of Greenland. We would roam through flowering-plant-dominated forests whose diversity approached the plant diversity we might find in the modern 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 walnuts, birches, sweet gales, beeches, chestnuts, chinkapins, and oaks.
The oaks themselves, however, were so few in number at that point that they left scant if any pollen in the mud and no acorns or leaves to be recovered by 21st-century 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 degrees Fahrenheit] worldwide and reaching even higher levels in the Arctic. The PETM may have been triggered by a massive and protracted period of volcanic activity. Magma gurgling up through a fissure at the bottom of the North Atlantic drove a wedge between North America and Europe and poured a trillion kilograms [2.2 trillion pounds] of carbon into the atmosphere every year for several thousand years.
Rising temperatures melted corpses out of the Antarctic permafrost, and the rotting sedges, sphagnum mosses, fungi and lichens, mollusks and marsupials returned greenhouse gases — carbon dioxide and methane — to the atmosphere.
Temperatures then crashed back to their original levels within about 120,000-220,000 years. That’s barely enough for a double take in geological terms: When you look at a temperature plot for the past 100 million years, the PETM looks like a fencepost driven into the hillside 56 million years ago. It goes straight up and almost straight back down.
The effects were dramatic. The PETM drove 30%-50% of deep-ocean-bottom foraminifera — single-celled organisms…
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