Source of the article: Science Compound
The impact of global warming on life on Earth has been detected by threats to polar animals such as polar bears and widespread coral bleaching. In fact, the influence of climate on organisms has always existed, and the warming of the climate in ancient times was inextricably linked to the evolution of mammals.
A mysterious heat event 56 million years ago
The story begins at the end of the Cretaceous Period 66 million years ago, when an asteroid descended from the sky without warning and slammed into Mexico's Yucatan Peninsula, triggering a series of major environmental changes that eventually led to the withdrawal of the 160-million-year-old dinosaurs who dominated the earth from the stage of natural history. (What happened on the day the dinosaurs went extinct, this article takes you to understand)
For 10 million years after the extinction of the dinosaurs (Paleocene), the climate on Earth was generally calm, but with the occurrence of a rapid warming event, the original silence was broken. 56 million years ago, the average global temperature rose abruptly by 5 to 8°C in less than 20,000 years (Figure 1).
20,000 years may seem long for human history, but for geological history that is often measured in billions of years, it is only a moment. The end of this high temperature event, like its beginning, was very sudden and mysterious, and the whole event lasted less than 200,000 years, and then quickly fell back to the previous temperature. This global warming event, which came and went in a hurry, was known as the Paleocene-Eocene Thermal Maximum (PETM). After this, the Earth ushered in a warm period of more than 20 million years, known as the Eocene (Figure 1).
Figure 1 Earth's climate change since 66 million years (Image source: adapted from reference 1)
The ancestors of modern mammals ushered in a major explosion
Time flies, and the bucket turns to the stars. In the blink of an eye, millions of years have passed since the catastrophe that ended the "Dinosaur Empire" at the end of the Cretaceous Period, the earth's ecosystem has gradually recovered, and the descendants of the survivors of the disaster have begun to emerge and strive to find their own world.
Throughout the Mesozoic period, mammals lived for a long time in the shadow of predominantly dinosaur reptiles. With the end of the era of dinosaur supremacy, those mammals that were once dismissed by dinosaurs gained huge living space, and their survival evolution ushered in a major turnaround, replacing the supremacy of dinosaurs on the earth. The earth has since ushered in the age of mammals , the Cenozoic Age.
Due to the severe damage to the fauna at the end of the Cretaceous Period, at the beginning of the Cenozoic, the earth ushered in a 10 million-year-long animal depression (Paleocene), and barrenness and desolation were the true portrayal of the earth at that time. It was not until the beginning of the Eocene (56 million years ago) that the hustle and bustle of the past was restored from a depression, when the climate suddenly warmed up, low-latitude plants migrated significantly to higher latitudes, subtropical, warm temperate forests even expanded into the Arctic, palm trees spread as far north as Alaska and Northern Europe, there were signs of ferns, sequoias and ginkgo bilobas in some parts of the Arctic Circle, and large tracts of temperate forests grew on the Antarctic continent, except for arid desert areas on Earth. Lush forests everywhere. In Fushun, northeast of China, the climate is as warm and humid as the south of china today, with forests and swamps and tall trees, which provide favorable conditions for the formation of coal, and this period has also become an important coal-forming period in the geological period.
The warm and humid climate and the expansion of forests have created good conditions for the reproduction and migration of mammals. This seemingly brief warming event profoundly affected animal evolution, and the rapidly warming climate prompted the rapid evolution of mammals, which became a critical period in the evolution of Cenozoic mammals.
Due to changes in the climatic environment, most of the paleocene mammals could not adapt to the environment, most of them went extinct, new species ushered in an outbreak period, a large number of modern mammals such as even-ungulates, odd-hoofed order, primates and comb-toed rat types of rodents, etc., began to appear in Asia, Europe and North America. Many of the progenitors of modern mammals that we are familiar with began to appear, such as the earliest horses, the earliest elephants, the earliest primates, etc.
The size of a domestic cat, Archaeopteryx
During the PETM period, with the rapid rise in temperature, the phenomenon of individual miniaturization was also common in mammals. Archaeopteryx (Figure 2) lost 30% of its body size during the first 130,000 years of PETM, reducing its weight to 4 kg, about the size of today's domestic cats, and then increased by another 76% during the 45,000 years of temperature drops, reaching a height of 50 cm and a weight of 7 kg.
Higher temperatures are more suitable for the survival of small mammals, and compared with large mammals, small mammals have a larger body surface area per unit weight and are more likely to solve the problem of heat dissipation (Bergmann's law).
Figure 2 Restoration of the horse's ancestors - Archaeopteryx (Image source: https://baike.baidu.com - Source: DeviantArt)
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Living in forests and swamps, Archaeopteryx prefers fruits, young leaves and grasses, and is only 30 centimeters tall, with slender limbs and a flexible body, which is conducive to their free movement through the forest. Forests are a natural barrier, and their dense branches allow them to easily escape the pursuit of other animals. The odd-hoofed animals, represented by archaeopteryx, were one of the most common animals in the northern hemisphere at that time.
The ancestor of whales, bucky whales: from land to sea
In the Early Eocene more than 50 million years ago, when pakistan is still located near the Gutetis Sea, the climate became unusually hot and dry, and the land under the scorching sun was almost deserted. A land animal about the size of a wolf or dog, called bucky whale (Figure 3), is facing an unprecedented existential crisis due to food shortages caused by environmental changes.
In order to obtain a new food source, the hungry and dizzy Bucky whale began to try to fish in the sea, and because it had not yet evolved its flippers, bucky whales could not swim as easily and freely as the current whales when fishing in the water, but used a "dog planer" swimming posture, slow and clumsy. But the attempts in the water gave them a taste of sweetness, not only to get enough food, but also to get rid of the heat, but also to avoid the threat of predators. The attempt to go to sea allowed the ancestors of whales to take the first step from land to the sea.
Figure 3 Bucky Whale (Image source: Bilibili Up Lord @Fontauff)
Bucky whales have a long conical head, a large mouth, a short neck, strong limbs, a slender tail, a hairy body, and a very ugly appearance, living mainly in shallow seas or on land on the shores of lakes, and living by preying on fish, small animals and eating plants. 49 million years ago, the more adapted to the life of the water chocobo whale appeared, chocobo whale is also called the wandering whale, that is, the whale that can walk and swim, is a semi-aquatic mammal, its living habits are similar to crocodiles.
Over time, 46 million years ago, a whale that was more adapted to life in the water, the Rhodes whale, appeared, with large, webbed limbs like a boat oar. About 40 million years ago, the first generation of marine behemoths that completely broke away from land and survived in the ocean, the Dragon King Whale, appeared, its body was slender (18m), its hind limbs gradually degenerated, its forelimbs and tail grew into fins, and its body shape also developed to a streamlined shape.
By the end of the Eocene (34 million years ago), the world was rapidly cooling, the Antarctic ice sheet began to form, sea levels dropped rapidly, the tropical shallow sea environment on which the Draca whale lived disappeared, and it eventually went extinct due to its inability to adapt to changes in the climate; a smaller spear-toothed whale survived. 30 million years ago, speartooth whales encountered the threat of the marine overlord megalodon shark, forced to travel to the depths of the polar oceans with low visibility and low water temperature, and gradually evolved sonar function back and forth to locate in order to adapt to the dark ocean depths, in order to search for prey and avoid attacks. 4.5 million years later, due to cooler climates and changes in lifestyle, whales began to grow in size and eventually evolved into the world's largest animal.
The overlord of the dark sky, the bat
With the advent of the Cenozoic, animals that survived the mass extinction at the end of the Cretaceous period sought a place for their own development. The bat's immediate ancestors still retain their original appearance: arboreal, insect-based, nocturnal.
With the advent of the PETM event, the ancestors of bats ushered in new development opportunities. In the interior of North America at that time, the climate became very warm and humid, and dense forests grew everywhere, and large lakes, rivers and swamps were scattered around the forests. In order to prey on flying insects, the ancestors of bats first evolved a "big slap" in order to prey on flying insects, and began to jump frequently between trees, and after continuous evolution, they eventually grew wings and had a certain ability to glide and even fly.
Since then, the Earth has ushered in a multi-million-year-long period of high temperatures (early Eocene climate adaptation, referred to as EECO, Figure 1), when the ancestors of bats have truly mastered the ability to fly and evolved into bats in the true sense of the word - the Islamic bat (Fig. 4) and the claw bat.
Fig. 4 Restoration of the earliest known bat, the Ishin Bat, fossil found in the Early Eliocene Strata of the Green River Formation in Wyoming, USA, 52.5 million years ago (Source: Paleontological Exploration)
Bats have poor eyesight and poor light at night. In order to catch agile flying insects without hitting trees, bats have evolved ultrasonic echo positioning functions to accurately catch flying insects, avoid obstacles and evade predators. Bats, as the only mammals that truly have the ability to fly, avoid competition with land mammals, and echolocation systems help bats avoid competition with most birds, thus spreading globally, becoming the second largest group of mammals after rodents.
Ancestors of mankind: primates were born
Of course, the most important animal evolutionary event of this period is the emergence of modern primates, which evolved our ancestors.
Recently, a research result of the Institute of Tibetan Plateau Research of the Chinese Academy of Sciences shows that the rapid warming of the PETM period led to the evaporation of the ocean surface, and the contrast between sea and land heat was more intense, prompting the ocean wind to blow to the Asian continent, carrying more marine water vapor from the tropical ocean surface to the East Asian continent, forming abundant precipitation, even in the original drought and little rain, desert in central China, the climate suddenly became wet; plants in arid areas could not withstand rain toss, desert and drought disappeared rapidly. A large area of dense virgin forest was ushered in.
Originally scattered forests were connected, and the primates that depended on forests for survival were able to rapidly expand to Europe and North America, becoming one of the most prosperous fauna in the northern hemisphere. In the dense forests of central China, an ancient primate, the Achillesian monkey, thrives here (Figure 5).
Figure 5 Restoration of Achillesian monkey, fossil found in Songzi, Hubei Province (painted by Ni Xijun)
The Achillesian monkey is a very small primate with a body length of 7 cm and a weight of only 20 to 30 grams. Its head is large, a pair of large eyes have better vision, at this time has evolved early tricolor vision, has a strong sensitivity to red, a glance from a green leaf can see the red fruit.
They have slender and flexible limbs, feed mainly on insects, and are diurnal animals, active during the day, jumping around on branches in search of food. The thumbs on its hands and feet can be held against the other four fingers, and this body structure can help it grasp branches and food. The long tail behind it can maintain the balance of its body when exercising. The Achillesian monkey is the earliest primate ever discovered,[4] and this ancient monkey may have diverged into two branches of tarsiers and great apes in later evolution, the latter of which differentiated into humans after tens of millions of years of evolution.
The release of methane from the seabed triggers a high fever on the earth
The causes of this bizarre global rapid warming event are also debated in academic circles. There are ideas such as the comet impact hypothesis and volcanic eruptions, but most scholars attribute it to the decomposition of natural gas hydrates (combustible ice).
With a slow, continuous warming process of up to 10 million years, 56 million years ago, the temperature finally broke through a certain tipping point, and the seawater at the north and south poles gradually became hotter, and the high temperature melted the natural gas hydrate buried deep in the cold seabed, decomposing to produce huge amounts of methane and releasing it into the atmosphere. Methane is a super-strong greenhouse gas that is more than 20 times more powerful than CO2. The rapid release and oxidation of METHANe into CO2 causes the Earth's temperature to soar instantaneously. Fortunately, these huge amounts of carbon are absorbed through plants, soil, rock weathering or marine life in the later stages, so that the climate quickly returns to pre-event levels.
Conclusion: Paleocene-Eocene extreme heat events may be reproduced
While there are still many unsolved mysteries about the PETM event, there is no doubt that rapid warming has had a profound impact on life on Earth. In recent years, extreme weather events such as floods, heavy rains, and high temperatures have occurred in the context of the current global warming. Global warming is a "double-edged sword" with pros and cons. What needs to be vigilant is that since the Industrial Revolution, human activities have released a large number of greenhouse gases such as CO2, exacerbating the greenhouse effect, and in recent years, the rate of human carbon emissions has been much faster than in the PETM period! Over the past hundred years, the average global temperature has risen by 0.7°C, while during petm, it takes thousands of years to rise the same temperature.
Clearly, the current rate of warming has far exceeded that of the PETM period. Once the global warming situation is uncontrollable, the earth may usher in another warming event similar to PETM, and the creatures on the earth may face replacement again. Perhaps the size of mammals will become smaller, and the fate of human beings in the context of warming climate, and how humans will cope with the challenges brought about by warming, are all worth pondering.
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