In his new book "A Short History of Life on Earth", author and editor Henry Gee tells the story of how simple and ancient life forms paved the way for all living things that exist today.
An excerpt from Gee's book explores how dinosaurs evolved the necessary structures and flight capabilities.
All birds can be traced back to a group of dinosaurs called theropods.
The following is an excerpt from the very short history of life on earth. Used with permission from the publisher St. Martin's Press. Copyright © Henry Gee 2021.
Dinosaurs have always been born to fly. It started with their commitment to bipedal walking, which has always been much larger than their many crocodile-like relationships.
The center of gravity of most habitual quadrupeds is in the chest area. They need a lot of energy to support themselves with their hind limbs. This makes it difficult for them to stand upright comfortably at any time. In contrast, in dinosaurs, the center of gravity is above the hips. The relatively short body in front of the hips is offset by the long, stiff tail behind. With its hips as a pivot, the dinosaur can stand on its hind limbs effortlessly. Unlike the stout limbs of most amniotic animals, the hind limbs of dinosaurs can be long and thin. If the legs are more slender towards the end, it will be easier to move. The easier the leg moves, the faster you run. The number of forelimbs no longer needed to run has been reduced, and both hands can free up space for other activities, such as grabbing prey or climbing.
The dinosaurs are constructed as a long lever to maintain balance on their long legs and have a coordination system that can constantly monitor their posture. Their brains and nervous system are as sharp as any animal that has ever existed. All of this means that dinosaurs can not only stand, but can also run, support, rotate and rotate in a posture and grace that the earth has never seen before. This is to prove a successful formula.
The dinosaur swept everything in front of them. By the end of the Triassic, they had diversified to fill every niche on land, just like the Permian zoology-but with perfect elegance. Dinosaur carnivores of all sizes prey on dinosaur herbivores, and their defenses either grow very large or wear armor thick like tanks. Among the sauropods, the dinosaurs turned back into quadrupeds and became the largest terrestrial animals in history. Some were more than 50 meters in length, while in the Argentinosaurus, they weighed more than 70 tons.
However, even they did not completely escape predation. They were preyed by huge carnivores: terrestrial sharks, such as Carcharodon and Megalodon, reached their peak in Tyrannosaurus in the last days of the dinosaurs.
In this single creature, the potential of the dinosaur's unique structure has been brought to the extreme. The hind limbs of this five-ton monster are composed of muscles and muscular double columns, in which the speed and elegance of its ancestors are exchanged for amazing strength and almost unstoppable power. The long tail supports its strong buttocks, its body is relatively short, and the forelimbs are reduced to only the remaining traces, most of which are concentrated on the powerful neck muscles and deep chin. The lower jaw is full of teeth, and each tooth is the same size, shape and consistency as a banana, if the banana is harder than steel. They have the power to crush bones and can pierce slow-moving but well-defending bus-sized herbivores such as Ankylosaurus and Triceratops. Tyrannosaurus and its relatives ripped blood clots from their prey and swallowed them whole-meat, bones, armor, etc.
But dinosaurs are also good at miniaturization. Some are small enough to dance in the palm of your hand. For example, Microraptor is only the size of a crow and weighs no more than one kilogram; the peculiar, bat-like Yi, is small in name and size, and weighs less than half of it.
The size of beasts ranges from large elephants to small terriers, but dinosaurs even surpass these extremes. How did dinosaurs become so huge-so small?
It starts with the way they breathe.
Deep in the history of the amniotic membrane, a rupture occurred. Among mammals-the last surviving animal, the return of the Triassic still persists in the shadow of the dinosaurs-ventilation is a matter of inhaling and exhaling again. Considered objectively, this is an inefficient way of letting oxygen enter the body and expelling carbon dioxide. Fresh air is sucked in through the mouth and nose and down into the lungs, where oxygen is absorbed into the blood vessels around the lungs, thus wasting energy. But the same blood vessel must discharge waste carbon dioxide into the same space, and it must be exhaled through the same hole through which fresh air enters. This means it is difficult to remove all the stale air at once or fill every corner and breathe fresh air in one inspiration.
Other amniotic animals—dinosaurs, lizards, etc.—also breathe through the same holes, but what happens between inhaling and exhaling is quite different. They developed a one-way system for air treatment, which makes breathing very effective. Air entered the lungs, but did not immediately come out again. Instead, the air is shunted and guided by a one-way valve through an extensive airbag system throughout the body. Although it can be seen in some lizards today, dinosaurs have taken this system to the extreme. The air gap-ultimately an extension of the lungs-surrounds internal organs and even penetrates bones. Dinosaurs are full of air.
This air treatment system is both elegant and necessary. Dinosaurs have a strong nervous system and active life that requires a lot of energy to acquire and consume, so it is very hot. This energetic activity requires the most efficient delivery of air to hypoxic tissues that may be artificially designed. This energy turnover generates a lot of excess heat. Airbags are a good way to get rid of it. This is the secret of some dinosaurs reaching huge sizes: they are air-cooled.
If an object grows but maintains its shape, its volume will grow much faster than its surface area. This means that as the body gets bigger, there is more body inside than outside. This can become a problem of getting the food, water and oxygen the body needs-as well as the excretion of waste and the heat produced by digesting food and simple life. This is because the area available for entry and exit of items is reduced relative to the volume of paper towels that must be provided in this way.
Most living things are microscopic, so this is not a problem, but for anything much larger than punctuation, it becomes a problem. The solution to this problem is, first, by evolving specialized transportation systems, such as blood vessels, lungs, etc.; second, by changing the shape, creating an extended or complex system that acts as a radiator from the sails and ears of Panosaurus. Elephants play an important role in the internal complexity of the lungs, in addition to gas exchange, but also heat dissipation.
When mammals were finally liberated from the world ruled by dinosaurs and were able to grow into something larger than badgers, they solved this insulation problem by shed hair and sweat during growth. Sweat secretes water to the surface of the skin. As the water evaporates, the energy required to convert the liquid sweat into steam is released through the tiny blood vessels under the skin to produce a cooling effect. But the air exhaled from the lungs also causes heat loss-which is why some fur mammals pant and expose their long, wet tongue to the vaporized air. The largest land mammal is Paraceratherium, a tall, slender, hornless relative of rhinos that lived about 30 million years ago, long after the dinosaurs disappeared. It grows to about 4 meters at the shoulder and weighs 20 tons.
But the largest dinosaur is much bigger than this. Huge sauropod dinosaurs, such as the 70-ton, 30-meter-long Argentinosaurus, are one of the largest terrestrial animals in history, and their surface area is very small compared to their size. Even changes in shape, such as elongating the neck and tail, are not enough to radiate all the heat generated in its spacious interior.
Although sauropod dinosaurs are very large, according to experience, the metabolic rate of large animals is easier than that of small animals, so they usually run cooler. It takes a long, long time to heat such a large dinosaur in the sun-but it also takes the same time to cool it, so once a very large dinosaur is heated, it can maintain a fairly constant body temperature as long as it is very large.
However, it was the legacy of dinosaurs that saved them-and made them grow so large. Because their lungs are already large and extend into an air sac system all over the body, these animals are not as big as they seem. The air sacs in the bones also keep the bones light. The bones of the largest dinosaurs are a triumph of bioengineering. The bones are simplified into a series of hollow load-bearing pillars to reduce non-load-bearing parts as much as possible.
But the point is that the internal system of the airbag does more than conduct heat from the lungs. It absorbs heat directly from the internal organs, without having to transport the heat to the whole body through the blood first, then to the lungs, and then radiate some heat along the way, which makes the problem more complicated. A considerable beneficiary is the liver, which generates a lot of heat, and for large dinosaurs, it is the size of a car. The air-cooled internal operation of the dinosaur is more efficient than the liquid-cooled mammalian version. This allows dinosaurs to become larger than mammals without having to boil themselves.
The Argentinosaurus is not so much a cumbersome behemoth, as it is a light-footed, four-legged, flightless...bird. Because birds, the heirs of dinosaurs, have the same lightweight structure, the same fast metabolism, and the same air cooling system. All of these are very conducive to flying, which is an activity that requires a light airframe.
Flying is also related to feathers. From the early days of dinosaur history, feather coats are characteristic of dinosaurs. In the beginning, feathers were more like hair, which is a characteristic of pterosaurs. Pterosaurs were the first type of vertebrates that learned to fly back in the Triassic. They were close relatives of dinosaurs. Even without flying, a layer of feathers provides an indispensable insulating material for small animals that generate a lot of heat. The problem faced by small, active dinosaurs is the opposite of the problem faced by large dinosaurs—preventing all expensive heat from dissipating into the environment. But this simple feather quickly grew leaves, barbs and colors. Smart and active animals like dinosaurs have very busy social life, and social display plays an important role in it.
Another key to the success of dinosaurs is laying eggs. Although most vertebrates always lay eggs—this was the habit of the first amniotic animals to finally conquer the land—but many vertebrates have restored the ancestral habit found in the earliest jawed vertebrates, that is, giving birth to offspring. The key is to find a strategy that can protect offspring without causing too much cost to parents. Mammals start from laying eggs. Almost everyone has become a living person, but at a terrible price. Carrying life requires a lot of energy consumption, which limits the size of mammals that can be achieved on land. It also limits the number of offspring they can produce at one time.
However, no dinosaur bred its offspring in this way. All dinosaurs lay eggs-so do all main dragons. As smart and active creatures, dinosaurs maximize the success rate of offspring by hatching eggs in the nest and caring for the cubs after hatching. Many dinosaurs, especially the more social herbivores, such as sauropods, and the smaller, more bipedal hadrosaurs that replaced sauropods during the Cretaceous Period, nested in public habitats that dominated the landscape. Extending from the horizon to the horizon. Female dinosaurs use their bones to provide enough calcium for their eggs, and birds retain this habit.
Given the advantages provided by spawning, this is a sacrifice worth making. Amniotic eggs are one of the masterpieces of evolution. It includes not only an embryo, but also a complete life-sustaining capsule. Eggs contain enough food for animals to hatch, and waste disposal systems to ensure that this self-sufficient biosphere is not poisoned. The behavior of laying eggs means that dinosaurs are free from the hassle and expense of breeding cubs in their own bodies.
Some dinosaurs did spend energy to take care of their offspring after hatching-but they were not bound by this obligation. Some people bury their eggs in warm holes or in the middle, allowing young people to take risks. Otherwise the energy spent on breeding and raising a few offspring could have been used elsewhere-for example, by laying far more eggs than any internal rearing allows. And, of course, through growth. Dinosaurs grow rapidly. Sauropods need to grow as fast as possible until they become too big for carnivores to cope. In response, carnivores must grow quickly. For example, Tyrannosaurus Rex has reached an adult weight of 5 tons in less than 20 years, gaining a maximum of 2 kilograms per day-much faster than its smaller relatives.
Dinosaurs and their immediate relatives have spent millions of years accumulating everything they need to fly: feathers, fast-moving metabolism, effective air cooling to maintain control, a lightweight body, and a unique love for spawning. Some dinosaurs use some of these adaptability to do things that are very unlike birds, such as growing to a size that land animals have not yet exceeded. However, in the end, the dinosaurs were allowed to take off. So, how did the dinosaurs take the last step and take off?
It began in the Jurassic period, when the already small carnivorous dinosaurs evolved even smaller lineages. The smaller they become, the more feathery their skin, because small animals with fast metabolism need to stay warm. These animals sometimes live in trees-best to avoid the attention of their older brothers. Some people have discovered how to use their feathery wings to stay in the air longer-thus becoming birds.
HENRY GEE is the senior editor of "Nature" magazine and author of several books, including "History of Life on Earth". He has appeared in "All Considerations" on BBC TV and Radio and NPR, and has written for The Guardian, The Times and BBC Focus. He lives in Cromer, Norfolk, England, with his family and many pets.