Why is the era of giant creatures running rampant in science and education films gone forever?

Anyone who is interested in prehistoric creatures must know that there were many giant animals in prehistoric times, but the animals around us now don’t seem to be very big. Why don’t those big animals exist? Let’s talk about it today.

This article is divided into three parts: prehistoric giant insects, prehistoric giant dinosaurs and contemporary giant mammals. There are some differences in the factors that make them huge, so let's introduce them one by one.

01. Prehistoric giant insects

A 1-meter-long dragonfly (Meganeura monyi) and a 3-meter-long giant centipede (Arthropleura armata)

A series of giant creatures began to appear widely on Earth during the Peaty Period.

If you traveled back to the Earth at that time, you might feel like you were in the movie "Journey to the Center of the Earth" where there were only giant creatures.

Why is this? One theory is oxygen. In the history of the Earth, there have been times when the oxygen content exceeded 30%. The increase in oxygen content has made it possible for insects to grow. This is because many insects absorb oxygen through tracheae that are spread throughout their bodies or parts of their bodies.

In the same environment, the larger the body size, the larger the trachea (respiratory system).

When oxygen concentrations are high, organisms do not need to increase the size of their respiratory system to provide more oxygen for their increased body size, so the limiting effect of the respiratory system on body size is reduced, allowing organisms to evolve to larger sizes.

Speaking of this, we have to mention the history of oxygen on Earth.

02 , Interlude: The History of Oxygen on Earth

Today we live in a green world with 21% oxygen, but at first, the Earth had no oxygen. The earliest life on Earth was purple instead of green. Ancient microorganisms may have used molecules instead of chlorophyll to capture sunlight, which gave the organisms a purple hue. At that time, blue-green bacteria that would photosynthesize and produce oxygen could only hide in deeper water to absorb the blue-purple light left over by purple bacteria.

The Great Oxidation Event refers to an event about 2.6 billion years ago when the free oxygen content in the atmosphere suddenly increased. The specific cause of this event is still unknown, and there are only a few hypotheses to explain it. Some people believe that it was due to the photosynthesis of seaweed plants that caused a rapid increase in oxygen on Earth, while the amount of nickel that methane bacteria that destroy oxygen rely on decreased sharply, causing a large increase in the oxygen content in the atmosphere, so it is called the "atmospheric oxidation event." The Great Oxidation Event changed the composition of minerals on Earth and made the emergence of animals possible in the future.

Simply put, before this, the Earth can be simply considered an oxygen-free/anaerobic environment (oxygen content 0.02%) , with oxygen mainly existing in the form of compounds in water or rocks. After this, the Earth generally became an aerobic environment (the highest is said to be more than 30%, and now it is 21%) [1 ].

The above picture shows the trend of oxygen changes in the atmosphere. The arrows point to the Great Oxidation Event.

3.8-2.45 billion years ago: There was basically no oxygen in the atmosphere

2.45-1.85 billion years ago: Oxygen was produced, but absorbed by the ocean and seafloor rocks

1.85-850 million years ago: Oxygen emerged from the ocean, was absorbed by the land surface, and formed the ozone layer.

850 million years ago - present: Oxygen continues to accumulate.

After that, a phenomenon that changed the earth's ecology appeared: plants began to land on the earth! The first plants to land on the earth were just for survival, but their unlimited reproduction caused an ecological crisis on the earth.

The original plants had no leaves. Due to the photosynthesis of plants, the oxygen content on the earth gradually increased. After about 100 million years of wild growth, by the Carboniferous period (360-280 million years ago), the oxygen content on the earth reached an unprecedented high, exceeding 30%. The high concentration of oxygen gave rise to countless giant animals and prehistoric monsters on the earth, such as dragonflies over one meter long.

However, the first crisis in the history of plants appeared! This crisis almost destroyed the natural world.

At that time, plants were absorbing carbon dioxide like crazy, and then, in an instant, the earth’s carbon dioxide was not enough!!! What the hell, not enough carbon dioxide? Yes, seeing the greenhouse effect today, don’t you think it’s weird?

But the fact is that there is not enough carbon dioxide on Earth for photosynthesis. Plants rely on carbon dioxide for survival (raw materials for photosynthesis: water, carbon dioxide). Once carbon dioxide is gone, plants cannot survive, and thus the first crisis for plants was born.

Raw materials for photosynthesis

Countless plants died. At this time, thanks to the emergence of fungi, the bodies were decomposed and the lack of carbon dioxide began to be compensated. At the same time, some plants began to find new paths, and leaves began to appear on the stage. Finally, a relative balance was reached today, that is, there is decomposition and there is production.

Being giant is a strength advantage, but it is also a pressure. For giant creatures, food is a core constraint and selection pressure. Ordinary people like us eat 2 kilograms a day, but a whale eats several tons a day. Therefore, food is a core factor in the formation of giant creatures.

03. Giant reptile

Of course, I believe many people want to talk about another kind of giant creatures, reptiles represented by dinosaurs (in fact, dinosaurs are an order, and the classification is very complicated. I just use the word dinosaur for the sake of popular science). In fact, dinosaurs are a very complex group, some big and some small, some carnivorous and some herbivorous. This article mainly discusses large herbivorous ones.

At that time, there were many giant creatures on Earth, such as the Argentinosaurus, which was 35-40 meters long and weighed about 90 tons. Of course, there were also many familiar Mamenchisaurus, which were all very huge.

Why were there so many giant creatures in that era? Two reasons

1: Plenty of food. This is the factor of plants I mentioned above. After plants landed on the earth, they lacked enough enemies, which led to an abnormal proliferation of plants. Herbivorous dinosaurs feed on plants. Therefore, there is no shortage of food, and naturally there is a lack of sufficient selection pressure. Otherwise, if the earth is short of food, for example, if dinosaurs were thrown into the Cambrian period, they would have starved to death.

2: Megathermia (mesothermia) advantage. When it comes to animal temperature, everyone must think of cold-blooded animals and warm-blooded animals, the former such as reptiles, the latter such as mammals. But which type are dinosaurs? In fact, dinosaurs should not be described by the above two methods, but by endothermia and ectothermia.

In layman's terms, what does a living thing rely on to maintain its temperature? Some living things regulate their temperature mainly through external temperature changes, typically cold-blooded animals, which are very easily affected by temperature, and even hatching males and females is related to temperature. Other living things maintain their temperature through internal metabolism, typically mammals, which can maintain a constant temperature for a long time, regardless of whether it is icy or scorching.

What kind of dinosaurs are they? They are in a transitional stage, and they may have all of them, because dinosaurs are an order with countless types, just like today's earth ecosystems, some are endothermic, some are ectothermic, and there is also a type of giant, which should be described by another word: mesothermic (also called mesothermic) . What is megothermic?

I believe everyone must remember a biological theory called Bergmann's law, which states that animals living in cold places are larger. The principle is the problem of specific surface area . Take a theoretical spherical chicken as an example.

The surface area of ​​a sphere is calculated by the formula: 4πr^2 , where r is the radius of the sphere.

The volume calculation formula of a sphere : Vsphere = (4/3)πr^3 , r is the radius of the sphere

The relative surface area of ​​a sphere is 3/r. In other words, the relative surface area is inversely proportional to the radius . As shown in the figure below, as the radius increases, the rate of increase in volume is significantly higher than the rate of increase in surface area.

The ratio of the two, that is, the trend of relative surface area change, means that as the radius increases, the relative heat dissipation area decreases significantly.

For dinosaurs, their huge size created a scale effect, which is gigantic temperature. For some dinosaurs, their huge size advantage allowed them to reduce heat dissipation and maintain a good body temperature. They did not need to increase metabolism to maintain body temperature like homeotherms, nor did they change with the change of daylight like poikilotherms.

Therefore, the giant dinosaurs had huge temperature due to the volume effect, which actually reduced their energy consumption.

04. Contemporary giant creatures

As for the large creatures of today, there are fewer reptiles, but mammals, such as whales and elephants. In fact, if you compare them carefully, you will find that these giant animals are basically herbivorous, or they consume creatures that are particularly easy to catch. Why? The key is: food.

A giant creature has a huge demand for food. Only an elephant, which eats all day long, can maintain its giant size. But elephants can only grow so big, and it is difficult for them to grow any bigger.

Whales are an exception. Why? Because of the ocean.

Remember the concept of specific heat in physics? The specific heat of water is 4.2*10^3 joules/kilogram, which is much higher than many solids. The result of high specific heat is that the temperature of water changes less, and it has a strong temperature regulation ability. Even the temperature maintenance of the entire earth is related to water to a certain extent (of course, the sun is the root behind it).

The evolutionary path of whales

For mammals like whales, living in the ocean can take advantage of the small changes in water temperature, so they don't need to increase their metabolism to maintain their body temperature. Of course, I think whales themselves are also somewhat megathermic.

Okay, that’s it, let me give you a conclusion.

For prehistoric insects, one of the factors that led to their huge size was oxygen; for prehistoric dinosaurs, the factors that led to their huge size were food and huge temperatures; for modern mammals, the factors that led to their huge size were still food and temperature. Of course, the oxygen concentration in the era when dinosaurs lived was slightly higher than it is now, so there must be some advantages involved.

1. Holland H D. The oxygenation of the atmosphere and oceans[J]. Philosophical Transactions of the Royal Society of London B: Biological Sciences, 2006, 361(1470): 903-915.