From whales that live like fish in the ocean, to bats that fly like birds in the sky, to naked mole rats underground like ants centered on a queen, to humans who set their sights on habitats beyond Earth.
These animals, which vary greatly in appearance and body size, as well as in their living habits, are called mammals, and the genetic gap between humans and whales, bats, and naked mole rats is much smaller than we think, because all mammals are related by blood.
Pictured: Blue Whale
Mammals are now distributed all over the world and have an absolute dominance on the earth, but like the "rulers" of any era, the achievements of mammals today are not achieved overnight.
So, how did mammals take over the "dominion" of the earth step by step?
Pictured: Morgan Beast
The separation of Pangea
The oldest known fossil of a mammal is Morgan, which is small — a little smaller than a shrew — and lived about 210 million years ago or more.
Morgana is classified as Morganodont, and it is only one of several different mammalian lineages that have emerged at the time, including us humans, and all mammals today are descendants of one of the lineages that survived at that time.
In the late 1960s, there was evidence that Earth's land mass was once a huge continent that came together — known as Pangea.
Pangea, Source: Fama Clamosa
About 225 million years ago, Pangea began to split into the northern continent – known as Lauya , and the southern continent – known as Gondwana.
Although the ancestors of mammals were separated and developed independently like other animals, many researchers believe that the early mammals of the northern continent of Lauya survived.
These survivors migrated southward intermittently over the next few days along with land bridges between continents, all of which are distributed in Australia and Antarctica today.
But before their mammalian ancestors set out on their journey, they had an epic rival on land, and that was the dinosaurs.
In morgan's time, and for more than 100 million years, dinosaurs on land had an absolute advantage, and mammals did not fare well under the suppression of dinosaurs.
In addition, the oceans were full of crises at that time, giant sharks and marine reptiles swam around, and mammals that evolved on land had no chance to go to the sea.
Therefore, the size of mammals at that time basically did not exceed the size of cats, and most of them were mainly plant-feeding.
However, it is well known that about 65 million years ago, an asteroid hit the Earth, ending the age of dinosaurs, and climate change has made it easier for small animals to survive.
There is no doubt that mammals are the beneficiaries of the changes caused by the asteroid, but unlike many people may think, mammals did not rise up after 65 million years.
In fact, before that, the existing dominant characteristics of mammals had been evolved, and at least 18 of the existing 28 orders of mammals had appeared before the extinction of the dinosaurs.
So, it should be said that asteroids only helped mammals clear the hurdles and let them take over the Earth more quickly, not the main reason.
If we are to discuss how mammals have emerged, we can only start with when and how the existing dominant characteristics of mammals appear.
Constant temperature changes in the body and bones!
In the era of dinosaurs, animals have begun to try to survive at a constant temperature, and the body temperature of dinosaurs is likely to be similar to that of today's birds, rather than cold-blooded like other reptiles.
Constant body temperature has some disadvantages for animals, such as they need more food to maintain body temperature, but for land animals, the advantage is obviously more obvious, and their survival can not be limited by the environment.
The earliest ancestors of mammals that evolved on land clearly inherited this advantage, which is the basic factor for their future rise, and it can even be said that with a constant body temperature, the survival of mammals on land is already guaranteed.
Over the next hundred million years, mammals under pressure from giant reptiles changed the bones of their heads.
Skull and jaws of the Morgan beast, Source: Hemiauch
The most important feature of the Morgan is a small jawbone, which is the form of modern mammals and is separated from reptiles.
The jaws of reptiles are made up of several pieces of bone, while those of mammals are fused together, which causes the rest of the jawbone of mammals to migrate backwards and become small bones of the middle ear– the ear bones, thus allowing mammals to have better hearing.
At the same time, the jawbone of Morgan was accompanied by teeth, another major innovation in mammalian improvement.
Morgan's upper and lower molars are interconnected, allowing them to cut food into pieces, allowing food to release more calories and nutrients than reptiles can.
In fact, whether it is ear bones or teeth, the evolution of these characteristics is most likely because mammals are not easy to survive at that time, better hearing is likely to avoid powerful reptiles, and higher food utilization may be because they get very limited food.
However, the most important role of the fusion of the jawbone and the separation of the ear bone is not to make the teeth and hearing better, it is the key to allowing the mammalian skull to expand to the sides and rear, so that the mammal can develop a larger brain and make us smarter.
Excellent Mother: Placenta Appears!
Mammals are absolutely the best mothers than others, and all female mammals are responsible for their offspring.
They do not give birth to cubs at will, but allow them to develop in the womb for a long time, so that the cubs can better face the crisis from the moment of birth.
Even after the cubs are born, they do not ignore them, but will breastfeed for a long time until the cubs are strong enough.
Animals have two options for genes to continue: they either focus on the birth rate of their offspring or the survival rate.
There may have been multiple choices in the earliest lineages of mammals, and it is difficult to determine which option is better, but there is only one option left today — and that is to try to ensure the survival rate of offspring.
In order to preserve survival rates, mammals have made huge changes, and these changes are often aimed at keeping their cubs by their own side as much as possible.
Photo: Platypus has ancient mammary glands, but no RT
The first change was the appearance of the mammary glands. Scientists believe that the mammary glands originate from the sweat glands at the root of the hair, and both the sweat glands and the mammary glands produce water, salt and protein, all of which are necessary for newborn survival and are easily exploited.
Although the mammary glands can take good care of the cubs, the most important trick of mammals is not the mammary glands, but the placenta.
No animal other than mammals is able to keep an embryo in its body for long periods of time because it is a foreign body to the mother — after all, half of its genes come from the father and are attacked by the immune system.
Ancient placental fossil, source: Franzen/Sencke
But the placenta can isolate the developing fetus from the mother's immune system, making better use of the mother's nutrition.
In fact, not all mammals have placenta, kangaroos have marsupials that do not have a placenta, they can only let the fetus be born earlier, and then put into the nursery bag.
This brings a crucial limitation, as marsupials develop their forelimbs very early in order to be able to climb into the bag.
Image source: Frankfurt Zoo
Imagine what it would be like for all animals if they needed to develop their first forelimbs.
The answer is simple, they will never get wings that fly into the sky, fins that swim in the water, and all the other flexible specialized forelimbs.
Therefore, the emergence of the placenta already means that in the future, breastfeeding will face more and more complex habitats.
In 2002, the fossil of the oldest placental animal was found in Liaoning, China, and named it Archaeopteryx, and it is not difficult to find from the naming that it is a placental mammal - the earliest known ancestor of the order Euptaceae, to be precise.
Photo: Archaeopteryx fossil
Fossils of Archaeopteryx can be traced back at least 125 million years, and if the smarter brains were predestined 210 million years ago, the more diverse ones may have been present 125 million years ago.
If dinosaurs really didn't go extinct, perhaps more "advanced" mammals could take their place over time.
It's just that the extinction of dinosaurs did accelerate the rise of mammals, and in just 270,000 years after the extinction of dinosaurs, mammals developed rapidly, occupying completely different habitats around the world, and some of their species became more and more massive.
Pictured: Chinese Akebono
At last
At the end of the last century, Chinese paleoanthropologists discovered a series of ancient mammal fossils— including four higher primates— in ShanghuangZhen, Liyang County (now upgraded to a county-level city), and named one of them the Chinese Akebono.
Akebono is called because paleontologists believe it is the dawn of the apes, and its fossils date back 45 million years, older than any known ape fossil.
These great apes with flexible limbs and well-developed brains will open a whole new chapter in mammals in the future.
The core content of this article is mainly from National Geographic magazine: The Rise of Mammals. Rick Gore