How did mammals rise? These animals living in China 125 million years ago were the key

From whales that live like fish in the ocean, to bats that fly like birds in the sky, to naked mole rats that live like ants underground with a queen as the center, to humans who are setting their sights on habitats beyond Earth.

These animals, which are very different in appearance, body shape, and living habits, are called mammals. The genetic gap between humans and whales, bats, and naked mole rats is much smaller than we imagined, because all mammals are related.

Image: Blue Whale

Mammals are now distributed all over the world and have absolute dominance on the earth. However, like the "rulers" of any era, the current achievements of mammals were not achieved overnight.

So, how did mammals gradually seize "dominance" over the earth?

Figure: Morganu

The separation of Pangaea

The oldest known fossils that could have been mammals are Morganucodon , which was very small - slightly smaller than a shrew - and lived about 210 million years ago or more.

Morganucodon is classified as a Morganucodont, which was just one of several different mammal lineages that had emerged at the time, and all mammals today, including humans, are descendants of one of the lineages that survived.

In the late 1960s, evidence emerged that Earth's landmass was once a massive continent called Pangaea.

Pangaea, source: Fama Clamosa

About 225 million years ago, Pangaea began to break up into a northern continent, known as Laurasia , and a southern continent, known as Gondwana .

Although the ancestors of mammals separated and developed independently like other animals, many researchers believe that it was the early mammals of northern Laurasia that survived.

In the following days, these survivors migrated southward intermittently along the land bridges between continents, until they are distributed in today's Australia and Antarctica.

However, before the ancestors of mammals set out on their journey, they had an epic competitor on land: the dinosaurs.

In the era of Morganucodon, and for more than 100 million years thereafter, dinosaurs on land had an absolute advantage, and under the suppression of dinosaurs, mammals did not live well.

In addition, the oceans at that time were also full of dangers, with giant sharks and marine reptiles swimming around, and mammals that evolved on land had no chance of going into the sea.

Therefore, the size of mammals at that time basically did not exceed that of cats, and most of them were mainly herbivorous.

However, it is well known that an asteroid hit the Earth about 65 million years ago, bringing an end to the age of the dinosaurs, and climate change made it easier for smaller animals to survive.

There is no doubt that mammals were the beneficiaries of the changes brought about by the asteroid, but contrary to what many people may think, mammals did not rise 65 million years later.

In fact, the current advantageous characteristics of mammals had already evolved before that, and at least 18 of the 28 orders of mammals currently in existence had already appeared before the extinction of dinosaurs .

So, it should be said that the asteroid simply helped clear the obstacles for mammals to take over the Earth more quickly, rather than being the main cause.

If we want to discuss how mammals rose, we can only start from when and how the current advantageous characteristics of mammals appeared.

Changes in the body and bones of a thermostatic body!

In the era when dinosaurs appeared, animals had already begun to try to survive at a constant temperature. The body temperature of dinosaurs was probably similar to that of modern birds, rather than being cold-blooded like other reptiles.

A constant body temperature has some disadvantages for animals, such as they need more food to maintain body temperature, but for terrestrial animals, the advantages are obviously more obvious, and their survival is not restricted by the environment.

The earliest ancestors of mammals that evolved on land clearly inherited this advantage, which was the fundamental factor for their future rise. It can even be said that with a constant body temperature, the survival of mammals on land is already guaranteed .

Over the next eons, as mammals came under pressure from the giant reptiles, the skeletons of their heads changed.

Skull and jaw of Morganucodon, source: Hemiach

Morganucodon's most important feature is a small jawbone, which is the morphology of modern mammals and separates them from reptiles.

The jaw of reptiles is made up of several bones, while the jaw of mammals is fused together, which allows other parts of the jawbone of mammals to migrate backwards and become the small bones of the middle ear - the otic bones, giving mammals better hearing.

At the same time, the changes in Morganucodon's jaws also included its teeth, another major innovation in the improvement of mammals.

Morganucodon's upper and lower molars were connected to each other, which allowed them to cut food into pieces, releasing more calories and nutrients from the food, something that reptiles could not do.

In fact, whether it is ear bones or teeth, the evolution of these characteristics is most likely because it was difficult for mammals to survive at that time. Better hearing is probably to avoid powerful reptiles, and higher food utilization may be because the food they get is very limited.

However, the most important function of the fusion of jawbones and separation of ear bones is not to make teeth and hearing better. The key is to allow the skull of mammals to expand to the sides and back, so that mammals can develop a larger brain and make us smarter .

Excellent Mother: Placenta Appears!

Compared to other animals, mammals are definitely the best mothers, and all female mammals are very responsible for their offspring.

They do not give birth to their cubs randomly, but let their cubs develop in the womb for a long time so that the cubs can better face crises from the moment they are born.

Even after the cubs are born, they will not ignore them, but will breastfeed them for a long time until the cubs are strong enough.

In order to ensure the continuation of their genes, animals have two choices: they can either focus on the birth rate of their offspring or on the survival rate .

There may have been multiple options in the earliest lineages of mammals, and it is difficult to determine which option was better, but now there is only one option left - that is to try to ensure the survival rate of offspring.

In order to preserve their survival rates, mammals have made dramatic changes, many of which are designed to keep their young by their side as long as possible.

Image: The platypus has ancient mammary glands, but no nipples

The first change is the appearance of mammary glands . Scientists believe that mammary glands originated from sweat glands at the roots of hair. Both sweat glands and mammary glands produce water, salts and proteins, all of which are necessary for the survival of newborns and can be easily developed and utilized.

Although mammary glands can be used to take good care of young, the most important weapon for mammals is not the mammary glands, but the placenta .

Apart from mammals, no animal can keep an embryo in its body for a long time. This is because the embryo is a foreign body to the mother - after all, half of its genes come from the father and will be attacked by the immune system.

Ancient placenta fossil, source: Franzen/Sencke

But the placenta can insulate the developing fetus from the mother's immune system, allowing it to better utilize the mother's nutrients.

In fact, not all mammals have placentas. Marsupials such as kangaroos do not have placentas. They can only give birth to the fetus earlier and then put it in the pouch.

This poses a crucial limitation, because marsupials develop their forelimbs very early in order to be able to crawl into their pouches.

Image source: Frankfurt Zoo

We can imagine, if all animals need to develop well-developed forelimbs first, what would they look like?

The answer is simple: they would never have acquired wings to fly, flippers to swim in the water, and all the other flexible specialized forelimbs.

Therefore, the emergence of the placenta means that breastfeeding will be available in more and more complex habitats in the future.

In 2002, the oldest placental animal fossil was discovered in Liaoning, China, and it was named Archaeotherium . From the name, it is not difficult to find that it is a placental mammal - to be precise, the earliest known ancestor of the Eutheria.

Figure: Archaeopteryx fossil

The fossils of Archaeotherium can be traced back to at least 125 million years ago. If smarter brains were destined 210 million years ago, then greater diversity may have appeared 125 million years ago.

If the dinosaurs had not really become extinct, perhaps more "advanced" mammals could have replaced them over time.

However, the extinction of dinosaurs did accelerate the rise of mammals. In just 270,000 years after the extinction of dinosaurs, mammals developed rapidly, occupying completely different habitats around the world, and some species of them became increasingly large in size.

Figure: Eosapiens sinensis

at last

At the end of the last century, Chinese paleoanthropologists discovered a series of ancient mammal fossils in Shanghuang Town, Liyang County (now upgraded to a county-level city) - including four species of higher primates, one of which they named Chinese dawn ape.

It is called Dawn Ape because paleontologists believe it is the dawn of the great apes. Its fossils can be traced back to 45 million years ago, which is older than any known ape fossils.

These apes with flexible limbs and developed brains opened a new chapter for mammals in the future.

The core content of this article is mainly from National Geographic magazine: The Rise of Mammals. Rick Gore