FunNature & AnimalWhat was the first mammal?

What was the first mammal?

We are sorry to disappoint you, but we can say that we do not have the name and ID of the first mammal. In fact, the question itself has complications that lead us to an exciting scientific debate from which, more than conclusions, we draw several answers to the question that titles this article. Do you want to know the fascinating historical journey of mammals? Go for it.

The first thing we must be clear about in order to know which was the first is: what is a mammal? Scientists often take one of the characteristics by which one species differs from another and thus name them to identify them. In this case, mammals feed their young with breast milk and hence their name. To this characteristic we must add other important ones: they are endotherms, that is, they have warm blood ; they have a single bone in the jaw ; and they have the body covered with hair . This class of Mammalia includes specimens as diverse as dolphins, bats, lemurs and you who are reading this (so far the Very Interesting data has only detected human readers).

Who plays on our team?

With this definition we already have a first debate : do we accept as mammals those that have some of these characteristics or do they have to have all of them? The scientific discussion objects that if we take the first option as valid, we include species very different from mammals in the group and if we take the second, we leave out very close taxa. Since virtue is at the midpoint, by scientific convention the species of the Mammalia clade were taken as those that met all the requirements. Those that only have some characteristics we take as a primitive stem group, let’s say they are protomammals.

And, then, of those who do enter the group, which was the first? Not so fast. As we have said, there is no single answer to this question. To better understand the different options, let’s see how we mammals came to the world.

Millions of years of evolution

Obviously, we are not going to go back to the creation of the universe. Not even when life on Earth was reduced to bacteria. We will go “only” at the end of the Devonian , 360 million years ago. At that time the tetrapods emerged, vertebrate animals with four limbs. From this group, amphibians, reptiles, birds and mammals will come out. As you can imagine, it was not a matter of a little while. Over 100 million years, tetrapods evolved, and during the Carboniferous , a group of tetrapods evolved into so-called amniotes : animals that could lay eggs out of water. This served them to conquer the land and not depend on water to spawn. These amniotes, over time, were divided into three large groups: the anapsids, synapsids, and diapsids . They differ in the number of pits they have on their skulls, behind their eye sockets. Anapsids do not have pits, synapsids have one, and diapsids have two. Mammals emerged from the group of synapsids, but there were still a few more steps to go.

It was in the Permian , 300 million years ago, when synapsids were represented by pelycosaurs and other mammalian reptiles. Mammalian reptiles , what monstrosity is that?, you may ask. Well, they were animals that looked like reptiles, but with some characteristics of mammals. We have the example of the Dimetrodon grandis, a pelycosaur that had a kind of sail on its back that would serve to regulate its temperature. These species were overtaken by the therapsids , the dominant amniotes until the dinosaurs arrived. These therapsids are the middle ground between mammalian reptiles and mammals. An example of these ancestors would be the cynodonts , which began to have warm blood, hair and developed a palate on the roof of the mouth, which allowed them to chew and breathe at the same time, a characteristic of mammals. Among the species of cynodonts we can mention the “dog-head” Cynognathus , key in the evolution of the first mammals.

And, finally, mammals

Some carnivorous forms of small synapsids evolved into the first mammals during the Late Triassic . Throughout the Mesozoic the earth was dominated by dinosaurs. Mammals dedicated themselves to abide by their rules. They waited in the shadows, expanding and diversifying from the safety of their burrows. The smaller and more hidden, the more opportunities to prosper and not end up in the mouth of a dinosaur.

In the late Triassic and early Jurassic, the closest relatives of mammals ate insects and moved around at night , avoiding dinosaurs. An example of this way of life is found in Morganucodon, a species similar to a mouse or a shrew that would not exceed 15 centimeters in length without including its tail. Living at night is not easy. Mammals that gained senses to help this prospered, such as better hearing and hairs to keep warm in the absence of sun.

These advantages allowed us to diversify during the Mesozoic despite having dinosaurs as predators. This was how they were divided into the main groups of mammals: tricodonts , multituberculates , the placentals , the monotremes , and the marsupials . The first two became extinct, the next three groups are still here. Marsupials are, for example, kangaroos and koalas. Of the monotremes we only have two weirdos: the echidnas and the platypus. Among the placentals we are, the humans, and all the animals that are gestated in the placenta of their mother and are born already formed.

In China, the fossil of a species called Juramaia sinensis was found, a small rodent that would be “the great-great-grandfather of all the placental mammals that exist today on the planet”, in the words of Zhe-Xi Luo, Professor of Biology and Anatomy of Organisms at the University of Chicago.

The Cretaceous-Paleogene mass extinction wiped out the dinosaurs. The mammals came out of their burrows without fear of their already extinct predators. They populated the Earth completely and became the ruling class until today.

And then, what is the first mammal? We almost prefer that you choose: the Morganucodont, the Juramaia sinensis or you can simply keep an eye on the new research that paleontologists bring us.


Barnosky, A. D. et al. 2007. The Role of Climatic Change in the Evolution of Mammals. BioScience, 57, 6, 523-532. DOI: 10.1641/B570615.
Kemp, T. S. 2005. The Origin & Evolution of Mammals. Oxford University Press.
O’Leary. M. A. et al. 2013. The Placental Mammal Ancestor and the Post–K-Pg Radiation of Placentals. Science 339, 6120, 662-667. DOI: 10.1126/science.1229237.
Vicente, A. et al. 2018. En busca del origen perdido. Paidós.

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