To say that we are mammals is a statement that is not controversial. However, we tend to overlook everything we have acquired throughout our evolution and that we share with many of the species in this group. Being mammals affects our behavior and our day to day.
Our story begins more than 300 million years ago in the Primary era. Earth was a hot and humid place, with a single continent (Pangea) covered in thick forests of ferns and mosses. It was then that a fundamental split occurred in land animals. A branch would give rise to mammals. The other, to modern reptiles, dinosaurs and birds. These two lineages evolved head to head, alternating in the domain of Earth.
During this initial period, proto-mammals acquired two fundamental characteristics, present in modern mammals: their “warm blood” and hair.
The term “warm-blooded” is inappropriate, as a lizard can reach 50 ⁰C if it lies in the sun. It is correct to talk about homeothermy. It is a sophisticated metabolic control system that allows maintaining constant body temperature, regardless of whether it is hot or cold.
The reasons for the appearance of this trait are not entirely clear, since it entails a very high energy cost: a mammal has to eat the same amount in a single day as a reptile of equivalent size in a month. The reasons for their evolutionary success seem not to be related to temperature itself, but to the ability to perform sustained physical exercise over time. Reptiles can move fast, but for a short time. Then they need a long recovery period.
It takes a high metabolism to keep running for hours like wolves do when hunting.
Hair is a formidable thermal insulation material. Scientists believe that it appeared during the Primary era, although it is quite possible that its initial function was related to perception and not isolation.
Fossils have been found that have a cranial slit to house the trigeminal nerve, indicating that there must have been sensitive hairs around the muzzle, as modern cats have. Once the biochemical machinery to manufacture sensor hairs appeared, these should have been generalized to the whole body with the new function of thermal insulation.
Survivors of the Permian extinction
The Primary era ended with one of the most significant natural disasters in Earth’s history: the Permian extinction. It is estimated that 70% of terrestrial vertebrate species disappeared in a short time.
The culprit behind this catastrophe was a period of volcanic activity that caused global warming by expelling millions of tons of CO₂ into the atmosphere.
The Secondary era that followed saw the protomammals lose influence to a new group of animals: the dinosaurs. Despite having lost their preeminence, protomammals acquired important new features throughout this era. The most important were the teeth.
In general, reptile teeth are all the same. Mammals, on the other hand, exhibit tremendous specialization: from incisors to canines, each type serves different purposes and is vastly more efficient at processing food.
The highlight was the development of the molar, endowed with a structure of ridges and valleys that allow chewing with tremendous efficiency. In addition, the upper and lower teeth of most mammals allow for perfect mating, which means great ability to bite, tear, and shred. The counterpart is that mammals have two sets of teeth for life, milk and adult. On the other hand, if a reptile loses a tooth, it simply grows another.
The ability to produce milk to feed the young is the characteristic that gives mammals their name and arose in the Secondary era. Experts are inclined to think that the milk derives from fluids rich in antimicrobial substances, whose function was to defend the young from infections, rather than to feed them.
This trait must have been very advantageous, since it evolved towards the development of the mammary gland and the production of a substance rich in food. Breastfeeding makes it possible to temporarily isolate the young from the uncertainties of the environment. For example, if insects are suddenly scarce, the baby birds are in serious trouble. The downside is that it imposes considerable metabolic stress on the mother, as she is forced to take care of her own energy needs at the same time as the enormous demand that milk production implies.
About 200 million years ago, the first animals recognized by science as mammals appeared: they were homeothermic, had hair, specialized teeth, and produced milk.
The placenta, the latest invention of mammals
The other great invention of mammals was the placenta. It was produced somewhat later, towards the end of the Secondary era, and not all current species possess this trait. Marsupials have only a basic placenta, hence baby kangaroos and opossums have to develop to a large extent after birth.
Monotremes, a very ancient group of mammals of which only two species remain (platypus and echidna) still lay eggs.
The Secondary era came to an abrupt end the day a meteorite struck what is now the Gulf of Mexico. The survivors suffered earthquakes and hurricanes never seen before. Plants died from lack of light and ecosystems collapsed like a house of cards.
It is estimated that more than 50% of species became extinct, including all the dinosaurs that had dominated the planet for almost 200 million years. When the ecosystems were able to recover, the mammals found that their main competitors were gone.
With all the achievements they had accumulated in their long journey, they diversified and occupied all the ecological niches. Without competition, mammals quickly diversified into the major groups we know today: primates, rodents, carnivores. This gave rise to creatures as different as the blue whale and the bat in record time.
Thus, we are descended from a very ancient line that has waxed and waned and has survived two global extinctions.
We can be proud to be mammals.