Flexible body was ready for success
Seven characteristics provided mammals with the perfect starting point for taking over from the dinosaurs. Even today, the features ensure mammal world dominance.
moment that the large dinosaurs disappeared from Earth’s surface.
BIZARRE MAMMALS CONQUERED EARTH
Over a period of a few million years, almost all the groups of mammals that we know today evolved – along with a long series of odd groups that have long disappeared again. The mammals were the fastest starters, and though they faced steep competition from awesome, 170 kg birds and huge, 14-m-long snakes, they always came up with a powerful response. The tapir-like Barylambda weighed 650 kg and dominated the world of herbivores,
while the Arctocyon, a predator the size of a bear, was at the top of the food chain. In the trees, you would encounter the Ocepeia: a small, mysterious animal that may have been the ancestor of hyraxes, sea cows, and heavy elephants. In the oceans, you could experience bizarre, cloven-hooved animals that evolved into the rulers of the oceans, the whales. Mouse-like animals developed membranes between their long fingers and began to fly awkwardly from tree to tree, until they fine tuned their capacity, turning into modern bats. Bears, dogs, and cats, which now include the largest terrestrial predators, can also date their common ancestor back to this era in the shape of the Dormaalocyon, which weighed about 1 kg. And in the trees, you would find the Purgatorius: a small, ratlike creature, whose descendants were monkeys - and humans.
FUR ENSURED EXTRA ENERGY
The overwhelming success of mammals from early Paleogene was the result of an extremely adjustable body. One of their most unique features was the ability to produce milk for their young. The whitish liquid
probably originated among reptile-like mammal ancestors, which used it to keep their eggs moist in dry surroundings. When they subsequently began to make the eggs develop inside the womb and gave birth to living offspring, the milk obtained another function. Its contents of nutrients was optimised to give the young the best start in life, and the liquid allowed the mother to transfer her immune bodies, so the offspring was protected against diseases. The parents were no longer forced to collect food for the young, i.e. they were less dependent on the constant availability of energy-rich food.
The fur was also very important for early mammals, helping them keep warm, when temperatures fell, and keep out heat, when it was hot. The body temperature was determined by the high metabolism that ensured constant body temperatures. The constant temperature provided optimum conditions for body cells and allowed the animals to keep up high energy levels throughout the day. Unlike cold-blooded reptiles, which depend on sunlight, mammals could adjust to cold regions or be active during the night. The extra energy also made it possible to forage across larger areas and hunt prey more efficiently.
The high activity level was only possible, because the mammals had developed a more sophisticated heart. Whereas a reptile heart only boasts three chambers, mammals have developed a heart with four chambers. The extra subdivision means that oxygen-rich blood from the lungs is not mixed with oxygen-poor blood from the body, before it is pumped into the blood vessels, so the cells of the body constantly receive large quantities of oxygen that can power the animals’ high metabolism and keep them active.
SENSES MADE THE BRAIN GROW
An impressive set of teeth provided mammals with yet another advantage in the evolutionary race. Unlike most other animal groups, every single mammal has a number of different types of teeth in its mouth, and the tooth types vary from species to species. Humans have front teeth, canine teeth, and molars that each have their own functions. Like in many other mammals, our teeth allow us to eat a broad range of different food and pulverize it, so we can consume the nutrients faster and more efficiently.
The ability to adjust to almost any habitat and any type of food was boosted by mammals’ flexible limbs. Whereas bird hands had been reduced into a set of feathered fingers, mammals could use all four limbs freely and were able to use their front legs to crawl, dig, handle food and tools, and support their heavy bodies. The freedom allowed mammals to take better advantage of their surroundings and resources, growing to giant sizes.
However, the key to mammals’ explosive conquest of the planet is primarily found inside their heads. The first mammals developed a set of new bones in their ears known as the hammer and the anvil. Together with a third bone, the stirrup, which also exists in reptiles and birds, the new bones allowed mammals to hear more high-pitched sounds than other animals. Moreover, ancient fossils show signs of improved olfactory and sensory senses – boosted by the development of fur that allowed sensitive whiskers. A thorough study of the skulls of mammal ancestors has revealed the effect
of the improved senses. To handle sensory impressions, mammals developed larger brains, and the extra brain capacity was quickly used. From being confined to interpret sensory impressions, the new brain centres were increasingly used for problem solving. In humans, the vast majority of the cerebral cortex has developed from these new innovative features. Intelligence is one of the most vital factors, when animals are to adjust to new habitats, and the mammals of Paleogene were more than smart enough to conquer the entire planet.
COLD MEANT NEW OPPORTUNITIES
Once mammals had become the dominant species, they never gave up the position. About 20 million years after the meteor strike, the climate started to change, benefitting mammals even more. Continental drift tore Antarctica and Australia apart, causing new ocean currents around Antarctica, which stimulated slow cooling of Earth.
As temperatures fell, the ice caps of the poles grew. The water was tied up, ocean water levels fell, and new land bridges were produced between territories that used to be isolated. Moreover, India collided with Asia, Africa hit Asia, and Europe and North Amerrica got in contact with South America, allowing mammals to spread more easily.
The cold climate also changed the makeup of the ecosystems, which were no longer dominated by dense forests. Earth now included deserts, tundra, and vast grass plains, and the new environments paved the way for even more variation among mammals and triggered the development of huge herbivores and fast predators.
DIVERSITY ENSURES DOMINANCE
The mammals won the race both on land and in the oceans. The success is not about the number of species, as both birds and reptiles beat mammals in this respect. Instead, the global dominance is based on a unique diversity of body shapes, life styles, habitats, and sizes. Mammals come with wings, fins, and legs. They live in mountains and in the ground. They include some of the smallest terrestrial vertebrates and whales: the largest animals that ever existed on the planet.
The considerable diversity was possible because of a meteor strike that changed Earth’s destiny in a matter of a few seconds. Had the meteor not struck, or had mammals been slow to begin their evolution, the world might have been a different place today. But a combination of random factors meant that our ancestors became the winners.