The Universe and Me: Where Did I come From?
Part 6: Mammals
Mammals lived, for 135 million years, in the shadows of much larger reptiles. Dinosaurs dominated the land, giant marine reptiles owned the seas, and pterosaurs patrolled the skies. Mammals were forced into the roles of scavengers and insectivores, usually nocturnal. Although there were some exceptions, this was the basic state of affairs throughout the Mesozoic. Mammals did not rest on their laurels, however. They continued to develop the traits that define them today. They covered their bodies with fur. They evolved the behaviour of nurturing their young with milk; and, instead of laying eggs, they began giving birth to live young (although monotremes, the group that includes platypuses and echidnas, who still lay eggs, diverged prior to that). They evolved separate passages through which to mate and pass waste. After the divergence of the marsupial group, another group of mammals evolved a reproductive system in which embryos attach themselves to the uterus of their mothers via a temporary organ call the placenta. Through the placenta, they receive nutrients and oxygen and eliminate waste. They became placental mammals. These mammals also had much longer pregnancies, and gave birth to young that were already quite well-developed. This is unlike marsupials, which give birth very soon after conception to relatively undeveloped young, and have to protect them in pouches until they have grown enough to survive on the outside.
Placental mammals did not diversify much, restricted by the domination of most ecological niches by dinosaurs. But the reptilian grip of the dinosaurs was broken, spectacularly and catastrophically, about 66 million years ago.
An asteroid or comet at least 10 kilometres wide, one of those leftovers from the solar system’s birth, slammed into North America. It delivered an explosion two million times more powerful than the most powerful bomb humans have ever made, causing huge megatsunamis, ejecting a massive cloud of dust into the atmosphere, and triggering earthquakes and volcanic eruptions with its shockwaves. Fragments of the asteroid and the planet would have been launched into space, only to be pulled back by gravity and re-enter the atmosphere. The heat from their return would have created firestorms across the planet.
The dust that was pushed into the atmosphere spread across the whole planet, throttling the amount of sunlight reaching the surface. This caused a huge die-out of plants, which relied on the sun for energy; in turn, the herbivores that fed on plants died out, and the predators that fed on them. After the initial die-out, there would have been a devastating greenhouse effect caused by the carbon dioxide vaporised by the asteroid, wiping out other vulnerable species.
The only dinosaurs to survive the extinction were birds. The rest were completely erased, along with pterosaurs. The large marine reptiles that had been the prime predators of the seas disappeared. Both land and marine invertebrates were heavily affected, as were plankton. Mammals did not escape unscathed either. All marsupials outside of Australia became extinct, and all other groups lost species. But mammals endured, probably because of their small size at the time. This and the fact that many of them were burrowers would have enabled them to take shelter from the stresses they faced in their new environments, and meant that they required less energy to survive.
When the dust had cleared, and life began to recover, mammalian diversity exploded. They diverged into a huge number of forms and functions, racing to fill in the niches that the dinosaurs had left. Although they endured a brief period of competition with large, flightless birds, they became the dominant predators in most land environments. They also became the prey, evolving large herbivores. Some were familiar, while others would be completely alien to anybody alive today. They colonised every environment, including the air and the sea. Among them were the first primates.
These creatures lived in tropical forests, and were adapted for a life in the trees. They had strong binocular vision for depth perception, hands and opposable thumbs for climbing, and were beginning to develop larger brains. They were mainly fruit-eaters. This group gave rise to monkeys and apes about 40 million years ago. Apes split from Old World monkeys some 25 million years ago, having gained a large amount of freedom of motion in their shoulder joints, which they used for swinging from branch to branch. They had also evolved very large, complex brains, with which they were able to communicate and develop complicated social orders, and even to use tools for foraging. After the divergence of gibbons at eighteen million years ago, orangutans at fourteen million years ago and gorillas at seven million years ago, they began to hunt for meat in order to supplement their diet of fruit. Chimpanzees then split off, leaving the Australopithecus. The brain of Australopithecus species continued to grow. As they moved further and further from their original habitat in the trees, they stood upright on two feet, giving them a wider field of view and allowing them to run and hunt more easily, while also being able to carry food. One of these species diversified into a group of species that used tools widely, making them from bone and stone. Some of these species also used fire to cook their meat, the first and only time this behaviour has ever evolved. More frequent consumption of meat provided more protein for brain development, which continued unabated. This new genus began to lose the hair across its body, replacing it with furs from animals it had hunted. By two hundred and fifty thousand years ago, Homo sapiens had emerged.
To be continued!