Although our addiction to coal and oil is changing the world and is a contributing factor to the very large number of extinctions that have happened in the last 300 years, without the coal and oil that formed in the Carboniferous geological epoch, modern-day civilization would not exist. We would not have built up the vast amount of knowledge we have of the universe, and nor would we have invented computers, cars and electric ovens. Coal and oil are both a blessing and a curse.
The mass extinction humanity is causing is referred to as the sixth great extinction our planet has experienced, although there may have been more. The most famous happened 66 million years ago when a meteor killed off the dinosaurs, but other mass extinctions we know of occurred 440, 360, 250 and 210 million years ago. Scientists can read the devastating impact of these events from the fossil record, and each time a significant fraction of life on Earth died. The biggest mass extinction was the one that occurred 250 million years ago, marking the end of the Permian geological era. It is commonly referred to as ‘the great dying’. It wiped out over 80 per cent of species in the ocean, and 70 per cent of land-living vertebrates. It was caused by the huge amount of carbon dioxide released by the formation of the Siberian traps we encountered previously.
Mass extinctions often wipe out dominant species, opening the way for a new group to thrive, and the great dying was no exception. At the end of the Permian, our ancestors, a group called the synapsids, were the dominant species on the planet. The ancestors of dinosaurs, called sauropsids, were both rarer and smaller. The great dying killed off many though not all of the synapsids, including the two-ton herbivore Tapinocephalus, the largest animal of its time. It also killed several species of sauropsid, but those that survived got the upper hand in the new world, dominating it for the next 185 million years as dinosaurs, while relegating our synapsid ancestors to a life on the margins.
The synapsids had been the dominant group on Earth for over 100 million years, and, like all life forms, they evolved from an ancestor. The mass extinction prior to the great dying, which happened 110 million years earlier, had given a group of vertebrates called the amniotes a shot at being the dominant vertebrate fauna. Amniotes had evolved from fish and amphibian ancestors, with the first appearing 330 million years ago. They had the advantage of being less dependent on water than their ancestors, which helped them thrive in warm, dry climates. Their name comes from their ability to produce embryos surrounded by an amnion or amniotic sack, a membrane that prevents the developing foetus from drying out. Reptiles, birds and mammals are all amniotes, even though some lay eggs while others give birth to live young. A few million years after the first amniotes evolved, their branch of the tree of life split into the synapsids that were to evolve into mammals, some non-mammalian synapsids that subsequently died out, and the sauropsids that included the ancestors of dinosaurs, reptiles and birds.
Some of the extinct synapsids looked a little like modern-day carnivores, and they shared many traits with modern-day mammals. They were warm-blooded and had primitive mammary glands for provisioning their young, even though they laid eggs. Their jaws contained molars, canines and incisors, and some species appear to have had fur. The big synapsids including Tapinocephalus are not seen after the great dying, but some smaller species survived it, and by 160 million years ago had evolved into the first mammals. These mammals did not immediately go on to evolve into modern-day mammals like rhinos, hippos and humans because the surviving sauropsids, for reasons we do not understand, stole a march on the surviving synapsids following the great dying, growing to become the dominant class.
The dinosaurs that evolved from the sauropsids that survived the great dying roamed the Earth from about 240 million years ago. Like their famous descendant, T. rex, the first dinosaurs walked upright on their two back legs. They quickly rose to dominance and, once there, they enjoyed that position until 66 million years ago. Many dinosaurs were successful large species, and their success prevented mammals evolving to the sizes they achieve today. Mammals couldn’t effectively compete with the tyrannosaurs, sauropods, triceratops and other large species. Interestingly, there were no small dinosaurs. It is possible that the small mammals that survived prevented dinosaurs evolving small sizes, while the large dinosaurs prevented mammals evolving to large sizes.
There are relatively few ways of making a living if you are a vertebrate. You get your food by feeding on other forms of life. Species can feed on vegetation, on seeds or fruits, on other animals they have hunted, or those they find dead, and, in the case of vampire bats and occasionally the vampire ground finch of the Galapagos, on the blood of living animals. No vertebrates are able to survive by consuming decaying plant matter, a way of life termed detritivory, but many vertebrates are omnivores eating a varied diet. Grizzly bears, for example, eat fruit, vegetation, insects, fish and meat. Regardless of what their favoured food is, evolution has produced a plethora of ways for species to detect, acquire and then utilize food.
The first step in finding food is to detect it. Animals have senses to help them do this, and these range from vision, smell, and touch, which are very familiar to us, through to echolocation, used by some bats. Each sense requires a specific organ – eyes for vision, ears for sound, a nose for smell – plus an area of the brain to make sense of the information the sense organs collect. Some food types are easier to detect than others, with it being easier for a bison to find grass than it is for a woodpecker to find bugs in bark. When resources are hard to find, evolution hones sense organs and brain areas to be exquisitely sensitive. Elephants, for example, can smell water from up to 12 miles away, while cheetahs can spot potential prey up to 3 miles away.
Echolocation requires more than just hearing; it also requires the bats that use it to produce sound waves that bounce off objects before they detect them. Their brains then produce a form of map in reflected sound that the bats use to spot insect prey. Although producing sound to detect food is relatively unusual in the animal kingdom, it is not a trick unique to bats. The aye-aye is a lemur, a species of primate, that is found only on Madagascar. It survives on insect larvae it extracts from tree trunks. It finds them by tapping a specially adapted finger on the trunk and listening with its oversized ears until it locates a cavity underneath the bark where a tasty morsel may be hidden.