Once the food has been detected, it needs to be acquired. Some herbivores have it easy, with the bison needing to simply put its head down and graze. In contrast, wolves need to chase down their prey before killing it, while orang-utans need to track down fruiting trees, pluck fruit from them, and sometimes peel the fruit, before consuming it. Killing prey can be dangerous, with buffalo and giraffe sometimes killing lions, and bison mortally wounding wolves. All animals have adaptations to help them acquire their food. Having detected a cavity within a tree, the aye-aye uses its forward-pointing incisors to drill a hole into the trunk before using its significantly elongated middle finger to prise the insect prey from out of its hiding place. Both the angle of teeth and the extension of a single digit on each hand are unique to the aye-aye, being found nowhere else in the animal kingdom. These phenotypic traits have evolved to enable the aye-aye to thrive. Aye-aye ancestors with these traits succeeded in avoiding death and failure to reproduce by detecting, acquiring and using food, and the phenotypic traits, and the genes that determine them, spread through the aye-aye population.
Even if acquiring food can be relatively straightforward, it needs to be done without becoming food for something else. The antelope on the African plains needs to be vigilant for cheetah, lion and wild dog, while the mouse searching for seeds, fruits and insects in the leaf litter must do so while avoiding snakes. The impressive 100 km/h top speed of the pronghorn antelope, the colour-changing skin of the chameleon and the hedgehog’s resistance to snake venom are all adaptations to reduce the risk of death while foraging. Evolution has been inventive, with arms races between predators and prey resulting in remarkable camouflage, staggeringly fast top speeds and senses honed to detect the minutest smells.
Once food has been acquired, it needs to be utilized. Things can now become more challenging for herbivores like the bison, which need to ruminate. They initially swallow the vegetation they consume, before regurgitating it and further chewing it. As they chew their cud, they release an alkaline saliva that counters the acid in their first stomach, the rumen. They then swallow their twice-chewed meal, which can be more easily digested by the chemicals and bacteria in their guts. A bison can spend up to eight hours a day chewing its cud. Energy from grass is hard won. Koalas arguably have it even harder. They survive on a diet of eucalyptus leaves that contain many protective chemicals to keep insects at bay, and this makes them extremely hard to digest. The koala has a longer gut than other animals its size to aid with digestion. The longer it takes for the chewed leaves to move from one end of the gut to the other allows more time for leaves to be digested and goodness extracted. Still, despite this adaptation, the koala sleeps for up to twenty hours a day, during which it does nothing but digest.
Once the food is digested the energy needs to be allocated to all the different tasks an animal must do. Some energy will need to be dedicated to reproduction, some to fighting disease, some to detecting and acquiring the next meal, and some to just keeping the metabolism ticking over. Some species of animals use energy sparingly, eking it out over a long period, while others use it more quickly, often living a ‘live fast and die young’ lifestyle.
The guppies in Trinidad where I work occur with predators in streams and rivers on the floodplains at the base of the Northern Range mountains. Guppies in these streams will inevitably be predated. Individuals do not live long, and the predators keep their prey population sizes small, meaning there is plenty of high-quality food for the guppies in the form of nutritious stream invertebrates. The guppies are well adapted to live in this environment. They reach sexual maturity at a young age, when they breed they produce large litters of small young, they have a very high metabolic rate and are able to swim quickly, and they are very vigilant for predators. Their mouths are adapted to suck prey out of the water column, and their shortish guts allow them to digest their nutritious food effectively.
If you follow one of the streams up into the mountains, you soon encounter a waterfall. Predatory fish are never found above these waterfalls, but guppies sometimes are, yet they are different from their relatives below the waterfalls. Because there are no predators, their population sizes increase until there is insufficient food to go round. Instead of dying by being predated, these guppies starve to death. Their high densities mean high-quality food is scarce, and the guppies switch their diet to include algae and bacteria. The predator-free, low-food environment selects for different phenotypic traits than the high-food, high-predation streams. Guppies evolve new jaw shapes to scrape algae and bacteria from rocks, they evolve slower metabolic rates so they use energy more efficiently, and their gut becomes longer to allow them to extract energy from their less nutritious diet. Because they do not need to sprint away from predators, their maximum swimming speed is slower. They grow more slowly than their high-predation cousins, reach sexual maturity at a greater size and age, they produce fewer, larger offspring, and their maximum longevity increases. In the absence of predators, males become more brightly coloured, and they compete more intensively for females. Numerous phenotypic traits change, and the fish, although clearly still guppies, differ considerably between predator-rich and predator-free streams.
These changes have occurred because different genes are switched on and off at different times during development. Natural and sexual selection has moulded the guppy to thrive in environments with very different causes of death and reproductive failure. It does this by favouring sets of phenotypic traits that help guppies avoid an early grave and a failure to breed in contrasting environments, and as these traits evolve, different alleles on different chromosomes become favoured and high and low guppy populations start to genetically diverge. In time, perhaps, a new branch of the tree of life will emerge. The guppies are a microcosm of how evolution has sculpted life over the past 4 billion years.
In this chapter I have explained how competition for resources is the driver of evolution, and how on our limb of the tree of life it has produced complex multicellular organisms that sexually reproduce. I have also given a high-level summary of how animal life has emerged and evolved until mammals inherited the Earth. Along the way, I have summarized how evolution is a powerful process that has created strain MK-D1, E. coli, aye-ayes, guppies, wolves and you and me. Each species is a success story, surviving in a world where death or reproductive failure is never far away. The ancestors of each organism alive today have survived periods of food shortage, landscapes rich in predators, cold climes, and warm ones. They have lived in water and on land and have survived volcanic eruptions and meteor impacts. They have found many ways to thrive, some as single-celled wonders, others as complex animals consisting of trillions of cells. In complex animals such as you and me, we have evolved keen senses to detect, acquire and efficiently utilize a wide range of food, and phenotypic traits to allow sexual reproduction.
For mammals to exist, multicellularity, sex and a whole host of other attributes I do not mention had to evolve. These characteristics all evolved in response to the environments that ancestral species on our branch of the tree of life experienced. Modern mammals are a product of billions of years of evolution of competition for resources in both cold and hot environments, as well as wet and dry ones, in places where competition was sometimes intense and sometimes a little more relaxed. At times the journey was driven by volcanoes, earthquakes and asteroid and meteor collisions, and at others by predators, disease and limited food. Sixty-six million years ago, mammals became the dominant animal species on land. They spread and diversified, and the next step on our journey is how they became human. But before I discuss that, there is an important phenotypic trait I am yet to mention: consciousness. And that is the topic of the next chapter.