The theory of evolution has made a huge difference to the way we think about the universe. It is very useful to get a good grasp of its principles, so that we can think about it more intelligently. The two books that helped me most in this way were The Selfish Gene, by Richard Dawkins (Oxford University Press, new edition 1989), and Darwin's Dangerous Idea, by Daniel Dennett (Simon and Schuster 1995, Penguin 1995). Both of those were written before their authors became obsessed by atheism!
Let me mention a couple of important principles here. The first is that there is a continuous background of mutation in genes - they can occasionally change their structure just by chance. Most of these changes will be harmful, but from time to time one may give its possessor some advantage over its conspecifics, and in that case it will tend to spread through the population, as better adapted creatures are likely to leave more offspring.
The second is the idea of the arms race. Imagine a population of simple organisms which manufacture the chemicals they need for their lives from simple ones in their environment. The organisms themselves can be looked on as bags full of nutritious chemicals, and it is easy to see that if a predator evolved which could eat the organisms instead of bothering to manufacture the chemicals for itself, it would have a great advantage. We might imagine one of the organisms undergoing a genetic mutation which causes it to eat other organisms rather than its usual chemical diet. Even a failure of some gene might bring this about, a gene which helps the organism to recognise components of its regular diet. Of course a lot of organisms will end up being devoured by the new predator, but some may escape, perhaps because of greater speed in movement, or perhaps because of some chemical sense which enables them to detect a predator's presence. When their regular prey becomes scarce, many predators will die, but any that can move faster or remain undetected by the more elusive prey organisms will survive. After that, only even better adapted prey organisms will survive until they too fall victim to even more skilful predators. As time goes by, both predators and prey will evolve better and better mechanisms to further their survival. Evolution may be driven by such arms races.
Of course that last paragraph is merely a "just-so" story. In 1902 the author Rudyard Kipling published a book called the Just So Stories for Little Children, in which, for instance, the elephant's trunk was explained by an elephant with an ordinary sized nose having it pulled by a crocodile. A just-so story is one which is plausible, but has no direct evidence to support it. Although there is evidence for arms races involving various complex creatures, the origins of life on our planet and its earliest evolution are still rather a mystery, although tales like the one above make sense.
Anyway, it is worth pointing out two strategies for coping with predators. Many fish and insects produce large numbers of small and simply structured offspring, in the hope that a few of them will escape being eaten. The other strategy, adopted by mammals including humans, is to give birth to a small number of large brained young and to look after them carefully, so that in the end they have sufficient skills to avoid their predators.
It would probably be difficult to find a pet owner who did not feel an emotional link with their pets. It is so easy to tell if your dog or cat is feeling happy, and to make it feel happier by a show of affection. Social animals at least seem to feel a need to get along with their human companions, and regularly make their affection known. They respond to anger by trying to change their behaviour and make amends. This is true even in the case of animals we feel to be less intelligent, like hamsters or mice.
Cats are exceptional in a way, as they are not really social animals, although female cats sometimes cooperate in looking after kittens or hunting. We know that their wild relatives, and other members of the cat family like lions and tigers, may be friendly enough and playful as kittens, but soon lose that characteristic in adulthood. It appears that domestic cats remain mentally juvenile all their lives - they never reach the adult cat stage. They treat their human carers as they did their mothers, with respect and affection, in other words.
Quite complicated emotional states can be detected in our pets. One of our two cats is notable for her occasional sulkiness. If I greatly displease her in some way, by allowing her sister to sit with me for an unusually long time, for instance, she rejects any friendly gestures for a while. If I try to stroke her head, she pulls it away and sits with her back to me. Of course sulkiness is noticeable among human children, and is a much criticised emotional state, since children can explain their problems in words without having to show their displeasure by sulking. I can remember my mother often saying to me, "Don't sulk! Just tell me what's wrong!" But of course cats have no other way of demonstrating their annoyance.
Many scientists investigating animal behaviour hesitate to ascribe thoughts or emotions to their subjects, preferring to stick to measurable behaviour. However it is much easier now to observe brain activity in ordinary situations, and the similarities between animal and human are becoming more apparent.
From the evolutionary point of view, it would be very surprising if we did not share emotional states with other animals. Feelings of pleasure, for instance, can be seen as a "reward" for doing things of advantage to us, such as eating good food or pleasing others in our social group. Animals as well as humans will prefer to do things that bring them pleasure, and that will be good for them in the long run as well as for their genes. For most animals, of course, finding good food needs a lot of effort: it is perhaps unfortunate that many human beings can get that pleasurable feeling all too easily!
Negative emotions like fear are also useful from the point of view of evolution. They force us to be alert if there are likely to be predators about, or if we are walking near the edge of a cliff, for instance.
Another interesting point is that emotional responses are not confined to mammals. They must go further back in evolutionary history, if we accept the evidence of the social birds we keep as pets, like budgerigars and parrots. I have never kept a bird myself, but I well remember visiting a primary school friend's home where they kept one budgie. In the evenings, the family would carefully close all the living room doors and windows and let the bird out of its cage. It would fly from person to person in the room, perch on a shoulder and run strands of their hair through its beak. Budgerigars in the wild groom each other's feathers, and presumably human hair was the nearest thing it could find to feathers. My friend's budgie was very happy if you ran a finger through its feathers in return. But when the bird first came to me, her father told me just to sit still and ignore it. The budgie played with my hair for a bit, but when I didn't respond, it started to pull my ear harder and harder - a rather obvious display of anger! From tales I have heard about parrots, they seem to be just about as intelligent as cats.
Charles Darwin himself wrote a very interesting book on these topics, The expression of the emotions in man and animals (1872).