The way we are treating animals in our society is nothing short of appalling, and may well represent the greatest crime ever committed by our species. And this is a quite serious accusation, considering Homo Sapiens' genocidal track-record.
The ongoing mass extinction of species, one of the most significant in the history of Earth, is widely considered to be the result of human activity. Most scientists agree that we are now living in a new geological epoch, the Anthropocene, marked by significant human impact on Earth's geology and ecosystems, including anthropogenic climate change. The dating of the epoch is controversial, but some place the start of the Anthropocene as early as the beginning of the Agricultural Revolution 12,000–15,000 years ago. During this period, humans developed agriculture and animal husbandry to supplement or replace hunter-gatherer subsistence, which had become unsustainable due to overhunting. Such innovations were followed by a wave of extinctions, beginning with large mammals and land birds. This wave was driven by both the direct activity of humans (e.g. hunting) and the indirect consequences of land-use change for agriculture. The mass extinction is in progress, aggravated by human-made climate change.
The Agricultural Revolution, apart from driving to extinction about 50% of all the large terrestrial mammals of the planet, was responsible for another, much worse, form of animal suffering, unprecedented in scale and intensity. Of the mammals that did survive humanity's calamitous development, the large majority became domesticated, their lives completely subservient to human needs. These animals payed the price of unparalleled evolutionary success with tremendous individual suffering. You may object that chickens, pigs and cows in farms are cared for by humans, provided with food, water and shelter and protected from predators and natural disasters. What could be wrong with that?
A lot. The argument implicit in the question completely disregards the individual subjective experience of each animal. Farmers take a young calf, separate her from her mother, put her in a tiny cage, vaccinate her against various diseases, provide her with food and water, and then, when she is old enough, artificially inseminate her with bull sperm. The calf has fulfilled her evolutionary role: she had plenty of food, didn't have to worry about predators, and reproduced, usually many times over. The evolutionary needs were met, therefore the calf lived a good life. But the logic in this case is fatally flawed. Ask yourself a simple question: is that how you judge the value of a human life? Would you be happy to live like that?
Of course not. And the reason is simple. The subjective experience of the calf is not shaped by the evolutionary pressures of survival and reproduction now, but as they were tens of thousands of years ago, when her ancestors roamed the land. I am going to quote a well-known article by Harari, because I could not explain this better myself.
It is certainly true that all instincts and drives evolved in order to meet the evolutionary pressures of survival and reproduction. When these pressures disappear, however, the instincts and drives they had shaped do not evaporate instantly. Even if they are no longer instrumental for survival and reproduction, they continue to mould the subjective experiences of the animal. The physical, emotional and social needs of present-day cows, dogs and humans don’t reflect their current conditions but rather the evolutionary pressures their ancestors encountered tens of thousands of years ago.
Ancient wild cattle were social animals. In order to survive and reproduce, they needed to communicate, cooperate and compete effectively. Like all social mammals, wild cattle learned the necessary social skills through play. Puppies, kittens, calves and children all love to play because evolution implanted this urge in them. In the wild, they needed to play. If they didn’t, they would not learn the social skills vital for survival and reproduction. If a kitten or calf was born with some rare mutation that made them indifferent to play, they were unlikely to survive or reproduce, just as they would not exist in the first place if their ancestors hadn’t acquired those skills. Similarly, evolution implanted in puppies, kittens, calves and children an overwhelming desire to bond with their mothers. A chance mutation weakening the mother-infant bond was a death sentence.
Therefore, from a subjective perspective, the calf feels a strong urge to bond with her mother and to play with other calves. If these urges are not fulfilled, the calf suffers greatly. Consider this for a moment: if mammals are conscious, what is the nature of our sin? The kind of pain they are subjected to is unimaginable, one that our most infamous prison systems wouldn't inflict on the worst criminals.
Our society treats cattle like machines, as if they didn't have any consciousness, and indeed that is the only way to ethically justify what we are doing on a daily basis. This all goes back to Descartes, who maintained that only humans are conscious, have minds and souls, can learn and have language and therefore only humans are deserving of compassion. Animals are more like automata, some kind of mechanical robots with no awareness or feelings. Descartes' judgement is what shapes humankind's view on non-human animals to this day.
Descartes' assertion, notwithstanding philosophical problems with the definition of consciousness even in humans, is a falsifiable statement that can be tested against experimental data. And the overwhelming majority of neuroscientists now rejects Descartes' view and agree that awareness also exists in nonhuman animals. The philosophical problem in determining animal consciousness is just the old epistemological problem of other minds: given that I can only observe the behaviour of others, how can I know that others have minds? This is the central issue of a philosophical idea, strongly supported by Descartes, known as solipsism: the notion that for any person only one's own mind is known to exist. Solipsism maintains that no matter how sophisticated someone's behaviour is, behaviour on its own does not guarantee the presence of mentality. The problem with this argument is that if it is used to dismiss animal consciousness, then it also automatically rejects human consciousness, given that for animals and humans alike all that is accessible to us is behaviour. This is clearly preposterous, unless you really want to believe that you are the only conscious being in the universe, and in fact the only thing that exists, given that anything external is just a creation of your mind.
Critics will surely object that, while we can never know if other people are conscious, it is reasonable to assign sentience to them because they are similar to us. In particular, the brain structure is identical between members of the same species, therefore it is sensible to assume that they will have the same type of subjective experiences if consciousness arises in the brain. The same cannot be said about non-human animals because the neurophysiology is different.
This position has been (recently) refuted by extensive neurobiological evidence. In 2012, a group of neuroscientists attending a conference on "Consciousness in Human and non-Human Animals" at the University of Cambridge in the UK, signed The Cambridge Declaration on Consciousness, asserting that
The absence of a neocortex does not appear to preclude an organism from experiencing affective states. Convergent evidence indicates that non-human animals have the neuroanatomical, neurochemical, and neurophysiological substrates of conscious states along with the capacity to exhibit intentional behaviors. Consequently, the weight of evidence indicates that humans are not unique in possessing the neurological substrates that generate consciousness. Non-human animals, including all mammals and birds, and many other creatures, including octopuses, also possess these neurological substrates
Instead of agonizing over the meaning of consciousness, scientists at the conference asked a very simple scientific question, namely whether the neural substrates that are crucial to the emergence of consciousness in humans are also present in non-human animals. And the available data overwhelmingly showed that this is the case. In particular, the neural substrates of emotions do not appear to be confined to cortical structures (the neocortex is the part of the mammalian brain involved in higher-order brain functions, most developed in humans). In fact, subcortical neural networks aroused during affective states in humans are also critically important for generating emotional behaviours in animals. What this means is that many non-human animals, including all mammals and birds, are indeed similar to us, in the sense that they have the same neurological structures that allow for consciousness. The reasonable conclusion then is that they are conscious and should be treated as people.
The extension of fundamental rights to animals (at least mammals and birds that have been proved conclusively to possess conscious states) is one of the most pressing ethical issues of our time. Peter Singer, moral philosopher, once claimed that industrial farming is responsible for more pain and misery than all the wars of history put together. If we believe our science, this is not so far from the truth.