摘要:The conscious experiences of non-human animals, from whales and birds to octopuses and bees, are revealing fresh clues about when consciousness evolved and what it’s for

Children know the fun of throwing a ball into the sea, only to watch the waves fling it back. Jennifer Mather and Roland Anderson at the Seattle Aquarium were surprised to find octopuses playing similar games. Their toy was a floating pill bottle, which they were free to ignore or explore as they wished. Six of the aquarium’s octopuses soon lost interest, but two showed childlike curiosity, pushing it with their arms or shooting jets of water to move it against the tank’s current. It is hard to interpret this as anything other than play, which many researchers argue requires some form of conscious awareness.

Many animals exhibit behaviours similarly suggestive of an inner life. Conscious creatures may include our primate cousins, cetaceans and corvids – and potentially many invertebrates, including bees, spiders and cephalopods such as octopuses, cuttlefish and squid. The challenge, of course, is to understand how the inner lives of these creatures differ from our own.

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In the past, scientists spoke about “levels of consciousness”, as if there were a hierarchy with humans on top. But in a paper published in 2020, Jonathan Birch, a philosopher at the London School of Economics and his colleagues argue that we would do better to consider five separate elements of conscious experience. The first is perceptual richness, or how well an animal can discriminate different details in each of its senses. The second is evaluative richness – broadly speaking, the capacity to differentiate between positive rewards and noxious stimuli, which could be analogous to human emotions such as pleasure or pain. The third, unity, concerns the extent to which an animal integrates the information from its sensory organs into a single experience. Then there is temporality: does the creature’s past experience influence its present behaviour, and can it plan for the future? The fifth dimension is selfhood. This may be tested by assessing whether an animal recognises itself in the mirror, or has so-called theory of mind – the ability to understand that another animal has its own mind.

Many ways of being

According to Birch and his colleagues, it doesn’t make sense to ask whether one animal is more or less conscious than another, since each species may score highly on some of these facets of consciousness, but low on others.

Research by Birch’s co-author Nicola Clayton at the University of Cambridge offers a good example. Scrub jays, a type of corvid, bury food to eat later, which demonstrates high temporality because it involves planning for future scarcity and remembering the location of caches. Their use of deception when hiding food in the presence of a rival bird, meanwhile, shows theory of mind, which suggests a relatively sophisticated sense of selfhood, too.

Cephalopods, by contrast, haven’t yet shown evidence of self recognition. But the octopuses’ enjoyment of play may be a sign that cephalopods can experience something akin to pleasure – evidence of some evaluative richness. They also have extraordinary perceptual richness, with complex vision that can detect polarised light and the ability to taste-by-touch with their suckers.

Perhaps the most startling difference between species concerns the unity of their conscious experience. We have two eyes but we seamlessly integrate the two visual fields into a single conscious experience, thanks to the thick nerve tract connecting our left and right brain hemispheres. Birds lack that connective structure, leading Birch and his colleagues to speculate that within each individual there may be “a pair of conscious subjects, intimately cooperating with each other”.

Or consider the octopus. Two-thirds of its neurons are located in its arms, and there is some evidence that each limb operates semi-autonomously. “You could conceive of there being eight conscious experiences associated with the different arms, that are partially unified with the experience associated with the brain,” says Birch.

That is a consciousness so alien, it is almost impossible for us to imagine.

When did consciousness evolve?

Given that there is a variety of conscious experience across the animal kingdom, it is natural to ask when and how consciousness first emerged. Can we thank a single common ancestor? Or did it evolve independently in different lines?

Andreas Nieder at the University of Tübingen in Germany favours the latter explanation. He recently conducted an ingenious experiment to discover the brain processing underpinning visual consciousness in corvids such as crows. The birds were trained to respond to different coloured squares, some of them almost imperceptibly faint. Neurons in a region called the pallial endbrain lit up whenever the crows reported seeing the squares, but not when they failed to spot them, suggesting that this area is essential for their conscious visual perception. In humans and primates, a different part of the brain, the prefrontal cortex, performs the same job. For this reason, Nieder thinks that consciousness probably emerged separately on multiple occasions, in much the same way that wings appeared separately in insects, birds and bats.

Eva Jablonka at Tel Aviv University in Israel and Simona Ginsburg at the Open University of Israel take a different view. In their book The Evolution of the Sensitive Soul, they argue that birds, mammals and other vertebrates inherited consciousness from a common ancestor. Their argument hinges on a type of thinking known as unlimited associative learning, which involves learning about novel stimuli and linking them together even if time has passed between experiencing two stimuli. It also entails forming chains of such links so that the same stimulus can be interpreted in different ways depending on the context.

Importantly, these associations can be updated: something that triggered danger may come to be a sign of reward if circumstances have changed. Jablonka and Ginsburg believe that this kind of complex learning is impossible without consciousness to direct attention, evaluate different cues in the environment and integrate experiences over time. So, what can it tell us about the origins of consciousness?

Unlimited associative learning has been documented in many diverse species. “Even little fish are able to do this,” says Jablonka. That leads her and Ginsburg to propose that consciousness must have emerged in early vertebrates around 530 million years ago.

They suspect certain groups of invertebrates, including arthropods like honeybees and spiders, can also think in this way. If so, their earliest ancestors evolved consciousness independently of vertebrates, around 500 million years ago. The shell-less cephalopods are relative latecomers to the table. The fact other molluscs don’t appear capable of unlimited associative learning suggests to Jablonka and Ginsburg that consciousness in octopuses, cuttlefish and squid originated around 300 million years ago. “I think it is one of the most promising approaches we have at the moment for how to study the evolution of consciousness,” says Jonathan Birch at the London School of Economics.

What is consciousness for?

Consciousness has deep origins and appears to have evolved several times. This alone suggests that it must provide important benefits when it comes to survival. But what might these be?

Biologists hadn’t grappled with this question until quite recently. Many of those who are now considering it, think one probable answer is flexibility of behaviour. “We are better able to react to the contents [of the conscious mind], as opposed to the unconscious content that will usually trigger automatic reflex-like responses,” says Andreas Nieder at the University of Tübingen, Germany. Being conscious also offers a way to evaluate the world – through feelings that make an individual aware of whether something is good or bad. And consciousness entails selective attention, which allows the brain to focus its processing activity on what is most important.

As a result of all this, instead of reacting in set ways to given stimuli, conscious animals can make complex decisions. “That would be a survival benefit that helps an individual, and then the entire species, to survive better in the changing world,” says Nieder.

Greater behavioural flexibility also fits with the idea that consciousness is associated with a particular type of thinking called unlimited associative learning – the ability to combine multiple cues into a single perception. This allows animals to respond flexibly to the challenges they face, rather than relying on hardwired behaviours. It means, for instance, that they can better discriminate between a healthy and a poisonous source of food based on small perceptual differences. “Associative learning is one of the greatest adaptations on the planet,” says Eva Jablonka at Tel Aviv University in Israel.

According to Jablonka and Simona Ginsburg at the Open University of Israel, consciousness first emerged around 530 million years ago and the resulting behavioural complexity it brought had huge knock-on effects. It created greater competition among species, driving the rapid diversification of animal species known as the Cambrian explosion. Predators, for example, would have been better at detecting their prey, which, in turn, would have needed to find new ways to avoid detection – pushing the predators to become even more sophisticated in their strategies. “There is a kind of ongoing co-evolutionary arms race,” says Jablonka.

But the emergence of consciousness didn’t just allow animals to adopt more complex behaviours. Jablonka suggests that it is responsible for much of the beauty that we see in nature too. It led different species to evolve camouflage, for example. And it pushed plants to evolve colourful flowers that would stick out from the competition to attract pollinating insects. “It changed the world completely,” she says. “The world would have been a very different place, and a very much more boring place, without consciousness.”