A fossilized trilobite with a bite mark. Evolutionary neuroscientists suggest that the brain only developed after animals developed a taste for eating animals. Pity the species of the planet Vegetaria.
This is the third of a series of posts about the evolution of consciousness. In the first post
, I laid out a basic theory that goes something like this: consciousness began to evolve about 350 million years ago, when we emerged from the water on to land. Why? By enabling vision to work over distances many times greater than in water, this move gave us the ability to perceive multiple futures. As a result, the ability to consciously plan ahead became important. In my last post
, I detailed why long distance vision reigns supreme when it comes to planning (as opposed to other long distance senses such as hearing or sense of smell). In this post, I want to make the argument more comprehensive. The crucial environmental condition for evolving neural structures to support planning is that there is an interlude--- space to breathe--- between perception and action. Without such a gap, only simple, fast, and direct transformations between sensory input and motor output can keep an organism safe from predators. But the long-range sensing abilities discussed in the last two posts are just one category of possibilities for such a gap to open: there are other fancy brain abilities unrelated to sensing that can also open this gap. Here, I consider two such capabilities: memory and communication. An animal can plan to do something based on memory ("I remember good breakfast was always in this direction"), communication (“hey buddy, around the corner is a good place for lunch”), and, as discussed already, perception ("I see something tasty looking over there"). Let’s go through planning via memory and communication, and compare these to the perceptual route. Combined, the three different mechanisms are the very grist of the mill of consciousness-as-planning
. Remembrance of possible futures. If you have an accurate mental map of a space containing memorized landmarks, then you can devise multiple plans without sensing and execute them by going from landmark to landmark, where those landmarks are spaced no further apart than your sensing range (which could now be very short, and even work through touch). For example, imagine the landmarks are bushes of berries, and they are spaced apart a distance equal to or less than the range of the sensory system you are using to perceive the bushes. You’ve visited these bushes so often, you’ve memorized each bush’s position with respect to the others. (Such maps exist in all animal brains where they’ve been looked for. Their neural basis is under intensive investigation. Fairly elaborate ones have even been found in honey bee
s.) Now, before you make your first move, you devise a plan for harvesting efficiently: 1) You know that you will be out until dusk, and you want to be able to see your home before it gets too dark, so you decide to start with the furthest bush and end with the bush closest to home; 2) You typically remove all the berries from a bush before moving on, so it’s important not to waste time in revisiting bushes you’ve already picked. So you devise a trajectory through the bushes that has no overlaps. Both aspects of this strategy can be provided by remembering a bush’s position. In fact, birds and bees use strategies of harvesting from plants that avoid revisits, and need to use memory for this. Communication of possible futures. Bees have fantastic navigational systems that let them roam hundreds of meters from their hive to find a food source and describe its location to their nestmates back home. They use their relatively coarse visual system to obtain a local cue (optic flow) that lets them detect how far they’ve gone, and they sense direction using their ability to sense the angle of polarized light. They come back, and then communicate distance and direction to nest mates via their dance language. This means the hive mind has an extended sensory range and can collectively explore multiple places to find food. The same is true for humans, with their symbol systems. We can go over the hill, come back and tell our friends that there’s an ice cream stand beyond where we can see. (Our ability to review this ice cream stand on Yelp, thereby enabling anyone in the world to find it on Google Maps, increases humanity’s possible futures exponentially to the point of creating a new phenomenon of choice anxiety. But that’s another post entirely…) Both memory and communication, then, can extend our perceptual capabilities, and thereby give us the room for multiple possible futures. One of the core parts of the idea I’ve been discussing here about how/why consciousness emerged in animals is that the neural basis for planning would have really been pushed once we had long range vision (after we came up on land). Could the ability to plan have come about because of improved memory or communication abilities, rather than long distance sensing? While possible, this seems unlikely. Here’s why. A problem for both memory-based and communication-based planning is that they depend on the goal being relatively stable in spatial position. For example, you can plan to go hunting in a place where tend to be are lots of antelopes, but to kill a particular antelope, you can’t hunt purely on the basis of memory---unless it happens to be paralyzed or dead, which is generally not the case. Similarly, bees would not do so well to come home to their nest and communicate the position of a source of nectar if that source of nectar happened to be a very strange plant---a plansect, if you will---that had wings and was constantly moving around. The point is clear: for stationary food sources or goals, both memory and communication work well in support of planning. But for unpredictable food sources, like the very nutrient rich body of another moving animal, memory and communication can get you part of the way (“antelope over the next hill!”) but can’t close the deal. Planning different possible paths to the most nutritious sources of energy requires long-distance perception. Perception of possible futures. I therefore hypothesize that the biggest payoffs to our early land-based ancestors came from advances in long-range perception combined with small buffer of working memory to hold some different possible futures being considered. This combination lets you hunt a moving animal that may be devious and require rapid contemplation of multiple possible approaches to capture. If this logic is correct, then consciousness may only come about in a world where animals developed a taste for eating other animals. Interestingly, experts in the evolution of the nervous system have suggested that it was only after animals started preying upon one another that diffuse neural nets (similar to those in sponges and jelly fish) condensed into what we now know as the brain over 500 million years ago (e.g., Northcutt and Gans’ “New Head Hypothesis” from the early 1980s). However, by my argument, carnivory alone would not have been sufficient for the birth of full-fledged awareness: you and your prey need to move onto land, where you can see it from a distance and envision several ways of successfully capturing it. Once weighing these various options becomes useful, evolution can work its powerful ways in slowly accreting the necessary neural structures for thinking about these futures. Image courtesy of GeoKansas
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