In our last post, we looked at some of the many ways life could be prevented from giving rise to a pre-intelligent species–say, an animal like Proconsul, the earliest known ape, living about 25 million years ago. In doing so, we knocked f_i down to 0.02. Here, we’ll look at what could prevent the descendants of a pre-intelligent species from evolving intelligence.
First, we have to set aside our self-centered bias and admit a blunt truth: Evolution has no goal. It blindly pursues local optima. There’s no guiding hand ensuring that evolution reaches an unsurpassable pinnacle, i.e., us.
But the evolutionary record shows ever more complex organisms, you might protest. Sure. But at the cellular and below-the-neck anatomical levels, human beings aren’t that much more complex than a mouse, or a sperm whale. And while we are in love with our big brains, here are two severe constraints against the emergence of a big-brained life form:
- Our brains make up about 2% of our body mass, but consume about 20% of all the energy our bodies take in from food.
- Our brains are too big to fully develop in utero, meaning that human offspring are very immature, and require significant care, for a long time. By extension, their mothers require support from other members of their band in order to care for those infants.
To overcome the first constraint, we need an environment where the cost of having a big brain would be outweighed by the benefit. Regarding environment in the sense of the climate, terrain, and ecosystems around our pre-intelligent species, we need a medium level of change. A constant environment, barely changing during an individual’s lifespan, makes it possible for the individual to rely on instinctual behaviors. There’s no benefit to noticing seasonal or other short-term changes and acting on them. At the other extreme, if the environment is too much in flux, there’s no benefit to learning about it and storing that learned knowledge in a big brain. If a food source vanished or the climate drastically changed, that big brain is an expensive boondoggle. It’s only the sweet spot of medium change in the external environment that makes a big brain worthwhile.
Pre-intelligent life on a planet without an axial tilt, or in the twilight zone between the light and dark faces of a planet tidally locked to its star, would face a far more constant environment than did our ancestors. Result: no intelligence evolving in those circumstances. Considering that the Earth only has its massive axial tilt thanks to the precise dynamics of the giant impact that gave us the moon, and that most of the stars long-lived enough to let life evolve pre-intelligence are M type stars, so small that their habitable zone is inside their tidal locking radius, f_i may drop 95%, down to 0.001.
But climate, terrain, and ecosystems aren’t the only environment facing a member of our pre-intelligent species. Its other environment is other members of its species. Remember, the biggest rivals are competitors for the same niche. Social creatures need large brains to model and predict the behaviors of their peers, in order to outthink them. A creature living in small groups, or being entirely solitary, has very few peers and can get by on social instinct.
And not just any social environment. A common social structure among animals is a group of females with their immature young, with a solitary powerful male either a full-time resident (e.g., gorillas) or a visitor during the mating season (e.g., elephants). Again, social instincts can guide most behavior in this situation. The male (and rival males outside the group) ignore the females until they enter estrus, then they try to mate with them. The males fight as needed until only one remains to mate. If the dominant male loses, the new males commit infanticide to get rid of the dominant male’s offspring.
Human beings are one of the few animals to form long-term social groups of multiple males and females. Related to this, women are among the few females to have concealed estrus. In other words, you can’t tell a woman is fertile by glancing at or smelling from a distance her engorged genitals. This greatly complicates human social behavior, as both sexes have to engage in even more behavior modeling and coalition building.
Is she only mating with me? Can I trust him to spy on her without trying to mate with her? Should I offer to spy on his mate for him? If so, should I try to cuckold him?
Should I mate with that male, that other male, or both? Should I tell either one about the other? Is he as high in status or as successful at hunting as he says? What do the other women say? What do the other men say? And if I’m talking to them, should I mate with one of them in addition, or instead?
You see the benefit of a big brain at navigating the human social world. Given the rarity of this kind of social structure in the animal kingdom on Earth, f_i may drop another 95%, to 5e-5. (The numbers are getting small enough to switch to scientific notation).
All that, and we still haven’t considered the rarity of language (requiring co-evolution of brain structures to listen and speak, and anatomical features to make speech possible). Perhaps non-speaking animals could be intelligent, but how would we know? We’ll drop f_i another 90%, to 5e-6.
And how many other happenstances had to break just right for human beings to arise? We’ll drop f_i another 90%, to 5e-7.
We’re now at N, the number of detectable civilizations in the galaxy, at [5e-7 to 8e-6] * f_c * L. Is it likely that intelligent life will evolve high-technology civilization? Stay tuned for our next discussion, of the value of f_c.