A Brief Summary of Intelligence — Evolutionary Pressures
Intelligence only makes sense in the context of why it evolved in the first place
I have increasingly seen that for us to make true progress in understanding human intelligence and consciousness, we need to ask why it exists in the first place. This question can translate in a few ways, the least of which is what function it serves today. The more important translations include: i) what problems does it solve that could not be better solved through other means, and ii) what evolutionary pressures were around at the time of its various stages of evolution?
This is why I was so enthralled to read Max Bennet’s A Brief History of Intelligence (Mariner Books, 2023). The author reviews our current best understanding of how modern human intelligence evolved all the way from our humble beginnings as microbes. Through a journey of “5 Breakthroughs”, he explores the evolutionary pressures facing the organisms at various times in our evolutionary past, explains the different strategies taken by those organisms at the time, and shows why our particular lineage took the strategies that it did.
The knowledge presented in that book is such a good reference, and I encourage everyone with an interest in the area to read it. The book follows the evolutionary journey through time, and I found myself wanting a reference summary of the key points, ordered by topic rather than time.
So this is the first of what might later become a series, to summarize those points. In this post I am going to focus on why certain forms of intelligence appeared at different stages, but summarizing the evolutionary pressures.
Overview
Bennet distills the evolution from micro-organism to human into a simple linear progression of 7 different stages. The stages will be discussed in detail in what follows, but briefly they are:
- Single-celled organisms
- Large multi-celled radially symmetrical animals (Radiatans)
- Bilaterally symmetrical animals (Bilaterians)
- Vertebrates
- Mammals
- Primates
- Humans
While that progression is overly simplified, it is still sufficient to tell a very compelling story. Furthermore, it turns out that each of those steps have representative ancestors living today that we can study:
- Single-celled microbes: I’m not aware of any specific exemplar for these, but there’s surely plenty of them
- Radiatans: Coral polyps
- Bilaterians: Nematodes such as C. Elegans.
- Vertebrates: Fish
- Mammals: Mice
- Primates: Chimpanzees
- Humans: us
In each of the cases, scientists believe that these exemplars are sufficiently close to the first animals of each stage, way back then. This means that we can gain a detailed understanding of what the next stage evolved from.
Context is Everything
Having modern exemplars means that that we have a fighting chance to figure out why each stage evolved as it did. And that’s important if we want to understand why humans have the kind of intelligence that we do.
But to understand the “why” at each stage of our great evolutionary progression, we need to consider more than just what came before and after. We also need to consider what other organisms were around and what was happening at the time.
So here’s a more detailed story — a phylogenetic tree of our ancestors and some of their competition:
To pull this apart, I will go through each of the major pre-historic periods highlighted in the above diagram. As this article is intended as a summary of Bennet’s book, I’ll be skipping much of the detail.
Our Microbial Past
3.5 to 1 billion years ago
The environment at the time: ocean life, single-celled organisms, and small multi-celled organisms.
In the earliest stages, everything lived in the ocean. The first microbes got around through something called chemotaxis. Many single-celled microbes have cilia for motility. These are thin filaments that independently respond to the chemicals that they sense. The microbe moves according the gradient of attractant and repellant chemicals that its cilia sense and respond to: creating movement towards increasing attractant and decreasing repellant.
In fact, many microbes actually had slightly elongated bodies and tended to move according to patterns very familiar to us: forward/backward and turn. This enabled a very simple strategy for navigation:
- in a neutral environment or when moving away from toxic substances, move quickly and in a straight line, turning occasionally
- when the presence of food is detected, slow down and wriggle in different directions (ie: search)
Studies have shown that this simple strategy is optimally efficient for an organism that has no ability to sense or infer anything about its environment other than the chemical composition of its immediate vicinity.
The First Animals
1 billion years ago to 640 million years
The environment at the time: ocean life, single-celled and multi-celled micro-organisms, large multi-celled organisms such as plants and fungi.
What evolved: Radiatans, eg: coral polyps.
Why they evolved: to better consume other organisms as a source of sugar to support respiration.
Adaptations needed to support their evolution: reflexes, basic senses, nerve nets.
During the earliest stages of evolution, nature settled upon a beautiful balance between two kinds of energy production means:
- Photosynthesis: uses sunlight + CO₂, expels O₂
- Respiration: uses sugar + O₂, expels CO₂
Both processes need a gas plus something else to provide fuel for the reaction. While CO₂, O₂ and sunlight are readily available almost anywhere, sugar is not. So photosynthetic plants can settle down in one place and just let their needs come to them, while respiratory animals must do something in search of sugar, such as hunting.
The best source of sugar is other organisms. Single-celled microbes consume other single-celled microbes by wrapping themselves around their victim. Usually size is factor, with the larger microbes consuming the smaller ones. Thus there was an evolutionary pressure for single-celled microbes to evolve into multi-celled organisms in order to be larger.
As single-celled organisms are basically just round blobs, so were the first multi-celled animals. We don’t know exactly what the first animals were, but they were probably something like a coral polyp. These are the individual organisms that together form the colonies known as corals. Their lifecycle is similar to a plant: they start off as a tiny bud that might float around for a little bit before settling down and then stays put. Polyps don’t really hunt for food as such. Rather, they have a mouth part with tentacles that detect the presence of food. When food is detected, the tentacles contract around the food, and pull the food down towards the mouth itself, which is then pulled further down into the stomach through muscle contractions.
Importantly, all those actions need to be coordinated. And that coordination was done through nerve-nets, the diffuse and disorganized precursor to central nervous systems. Nerve nets are largely genetically hard-wired with minimal to no learning over the lifetime of the individual. Thus nerve nets provide a reflex-based form of behavior — the behavior is genetically hard-wired and inflexible. The only way that behavior changes over the course of the lifetime of an individual is due to developmental processes.
The Ediacaran Period
635 to 540 million years ago
The environment at the time: ocean life, microbial mats, multi-celled radiatans that mostly stayed put.
What evolved: Bilaterians, eg: C. elegans.
Why they evolved: in order to better prey upon microbes.
Enabled by adaptations: controlled movement, central nervous system, conditioned responses.
The Ediacaran period is known for its serenity. There was a general lack of the prey/predator dynamics that we see today, as best we can understand. Most of the eating was of microbes. In particular, the ocean floor was covered in a rich source of food in the form of “microbial mats”, such as produced by cyanobacteria.
The bilaterians were the earliest and simplest form of what we would now call invertebrates, but they initially appeared without any of the hard shells that we see today on most invertebrates. Bilaterians solved a problem faced by stationary radiatans: you may starve if you can only consume the food in your immediate vicinity. And so they started to move, at first to take advantage of all those yummy microbial mats. In fact, movement was so successful that now about 99% of all animals on Earth are of the moving bilaterian sort.
The simplest form of movement had already been discovered by microbes in the form of chemotaxis, with its simple forward/backward and turn rules. But those actions are much harder to coordinate in multi-celled organisms, so it took a while for the right body-shape and underlying processes to evolve.
The most studied simple bilaterian is C. elegans, a species of nematode. It is studied because it is believed to be very similar to the earliest bilaterians. And because its nervous system is simple. Really simple. It only has 302 neurons. And yet the evolution from radiatan to C. elegans was a big leap, and required the evolution of a number of key capabilities. That included centralizing the nerve nets into a central nervous system, and the introduction of the very first and simplest form of learning.
Evolution enables organisms to adapt; but it is slow and applies equally across the whole species or large groups of it. By moving, the earliest bilaterians became exposed to a wider variety of circumstances than their stationary ancestors. They needed to adapt more rapidly, and even more importantly, individuals needed to adapt in ways that was specific to their particular circumstances. This led to the evolution of conditioned responses, where reflexive behaviors become triggered by sensations that were previously associated with the reflex occurring. Now a nematode that wanders into a new territory filled with toxic algae can learn to trigger its withdrawal reflex whenever it senses tell-tale chemicals associated with that algae.
The Cambrian Period
540 to 485 million years ago
The environment at the time: ocean life, nematodes, arthropods, explosion of predation.
What evolved: Vertebrates, eg: fish.
Why they evolved: escalation of prey/predator dynamics.
Enabled by adaptations: learning through trial and error, habitual behavior control.
The Cambrian period saw such an escalation in the number of species that it’s referred to as the Cambrian Explosion. This probably occurred as the result of the invention of predation of multi-celled life.
Nematodes were like slugs, and lacked any form of protection or defense mechanism. Once animals started to prey upon other animals, they started to evolve various defense mechanisms, such as hard-shells. This led to further predatory evolution, and so forth. During the Cambrian period, arthropods ruled (the group that started off as crustaceans and now includes insects and spiders), and one of the most prolific such groups was the hard-shelled trilobite.
The earliest vertebrate were fish, and they were small and few during this time. Bennet doesn’t offer a theory for why vertebrate evolved, but it’s likely that they found a niche for survival by being smarter and faster. While a few of the modern invertebrate species, such as spiders and bees, have evolved some nifty levels of intelligence, for the most part invertebrate have followed the same basic brain plan as the nematode, and thus are limited to a reflex-based behavioral control system with its limited ability to adapt.
To be smarter, the fish had to significantly improve in a number of ways: learning, senses, and mapping. It was fish that first evolved trial-and-error based learning, and with it behavioral control based on learned habits rather than simple reflexes. While even the trilobites appear to have had compound eyes, antenna, and possibly other sensory organs, it is believed that fish have a far greater ability to learn sensory patterns than any invertebrate. Lastly, fish were the first to track the locations of things. This probably enabled them to quickly retreat to known safe areas when predators were nearby.
Thus the ability for an individual to adapt to its specific circumstances, which began in a nascent form in the earliest bilaterians, really took off with the first vertebrate.
The Devonian to Cretaceous Periods
420 to 65 million years ago
The environment at the time: transition to land life, amphibians, reptiles including dinosaurs, explosion of land-based invertebrates, two mass extensions.
What evolved: Mammals, via therapsids and cynodonts.
Why they evolved: opportunity, followed by heavy predation by dinosaurs.
Enabled by adaptations: learning through simulation, goal-oriented behavior.
It was during the early Devonian period that life started to spread onto land. By this time fish had become the predominant predator in the oceans, creating an evolutionary pressure for the invertebrate to move onto land to escape predation, later becoming our modern insects and spiders. Land-based plant-life also started to flourish during this time. However, CO₂ levels dropped during the Late Devonian Extinction, possibly caused by the increase in plant-life. This led to a dramatic drop in temperature, causing extinction for many, but creating niches for those that did survive.
With the increased food on land in the form of plants and insects, vertebrates had also made the beginnings of a land migration. Those land-based vertebrate that remained survived by living in warm puddles near the shore, living on insects. These were the earliest form of amphibian, and they diverged into three groups: i) the modern amphibians, such as frogs, ii) the reptiles, and iii) the therapsids, which evolved to be warm blooded.
Like their fish ancestors, amphibians and reptiles were cold-blooded. Consequently, during night reptiles become immobile because temperatures are too low for their muscles and metabolisms to function properly. Warm-bloodedness in therapsids comes at the cost of an increased energy requirement, but they could easily meet that by preying on the bountiful supply of smaller reptiles, particularly when they lay still during night
For a period of time this worked well for the therapsids, which grew to the size of a modern tiger and became the most successful land animal at the time. However, the Permian-Triassic mass extinction significantly cooled the planet again, and the energy hungry therapsids all but went extinct. In contrast, the energy-conservative reptiles returned in number and variety, and even took to the skies in the form of the pterosaur. The subsequent rule by the dinosaurs and other reptiles lasted 150 million years.
The cynodont was a form of therapsid that originally evolved to live in burrows to escape the larger therapsids. Following the Permian-Triassic extinction, these were the only therapsid to survive, but they had it tough living with the dinosaurs. They survived by becoming smaller and spending most of their time in burrows, leaving only at nighttime to quietly sneak past the other animals in search of insects. This smaller form of cynodont would have resembled a small modern rodent, and was the first mammal.
Apart from their size, a few other innovations helped to keep the first mammals alive. The most significant of those was the addition of the neocortex, the outermost layer of the mammalian brain. This radical addition to the brain structure enabled an equally radical new ability — simulation. The earliest mammals were able to consider possible courses of action and to predict their likely outcomes, including the likelihood of whether a predator would notice them sneaking past or not. Thus they were able to plan ahead; to choose the best course of actions. This is known as goal-oriented behavior.
In contrast to reflexes and habitual control, goal-oriented behavioral control is by far the superior form in terms of adaptability.
When referring to simulation being used to adjust future behavior, Bennet refers to this as learning through “vicarious trial-and-error”. The implication is that simulation doesn’t just help in the moment. The ability to simulate multiple alternative possibilities means that the animal’s habitual behavior can be trained by its goal-oriented action selections a) in preparation for a future plan, and b) as a result of realizing a better alternative following a mistake (ie: counter-factual learning).
Just as for the fish during their time, the evolution of mammals coincided with a strategy of avoiding being preyed upon by being small, nimble, and smart.
The Paleogene Period
65 to 23 million years ago
The environment at the time: dinosaur-free due to extinction, explosion of mammalian life
What evolved: Primates, eg: chimpanzees.
Why they evolved: niche in tree-dwelling, frugivore, large multi-male group life.
Enabled by adaptations: socializing, planning for future needs, learning by observation.
The Cretaceous-Paleogene extinction event wiped out the top of the land-based food chain: the (non-avian) dinosaurs. This created an opportunity for mammals, leading to the huge diversification of mammalian life that we see today, including the primates.
Each species follows a particular survival strategy, and each strategy carries with it certain advantages and disadvantages that requires different long term adaptations. Within the huge variety of life that existed at the time, primates found their niche by switching to a fruit-based diet, living in the trees, and forming large groups. Living in the trees and in large groups gave protection from predatory mammals. It also gave them ready access to the fruit that tends to grow in trees, which provided a much richer source of energy than a plant-based diet.
The problem with group life is that it tends to lead to aggression, particularly in the multi-male groups preferred by primates. High aggression isn’t good for survival. With the increased energy supply from fruit, primates were able to evolve larger brains to solve aggression in a unique way: politics. By socializing and forming alliances, primates reduced their self-imposed deaths. Thus having larger and more complex brains was an evolutionarily stable strategy. It is believed that this led to a positive feedback loop between increases in brain size, sociality, and group size.
There is a problem with a fruit-based diet, however. Fruit is more time-sensitive than any other source of food. There is a very narrow window between a fruit becoming ripe and when it drops to the ground, becoming less accessible due to rotting, other animals eating it, or the threat of predators. Thus primates had to follow the ripening of their fruit supply, moving from tree to tree, and tracking when each fruit source would become available. This led to the evolution of the cognitive ability to plan ahead in order to meet a future need. Primates regularly plan their location of sleep to be near where they plan to hunt in the morning, and will start earlier when hunting fruit that is scarce or in high competition.
Thus, in primates, the goal-oriented behavioral control systems, that first evolved in mammals, became significantly more advanced to meet the particular circumstances of the niche that they evolved into.
The Arrival of Humans
10 million to 100 thousand years ago.
The environment at the time: huge variety of animal life, predators that could harm primates, tectonic activity creating a separation between east and west African primates
What evolved: Homo- genus, eg: homo-sapiens.
Why they evolved: food scarcity, evolutionary positive feedback loops.
Enabled by adaptations: meat-eating diet, tool-use, language, altruism, premature births.
While primates originally evolved in the fruit-filled rainforests of Africa, 10 million years ago saw tectonic plate activity split Africa into distinct east and west regions. The west remained the same, but the east became drier and developed into what we now call the Savannah. This started a significant evolutionary change in the eastern primates. For reasons that we still don’t know, we evolved to walk upright. Our shoulders changed, enabling us to efficiently throw rocks and spears for hunting. And our bodies became extremely efficient at cooling via sweat, enabling long distance running.
A key driving factor was the switch to a meat-based diet, due to the scarcity of fruit. Some estimates put initial primate meat consumption at 10% of their diet. In contrast, the earliest pre-homo human ancestors consumed about 30%, and homo-erectus a whopping 85%. At some point we also invented cooking, and our bodies further changed to the point that the modern human body is unable to survive properly without the pre-processing provided by cooking.
Language evolved somewhere between 500 and 100 thousand years ago, but only as part of an elaborate positive feedback loop, illustrated here:
Primates were already a social species, with all the skills to enable cooperation between large groups. With the initial food scarcity caused by the east/west split, and the switch to hunting, it’s likely that we continued to solve such problems by taking advantage of being able to cooperate to surround prey. But we also had to invent tools in order to help with the hunting and capture of prey, and to tear the meat from the caught animals. This would have created pressure for larger brains to evolve that could construct better tools. It was perhaps tool-use that triggered the first development of language, initially with mothers teaching children the difficult skills required to create and use those tools.
With better tool-use and thus better hunting results, and with the increased energy supply from cooking, we likely had a surplus of calories that enabled the growth of larger brains, which further improved all those things mentioned already. But it also created a problem. The heads of children became too large to be born in a fully developed form. Thus we evolved to birth prematurely, with an extended infant period where the newborn is totally dependent. This required the fathers to take an active role in parenting.
Altruism and language likely initially evolved around the family unit, but they eventually led to increased cooperation that improved hunting and enabled larger groups. Larger groups, larger brains, and language, further improved tool-use and hunting results, which further provided better calorie supply and enabled larger brains still.
Wrap-up
Scientists have been trying to piece this story together for a long time. It’s hard work extracting what little information we can from fossils and geological evidence. Much of the supposed reasons for traits to evolve are still being argued about.
Nevertheless, with the goal of understanding what makes up intelligence in humans and the rest of the animal kingdom, I find this to be a very useful start.
For example, while mammals, primates and humans each made significant evolutionary contributions, we now know that many aspects of our brain function today can be attributed to brain structures that evolved in earlier species such as that of fish. Even the basic functioning of the neuron appears to be the same in humans as it was in the nerve nets of early bilaterians.
With that backstory, Bennet is able to put into context the evolution of hormones for internal state, integration of processing into a central nervous system, how hormones were adapted for reinforcement learning, and much more.
Further Reading
While preparing for publishing of his book, Bennet also published a small handful of journal articles. These contain extra information and references not included in the book:
- Bennett, M. S. (2020). An Attempt at a Unified Theory of the Neocortical Microcircuit in Sensory Cortex. Frontiers in Neural Circuits, 14,40. https://doi.org/10.3389/fncir.2020.00040
- Bennett, M. S. (2021). Five Breakthroughs: A First Approximation of Brain Evolution From Early Bilaterians to Humans. Frontiers in Neuroanatomy, 15, 693346. https://doi.org/10.3389/fnana.2021.693346
- Bennett, M. S. (2021). What Behavioral Abilities Emerged at Key Milestones in Human Brain Evolution? 13 Hypotheses on the 600-Million-Year Phylogenetic History of Human Intelligence. Frontiers in Psychology, 12, 685853. https://doi.org/10.3389/fpsyg.2021.685853
Bennet’s X post announcing his book:
