Also see: Intelligence on Earth

We don't know exactly what mechanism brought to the appearance and development of an intelligence similar to ours, since we're the only species known to us that has it, but there are some factors that are likely to have had a significant role.

See here for a discussion of sapient evolution on the forum.

Nervous systemEdit

The nervous system is the structure that transmits signals and coordinates action in a body, and is therefore crucial to the development of intelligence. Many animals on Earth (mostly arthropods and mollusks) have a nervous system composed by many ganglia located throughout the body and connected by nerves: each ganglion controls, in great autonomy, a single limb or organ. This arrangement is not optimal for intelligence: the needed interconnections take up much room and block bodily functions (the largest ganglia in mollusks are wrapped around the throat), while each ganglion on its own can manage only very simple, stereotyped actions, such as lobsters' caridoid escape reaction. Spiders, among the most intelligent arthropods on Earth, have derived from their frontal ganglia a mass very similar to a brain.

Spider internal anatomy-en.svg

Internal anatomy of a spider (nervous system in blue).

A centralized nervous system, as present in all vertebrates, allows instead a quick comparison of information coming from all the sensory inputs, and makes the coordination of bodily functions much more efficient. This central "super-ganglion" (the brain, and the associated organs) is connected to every other organ by a simple net of nerves. Anyway, since the maximum speed of neural impulses is estimated to be about 100-120 metres per second[1], an organism more than a few metres long could still need peripheric ganglia in the bodyparts farthest from the brain to take care of simple emergency or routine reactions, while voluntary activity would still be managed by the brain.

Encephalization quotient is a simple measure of how large the brain of a vertebrate (or vertebrate-like organism) is compared to what it needs. Another, even simpler estimate for expected intelligence is to take brain mass (1350 g is the human average) and subtract from it the spinal cord mass (35 g for humans): the greater the result is, the more intelligent is likely the animal.

Alien nervous systemsEdit

Because nerves only evolved once on Earth, any speculation on exactly what they may look like on other planets must be based purely in theoretical constraints and chemistry. Based purely on the idea of setting up an action potential—that is, forcing propagation of an electrical pulse in one direction—we can see from Earth bacteria that potassium can be used instead of sodium. But other forms of life may well use completely different chemistry.

One might still like to assume Earth-like nerves would be common, even given they may use different methods to control electrical propagation. The best way out of this way of thinking is to take a look at properties shared by all Earth nerves which aren't necessary to the core function of quick communication across the body. Such functions may be frozen accidents.

  • Nerve signals are binary, like computers: a nerve either sends a pulse or does not. This is due to the nature of the sodium switch action potential.
  • Nerves interact either by stimulating each other to fire, or inhibiting. One nerve is either wired to stimulate another or inhibit it.
  • Individual nerves choose to fire after a certain threshold based on how many pulses were received and how recently. Frequency of firing provides a more analogue, non-binary element to nerve activity.
  • Nerve fibers do not change position very quickly. In the brain this means short-term cognition is in the pulses while long-term is in neuron connections. In the body it means nerves are poor at healing.

Any of these may be different in a separately evolved system, on Earth or elsewhere.


The nervous system is extremely energy-consuming: despite comprehending only the 2% of the mass of an adult human being, the brain requires the 20% of all the energy and the 25% of all the glucose acquired by the body. The basic biochemistry of the organism has to permit high-energy reactions: particularly, it needs to breath a oxidizing atmosphere, better if containing oxygen (other common oxidizing agents are not quite as efficient, though they don't necessarily exclude intelligent life).

Also, the organism shouldn't perform other laboriously energy-consuming activities, such as flight, except on a planet with very low gravity or a very thick atmosphere - or in the case of floating lifeforms, which would not require energy at all to keep themselves in air. For example, Barlowe's Eosapien has developed an almost human-like intelligence floating with bladders full of methane and manipulating tools with ventral tentacles. Furthermore, a well-developed brain can be very heavy and likely shifted towards the frontal end of the body (with the main sense organs), unbalancing the organism.

To direct most of the body's energy to the brain would leave the rest weakened, and inhibit healing of the wounds; a species with this feature wouldn't be much aggressive and will develop good passive defences (shells, spines, poison, large size, etc.); if carnivorous, it'd probably be an insectivore or a filterer. This is not a good strategy for evolving intelligence, except for organisms specialized to a diet very poor in energy - such as elephants.

Intelligence would more easily arise with a diet rich in highly energetic foods[2]:

Food Energy Content
Leaves, grass, sprouts 130 kcal/kg Carbohydrates, mostly cellulose
Roots and tubers 300 kcal/kg Carbohydrates, mostly starch
Fruit 500 kcal/kg Many carbohydrates, such as fructose
Milk 800 kcal/kg Proteins (caseine), lipids, lactose
Blood 900 kcal/kg Carbohydrates, some lipids and proteins
Beans 1000 kcal/kg Many carbohydrates and proteins
Fish and shellfish 1000 kcal/kg Many proteins and lipids
Bird meat 2000 kcal/kg Many proteins and lipids
Mammal meat 3000 kcal/kg Many proteins and many lipids
Grains 3000 kcal/kg Much starch and other carbohydrates
Honey, nectar 3000 kcal/kg Almost pure sugars
Insects 5000 kcal/kg Many proteins and carbohydrates
Nuts and hard fruit 5000 kcal/kg Carbohydrates and many lipids
Butter, fat, vegetable oils 8000 kcal/kg Almost pure lipids

There are two ways to gain a high amount of energy from food: specializing to extract every calorie from a single kind of food or adapting to eat a wide range of different foods. The first strategy, while effective, is easily invalidated by climate change, but it can be a strong incentive for at least a simple form of agriculture or breeding. In the second case, the main purpose of intelligence is to find a variety of new foods sprinkled in the environment, and to make edible things that are not: human ancestors learnt early to break open bones with rocks and cook meat with fire.

Fruit is an example of a food that stimulates intelligence more than others, being scattered throughout the environment and short-lasting, therefore requiring good memory and spatial cognition. In the forest, fruit is hidden in a three-dimensional space; in the open plains, one has to know in great detail very wide areas. Moreover, the colour change typical of fruit is also an incentive to develop a good eyeseight, which in mammals is thought to be related to neocortex growth. Hunting active preys requires instead a number of behaviours, such as strategic thinking, especially if the prey is larger than the hunter; generally speaking, carnivores tend to be smarter than herbivores (meat is also much richer in energy than plant tissues).

Rapidly shifting sources of food, and environmental conditions as a whole, can force organisms to elaborate different strategies in their lifetime to react effectively to changes (thereby favouring individual sentience over genetic sentience). In fact, while apes existed for over 20 million years, humans emerged only during the wild climatic swings of Pleistocene. Any kind of food that can be difficult to obtain is also useful, whether it is hidden, shielded, dispersed or partially inedible. Baboons search for buried food; capuchin monkeys get it from eggshells, nuts, hard or prickly fruits, bark; chimpanzees extract termites from their nests and honey from beehives.

Manipulation organsEdit

Also see: Tool use

The development of complex technology requires some organ apt to hold and manipulate tools. Intelligent organisms that lack manipulators, such as dolphins, crows and parrots, hold tools in their beak, but this provides only a very basic manipulation. Feet with hooves or curved claws are probably useless in this regard, but soft paws like those of bears can primitively hold objects. Truly prehensile organs have arisen many times on Earth:

Prehensile part Examples
Opposable digits:

1st finger: Humans

1st and 2nd fingers: Many marsupial (phalangerids, koala)
1st toe: Maned rat, opossums
1st finger and 1st toe: Primates (except for man, colobi and spider-monkeys)
Partially opposable digits:

Sesamoid bone: Giant panda

1st finger (?): Some coelurosaurs (Nqwebasaurus, Troodon, Bambiraptor)
Tongue: Giraffe and okapi
Proboscis (trunk): Elephants, tapirs
Lips: Lake sturgeon, horse, rhinoceroses, manatees, orangutans
Tentacles: Cephalopods (squids, cuttlefish, octopi)
Tail: New World monkeys, New World porcupines, some anteaters, rats, many possums, Solomon Islands skink, crested gecko and other squamates, drepanosaurs (extinct), Aneides and Bolitoglossa salamanders, seahorses

The animals in this table use either bodyparts close to the mouth (lips, tongue) to manipulate food, or limbs (hands, feet, tail) to move among the treetop: life in dense forests seems an excellent incentive to develop manipulators (but seahorses hold on seaweed). Parts that lack rigid elements (lips, tongue, trunk, cephalopod tentacles) can be much more refined and versatile manipulators, but they'll be much less strong.

These discussions on the forum contains some speculations about extraterrestrial manipulators, including prehensile mouthparts and genitals, cerci, gripping bulbs, tentacles with boneless fingers and fractal digits.

Social organizationEdit

While solitary species can develop intelligence too (as is the case for crows and octopi), an actual civilization, or even just complex technology, requires a degree of social interaction to pass knowledge between generations.

  • Asociality: individuals of the same species do not interact at all, except to procreate.
  • Subsociality: parents raise their own offspring.
  • Presociality: complex social interactions between individuals.
    • Communality: individuals of the same generation live together, but raise their offspring separately.
    • Quasisociality: all individuals raise cooperatively the entire brood.
    • Semisociality: only a specific group of organisms can reproduce; the whole group raises the brood.
  • Eusociality: cooperative brood care, generation overlapping, labor by non-reproductive castes.

While more extreme organizations such as eusociality are usually genetically pre-programmed, intermediate forms (primates belong between communality and quasisociality) strongly favour intellective development by forcing each individual to keep a record of others' identity, relationships, hierarchies and alliances, and - in some cases - beliefs and desires. Baboons and capuchin monkeys can compare their relationship with different individuals, and relationships between these; chimpanzee males offer food to whom they consider a likely political ally.

The most basic form of social interaction, offspring care, is also the most effective way of passing on knowledge in a species with few instinctual behaviours - better if performed by many adult individuals in the group.

Other factorsEdit

  • Size: more often than not, highly intelligent species are unusually large, and none of them is less than a few kg heavy. For example, cephalopods are larger than other mollusks, whales and elephant are larger than most mammals, humans are larger than any other primate except for the gorilla, and so on. This does not apply to swarm intelligence.
  • Activity: a highly active lifestyle requires more cognitive ability than a sedentary one. None of Earth's sessile species (sea anemones, sea squirts, etc.) has a complex brains, and many of them (sponges, mushrooms, plants) lack a nervous system altogether. Perception of balance and movement, coordination of movement, active search for food and mates, etc. all require a certain degree of intelligence.
  • Senses: highly developed senses are correlated with high intelligence, such as the excellent sight of birds, primates and cephalopods, the echolocation of whales, the tactile sensitivity of elephants' trunks and octopodes' tentacles, and so on. A high sensory resolution provides a constant flux of new information, the raw material for cognitive processes. The same is true for motor skills such as the vocal ability of crows and parrots and the hand-eye coordination found in primates.
  • Environment: challenging environments beget intelligence more than static ones; for example, human intelligence developed mostly in the climatic severity of Pleistocene. Frequently changing or highly fractionated environments (especially if the changes are not too regular or predictable) force organisms to modify constantly their strategies, rather than rely on instinctual, "pre-programmed" behaviour.
  • Time: over the course of evolution, a trend from lesser to greater intelligence is stronger than the opposite one. Most highly intelligent species are relatively recent: corvids and modern elephants have existed for 20 Ma (million years), apes and dolphins for 30 Ma, parrots for 50 Ma; octopodes are known to exist since the late Jurassic[3], but how intelligent they were back then is up to speculation. However, Cenozoic fauna seems overall more intelligent than the Mesozoic fauna, which is more intelligent than the Paleozoic fauna.


  • Cosmic Biology (Louis Neal Irwin and Dirk Schulze-Makuch, 2011), pag. 281-283
  1. David Parizh, "Speed of Nerve Impulses", The Physics Factbook, 2002. <>
  2. From Elizabeth Viau, "The Caloric Contents of Food", World Builders, 2003 (modified). <[>
  3. Phil Eyden, "Fossil Octopuses", May 2010. <>

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