Human existence

Human origins and SPEND Bookmark and Share

You exist because your big brain was "hot" as a child. Simply, your 20W adult brain used 30W of energy between four and nine years of age. This is due to SPEND (Synaptic Prolonged Expensive Neurodevelopment). Paleoanthropologists ignore this neuroscience, and its key role in making us smart. Here the jig-saw puzzle of your origins is explained. 

Why your human brain is uniquehuman brain

It is big—particularly its cerebral hemispheresfor the size of your body.

You know that.
What you do not is that it under went when you were a child, SPEND (Synaptic prolonged expensive neurodevelopment)


Some background details.

Brain size--important--but not the complete story

elephant brainelephant and its brainpilot whale brainpilot whale outline It has to do with big brains in a fairly small body. Elephants and  whales and dolphins can have larger brains than humans but their brains make up only a tiny part of their massive bodies.

Callithrix pygmaea It is not just relative size. Some small monkeys and rodents in percentage terms have as large brains relative to their bodies. The pygmy marmoset has a body of 140 g and a brain of 4.5 g (3.2%) while a typical human might have a body of 79 kg and a brain of 1.5 kg (2.1%). 

allometry relationship But these high brain size to body size brains in small animals fail to take into account allometrics--the fact larger animals generally have smaller organs relative to their body size. Scientists use allometric exponential calculations to take this into account to create an encephalization index. So adjusted, humans do have large brains for our body size. 

marmoset brain-marmoset brainhuman brain outline
human brain
Perhaps, more importantly, and intuitive is that the large brains relative to bodies or small animals are more  subcortical than cortical. In contrast, human brains are mostly cortical--higher brain.

microcephalic brain
But even this does not explain our unique smartness. Brain size variation in humans correlates only modesty with intelligence as measured by IQ. Indeed, the clinical literature contains many cases of humans (microcephaly, hemispherectomy) with normal levels of IQ with brains around half that of most people.

For a literature review and explanation, "Human evolution expanded brains to increase expertise capacity, not IQ", Psycoloquy: 10(002) 1999, pdf or hmtl.


Brain size helps but it SPEND that makes us smart. SPEND is the missing piece in the human origin jig-saw puzzle. This is because SPEND allows the integration in an empirical context of diverse previously unexplained facets of human existence such as why we have childhood, why we are smart, why hunter-gatherers pool food and why Neanderthals went extinct. It is a hub fact that brings together previously unlinked issues in neurodevelopment, cognitive development, child exercise physiology, human evolution and anthropology.

Only humans have childhoods
Other animals have a juvenile stage but only humans have childhood—the period of slow growth in our lives between infancy and puberty.
Body growth during childhood is not only the slowest amongst mammals but is so retarded that it matches that of cold blood reptiles.
Young Neanderthals lacked this extreme period of slow growth. This makes its existence unique to our particular type of human species: modern anatomical humans (Homo sapiens sapiens).

Childhood gives rise to your human mind

Childhood from kindergarten to high school is a period of very rapid mental growth. During this period, we acquire our species unique cognitive abilities such as language. This growth in mental ability is due the refined wiring up of synaptic links between neurons that make up the neural circuits in our brains.
Key to this writing of neural circuits is starting big with a two-fold excess in the number of synapses and then the winnowing them down so they join neurons into effective information processing circuits. It is important to note that the neurons and axons of the adult brain already are in place by infancy—the brain development in childhood is thus critically concerned with this network refinement by elimination of synapses.
SPEND (Synapse Prolonged Expensive Neurodevelopment) is the story
75% of the brain's energy is burnt by synapses. Their doubling in the childhood brain causes childhood cerebral gray matter to consume twice that of adult cerebral gray matter.
As a result, the childhood brain in total burns 30 W of energy compared to 20 W in an adult. In adolescence, this declines to adult levels.
High energy consumption shifts across the cortical mantle with age. This can be seen when MRI snaps shots of the brain are compared because changing local energy use causes a changing thickening/thinning of the cerebral cortex. Increased energy consumption, for example, requires extra capillary vasculization and glial support cells and this expands the nearby cortex.
This cortical thickness/thinning has a complex pattern of change across the brain as children grow older and mentally mature. Significantly, the longer this period and the more complex pattern of change, the more intelligent the cognitive ability that is acquired.
The child's body is small due to SPEND
The high energy burn of the childhood brain makes it more sensitive to  metabolic disruption. Such disruption occurs when there is reduced blood levels of glucose and oxygen; dehydration; increased body temperature, and the build up of waste products from protein breakdown. These happen when the body is strenuously exercised for long periods both for adults and children but are much greater for the latter.

Young humans have been selected to miminize such metabolic disruption
upon the childhood brain by limiting body size as this reduces the amount of skeletal muscle that produces this disruption. This is the adapative reason why children's bodies have been selected to grow slowly when their brains are undergoing their most intense synapse remodeling of neural circuits.
SPEND answers the question why humans are so smart
The lack of such a slow growth period in Neanderthals—and so lack of a proper period of small body size childhoodstrongly suggests that SPEND—Synapse Prolonged Expensive Neurodevelopment—is unique to modern humans.

Anothe jig-saw puzzle fits in here. Hunter-gatherers—the form of life in which humans evolved—pool high-energy foods between nonkin. This was a critical innovation since it supports the prolonged childhood period of energy dependence caused by SPEND. Further, anthropologists find that nonkin food pooling links to language, for example, the social sanctions made possible by gossip. This suggests the existence of a self-sustaining loop that makes our species possible:  nonkin food pooling supports SPEND, that enables human language, that enables nonkin food pooling. Neanderthals have been shown to have similar hunting skills to early modern humans and it is this new cooperation that came with language that advantaged our species over them. Thus, the key to understanding our biological uniqueness is a cycle between energy, neurodevelopment, cognitive abilities and cooperative social behavior .
Where to read the story in greater detail

Full long review article: "Human metabolic adaptations and prolonged expensive neurodevelopment: A review"  45,567 words, 10 figs, 445 refs, 9 appendixes pdf, online pdfcomments subject to review here  

London School of Economics, Department of Anthropology, Seminar Series on Culture and Cognition, 30 Jan 2008, 78 slides  Powerpoint

Apart from brain size and SPEND do other important traits exist that might effect the intelligence capabilities of the human brain?

Yes, in  "Human metabolic adaptations and prolonged expensive neurodevelopment: A review", I discuss some of them in section 6 and appendix 3. These including possibly genetically determined increased adult synapse numbers in the prefrontal cortices (but not visual ones), enhanced developmental heterogeneity in synaptic neurodevelopmental timing across the cortical mantle (see section 6); changed axons connections, cerebello-cerebral reorganization, late developing neuron types, late developing dendritic arbor re-sculpturing, altered dendritic arbor geometry and the presence of novel internal stimulation as a result of language (appendix 3).