Outline of brain development: mammals to humans
See EHE p. 107 -123 for overview and more details.
EHE, p.110. The sensori-motor "homunculus"
mapped out during the 19th and early 20th century demonstrated a very similar
pattern of gross connectivity and organization. Leyton and (1917) Sherrington
mapped out the neocortex of a gorilla, orang, and chimp. Later Penfield (1975)
found corresponding patterns in humans.
While brains themselves can vary considerably in size, within a species and between related species, the cells comprising those brains (glial and neurons) are very similar in size.
Many of our basic ideas about neurons begain with the famous Spanish histologist, Santiago Ramon y Cajal (1852-1934). Cajal discovered that neurons were the basic unit of the nervous system, how they connected into circuits, their overproduction and pruning in development, and laid out the life history of cells: birth, differentiation and growth, migration, maturation and death.
The most obvious change is in size -- "encephalization" -- but this covers up changes in relative organization and interconnectivity. There also is a problem of interpreting these brain -body size correlations (allometric relations): which, if any, is the driving factor in natural selection?
Primates, elephants, and cetaceans all display an increase in encephalization over ancestral mammals. (Deacon, 1990).
Keep in mind no single factor can explain the various species and individual differences in brain and consequent behaviors. Also remember there are costs and benefits for every feature and these can only be assessed with respect to a given set of conditions, e.g. climate variables, social structures, food supplies.
Even if there were no changes in numbers of neurons, the interconnections among
them can vary greatly. Expansion of brain volume, even with same numbers of
neurons, can make for increased interconnectivity. Variation in connectivity
during brain development provides a rich opportunity for emergence of novel
behaviors.
(Note that even neurons follow the "form- function" principles, being adapted
for their various tasks.)
Expansion of brain also may foster independence of cerebral hemispheres as local inhibition and other interaction is lessened by distance.
Packing an increasingly large brain into a skull causes a greater surface area to be compressed, like a wrinkled newspaper, into that space. This may also facilitate novel connections, along with different developmental rates of brain regions.
Large brains are not without problems -- birth difficulty, sore necks, and importantly, the energy they demand detracts from their presumed benefits in information processing. Even though our brains may be just 2% of body weight, they use close to 20% of our calories -- and for children it may be 40%.
In general, all of the above may be seen as consequences of selection for larger
body size. One might also expect to see increased modularization (independent
functioning) as distances between "circuits" increases, making communication
more difficult. This may have be a factor in the evolution of the differential
right and left hemispheric functions in humans. In addition changes in myelinization
may be expected.
(Myelin comes from a type of neural glial cell that wraps around long axons
-- a kind of fatty insulation that in vertebrates enables efficient transmission
and integration of neural signals over "long" distances. Myelin's importance
is evident when it degenerates, resulting in the symptoms of multiple sclerosis.)
4.5 The human environment plays a role in constructing its brain!
The human brain, born 12 months earlier than expected for a typical primate brain of our size, is exposed to a rich social, perceptual, and cognitive environment that plays a role, probably an extremely important one, in the construction of that brain. This is quite unlike any other primate.
The blood supplies nutrients, oxygen, and serves to cool the brain. Bipedalism already put an additional burden on bipedal hominid blood supply just to pump enough blood up against gravity; increasing brain size compounds the problem. Temperature regulation within the brain is a crucial function with little room for error.
(See EHE for drawings.)
As overlapping visual fields became important, olfactory structures lost out in competition for brain resources; moreover a large snout may get the way!
The eye in primate binocular vision sends information to both left and right visual hemispheres from the central overlapping (foveal) fields. (See EHE, p.112)
While many non-mammals have color vision, early primates probably lost it and then in a shift to diurnal species, regained color vision with subsequent brain reorganization as well as receptor development. Color vision is mediated by the fovea, rich in cones and necessary for sharp color vision and depth perception. (See Nordby reference).
While overall size increased, in each species specific adaptive changes occurred in various parts of the brain. In some cases these adaptive changes were made possible by correlative changes in brain due to size changes, i.e. increases in size "pre-adapted" the brain to better serve other functions as opportunity arose. See the homunculus sketch in EHE.
Human brains are specialized for language in the regions around the Sylvian fissure of the left hemisphere -- in over 90% of humans anyway. Our overly large cerebellum may also be involved in acquisition and production of rapid, fine-grained precise movements necessary for language (speech and sign.)
Non-human primates show little capability for such language movements, even with great training efforts.
This asymmetry (lateralization or specialization) includes behavioral factors as well as structural ones. Thus for over 90% of humans, the left hemisphere controls most language functions as well as our preferred right hand. (EHE, 121-122). Inspection of the brain reveals that in most humans the left hemisphere structures are larger than the corresponding right ones. Lesser differences have been reported in apes and apes show little of the species specific hand preference shown by humans.
The great functional asymmetry in humans contrasted with other apes may be a consequence of the great distance between the two hemispheres.
The evolution of functions of the left hemisphere seems to be the key to much of human distinctiveness. (Recall Gazzaniga video.)
Primates have an expanded prefrontal cortex; humans have one over twice that expected for a primate brain of our size --which is already three times the brain size of a primate with our body size. This prefrontal cortex is involved in many important primate skills including planning, social interactions, emotional regulation, and in humans, language use. (cf. the lobotomy history and the Phineas Gage story.)
Each individual has its brain structure determined in a number of different ways -- some unique, others common to all members of the species or order.
Synapses connecting regions of the brain are directed genetically during development.
Programmed cell death (apoptosis) and patterns of interconnectivity among cells is one of the most important and least understood aspects of brain development.
Some cells and connections die off when not stimulated by their peripheral sources of stimulation. Thus while a fetal human and fetal elephant share many similarities in forelimb structure and cortical connections, as the animals develop into their respective structures, feedback from the "fingers" differs dramatically affecting cortical organization.
Feedback from peripheral stimulation and movement is a very important factor in initial organization of brains as well as reorganization of brains after injury in adults.
Even if cells do not die from restricted stimulation, they may end up linking into different circuits as a consequence.
Learning is literally embodied by the formation of new dendritic connections among neural synapses. This occurs most importantly in the first few years after birth but substantial reorganization can occur in adulthood even though new neurons typically are not formed. (see "monkey hand" mapping overhead).
Darwin himself (1871, p. 146 demonstrated that "the brains of domestic rabbits are considerably reduced in bulk, in comparison with those of the wild rabbit..." Research today suggests learning throughout the life span of many species is manifest in increased synaptic connections reflecting that learning.
Like the cells involved, the chemistry of mammal brains is quite similar in terms of basic building blocks. Timing and quantities of essential "humours" can make considerable difference in impact --both developmentally and in adult behavior.
Neurons communicate amongst themselves with a complex chemistry that variously enhances, inhibits, and otherwise modify their functions.
Such "neurotransmitters", e.g. acetycholine, serotonin, GABA, dopamine, norepinephrine, nitric oxide, etc. serve specific functions in specific neurons and can be expected to differ in effect from species to species, as well as individual to individual.
(Psychoactive drugs, e.g. Prozac, amphetamines, have their effects at this level.)
Transmitted via blood supply, these chemicals play a critical role in behavior. Prenatally hormones play a role in differentiating male and female brains just as they do in forming the reproductive organs.
The older brainstem develops first and the neocortex (new brain) last!
During prenatal and early postnatal development, more neurons and axons are produced than are found in the adult. Infant primates may have up to twice the adult number of neurons. In a real sense, circuits are sculpted from the brain by pruning away cells and synapses.
Cells that are not stimulated by inputs may die off or be recruited into the different activities of their neighbors.
At some point the physical structure of the brain is modified when something is learned. The structure of neurons changes in these processes (and perhaps even new neurons are produced -- see below.)
Brain structure and function is most flexible early in the life of the organism. The most astonishing example of this comes from epileptic infants who have had one of their entire hemispheres removed (hemispherectomy) and develop nearly normally in terms of function.
Adult primates with large brains require fast flowing information. Myelin coated axons make this possible.
Two different chemical communication channels exist, hormones operating in time scales of hours to months, while neurotransmitters operate in fractions of seconds.
Hormone levels controlled by M & F genes begin changing body and brain structures weeks after conception.
These processes are barely understood as yet!
Developmental questions have always been at the forefront of brain unknowns. Recently these have been linked to questions of brain repair and rehabilitation. Here are several questions about primate brains that will probably be answered in the near future.
Under what conditions do neuron precursor cells become specific types cells? Can precursor cells be "transplanted" and shaped somehow to serve needed local functions?
The phenomena of "phantom limbs" had led to the suspicion that after trauma, certain regions of the nervous system undergo reorganization. Thus demonstrations of stimulating a lost "phantom" limb by tickling a cheek was thought to show how some of the limbs cortical receptor neurons had synapsed with nearby facial receptors. (e.g. Ramachandran, 1998)
Somewhat similarly there was speculation that unused regions of sensory cortex might organize themselves with nearby regions, changing the normal cortical organization of deaf or blind individuals. (See a report on this in Nature, 1/14/99)
Experimental work with monkeys has shown explicitly how this reorganization can occur. Cells surgically losing their nerve inputs from a finger acquired new inputs from nearby nerves as the monkey was forced to use the injured forelimb in experimental tasks. (e.g. Kass, 1995 in Gazzaniga, M. ed.)
Until recently, it was widely accepted that no new neurons formed in adult mammal brains. Birds had been shown to form new neurons in the process of learning new songs, but there had be no suggestion that anything similar happened in mammals and in particular primates. Yet recently it has been found that in limited circumstances new neurons may be formed in the hippocampus -- an area involved in primate learning and memory. Thus an increase in brain weight due to learning may not only be from new synapses but new neurons as well.
