Evolution of the Brain Structures

On with the old, in with the new

Things are effectively explainable if they follow the axiom out with the old, in with the new and they gets complicated in case of on with the old, in with the new. Any mutation must be applied to a DNA coding that already exists. It cannot be applied to coding that does not exist. It leads to the way that evolution changes an organism. Mutations are always applied to the existing DNA coding. Evolution makes something new out of something that already?? exists.? This has resulted in new structures being layered upon more primitive ones. Therefore, the human brain can only be truly understood by tracing its evolutionary pattern. Evolution usually works in an incremental fashion. It's a Quasi-Static process.

Example- if a person becomes distressed in a given environment. It doesn't sprout wings and fly.

Evolution of Organisms

1. Around billions years ago, life existed only in the form of single celled organism

2. These single celled organism then evolved into multi celled being through some chemical process.

3. More complex animals like humans evolved from these multi celled beings

 
Evolution of the human brain
It also moved from simple to complex over an extended period of time. In the term of multifunctioning brain leads the
 race by far among the various other organs of the human body like lungs, liver, kidney etc. The billions of cells that work
 together to make the brain and body function harmoniously have numerous critical functions. Structure has changed
 itself over time to perform a variety of vital functions, while at the same time being compelled to survive by selectively
 ridding itself of its more unwanted functions. This selection procedure enhanced the capabilities of the nervous system
 in accordance with the changing needs of mankind.
 
Relative to body mass, brain volume tends to increase gradually over time. Humans do have the largest brain size in
 proportion to the body when compared to all other mammals. There is no disputing that larger animals have larger
 brains than smaller animals of comparable construct and functionality. This is primarily because additional neurons
 are needed in order to increase the body tissue necessary to sustain them.

Add-ons are added as and when the need arises. The brain-evolution follows this principle as new and advanced structures are added with the need.

Evolution and the neural system

1. Protozoa (unicellular): No nervous system. The mechanical stimuli generate an electrical impulse by altering the Ca-ion and K-ion concentration across the cell membrane. Eg., amoeba, paramecium, euglena and so on.

2. Multicellular: These had specialized cells to form the nervous system as the cells underwent differentiation.

a) First neurons: The neurons first appeared in cnidarians (hydra-like) as a nerve-net with almost no concentration of nerve cells in any particular part of the body.

Hydra nerve-net

b) Position of the brain: With the bilateral symmetry of animals evolving, the position of brain became a concerning factor which was not so with the radially symmetric ones (hydra, star-fish).

This led to cephalization i.e., clustering of neural cells at the anterior end of the body in the form of a ganglion. Along with this 'master' ganglion several ganglia appeared throughout the body as in annelids and arthropods. These ganglia were inter-connected for co-ordination and formed a ventral nerve cord.

Ganglia and nerve cord in an arthropod

This continued cephalization led to huge mass at the anterior end of the body and a nerve cord forming the brain and the spinal cord respectively. Some even hold the view that the huge growth of fore-brain in humans is the result of continued cephalization as the fore-brain is the most 'anterior part'.

c) Position of the nerve (spinal) cord: The above mentioned series of inter-connected ganglia in the annelids and arthropods forms the nerve cord which is ventral in nature. But the vertebrates developed a dorsal nerve cord. This is supposed to have happened because of some mutation that inverted the position of the mouth which led our ancestral creature to flip-over thus making the ventral nerve cord dorsal.

Ventral nerve cord (earth-worm) Dorsal spinal cord (human)

Triune Brain

MacLean proposed a three-stage brain evolution in the vertebrates, namely

1. Reptilian complex(mid & hind brain): consists of brain structures(brain-stem, cerebellum) that helped territoriality, ritual behaviour and other "reptile" behaviours (balance, hunger, breathing, reproduction & survival) i.e., mostly the sub-conscious human activities.

2. Paleomammalian complex(fore-brain): consists of the limbic system (amygdala, hippocampus, hypothalamus, septum, cingulate cortex). These areas (in mammals) are the seat of emotions, parental nurture and memory. Reptiles are known to eat their young-ones!

3. Neomammalian complex(pre-frontal cortex): consists mainly of the cerebral neocortex which plays vital role in planning, creativity, prediction, decision making and ability of language.

                                                                         CNS in early embryonic phase (mammal)

Evolutionary changes in the limbic system

1. Hippocampus: In lower vertebrates, it organizes behavioural responses to olfactory stimulus i.e., to flee or mate.

In mammals, it performs a major role in the formation of memories, spatial navigation.

2. Amygdala: In lower vertebrates, it is concerned with the sense of smell (olfaction).

In mammals, this area is mainly concerned with the emotions, while the olfactory function is minimized and partially taken up by the hypothalamus.

3. Hypothalamus: comes up mammals for the first time.

Marsh Tit                        Great Tit

(food-gatherer)               (not food-gatherer)

                            Larger and seasonally expanding hippocampus

It grows and shrinks the brain every year! It is indeed not economical to keep and maintain the costly brain when not needed.

A study done on London cab and bus drivers showed variations in their hippocampus sizes too. The cab drivers having to drive all around the city had to have a larger hippocampus-the seat of spatial navigation than the bus drivers who ply on fixed routes.

London cab drivers London bus drivers

Larger hippocampus

Why did the fore-brain grow so much?

1. The fore-brain, mainly the neocortex, is concerned with creativity, planning, predicting, ability of language and expression, personality and many more. It is believed that these qualities of ?intellect? must have proved advantageous in courtship with the opposite sex and hence propagated to the next generation leading to a ?sexual selection? of a larger neocortex.

2. Also predicting the actions or behaviour of other individuals must have proved a boon in survival and hence neocortex grew.

3. Socialization brought with it the need of the animals to have a bigger social network which acted as a positive feedback for the neocortical growth.

Mean group size vs. neocortical ratio (monkeys and apes)

Important to note

It is still common to hear other animals discussed as if they were some inferior form of human beings ? as if there were some kind of natural ladder on which human beings obviously occupied the top rung.

But that is not what we actually see in nature. Every evolutionary line has developed independently, and rats, for example, are perfectly adapted to their environment. They are not in the process of extinction, and they live in perfect harmony with their surroundings. The same could be said for most of the species that populate the Earth, even though their brains are far smaller than ours.

Growth in size and importance of the associative areas of the brain, from rats to cats to humans. Green = sensorimotor area, red = visual area, blue = auditory area

Some Interesting facts about Brain

1) There are no pain receptors in the brain, so the brain can feel no pain.

2) The human brain is the fattest organ in the body and may consists of at least 60% fat.

3) Neurons develop at the rate of 250,000 neurons per minute during early pregnancy.

4) Altitude makes the brain see strange visions.

5) The capacity for such emotions as joy, happiness, fear, and shyness are already developed at birth. The specific type of nurturing a child receives shapes how these emotions are developed.

6) The left side of your brain (left hemisphere) controls the right side of your body; and, the right side of your brain (right hemisphere) controls the left side of your body.

7) Children who learn two languages before the age of five alters the brain structure and adults have a much denser gray matter.

8) Information can be processed as slowly as 0.5 meters/sec or as fast as 120 meters/sec (about 268 miles/hr).

9) While awake, your brain generates between 10 and 23 watts of power?or enough energy to power a light bulb.

10) A study of one million students in New York showed that students who ate lunches that did not include artificial flavors, preservatives, and dyes did 14% better on IQ tests than students who ate lunches with these additives.

11) Laughing at a joke is no simple task as it requires activity in five different areas of the brain.

12) Differences in brain weight and size do not equal differences in mental ability. The weight of Albert Einstein?s brain was 1,230 grams that is less than an average weight of the human brain.

13) A living brain is so soft you could cut it with a table knife.

14) The brain can live for 4 to 6 minutes without oxygen, and then it begins to die. No oxygen for 5 to 10 minutes will result in permanent brain damage.

15) Our brain often fools us. It often perceives things differently from the reality. Look at those pictures. Square A and B are actually the same shade of gray.

References

1. Michael O?Shea, ?The Brain: A very short introduction?.

2. Anna Dornhaus, Lecture on ?Evolution of mind and brain? http://www.youtube.com/watch?v=H_jODjsFt6w

3. Triune brain, http://en.wikipedia.org/wiki/Reptilian_complex

4. The evolution of the brain, http://www.onelife.com/evolve/manev.html#I

5. http://thebrain.mcgill.ca/flash/i/i_05/i_05_cr/i_05_cr_her/i_05_cr_her.html

6. http://www.essortment.com/all/evolutionhuman_rmow.htm