🚀 go-pugleaf

  New Scientist (5-Dec-92) had a seven page supplement "The great Brain 
race", which was one of the many supplements they have had on the Secret
Life of the Brain.

   As its relevent to a discussion currently going on in sci.bio
(Evolution of the human brain's cognitive capacity) I thought I 
would summarise it.

   To discuss how humans evolved such big brains it is maybe useful
to consider the following topics:-
   Why did mammals evolve large brains 250 Ma?
   Why did mammals evolve even larger brains 65 Ma.?
   Why did primates evolve brains large compared to other mammals?
   How do primates grow such large brains?
   Why did humans evolve such large brains compared to primates?
   Is size the only way humans brains differ?

REferences to brain size refer to Harry Jerison's encephalisation which
is a measure of brain mass relative to body mass.
     (Ma = million years ago)

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   Why did mammals evolve large brains 250 Ma?
Reptiles of 300 Ma, dinosaurs and modern reptiles all have the same 
brain size.  Mammals evolved 200Ma and they had brains 4 to 5 times 
bigger than the reptiles.  Most of the increase was due to the 
cerebral cortex.
                  WHY?
       {my comment - the ancestors of mammals lost out in competition
                  with ancient reptiles and so were pushed to the 
                  marginal niches.  This forced rapid evolution 
                     ie adapt or die }

REptiles are mainly visual animals and much of the processing of this
data is done at the periphery of their nervous systems - the retina.

Mammals were small nocturnal creatures, so forced to depend on
smell and hearing.  To extract spacial and temporal data from sounds and 
smell required processing of data at a higher level and this was done 
inside the brain.

They suggest the increase in brain size  was due to 3 reasons, 
the first 2 by Jerison and a 
third by John Allman, though the third is disputed by some.

resaon 1 - Mammals had to handle different types of information
reason 2 - Having a centralised processing system meant data from sight
           sound, touch and smell could be integrated to create a more
           detailed mental model of the world
reason 3 - Endothermy and the cortex evolved together.  Endothermy means 
           early mammals could occupy certain niches better than reptiles
           but they required much more energy (5 to 10 times more than a 
           similar sized reptile).  This was only possible if they were
           more efficient food and resource users, this in turn required
           better mental maps and this is what the cortex provided. 

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   Why did mammals evolve even larger brains 65 Ma.?
Another increase in mammal brain size occured starting 65Ma.  Within
about 30 million years average mammal brains increased to become
 4 to 5 times larger,
with the biggest increases occuring with the evolution of ungulates,
carnivores and primates.  Insectivores and marsupials did not change
much.

Jerison argues:-
  With the exinction of the dinosaurs the mammals were able to move
into the niches they had occupied.  But mammals were night not day
animals so they had to evolve improved vision.  They could possibly 
have returned
to the reptile-like peripheral processing system their ancestors had 
had but instead they adopted a system where the vision processing was
done in the forebrain (the thalamus and cortex).  This revamped vision
was then integrated with the smell and sound systems.

Allman's arguements include:-
  A larger brain provided "behavioural buffers" - more efficient 
resource use required better mental models which required a bigger
brain.


   Both Jerison and Allman while disagreeing on the causes agree that
the brain became larger to provide a fuller model of reality.


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   Why did primates evolve brains large compared to other mammals?

Monkeys have brains 2 to 3 times larger than the mammal average.
The article mentions that many explanations have been put forward
but there does not yet seem to be a concensus.  Among some of the 
reason are:-
 1 Allman's behavioural buffer
 2 All species have the biggest brains thay can afford and primates
   have such large brains because they are able to maintain a sufficiently
   rich diet.
 3 Social intelligence - species in larger groups have bigger cortices

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   How do primates grow such large brains?
The brains of monkeys and apes grow no faster than that of 
average mammals.  The trick they use to develop larger brains is that
they bodies grow more slowly.

 Humans use the same trick but in addition their brains continue to
grow for a year after birth.  The result, effectively, is a 21 month
gestation period and a brain 3 times bigger than a human-sized ape.

    {my comment = As humans and dolpins have the same sized brains
                  does anyone know what method dolphins use?}

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   Why did humans evolve such large brains compared to primates?

Humans (and dolphins ) have brains 6 times bigger than the mammal average.
the main reason human brains are bigger than chimps is that humans have
a massive cortex - 70% to 80% of the total size.

  Hominids evolved 7.5 to 5 Ma but brain size remained in the ape range
 until about 2.5 Ma.  It has been found that 2.5Ma the following began
    - the rapid increase in brain size
    - the earliest found stone tools
    - dramatic global cooling and the beginning of a cycle of Ice Ages
      followed by interglaocials which has continued till today.

  2.5 Ma two types of hominids evolved the small toothed large brained 
Homo and the large toothed small brained Australopithicines.

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   Is size the only way humans brains differ?

Not only do humans have a much bigger brain than the apes but it is 
"wired-up" differently.  They have a much bigger cortex and less of
it is occupied with vision, auditory and olfactory data.  so in the
wiring up phase of brain growth when the inter-neuron fibres grow, 
the non-sensory bits of the cortex face less competition from the 
sensory bits.

Aso in humans the prefrontal zone of the cortex is 6 times larger 
than in the apes.  So its actual and realative size allow it
to form more links to the rest of the brain.  This prefrontal 
zone is home to certain speech centres.

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