Michael writes: 

>Why nobody (that I'm aware of) hasn't emphasized the fact the third
>evolutionary stage of our brain development (the cerebral cortex) has
>only occurred in the last 100,000 years and the two previous stages took
>millions of years, is puzzling. 
>
>This is highly significant. One could convincingly argue that since we
>have only been in the third evolutionary stage of the development of our
>cerebral cortex for a fraction of the time the two previous evolutionary
>stages occurred, the evolutionary influences of the reptilian and
>leopard parts of our brains (which lasted for millions of years) are
>bound to take precedence (and even overwhelm) this most recent third
>evolutionary stage of our brain development which has only lasted about
>100,000 years.

I suspect that the primary reason that people haven't emphasized this very
recent evolution of the neocortex is that it isn't true. No matter how you want
to begin counting, the basic mammalian neocortex is at least 200 my old, with
the primate brain being ca. 90 my, the hominid brain being 6 my, and modern
Homo architecture ranging from 2 to .7 my.

The following paragraph is from an introductory biology course outline, but it
accurately recapitulates current thinking regarding the evolution of mammals:

"Mammals probably evolved sometime during the early Triassic (about 240 mya),
and likely from a Carboniferous (360 - 286 mya) synapsid reptile. We passed
through a therapsid reptilian stage as it is these became the first endotherms
(You would probably say warm blooded.). The earliest complete mammalian
skeletons are from the late Cretaceous period (about 75 mya) of the Mesozoic
era (248 -65 mya). So, mammals were around the whole time that dinosaurs ruled
the earth. But because of the dinosaurs, the little mammals were relegated to
play a minor role; a role that was likely nocturnal. But this had an important
effect! Being nocturnal they had to improve their ability to hear and probably
also their ability to maintain a high body temperature so that they could
remain active during the cooler nights. Typical mammalian features are (except
for monotremes, the egg-laying mammals): live birth (viviparous), endothermy,
appearance of a diaphragm, four chambered heart, expansion of neocortex, single
bone in jaw or mandible (the dentary bone), quadrate becomes the incus and the
articular becomes the malleus, development of elongated secondary palate,
single nasal opening in skull and heterodont dentition (i.e., teeth of
different shapes for different functions)."

  ---http://137.122.151.29/BIO1120/Topics/htm/17_Topic.htm

Wirt Atmar
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