X-Message-Number: 4529
Date: 21 Jun 95 01:17:55 EDT
From: Mike Darwin <>
Subject: brains

Bob asks what I mean when I say that humans took their brains out of the line of

First, I should point out that my speculation about why dogs and people have

such a singular and common lack of tolerance of cerebral ischemia was just that:
idle speculation based on little more than whimsy.

However, the whimsy was based on the following observations:

1) Humans apparently evolved on Savannah: grass land,  as runners and
hunter-gatherers.  In short they left the trees and became nonarboreal.  

2) Humans hunted in packs and used the same general "wear 'em down" and/or "get
the weak ones" technique that dogs do.  In fact there is a fair amount of
speculation in the literature of evolution that that is where humans and
wolves/dogs first crossed paths: certainly the dog has been domesticated for at
least 10K years!

3) Arboreal monkeys FALL from trees a lot.  Particularly juveniles.  They get

their heads wacked a lot from this too.  Ditto cats who will often climb up, but

fall down.  I only recently acquired a cat but one thing is sure: falls and good
plops on the head aren't all that uncommon.

The style of hunting and gathering described in #2 above does not expose humans
to much risk of head injury.  Indeed, many creatures that strike by ramming or

hitting their opponents head hard are only knee high to humans.  When I get into
an altercation with my pig it is NOT my head I am worried about.  This is
because I am heads taller than her -- and most animals I would have a
confrontation with.

Speed: starting with the horse and continuing through the automobile and
motorcycle are what puts humans at most risk of head injuries.  Otherwise we're
pretty good at covering our heads with our hands and arms if we see something

Finally, you will note that quite early in civilization (early enough to have
had an evolutionary impact based on observations of other species) humans began
to actively protect their heads: particularly as *war* became a major part of

their culture. Sheilds, helmets, armor, all were attempts at reducing the impact
of head injury.

Dogs are almost impossible to head injure without use of really heavy force or
the induction of counter-coup injury from sudden acceleration of the head (and
because of their thick, well muscled necks, inducing that kind of g-force is
hard to do).  Humans didn't become really susceptible to counter coup injury
until they started beating the shit out of each other and riding around at
relatively high speeds.

What are the real reasons for our poor tolerance to brain ischemia?  I  assure
you I don't know.  But, nevertheless, there it is.  Hossman reports one cat
surviving after an hour of laboratory-induced (and thus VERIFIABLE) global
ischemia.  Nobody, to my knowledge has reported anything like that for humans.
Macaque monkeys will usually have one or two out of 10 or 20 animals that will
survive 17-20 minutes of global (verified) ischemia, even in the control group.
Not so dogs or people.

Mike Darwin

Rate This Message: http://www.cryonet.org/cgi-bin/rate.cgi?msg=4529