Re: origins of endothermy

[TomHopp@aol.com]

In a message dated 2/16/99 7:46:54 PM, EctoDino@aol.com wrote:

<< Hi! Have there been any attempts to explain how endothermy could have
evolved
from ectothermy at the biochemical level? (Since that's where it would have
had to start, right?) Thanks in advance. >>

Human endothermy is the most well understood, and it is complicated.
Primarily, it is managed by the primitive center of the brain called the
hypothalamus. This is where signals as diverse as nervous impulses and hormone
levels are integrated to produce "wellness" or balance between environment and
the individual. How it all works isn't entirely clear yet -- let's just say
it's complex.
    The hypothalamus takes in sensory nervous input and sends out some axons
of its own. It controls the release of metabolic hormones such as somatotropin
and thyrotropin from the pituitary. And, among other things, it can react to
the immune system by raising the temperature set-point, causing fever. It can
induce shivering in the muscles to produce heat, and can "uncouple" the
oxidative burning of fat for more of same. My personal favorite is its ability
to produce what behaviorists call the "sociophobic" response, wherein animals
shun interactions and prefer to curl up in a corner (the fact that this is a
great way to get warm is in my opinion the real value in this behavior).
    Remembering that some PLANTS have heat generating ability, as do some
sharks and other evolutionarily interesting life forms, one might think of
endothermy as more of a recurrently chosen option than a requirement.
    All of the above having been said, I have begun to view the evolution of
endothermy as a gradual, matter-of-degree process, not an off-on switch that
occurs miraculously at some point. There are probably dozens of genes all
synergizing to produce whatever level of endothermy is appropriate for a given
organism. Furthermore, genes are notoriously subject to subtle up- or down-
regulation of their functional levels by simple mutations. Thus, lineages
might drift into and out of endothermy over evolutionary time, depending on
environmental and other needs, without catastrophic, do-or-die situations
being involved. Just tweaking for optimized performance, like most of the rest
of evolution.
    Hope this helps.
    Tom Hopp