wrote in message
news:c8l78i$1o5i$1{at}darwin.ediacara.org...
> It is well known that an organism has requirements for health.  For
> example, if humans don't get vitamin C they develop scurvy.  There may
> be hundreds of requirements for human health.  We know about many of
> them, we have partial information about many, and there may be many
> about which we know little or nothing.
>
> But my topic is: How did these requirements come to be?  They must all
> be the result of millions of years of evolution, beginning long before
> our hominid ancestors lived.
>
> I have never come across anything written on this subject.  In fact I'm
> not aware of anyone (besides myself) even talking about it.  Can anyone
> point me to anything written about the evolution of the requirements for
> health?
>
> Would anyone like to share a thought or two on this topic?

I don't have links to share, but I'm pretty sure that the thoughts
that I will share are fairly orthodox.

It is useful to break the requirement for a nutrient down into three
components:
* requirement to HAVE the molecule.
* requirement to MAKE the molecule.
* requirement to GET the molecule (from the environment).

Clearly, if you need to have, then you need to either make or get.

The general scheme for how a requirement to get arises is as
follows:  The "primitive" situation is that there is a requirement
to have, which is satisfied by an ability to make.  But the
molecule just happens to be available in the environment
(not too surprising for a heterotroph who eats things that also
are required to have the molecule).  So, the requirement to make
is relaxed higher up the food chain.  Natural selection can lead
to the loss of functionality if the requirement for that functionality
is relaxed enough.  So, the primitive requirement to make is
transformed to a new requirement to get.  That requirement leads,
over time, to specific adaptations targeted at that requirement,
such as dietary preferences and specific receptors for uptake.

But that is recent evolution.  More interesting to people like me
is the question as to how the "primitive" situation arose in which
there were requirements to have and to make.  There is a
chicken-egg problem - which came first, need-to-have or
need-to-make?  What follows is no longer completely
orthodox.

My guess is that ability-to-make arose first, though not as an
adaptation.  Given the presence of the molecule, natural
selection found an adaptive use for it.  Hence arises
need-to-have, and ability-to-make is transformed to
need-to-make.

The alternative is that the molecule was available from the
environment because it was produced in quantity by some
process of pre-biotic chemistry.  Thus, we start with an
ability-to-get, which transforms to need-to-have plus
need-to-get, which then in a few branches gets transformed
into the autotrophic need-to-have plus need-to-make.

Personally, I consider this scenario, in which hetertrophy
preceeds autotrophy, to be ridiculous.  But it is very common
in the OOL community to think in this odd way.
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