Re: Parasites and paleoanthropology
Paul Crowley (Paul@crowleyp.demon.co.uk)
Wed, 17 Jul 96 23:28:20 GMT
In article <glenn-1707960904550001@news.insync.net>
glenn@lexgen.com "Glenn A. Friedrich" writes:
> In article <837563874snz@crowleyp.demon.co.uk>, Paul@crowleyp.demon.co.uk wrote:>
>
> : Nearly all h.s.s. parasites and intermediates are associated with
> : bodies of fresh water.
>
> Do you have documented data that "most" of the 200 species (your number)
> of human parasites are "associated" (what does that mean?) with bodies of
> fresh water? Can you provide references?
I've consulted various textbooks, but they are mostly medical and I've
found little or nothing on the evolution of parasites. That's why I'm
asking. BTW I should have stated that I was generally referring to
"macroparasites" i.e. not bacteria or viruses. The smaller they are,
the shorter their lives, the faster their probable evolution, and the
less interest to this newsgroup.
Parasitology is an enormous field of study - from a medical point of
view. Millions die every year. Much has been done on their DNA and
could easily be studied from an evolutionary viewpoint. I'm looking
for references. Each parasite is fascinating and has its own peculiar
form of life. Many infest two or more hosts and the most common
alternate ones to h.s.s. are water snails.
> Put some meat on your argument
> and provide real numbers and use more exact terminology. Is it 85% of
> species or over 90%? Turn your speculation into something a bit more
> substantial.
This is not the place to recount the details of each species; and
there is too much variation to able to present meaningful percentage
figures. It would be like trying to say how many mammals are "closely
associated with water". And the life cycles of most parasites are far
more complicated than those of mammals. I think I'll withdraw "nearly
all h.s.s. parasites" and say that a high proportion of parasites are
definitely closely associated with standing water, e.g. the Guinea worm,
mosquitoes, malaria, bilharzia, most of the thirty-six or more species of
flukes (platyhelminthes) which have been reported as being parasitic on
humans.
> : In each case the h.s.s. population must
> : have achieved a minimum density in the area to justify speciation.
>
> Why? Can you quantitate what you mean by 'minimal'? What percent of any
> given parasite's life cycle is spent in a human host?
Dr Andrew Dobson of Princeton has an excellent article on page 412
of the Cambridge Encyclopedia of Human Evolution on Epidemiological
Theory with four sets of differential equations to calculate the
rate of transmission of a disease for a microparasite with a direct
life cycle. He states that "Similar equations may be obtained for
macroparasites or parasites with more complex life cycles involving
another species of host. Although the mathematics varies, the
equations for thresholds of establishment and instrinsic growth
rate of such pathogens are quite similar to those of simple micro-
parasites." Dobson's formulae can be used to calculate minimum
host populations, provided the other data can be estimated. He
states on page 411: " . population size is very important in
determining whether a pathogen can become established in a host".
> Why can't your strawman savannah theory call for some existence of
> humans near bodies of water? Heck, I can assume with fair confidence
> that our ancestors had to drink water. And why wouldn't this exposure
> to water be more than adequate to allow parasites to flourish? Malaria
> is alive and well without tribes of aquatic apes roaming the modern
> day wetlands.
Your ad hominen tone is not helpful. I certainly don't support any
idea of "tribes of aquatic apes roaming . . wetlands". I'm
suggesting that the evidence indicates that after about 500 kya fairly
large and stable hominid populations inhabited areas near lakes on a
semi-permanent basis. I know that this conflicts with PA tradition
which I suggest is "primitive" and long overdue for revision. In my
view, the development of language required complex social institutions
which could only have flourished in large populations. I am fairly
sure that a thorough study of the evolution of human parasites will,
in time, confirm this point of view.
Consider what is necessary for the establishment of a species of
malaria. First a species of mosquito which can regularly exploit
hominids has to evolve. This won't happen unless hominids can be
found close to the mosquito's lake on a permanent basis for tens of
thousands of mosquito generations. Throughout this period a minimum
number of mosquitos and hominids must live within a fairly small
radius. Dobson's formulae could be applied to get some figures.
Then a species of plasmodium (malaria) can evolve. It's going to
need larger populations as it parasitises both the mosquito and
the hominid. The variations in the h.s.s. genetic responses to
malaria indicate that it is a very ancient disease, probably in
excess of 100 Kyr.
Similar arguments can be applied in the case of each parasite.
The sum of such arguments could give us a detailed account of the
habitats and ecologies of hominid populations over the last few
hundreds of thousands of years.
> As an aside, your use of the word 'justify' in the above quoted passage is
> unfortunate. Speciation requires no justification because it doesn't see
> into the future. Which particular speciation events happen to occur is a
> matter of chance.
As you will gather, I disagree. The "justification" would be the
application of the differential equations to which Dobson refers:
" . . equations for thresholds of establishment . . ". Chance is
all very well, but if the new mutation has no basis for existence
it will disappear.
> I suggest you might want to flesh out your hypothesis with a little
> preliminary data before anyone embarks on a 'thorough and wide-ranging
> study' as you suggest. Such data isn't difficult to come by.
If someone isn't currently doing a "thorough and wide-ranging study"
on the implications of parasite evolution for late hominid habitats
and lifestyles then a lot of people aren't doing their jobs. But
someone must be. Does anyone know?
Paul.
|