Parasites and paleoanthropology
Paul Crowley (Paul@crowleyp.demon.co.uk)
Wed, 17 Jul 96 00:37:54 GMT
Does anyone know of a good study of the evolution of human
parasites with relevance to recent human evolution?
This should be a fertile area for study. There are, I understand,
about 200 species of parasites specific to h.s.s. In many cases
the parasite exploits intermediate hosts (such as water snails)
before using h.s.s as the final host. Occasionally two inter-
mediate hosts are used - as in the case of the Chinese liver fluke
which infests a snail and then a fish before passing to h.s.s. as
the final host. Another example is the broad tapeworm
(Diphyllobothrium latum) which infests various species of fish
and then a species of small water crustacean (Cyclops) before
passing to Man. Such parasites can occur only where there is an
intimate ecological association between all three host groups.
In addition to adapting to the general external environment,
individuals at each stage of the life cycle of a parasite must
adapt to the microenvironment provided by the host. Adaptations
include not only obvious features such as suckers or hooks for
attachment, but also those associated with the biochemical and
physiological conditions imposed by the host.
Each parasite and each intermediate host will tell us the nature
of the environment to which it is adapted. An investigation of
the DNA of the parasite and of its nearest relatives could give
us an estimate of the time taken for speciation. We would also
learn much about the nature of h.s.s. societies at, say, 50 Kya
or 100 Kya or, possibly, hominid ones as far back as 2 Myr.
Nearly all h.s.s. parasites and intermediates are associated with
bodies of fresh water. In each case the h.s.s. population must
have achieved a minimum density in the area to justify speciation.
For example, several species of mosquito are specific to h.s.s.
with special mouth parts adapted for cutting human skin. One
of these is Anopheles which spreads four species of malaria
(plasmodium). Let's say that an estimate is made that the
oldest form of malaria is, say, >50 Kyr old (such an estimate
could be based on studies of h.s.s. genetic adaptations such
as sickle-cell and thalassaemias) and let's say an estimate is
made that Anopheles would have taken 10 Kyr to speciate; this
would mean that at >60 Kyr there was a sufficiently dense
population of h.s.s. near standing water. Now this is just a
hypothetical example, so please don't criticise my figures.
A better case might be the Guinea Worm (Dracunculus medinensis).
This can be over 6 feet long and used to be endemic in all
tropical countries. The WHO has a program to eliminate it; its
target date was the end of 1995 -- the second elimination of a
human parasite (after smallpox). This has now, I believe, been
revised to the year 2000. I have not heard of it having any
close relatives and it may have speciated 100 kya or even 2 mya.
Someone should do research on it, while it's still around.
Again, whenever it did speciate, there must have been reasonably
dense and sizeable hominid population around the pools inhabited
by its intermediate hosts.
I suggest that a thorough and wide-ranging study of h.s.s.
parasites (and their evolution) would show that fairly dense,
localised and sizeable hominid populations have inhabited areas
close to standing bodies of fresh water in the tropics at or near
sea-level for many hundreds of thousands of years. In other words,
such a study would go to disprove hunting/savannah theories of
human evolution.
What work is being done in this area?
Paul.
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