Re: Speciation - how do you know?
Paul Crowley (Paul@crowleyp.demon.co.uk)
Sun, 20 Oct 96 19:11:32 GMT
In article <54de6s$hb8@news.utrecht.NL.net>
ghanenbu@inter.nl.net "Gerrit Hanenburg" writes:
> This is somewhat disappointing. Here we have a theory that explains
> almost every aspect of human evolution but fails to account for a
> major change in morphology in one of the most important functional
> complexes;the masticatory apparatus of the robust Australopithecines.
> While this theory maintains that shellfish was the main component of
> the hominid diet from A.afarensis (A.anamensis?) to H.sapiens we see
> something remarkable happen in the robust Autralopithecines such as
> A.boisei. In relation to A.afarensis:a strong reduction of facial
> prognathism,almost a doubling of postcanine tooth area,diminution of
> anterior teeth,hyper thick enamel,massive jaws,large anterior
> projecting zygomatic arch indicating a strongly developed masseter
> muscle,etc. In short,we see a masticatory apparatus that
> biomechanically relates to heavy chewing.
They could have specialised in a certain kind of shellfish
eating. Maybe they did scrunch up limpets and cockles in their
mouths, whereas the gracile forms used crude tools to do the
same kind of job. This is no more than speculation, but it's
probably as good as anything you've read so far about A.boisei.
The robust/gracile split may have paralled the gorilla/chimp
one. The oceanic littoral would have been difficult, if not
impossible, for the graciles to exploit as tides would have
allowed nocturnal predation. Robust forms could possibly have
coped with the leopards. (This is even more speculative.)
The robusts might have been driven into extinction once the
graciles acquired fire and became able to exploit the oceanic
littoral. (I hope you're no longer dissapointed. ;-)
> >The littoral ape theory has many explicit and implicit tests or
> >forecasts, such as:
> > a) shellfish remains will be found with hominid fossils to
> > the extent that will show they formed part of the diet
> > throughout hominid existence;
>
> So far zippo,except for the latest stages.
A lot of evidence can be missed, when you're not looking for
it. Lucy was found with freshwater crabs, which is along the
right road.
> > b) fossil hominids found on inland sites will show a strong
> > bias towards the early adult age range with indications
> > of early death resulting from excessive tooth wear
> > caused by unaccustomed diet;
>
> This is simply an ad hoc "prediction" based on a personal
> interpretation of data you already know. How does one distinguish
> tooth wear caused by an unaccustomed diet from wear caused by normal
> use?
Worn teeth in a young adult (say under 25 or 30) indicate an
unaccustomed diet.
> > c) fossil hominids found in sites further from the coast will
> > show a smaller proportion of pre-adults.
>
> Compared to the ones found in coastal sites? :-)
My impression is that the data so far supports my case (although
I'm first to admit that I've done little research - I have a job
to hold down). Fossils of infants have been found at Afar which
is likely to be as close to "coastal" as we'll get. (The Sea of
Afar probably existed then.)
> Anyhow this requires a statistical analysis which may not be very
> reliable at the moment due to very small samples.
> Something for the far future when we have discovered numerous coastal
> Australopithecines.
This rule should apply to _all_ hominid fossils; so infant/
juvenile H.? (H.erectus, H.s.neanderthalis, AMH, etc.) should
be much more rare than would be expected under the regular
"inland residence" hypothesis. There should be more than
enough data to test it now. IOW: Is the adult/juvenile/infant
ratio in the hominid fossil record similar to that for, say,
elephants, pigs, or crocodiles? Or is it distorted towards
adults and towards adult males in particular?
> > i) studies of the DNA of human water-borne and water-reliant
> > parasites will tend to show a very long evolution,
> > indicating the long existence of large, localised,
> > resident hominid populations;
>
> Which water-borne and water-reliant parasites are you refering to?
> And how does their DNA give any information about their past
> association with hominids?
Parasitology is a vast and complex field, about which I know
little. However, many human parasites have several hosts, all
of which have evolved together. Many species of mosquito have
evolved special mouth parts solely for human skin. A range of
complex questions have to be answered: How long would it have
taken? What would the minimum size of the resident hominid
population have had to be? How far away could it have been
from fresh water? Again the varieties of the plasmodium
parasite could only have evolved later, and similar questions
need answering. Again some estimate must be made from human
DNA of the antiquity of human responses to malaria.
Another interesting parasite is the guinea worm (dracunculus
mediensis). What is its nearest relative? When did it split
from it? What would the nature of the hominid population have
to have been to allow it to speciate?
When some of the answers are in about some of the parasites,
I believe that we will have a very different picture of human/
hominid evolution.
> There is another possibilty. The canine may have acquired a new
> function in hominids. Functionally it became an incisor.
It's hard to see how a canine can function as an incisor without
first coming right down in size.
> > Those individuals that keep the
> >best canines predominate and have more offspring. It's a
> >perpetual weapon race.
>
> That doesn't tell you that an organ is expensive,but only that it pays
> to have the organ.
It's not conclusive, but it is persuasive. The species will
go to some trouble to provide sound foundations for the organ.
And good weapons rarely come cheap.
> >So early, mid and late hominids had an abrasive diet. An abrasive
> >diet of what?
>
> Not necessarily all three categories.
> Several possiblities come to mind:vegetation with a high silica
> content,those parts of plants whose consumption may include some of
> the surrounding matrix (roots and tubers),hard fruits and nuts.
> And also less abrasive,low quality material that is consumed in bulk.
The human gut appears to be designed for a dietery mixture of
a high-protein food and some fruit. Given the long-standing and
continuous hominid trend in dentition, it is most unlikely that
earlier hominids consumed low quality bulky foods.
While certain local populations may have consumed particular
vegetables with a high-silica content, this would not account
for the high enamel found universally in all hominids.
Paul.
|