Re: Speciation - how do you know?
Paul Crowley (Paul@crowleyp.demon.co.uk)
Mon, 28 Oct 96 18:26:11 GMT
In article <DzvLzM.K73@inter.NL.net>
G.Hanenburg@inter.nl.net "Gerrit Hanenburg" writes:
> There's still the problem of how robust Australopithecines,who were
> only slightly bigger than graciles (ca.41 kg versus 37 kg),could deal
> with leopards,while graciles couldn't.
The size estimates are not reliable in view of the small numbers
of fossils and the difficulties in identifying sex, especially
for the robusts. Also, size might not be the crucial factor in
coping with leopards; an adaptation to better night sight could
be enough.
BTW does anyone know if gorillas (especially males) have a better
night sight than chimps? They frequently encounter leopards at
night.
> Then there's the problem of why
> robusts apparently lost the ability to use tools for food processing
> and took the costly road of a massive masticatory apparatus in order
The oceanic littoral would have been a very different environment
from the non-tidal one and would have had other species of edible
shellfish. These may not have been so amenable to the crude tools
used by the graciles at 3 mya. The shellfish may been too small,
and only consumable in quantity in the mouth. We are talking of
two habitats, each of which would be occupied by a species best
adapted to the most readily obtainable food.
> to crack the shells open with their teeth which,even if you have
> enamel as thick as A.boisei,will wear your teeth down long before you
> have reached maturity. No enamel cap can resist such an onslaught.
It would not have been cracking thick shells so much as crunching
up small ones.
> >It is somewhat far-fetched to make conclusions about gut dimensions
> >and structure from general abdominal shape.
>
> Who's calling the kettle black? As if your speculations have so much
> more substance.
The justification of each of our speculations is quite different.
You asked me to say how the robusts could fit in. (It was a
perfectly fair question and thanks for posing it.) They are
an extraordinary species and any reasonable theory should be
expected to account for them. The data exists and some account
of it has to be given. Whereas no data exists about australo gut
dimensions and it should not be created to justify a possible
theory about a presumed diet.
> There's Ardipithecus ramidus with thin enamel and post-canine
> dentition significantly smaller than A.afarensis.
How do you know this? Has it been published?
> A.afarensis with thick enamel and large postcanine dentition
> (Megadontia Quotient=1.30)
> Australopithecus boisei with hyper thick enamel and very large
> post-canine dentition (MQ=2.44)
> Homo erectus with reduced enamel thickness and reduced size of
> post-canine teeth (MQ=0.92)
> All this differentiation because of the same diet? Amazing!
> What mechanism do you suggest? Genetic drift?
Some account has to be given. It would be even more amazing
if the chrono-species (and A.boisei) had adopted completely
different diets, especially in view of the constancy of thick
enamel and the constant trend towards reduction in size and
towards a more parabolic dental shape.
What explanation do you give?
Within my framework it is only H.erectus that needs further
explanation. It must have found some method of preparation
which made the diet less rough; this could have been washing,
or soaking, in fresh water. In any case, H. erectus had better
tools, better techniques and, probably, use of fire. These
could well have come about from an ability to modify its food,
or possibly extend the range of its diet.
Paul.
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