Re: Speciation - how do you know?
Paul Crowley (Paul@crowleyp.demon.co.uk)
Sat, 19 Oct 96 10:31:23 GMT
In article <54901i$me2@news.utrecht.NL.net>
ghanenbu@inter.nl.net "Gerrit Hanenburg" writes:
> Paul@crowleyp.demon.co.uk (Paul Crowley) wrote:
>
> >It's sometimes a choice between a theory which explains the
> >data and no theory at all.
>
> Explaining the data is not enough.
> I'm sure the littoral ape theory has an explanation for the
> differences in tooth structure and facial architecture between
> A.afarensis and A.boisei that undoubtedly involves shellfish.
A theory does not have to explain everything, especially in
biological fields. A.boisei is so exceptional, that it may be
a long time before anyone has the beginnings of an explanation.
> Ad hoc explanations without testing will get you anywhere.
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;
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;
c) fossil hominids found in sites further from the coast will
show a smaller proportion of pre-adults.
d) the initial adaptation to bipedalism will be shown to be
early and fast;
e) fossils of infants and young will show fully bipedal feet
-- without an ability to grasp; these will long pre-date
any significant expansion in brain size;
f) adaptations for an arboreal existence will be shown to be
lost quickly;
g) early hominid fossils will not be found at any significant
distance away from large bodies of standing water;
h) the theory maintains that soft-body adaptations occurred
early; early hominid fossils may show indications of
subcutaneous fat, especially in females; nakedness and
sweating might also possibly be indicated;
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;
j) no significant evidence of hunting will be found before
about 200 kya.
I'm sure I could think of more, but that's enough for now.
> >All hominids (including H.s.s.) have thick enamel. It is absurd
> >to say that H.s.s. enamel is a primitive retention from an LCA
> >7 mya - which is implied by your argument. Enamel (and dentition
> >generally) is expensive and will evolve rapidly in new circumstances.
>
> How do you determine whether or not a certain tissue is expensive?
> I would say that brain tissue is metabolically expensive since it
> requires a relatively large supply of oxygen(3.5 ml/100gr./minute),but
> how is that in case of teeth?
Expense can be seen in many ways. Blood supply is good, but not
the only one. The cost of the change may be high. The Hss 20%
reduction in the last 20 kya has resulted in a lot of impacted
wisdom teeth, so the loss in size must have had significant
benefits - namely, a reduction in cost. Again the speed of
change in a new niche is an indication. The loss of large
canines occured quickly in early hominids. Presumably they did
not need them, so they lost them fast; so they must have been
costly. Also, observation of living primates and other mammals
can tell us that an organ is expensive. When a canine goes, the
animal often dies soon after. Those individuals that keep the
best canines predominate and have more offspring. It's a
perpetual weapon race.
> >The exceptionally thick enamel of the earliest hominids needs
> >an explanation; so also does the thick enamel of all hominids,
> >including H.s.s. How many explanations are there?
>
> Thick enamel may very well be a response to an abrasive diet,but a
> diet can be abrasive in different ways.
So early, mid and late hominids had an abrasive diet. An abrasive
diet of what?
> Crushing shellfish is only one of them. (and unlikely in the case
of primates)
Since late hominids had a diet of shellfish ("coincidentally"
abrasive) and were, in fact, primates, the "unlikeliness" goes
out the window.
> Why do Orangutans have thick enamel?
Who knows? Has anyone done a "Jane Goodall" on Orangs? They'd
need a head for heights.
Paul.
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