Re: Speciation - how do you know?
Gerrit Hanenburg (G.Hanenburg@inter.nl.net)
Tue, 29 Oct 1996 13:51:18 GMT
Paul@crowleyp.demon.co.uk (Paul Crowley) wrote:
>> 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.
These estimates are based on available data (McHenry,H.M.(1992),Body
Size and Proportions in Early Hominids. Am.J.Phys.Anth.87:407-431).
Whether or not they are right has to be decided on the basis of new
material. This is what we work with at the moment. These estimates are
our working hypothesis until it turns out to be wrong (by new data).
>Also, size might not be the crucial factor in coping with leopards;
>an adaptation to better night sight could be enough.
It's unlikely that robust Australopithecines had better nightvision
since none of the haplorhine primates has a tapetum lucidum,and no
orbital index has been reported that indicates the robusts had
relatively large eyes (as with the nocturnal Tarsiers and Owl
monkeys).
Since no other data is available that indicates better nightvision in
robusts I assume that it was comparable to that of extant diurnal
hominids
>> 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.
Another groundless speculation. Even worse than "fitting the data to
the theory" because you don't even have data.
>> 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.
In greater numbers...doesn't look like an improvement and brings with
it foraging theoretical problems such as increased gathering time and
diminished returns.
>> 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?
Yes,molar enamel thickness in A.ramidus ranges from 1.1 to 1.2 mm
(within the range of Pan troglodytes). In A.afarensis it ranges from
1.4 to 2.0 mm. (White,T.D.et al.(1994),Australopithecus ramidus,a new
species of hominid from Aramis,Ethiopia. Nature 371:309)
All specimens for whom postcanine tooth size is determinable are below
the A.afarensis mean (the two lower M1s more than 3 s.d.units). White
et al. interpret this as "a postcanine dentition significantly smaller
than in A.afarensis" (ibid.p.310).
>> 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?
There is no constancy in enamel thickness but a trend toward
increasing thickness in Australopithecines and a decrease in
Homo.Similar for size.
Combine this with information on microwear pattern and the suggestion
is dietary differentiation.
>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.
You're coming close. ;-)
Gerrit
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