Re: Speciation - how do you know?
Paul Crowley (Paul@crowleyp.demon.co.uk)
Wed, 30 Oct 96 15:07:31 GMT
In article <32760a5c.11657007@news.leiden.nl.net>
G.Hanenburg@inter.nl.net "Gerrit Hanenburg" writes:
> Paul@crowleyp.demon.co.uk (Paul Crowley) wrote:
> >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).
Estimates based on limited data are not "right" or "wrong". They
have a good or a poor reliability -- and in this case it's poor.
> Since no other data is available that indicates better nightvision in
> robusts I assume that it was comparable to that of extant diurnal
> hominids
I'd agree with you here. Nightvision could have been somewhat
better, but not by any order of magnitude. However, the problem
that I'm trying to face is not particular to my theory:
How did early hominids survive nocturnal predation by leopards?
"Sleeping in trees" is clearly not an answer.
Non-tidal seas would readily provide safe refuges, but so could
tidal oceans. Inter-tidal rock pools often have islands; and the
pools themselves could be a refuge in an emergency. The inter-
tidal area is not a natural one for leopards: the scent of prey
would be drowned by marine smells and the nocturnal breeze is
normally off the land; hearing would be drowned by marine sounds.
Hunting in such an area would be dificult and dangerous. Even
if predation prevented large populations, it would never eliminate
the species.
> >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.
We have data that needs explaining: two very slow bipeds that
don't sleep in trees; the first has large teeth with dense enamel
and apparently consumed a high-protein diet, while the other, with
the same ancestors and very similar morphology, presumably occupying
a closely related niche, has very large teeth with extremely dense
enamel.
You say (presumably) that the first lived on the savanna, competing
with chimpanzees and baboons, doing some scavenging and eating
"something rough" to account for the enamel; that the other lived
in the same habitat but chose to eat something like small seeds
(although you can give no reason why that diet needed hyper-thick
enamel). You say nothing about why they should have speciated;
nothing about why they became bipedal; nothing about how they
avoided nocturnal predation; nothing about the reasons for the
peculiarity of their dental arcade; nothing about their relation-
ship to later hominids and why they should have changed their diet
. . . nothing . . . nothing . . .
It's easy to accuse anyone who puts forward a possible explanation
of "groundless speculation". No one will ever accuse you of that.
Just keep on saying nothing.
(This is not personal, BTW -- all professional PA's are the same.)
> >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.
Theoretical. A mouthful of shellfish would have high nutritional
value. Another possible reason for the different feeding technique
could be that there were few stones of the right sort in their main
feeding locations. It could have been mostly sand, or mostly
boulders.
If predation kept Robust population densities low, we would have
another rough parallel with the chimp/gorilla split. This might
provide another explanation of respective morphologies. Robusts
would never be forced to exploit less accessible food resources.
OTOH population density would soon drive the Graciles to attack
harder shells, breakable only by stones. Population density would
also make the intra-species competion fiercer and emphasise the
use of weapons.
Consequently, I'd prefer to date the Robust/Gracile split early
-- before 3.5 mya. Is there any good evidence against this?
> 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).
Thanks. I missed this. A. ramidus would seem to be a very early
hominid -- perhaps on the cusp.
Paul.
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