David Lloyd-Jones <dlj@[EMAIL PROTECTED]
> wrote in message
news:<3E3581C8.60205@[EMAIL PROTECTED]
>...
> Tim Worstall wrote:
>
> >>
> >>I realise that you are not being entirely serious in this
> >>thread......however, I feel that you have slightly missed the point
> >>about marsupials. Foetal development largely takes place outside the
> >>womb, in the pouch, ( the abscence of the placenta being a major
> >>reason ), therefore hip size and head size are not related at all by
> >>the size of the birth canal.
> >>Unlike humans say, where an unusally large headed baby will produce a
> >>wince in recent mothers.
> >>
> >>
> >>
> Tim,
>
> OK, you've added one more reason for marsupial head size to not run up
> against evolutionary limits: not only is greater head size not limited
> by pelvis angle, but also there is the opportunity for head size to
> increase evolutionarily because of the pouch arrangement.
>
> So, again, why do marsupials have such small brains? Why don't kangaroos
> rule the planet? Or is becoming marsupial the next stage in humanity's
> evolution?
>
> Or, contrarily, is intelligence just not very important in evolution's
> scheme of things?
This is getting a long way off subjects where I would attempt to claim
any authority....but a few bits and pieces that might explain some
things.
I think marsupial is a less developed, not more so, method of
parturition. As far as I can remember, the placenta acts as a filter,
stopping the motherīs body from rejecting the foetus as itīs genetic
difference from the host becomes apparent. So I donīt expect being
marsupial to be the next stage of anything.
Secondly, evolutionary pressure only matters in the specific
environment in which the animals are found. Those areas with
marsupials ( Australia, S America, Antarctica and a few islands, PNG,
Mauritius ? Madagascar ? ) in the past were not competing against
those with mammals ( the rest ) in any valid sense. They were
competing amongst themselves. So if, intelligence, say ( or any other
thing that might increase reproductive success ) had not arisen in
marsupials, it would not be there as a pressure on the other
participants in the ecosystem, and so less likely to develop in them.
This of course was fine until S and N America met up and the mammals,
who had been subject to greater evolutionary pressure, poured over the
Darien Gap. I believe the only survivor of the S American marsupials
is the Possum ( Opussom ? ) which thrived and is now found as far
north as Canada. I donīt know whether the sloth you mentioned is a
marsupial or not.
There is a paradox in here somewhere....mammalian reproduction
obviously is more difficult than marsupial, but it does seem to be
more efficient, so the costs are outweighed by the benefits....even
when something like intelligence, and the associated large heads are
involved.
Iīm not anserwing your questions very well. Sorry, Too far off my
beaten track.
But I would posit that marsupials are an " earlier " stage of
evolution than mammals. So we would not expect that to be the next
stage for humans. And marsupials donīt have small brains. They are
perfectly adequate for the environment in which they developed. Itīs
when they compete with mammals that they have a problem. And
intelligence ? No, I donīt think that that is anywhere on evolutionīs
roadmap.....I donīt think that anything is on that roadmap, other than
whatever genetic attributes contribute to reproductive success.
Intelligence may be one of these things, as we humans seem to show,
for without it we wouldnīt be a successful as we are. But a road map ?
A destination ? A " scheme of things " ?
I pretty much go along with Stephen J Gould....Itīs all been random
chance within some pretty wide limits of chemistry and physics and if
we ran it again we wouldnīt end up with us. Although intelligence
probably would arise in some form, it appearing to be so useful.
Tim Worstall
>
> Cheers,
>
> -dlj.
|
