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From: buckley@reed.UUCP (Ken Buckley)
Newsgroups: sci.bio
Subject: Can unrelated twins exist (again)
Message-ID: <15289@reed.UUCP>
Date: 4 Aug 90 04:01:12 GMT
Reply-To: buckley@reed.UUCP (Ken Buckley)
Organization: Reed College, Portland OR
Lines: 34

To all those who responded to my posting ("Can unrelated twins exist" on
sci.bio), thanks.
The reason I posted was a pair of ideas in science fiction stories I
read recently: one in a P.J. Farmer *Riverworld* story and one in Asimov's
*Prelude to Foundation*; the answers to my posting allow a bit of fanciful
speculation.
The Farmer *Riverworld* series takes place on a planet where some being
has reincarnated all the people who have ever lived on earth -- about 
10 trillion, by Farmer's estimate. The story in question concerns Tom Mix's
adventures on this world, together with a Jew from Jesus's day and an
Egyptian woman (I think.) Almost as a just-for-fun afterthought, Farmer
throws in the fact that Tom Mix and his Jewish companion look precisely
identical -- this is ascribed to random genetic happenstance. In a population
of only ten trillion, this seemed quite unlikely to me, but I chalked it
up to Farmer's strange imagination.
Then I read *Prelude to Foundation*. Not having read any of the *Empire* 
series, I was struck by the huge size of Asimov's universe: 25,000,000
_worlds_. The largest, Trantor, has 40 billion people; a small backwater
world has around 500 million. Taking an average world to have around
1 billion people, Asimov has conjured up _25x10^15_ -- yes, that's 
quintillion -- people. Now here, it seemed, we had a sufficient population
for the implausible Farmer plot twist to occur. I am not a biologist, so
I posted my question to sci.bio to see if my hunch was right.
To my intense surprise, the answer is a definitive no! Using some of the
responses I got, a friend and I made the following conservative(!)
estimate: approximately 10^166 humans would be needed to make this an event
with even a remote chance of occurring.
Thus, it seems that Hari Seldon is safe from meeting his twin, while Farmer
is as usual off in space. Another interesting (?!) question poses itself,
though. Asimov's empire spans only our own galaxy. Assuming one Earth-type
planet per star, are there enough stars in the galaxy to support
10^166 humans? And can anyone write a science fiction story set in such a
universe? :^)
--Ken Buckley