On May 6, 2007, at 8:27 AM, Jones Beene wrote:
Horace Heffner wrote:
The Northern exposure idea would require a full galactic N-S
traversal (bobbing up and down) period, or about 130-200 M years,
while the period between extinctions is often roughly half that.
Hmmm ...Are you reading the same article I am? ... the complete
cycle is said to be half your ~130 my assumption and occurs exactly
4 times in one complete galactic 250 year revolution - which is
probably based on our Milky Way galaxy having two radial spiraling
"arms," as do many observable galaxies.
Yes, you are right. I just don't have the time to give proper
attention to this right now.
On May 6, 2007, at 1:38 PM, Robin van Spaandonk wrote:
In reply to Horace Heffner's message of Sun, 6 May 2007 02:41:17
-0800:
Hi,
[snip]
Further, the hypothesis doesn't give a reason the major
extinctions start about 600 My ago.
I didn't think there was much life around to extinguish prior to
600 MY ago.
(Or if there was, then probably mostly bacterial, and we wouldn't
notice an
extinction event anyway.)
Not much known yet about the Mesoproterozoic age yet, and radio-
dating has its problems back hat far, but
Eukaryota appeared starting about 1.8 By ago,
http://www.ucmp.berkeley.edu/precambrian/proterozoic.html
http://www.ucmp.berkeley.edu/alllife/eukaryota.html
http://en.wikipedia.org/wiki/Eukaryote#Origin_and_evolution
and they have structures fairly easily identified in the fossil
record. If there were major extinction events it seems like they
should show up. Maybe nobody has looked for extinction events? It
is for sure the speciation that took place was phenomenal, so maybe
the evolution rate was just too fast for the instantaneous
extinctions to show up?
On May 7, 2007, at 4:32 AM, Taylor J. Smith wrote:
Google `"snowball earth" BBC' to see a solid case made that
that the entire Earth had been frozen over for millions of
years prior to 600 My ago. An interesting question is
"Were there multicellular forms of life here that were
ground into oblivion during the years that Earth was a
ball of ice?"
Jack Smith
"Snowball Earth" seems to be a highly controversial theory. See:
http://news.bbc.co.uk/2/hi/uk_news/scotland/1857545.stm
A lot of evolution appears to have occurred through the period
without major extinctions, i.e. fast simultaneous extinctions of many
species.
Regards,
Horace Heffner
