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big dinos and death



[ This is largely off topic, but I'm allowing it because since Nick
  has submitted it I no longer feel like I need to address this aspect
  of Greg's over-simplified statements about longevity.  And I was
  planning to... -- MR ]

        As you are no doubt aware, cancer rates go up over time, but when
you stop and think about what this means, something becomes apparent.
Cancer is due to mutations in the bodies systems governing the growth
stimulation and growth restraints of cells primarily, and if you think
about it, a 50 yr. old and and a 5 yr. old are equally likely to developed
cancer if neither of them already has it, and they just need one mutation
(assuming mutation rates are the same). But this isn't the case. The only
way to reconcile the data is to assume multiple mutations are necessary,
and this is what researchers have evidence for. It seems to require about
five seprate mutations on average to cause cancer. This, like the
accidents Greg was referring to, can put an upper limit on an animal's
life. You can only go so far before you get too much damaged DNA.
        Interestingly, this limit is  variable. In humans, the
practical limit to a lifespan before mutations catch up is many years. In
mice, it's about two. The thing is, an animal is built to last about as
long as it is likely to last. Mice have a whole *#$^ing lot of things out
to eat them- coyotes, cats, owls, hawks, snakes, etc. and don't stand a
chance of lasting very long. So natural selection does not build mice
capable of surviving  for twenty years even though it probably could. So
probably, cell growth, anti-cancer and DNA repair mechanisms that would
enable mice to live for many years are not selected for.  It's the least
of their worries, apparently, the mouse with a tumor is probably lucky to
have lived that long. 
        Birds of the same weight may live ten years. This is because they
are up in the air and in the trees where it is very hard to get them. Your
parakeet may outlive your dog (dogs live a pretty high-risk lifestyle). no
surprise turtles are long-lived in captivity, they have few predators so
mechanisms that let them survive for 150 yrs.  are helpful to their
survival and selected for. The wandering albatross may live 60 yrs. or
more, and why not? I've been out there, and there isn't much that will eat
an albatross that far out at sea. Few predators, nothing to crash into,
just vast open water and waves and other birds. Perhaps it was the same in
the Niobrara sea for all I know, pterosaurs may have been pretty
long-lived too. Some big fish may live for decades, as well- apparently,
nothing goes after big rockfish and halibut except for people. This makes
fishing them out very easy (not true of salmon which reach large size in
only a feww years). Natural selection does not waste resources. an animal
is generally built to last as long as can be expected to. Jared Diamond
writes about this (third chimpanzee, I think) and asks why you would want
to own a very expensive, reliable car that will last you for twenty years
in a city where you are likely to get hit in the next few months. The same
principle applies in my hometown. You don't buy fancy cars if you have any
sense because you are right next to the ocean and the salt just eats them
up. 
        People are built to last a long time, we would be built to last
longer had we evolved in an environment like the developed world rather
than African savannah. There are other factors than cancer, like the cell
death Greg mentioned, but one wonders exactly what the ultimate limit
would be. There seems to be no reason that humans, if we had been selected
too, couldn't have lifespans far in excess of what we see today. All
things considered, life in the developed world is really  pretty tame.
Implications of social change for suc a society is best left to sci-fi
authors (but you can imagine the kind of generation gap that would exist.
"Why, a hundred fifty years ago, if we'd talked to our parents that
way..."). Kuhn recognizes death as one of the important forces of change
in science allowing for new ideas to move in. Progress, social and
scientific, might be pretty slow. 
        More on topic, I wonder if you could show predation by figuring
out how old dinos got. I mean, if Triceratops generally lived to a ripe
old age of seventy, you might be able to conclude "well, no, t. rex
probably wasn't hunting them". There are other factors, like drought
(actually, I think the areas was supposed to be wet) and intraspecific
combat which could contribute to mortality, but you might be able to
figure out if your duckbill and ceratopian and sauropod populations were
seeing any serious increase in "accidents" (soundds of theropods laughing
menacingly here) over what you would normally expect.