Thanks for clarifying your views. Let me try to clarify mine.
>Now, what I'm saying is that, as time progresses, evolution will create
>organisms in all available directions from the center point, and a great
>many of these will be very simple, and it will keep creating (and,
>sometimes, recreating) simple organisms through time, as new niches open and
>as niches change. But, it will also, in the upward direction, keep evolving
>new organisms of greater and greater complexity, so that, over time, the
>entire hemispherical "bush" expands, filling more and more simple-organism
>niches, but also working upward into more and more complex-organism niches."
I understand, but we must also remember that over 50% of the earth's
biomass is composed of microbes, 35% plants, and only 15% animals.
Your perspective on evolution seems heavily weighted by what has
happened with animals. Remember that for about 2 billion years
after life appeared, only single-celled organisms inhabited this planet.
Two billion years of evolution is not something we can ignore and
during this two billion years, there is a distinct lack of evidence for
any "upward direction" to evolution. If alien scientists landed on
earth 2 billion years ago, and studied life, do you think they would
characterize evolution as you have?
Furthermore, even if we focus on multicellular organisms,
it's not clear to me in what way the present age represents
the results of change in "the upward direction, so that, over
time, the new organisms of greater and greater complexity"
appear when compared to the Jurassic and Cretaceous periods.
In other words, I don't dispute the internal logic of your
views. It just seems to me that this logic is only weakly
attached to the evidence in the world.
You also added:
>It's not accurate in one important respect: The bush branches in many
>directions from each point it has so far reached, so that, simpler forms of
>life, even if they are far from the stem, will sometimes "spin off" forms of
>life that are more complex (i.e., further upward than they are themselves)
>IF a niche opens directly above them and stays open for a long enough period
>of time. And, sometimes more complex forms will become simpler if an
>especially promising niche opens below them. What organism gets the niche
>depends on local conditions and how easy it is for organisms to "move" into
Ah, and here we come to the point I am interested in. During the 2 billion
year history of bacterial evolution, apparently, only once was the niche
towards increasing complexity inhabited and exploited (one could argue
this happened many times, but those lineages all went extinct, but this is
ad hoc hand-waving in its purest). This extreme rarity strongly suggests
the evolution towards complexity was highly unlikely. Or to use your
example, during these two billion years (just contemplate how long this
is), apparently any niches that would allow for increased complexity
did not exist and/or opened for too short a time and/or were too difficult
to move into. It's only that one lineage "got lucky." Put it this way.
If we replayed the tape of life, Gould likes to argue that humans would
not exist. But I think it goes further than this. If we replayed it, it
is likely the planet would *still* be inhabited solely by microbes.