Denys asks:
[Please note: In order for this response to make sense, you will have to take
the time to look at each of the referenced pages or images. For some things,
pictures are worth a thousand words.]
> It is my understanding that Charles Darwin is, if not the originator, he is
> at least the scientist that first formulated this theory in his book "The
> Origin of Species." The basis of the theory is that all current life forms
> on planet Earth have evolved from prior forms that in turn evolved from
> prior forms and so on until the very beginning of life in the primordial
> soup. I believe the current theory is that evolutionary changes occur due
> to environment changes and the natural selection of the more adapted forms
> in the continuing struggle for existence.
That's basically correct, but with a few caveats. The notion of all life
being connected is at least as old as the Greeks, and the idea of biological
evolution was very much in the air prior to Darwin. Darwin's great accomplishment
was that he provided, for the first time, a plausible mechanism, natural
selection, by which lineages could adapt in form and behavior over time.
Aristotle, biologist/philosopher, spoke of the "scala naturae", or "the great
chain of being", which you can see schematized in this figure:
http://web.clas.ufl.edu/users/rhatch/images/greatChain.gif
Clearly, Aristotle had some of the basic order correct: minerals lead to
life, which lead to plants, which leads to animals, which leads to man, and then
inevitably to demons, angels and god, but Darwin's idea was much simpler and
much more obvious. He specified an algorithm:
o given self-reproduction, all populations expand in Malthusian fashion
to the limit of their boundaries.
o error in reproduction is inevitable, thus there will be variation.
o all competitive arenas are finite.
o when the limits of those arenas are met, the competitive exclusion of
the least appropriate variants must occur due to an inevitable competition for
resources, leaving the most appropriately matched individuals to
differentially reproduce and become the basis set for the next generation.
This simple statement of three premises and one inescapable conclusion came
as a thunderclap to the 19th Century scientific community, only because it was
so obvious, yet no one had so clearly outlined it before.
Darwin also perfectly well understood the philosophical implications of his
work, and it was his intent to have "Origin" published after his death, if for
no other reason than to spare his wife, who was devoutly religious, the
travails of a long public argument.
But he also understood that the exquisite perfection we see in nature was
purchased on the back of enormous destruction, and that may have bothered him as
much anything. In the final paragraph of "Origin," he wrote to both concerns,
the destruction implicit in the process and the physics of the algorithm that
is represented by natural selection:
=======================================
"It is interesting to contemplate a tangled bank, clothed with many plants of
many kinds, with birds singing on the bushes, with various insects flitting
about, and with worms crawling through the damp earth, and to reflect that
these elaborately constructed forms, so different from each other, and dependent
upon each other in so complex a manner, have all been produced by laws acting
around us. These laws, taken in the largest sense, being Growth with
Reproduction; Inheritance which is almost implied by reproduction; Variability from the
indirect and direct action of the conditions of life and from use and disuse:
a Ratio of Increase so high as to lead to a Struggle for Life, and as a
consequence to Natural Selection, entailing Divergence of Character and the
Extinction of less-improved forms. Thus, from the war of nature, from famine and
death, the most exalted object which we are capable of conceiving, namely, the
production of the higher animals, directly follows. There is grandeur in this view
of life, with its several powers, having been originally breathed by the
Creator into a few forms or into one; and that, whilst this planet has gone
cycling on according to the fixed law of gravity, from so simple a beginning endless
forms most beautiful and most wonderful have been, and are being evolved."
========================================
Reading this passage, it's easy to see why the robber baron capitalists of
19th Century America took such pleasure in his writings. It was only a small
step from this passage to their justifications of the exploitations of the lesser
fit. But Nazi Germany used much the same rationales to justify racial purity,
as did Marx and Lenin to justify the proletariat's overthrow of their
oppressors. George Bernard Shaw ultimately said that Darwin had the great fortune to
be useful to anyone with an ax to grind.
But Darwin intended none of this.
The first major reinterpretation of this Darwinian biological physics was
Boltzmannian thermodynamics. Boltzmann wrote that he dreamed of the becoming the
"Darwin of Matter," and that the 19th Century should be named the "Century of
Darwin." The second major reinterpretation was Shannon's information theory.
The connection between the three ideas is not accidental. Each part of the link
was purposefully intended by each of its participants.
> I believe the age of the planet is currently pegged at about 4.5 billion
> years old. IIRC, for the first two billion years or so, Earth was barren
> without either an atmosphere or with an atmosphere radically different than
> what we have now. I think the current theory is for one large ocean with
an
> original super continent or archipelago, called Columbia, which broke apart
> about 2 bya. The landmass was later reassembled into another
supercontinent
> called Rodinia. Then that one broke apart. The landmass reformed into
> Pangea which then broke apart about 300 mya.
>
> During this breakup and assembly cycle, life appeared somewhere on the
> planet, perhaps in the oceans and one of the by-product of this life is
> oxygen, which explains how the atmosphere was transformed over time.
>
> This life, which somehow started in the "primordial soup", evolved over
time
> into something like 100 million or is it a billion, species over about 2
> billion years. Of this huge number, about 98% is now extinct. For
> example, the dinosaurs were by all accounts, a very successful and diverse
> group of species which existed for over 400 million years, before hurriedly
> disappearing about 65 mya. If I remember, there have been "great dyings"
> going back eons. It's almost as if on some type of schedule, the slate is
> wiped almost clean and a bunch (sorry for the technical term here,) of new
> critters emerges, and these new life forms do not seem to have much
relation
> with the prior occupants. There seems to be a rather large jump and then
> ordinary evolution runs its course.
Again this basically correct, but with a few caveats. I have never heard of
Columbia. Nor can we track the continents back much further in time than 750 Ma.
[Ma = megannum, or millions of years before the present. Because this usage
is still relatively new, other people write it as Mya (million years ago).
Similarly, Ga is equivalent to Bya and ka is equal to kya].
The person doing the best work in this area at the moment is Chris Scotese.
He works as an independent scientist, thus he earns his living by producing
educational materials. Nonetheless, the pages you will want to look at are a
series of maps that he has produced:
http://www.scotese.com/earth.htm
It takes a little while to go through each of the eras that the maps
represent, but it's well worth the time.
[Here, btw, is a picture of Chris taken at a meeting that I attended a couple
of years ago:
http://www.biogeography.org/Mesquite%20IBS%20Photo%20Tour/image6.html ]
The earth is 4560 Ma old, but the crust of the planet is in constant motion.
As a consequence, our capacity to say something meaningful increasingly erodes
with the passing of each 100 Ma. Anything said about the shape of the earth
much older than 750 Ma becomes more speculation than known fact.
Although we can be more certain of which land mass was next to which one at
which time, and which land mass was submerged or not at any epoch (that record
is reasonably clearly written in the rocks), our understanding of the
lattitudes at which the continents appeared at any one time also grows substantially
fuzzier the farther back in time we go.
Nonetheless there is another record that's also very clearly written in the
rock: the impact that life has had on the geochemistry of the earth. Molecular
oxygen (O2) simply did not exist on the primordial earth, nor does it exist
now in the atmospheres of any of the other planets. On the earliest earth,
molecular oxygen represented only 0.0000000000001 of the atmospheric population it
does today. In an equilibrial chemistry, a chemistry characteristic absent of
life, oxygen, because its reactivity, immediately disappears. But life changed
that chemistry, several times. The following timeline is from David Catling
(Univ. Washington, Seattle):
http://67.41.4.238/lectures/earlymars/em-catling/resources/em-catling018.jpg
The chart is divided into the three primary epochs: the Archaen, the
Proterozoic ("first life", although don't take that interpretation literally), and the
Phanerozoic ("visible life," life large enough to be seen by the human eye).
The Phanerozoic encompasses the Cambrian Explosion of 540-570 Ma, the time of
the origination of the various body plans used by plants, animals and fungi,
and itself is broken into three eons: Paleozoic ("old life"), Mesozoic ("middle
life") and Cenozoic ("recent life").
Superimpose, at least in your mind, Catling's chart with this one from David
Des Marais (NASA Ames):
http://67.41.4.238/lectures/aslcnm/atmar-030919/resources/atmar-030919015.jpg
This is the biological clock for the planet Earth. From it we derive one
extraordinary fact: the machinery of life formed on the planet almost as soon as
it possibly could. For the first few hundred million years, life would have
been impossible. The crust/mantle/core of the earth was still in the process of
fractional distillation. Simultaneous to that time the earth and the other
planets were sweeping the inner solar system free of the debris of the initial
protoplanetary disc that formed simultaneously with the origin of the sun.
Life-extincting impacts would have been common, if life did have the fortune to
arise then.
We only have this one example of a solar system, and we only have this one
example of life in the universe, and we have to make as much of these two
exemplars as we can, knowing full well that we could easily becoming to erroneous
conclusions. Prior to the discovery of the first extrasolar planets in 1995,
based on what we knew about our on solar system, we believed that the planets
nearest their host stars would inevitably be stony-iron planets, planets that
have had their volatiles blown away by a vigorous solar wind. Planets that were
within about 2 AU of their central star would be like earth, venus and mars.
Planets further out would be similar to our gas giants, Jupiter and Saturn.
It came as a great shock therefore to find that the first extrasolar planets
we discovered were very close-in "hot Jupiters", orbiting just off of the
surface of their host stars. No one expected this result, but due to observational
selective bias in the technology used, these were the easiest planets to
detect, and thus were the first to be found. We still can't detect planets as
small as the Earth, but we're moving in that direction. But we're also having
trouble in explaining the origin of the hot Jupiters.
We have anticipations of being just as surprised at the kind of life we will
discover elsewhere in the universe, once we find it, but for now, we only have
the kind of life that we know on this planet, and we base all of our best
guesses on what we know of it.
DNA-based life extends back at least as far 3.6 Ga, and may go to 3.8 Ga.
This is the point in time at which the machinery of life that inhabits this
planet was invented. It was only bacterial, but it was the same kind of life that
we are now.
Two great events occurred in the history of life on this planet, and you can
see those events clearly marked in the geochemistry of the planet in Catling's
diagram. The first was the evolution of the eukaryotes. Bacteria are simple
cells, composed of an outer cell wall and loosely stuffed with a fair amount of
DNA. By contrast, eukaryotes are complex and highly organized cells,
containing a distinct nucleus and organelles, such as mitochondria, cloroplasts, Golgi
bodies, etc. The eukaryotes apparently evolved through the symbiotic
aggregation of a number of types of bacteria, first being only facultative symbionts,
but then becoming obligate symbionts and eventually so well integrated into
the working of the protoeukaryotic cells that they became organelles. This
evolution took an additional 2 Ga, and we have no idea why it took so long.
The next great step required another 2 GA, occurring only 570 Ma ago, and
that was the evolution of task-partitioned, complex multicellularity. This
evolution was the "Cambrian Explosion" of animals, plants and fungi, and again, we
have no idea why it took so long.
Des Marais' biological clock outlines these steps very clearly, but the
diagram is a bit misleading. The sun and the earth will last about 12 Ga. So far
we're about 1/3 of the way through that span, but life itself won't last that
long. One of two hells await us: either a wet hell, where the oceans evaporate
into the atmosphere and we have a Venus-like planet, or the oceans evaporate
and are lost to space, and we become a barren planetary surface, much like the
moon or Mercury. At the moment, we don't know.
In either case, life on the planet is expected to last only 0.5 Ga more. In
about the same time we've gone from trilobite to astronaut, it appears that
it's all going to be over.
> What I would dearly like to see is a solid chain that shows the ancestry of
> man going back a lot further than apes. The step from apes to man is, in
my
> estimation very small, compare to the whole concept. All the talk on
> evolution to which I have ever been exposed, seems to focus on small
> changes, like skin color, or teeth or bone formation. What I want to
> understand is the progression from something say, a half inch long swirling
> blindly in a mud puddle to a human being able to build computers, airliners
> and spaceships.
In this, you're in luck. An extraordinarily solid chain has come into
existence in the last 20 years. The process is called phylogenetics ("the origin of
families"), and it's a mechanism that any computer person would understand in
five minutes. It is the biological equivalent of a versioning control system
for code development.
Phylogenetics is possible because we've learned to reliably and rapidly
sequence the genomes of the life on this planet, and because of that, we can very
rapidly sort all life on this planet and say based solely on the content of its
code what is most like which else, and what changes must have occurred to get
us from this species to that.
The following diagram is a supertree of the extant mammals, humans included:
http://67.41.4.238/lectures/evolution/sanderson-04ev/resources/sanderson-04ev0
02.jpg
Like all supertrees, you can't even read the names of the species on the
chart. The deepest common ancestor lies at the center, while the most recently
derived species sit near the outer ring.
The process is completely objective, and it will work with any kind of code.
You don't even need to know what the code does, only what has changed. You
could just as easily create a phylogenetic tree of all of the UNIX variants and
backtrack to a hypothetical common ancestor.
Phylogenetics hasn't provided any real surprises. Based on other, more
physical characters, people had pretty much worked out these relationships over the
last century with surprising accuracy, but it has allowed us to settle some
long-standing squabbles, such as whether the macrochiroptera ("large hand
wings", the large bats such as flying foxes) were derived from the rodents as a
second, convergently evolved flying mammal or were merely an offshoot of the
microchiroptera ("small hand wings"). Although good evidence existed for both
points of view, it now appears certain that it was the second.
Nor is the process limited in time. We can create phylogenetic trees of the
most ancient relationships, as you can see in this enormously simplified
diagram:
http://67.41.4.238/lectures/earlymars/em-mckay/resources/em-mckay023.gif
These trees work because all life on this planet is descended from a common
ancestor. What we are attempting to do in our search for life elsewhere in the
universe is to find something that isn't on this tree, a true second genesis
of life.
Wirt Atmar
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