Level Head (level_head) wrote in how_life_began,
Level Head
level_head
how_life_began

The Story of the Universe So Far

These things are well understood, and supported by many independent lines of evidence:

-- The universe's origin roughly 13.7 billion years ago (bya),
-- the solar system's condensation from a cloud of stellar dust and gases about 4.6 bya,
-- the Earth's cohesion into a planet about 30,000 years after the Sun lit up 4.55 bya,
-- the then-slightly-smaller Earth getting whacked by another planet, creating the present day Earth and Moon,
-- the formation of very primitive life less than a billion years later,
-- the "poisoning" of Earth's atmosphere with oxygen more than 2 bya
-- the formation of early multicellular life about a billion years after that,
-- that the world was covered in ice around 700 million years ago (mya),
-- the beginnings of "modern" groups of organisms once ice released its hold around 600-550 mya,
-- the colonization of the land first by plants, then arthropods, then finally vertebrates about 350 million years ago.

The material that makes up the present day continents has been accumulating for a while -- it is the "froth" on top of the tectonic plates, light enough not to get drawn back down inside the Earth. The continents have swirled and collided and broken up and recollided in new shapes about two dozen times that we know of. Currently, the Atlantic is getting wider as new lava is inserted at a "seam" that runs north-south down its middle. This seam passes through Iceland, for example, which gets about 2cm (nearly an inch) wider every year.

Since the Earth's magnetic field affects rocks that harden at the time, we can tell that the poles have "swapped" many times (about 29 cycles). And we can see the "stripes" of rock spreading on the sea floor with the different magnetic orientation, and know that the Atlantic was a continental split that has been widening for about 54 million years. Earlier, the Iapetus ocean was absorbed when the continents drew together -- that old boundary runs through Greenland -- and England, as the new split crosses the old join.

I'd be pleased to take a shot at any "how do we know?" questions on any of this -- I'm working from memory, and it will be good exercise to look up good source material.

===|==============/ Level Head
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the then-slightly-smaller Earth getting whacked by another planet, creating the present day Earth and Moon

Why is this theory preferred to, say, the moon being an asteroid that got caught in our orbit?
First, it is nearly impossible to "capture" another object in an orbit, without a course change. An asteroid that approached us, and missed, would leave on the other end of the same parabola. Mars has some help from Jupiter's large influence, but even so the orbits of its two little moons are unstable, and they will be gone again soon and haven't been there long.

But there is more evidence here than probability.

The Moon is relatively light, but formed from the same material and isotopes as the Earth. Almost as though the Moon and Earth are the cream and milk, respectively, from the same jug. This separation took place about 50 million years after the Earth's original formation.

Had the Moon formed at a different radious from the Sun, its isotope composition would be different -- and it would have a rocky core.

But what would cause such separation by weight of a bunch of material? An impact, early on, by a body the size or Mars or thereabouts -- this would melt both and "splash" the lighter stuff into space, where it would collect and condense into what we now call the Moon.

Here's a bit of info with an interesting painting:
http://starchild.gsfc.nasa.gov/docs/StarChild/questions/question38.html

===|=============/ Level Head
Compare the Moon with asteroid 2002 AA29.

Current example: the asteroid Cruithne.
Since the Earth's magnetic field affects rocks that harden at the time, we can tell that the poles have "swapped" many times (about 29 cycles).

What does that mean, and how does it happen?
While we're waiting for an eloquent explanation....

"Magnetic field reversal" := magnetic north and south change around. The compass points the wrong way, so to speak. I'm not sure what the usual way of thinking about this is. I think of it as the inevitable side effect of theoretically reducing the entire Earth's core to an LRC circuit [Inductance-resistance-capacitance].

The intermediate stages are also interesting. At one point (within the Brunhes chron, which is currently estimated as starting about 780,000 years/ten ice ages ago), magnetic north was near India. I don't recall off-hand whether that was the Blake event, the Iceland Basin event, or the Jamaica/Pringles Fall Excursion.

I also should dig up the site (PBS documentary) where a lava flow was cooling during this sort of thing. If you assumed it cooled as fast as flows do now, it recorded a change in bearing of magnetic north of about 10°/day.
Lost another post. Irritating.

Ah, well. A "chron", the term used by Zaimoni here, is a period of time in which the magnetic field is relatively stable. This is true whether it's in the direction we now have -- which we chauvenistically call "normal" -- or if it's "reversed". Chrons are identified by (usually) a letter indicated the geologic period, the cycle number, followed by the letter identifing Normal or Reverse for the magnetic field.

It's not "clean", though -- as there are narrow reversals within some established chrons, and smoe boundaries that are quite turbulent.

We count "chrons" backward from the present. One aspect of a magnetic field reversal NOW, is that it will not only be a major problem for life on Earth, but it will also mess up our counts. ];-)

Here's an article on chrons and their appearance in deep ocean drill cores -- probably of more interest to Zaimoni:
http://www-odp.tamu.edu/publications/171B_SR/chap_09/c9_2.htm

One interesting aspect is that we can track the effects of the Earth's orbital cyles on organisms that have settled in the sediment -- and thus track how long each "chron" was. They've been averaging about a million years or so for the major ones, but are all over the map individually.

Hmmm. The Cretaceous-Tertiary impact occurred about 65 million years ago, and there have been about 30 full cycles (of two chrons each) since then. (The K-T impact is within chron 29R.) Yes, a million years average is pretty safe.

===|==============/ Level Head
Long comment killed by LJ. Ah, let's try again:

Briefly: Metallic particles in molten rock act like compasses, aligning themselves with and indicating the direction of the magnetic field until the rock solidifies.

The magnetic North Pole moves slowly around (in an area about 10 degrees wide)-- but fast enough that aviation maps need to be updated from time to time.

But every once in a while, on irregular timescales from thousands to millions of years, the magnetic north and south poles swap.

The seabed, spreading from the center rift line in the Atlantic, has a consistent record of these changes. The current magnetic field is "stored" right at the rift, but older ones are captured in the rock a little further away, producing "stripes" of magnetic reversals across the floor of the Atlantic. We have to be careful to note the original orientation of the rock, and also to track how it might have been moved by geologic processes since it first formed, to get the clear picture.

Taking rock core samples also shows this layering effect -- there are successive "north" then "south" layers, then "north" again and so on. We've cross referenced and dated these by other tools, of which radioactive decay is a good one.

===|==============/ Level Head
Let's see...to be quite honest I'd like to know where all the numbers come from, but... I think I'll try looking up some of these things for myself (unless you really want to explain each one of them).

The oldest bacterial fossile has been dated to about 3.8 billion years ago, right? That would mean that your date of formation of very primitive life is a little bit inaccurate (or has too much leeway).
Well, there's some argument about that 3.8 billion year old evidence, but at minimum 3.5 bya is strongly supported. Also, simply because we were lucky enough to find something doesn't mean that it's the oldest that could exist.

So I was intentionally fuzzy -- the boundary seems broadly to be between 3.5 and perhaps 4 bya -- maybe even a little earlier depending on what you'd call "life".

Is a self-replicating simplistic short-protein peptide that predates even the RNA world "life"? It seems likely that such molecules are in our ancestry -- but not everyone would feel comfortable calling them "Grandma". ];-)

I'll try to find the reference I read recently on potential problems with the Aussie bacteriofossils.

===|==============/ Level Head
Is a self-replicating simplistic short-protein peptide that predates even the RNA world "life"?

Why do you think that a self-replicating peptide predates the RNA-world?
It's far from certain, but there is good experimental support for early short peptide chain formation.

I've got -- let's see ... 493 papers and abstracts collected over a few years, and I am remiss in not posting these to this group. It was the reason I created the community in the first place.

Some are quite readable, some are long-haired esoteric jargon-rich sentences that are not at all easy to wade through. I've been favoring the more accessible reading material.

I'll put up one now from the Dec 1998 Nature.

===|==============/ Level Head
On the conditions of that time frame:
A permanent crust did not develop until around 4 billion years ago.
The original crust consisted of lava flows that erupted on the surface
long before the ocean basins filled with water....

The metamorphosed marine sediments of the Isua Formation in a remote
mountainous region in southwest Greenland suggest the presence of a
saltwater ocean by at least 3.8 billion years ago. Older rocks are
found in Antarctica and Africa, but few date beyond 3.7 billion years.

This implies that much of the early crust was recycled into the
mantle. Therefore, only a fraction of today's continental mass had
formed by this time.

No significant landmasses had yet formed, and only thin slices of
crust wandered across the watery face of the Earth....

The early continents were highly mobile crustal fragments, constantly
colliding with one another.

Jon Erickson, _Making of the Earth: Geologic Forces that Shape Our
Planet_ (Facts on File, Inc., 2000), pp. 4-5.


And I've just posted here (next entry) a somewhat controversial idea on how life might have begun, even more than 4 billion years ago. Deep sea vents are hot topics. ];-)

===|==============/ Level Head