Tuesday, September 26, 2017

pre-Cambrian survival

I never knew  -  ‘till Kalamazoo

What did I not know until Kalamazoo?  There is a tremendous amount that I didn’t know, and still don’t.  But that is probably true for most of us.  What I am talking about is some of the events related to how we evolved over the past two billion years to become the dominant species on earth.  Our knowledge of evolution is riddled with gaps and errors, even though the general story of evolution by natural selection is probably close to the truth.  The last section tells of three important ways marine species may have been able to survive the intense extended period of pre-Cambrian asteroid bombardment.

Many that believe in God reject evolution because the sequence of chance and random events are not acceptable as work of a creator conforming to their concept of God.  Many scientists, especially biologists, have found it difficult to accept God as the creator because acceptance is not compatible with their concept of the chance and randomness involved in evolution.  Both may struggle with the chaos, disasters, evil, and other bad things they see in the world and want to blame God, or see it as evidence of God’s non-existence.

Stop for a moment, and think about something in your life that seemed bad, but in retrospect you see that it helped make you who you are.  There are numerous such occasions in the evolution of life that may have been essential for evolution to take its peculiar course over the last billion years it took to produce Homo sapiens.  If that doesn’t inspire you, read on, find out how important extinction events were in the course of evolution leading to us.

THE FIRST TEN BILLION YEARS

The “Big Bang” begs a question like the one asking “is there is a noise if a tree falls in the wilderness and there is no one there to hear it?”  Astrophysicists have an answer for the projected rapid expansion from the first relative speck to the slowing expansion, formation of elementary particles, elements, and the first generation of stars.  Condensation of materials into later generations of stars (after the extinction of some early ones) was responsible for the formation of heavier elements completing the materials needed for the evolution of life on planets with favorable conditions around stars like our sun.

We don’t need to know the details of the formation of our solar system.  It was probably similar to a billion other places of the expanding universe.  Whether the protoplanetary disks around the stars formed by condensation of more diffuse matter, or from emissions or explosions from the newly formed stars, sufficient matter ended up forming planets spinning and orbiting the sun.  Some planets had sufficient gravity to retain gases in their atmosphere and retain space debris striking them.  Planets tend to collect most of the debris as well as larger objects in their orbits.  The larger objects sometimes were captured in orbit as moons or impacted the planets with varying results.  The sun, with its massive gravity, probably collected many objects, even planets, that lost speed due to some collisions and fell into the sun.

Our moon does not have seas and atmosphere like the earth, so erosion has not obscured the craters produced by impacting meteors, comets, and asteroids.  In fact, the moon may have provided some protection to us from some that would otherwise have hit the earth.  Craters are much more numerous on the far side of the moon than they are on the lava fields of the near side.

Some worry more than necessary about the danger of an asteroid causing our extinction as part of a phenomenon much like he events terminating the dinosaurs.  It is remotely possible, but the solar systems planetary arrangement suggests that a much more stable system exists than the state several billion years ago.

Where would we be without asteroids?
 The irregular structure of some asteroids and craters or “pock marks” shows they had been stuck many times.  The presence of many moons and smaller satellites around the four largest planets, all beyond Mars, suggest that some were captured as moons and perhaps the collision with potential moons and or planets may have provided the fragments (asteroids) of the asteroid belt orbiting in space between the orbits of Mars and Jupiter.

Some of those fragments provided extinction-type events in decreasing frequency as they were swept up by earth, the moon, adjacent planets and their moons.  In the beginning impacting asteroids contributed mass to the earth.  They probably sped up some of the potential chemical evolution steps in the pre-biotic earth.  Perhaps their craters filled with water to make many experiments in addition to the intertidal pools where the chemical steps leading to organic life may have started. 

How life survived asteroids

The late pre-Cambrian is thought to have experienced a long period of more intense asteroid impacts.  The abyssal sea was so great a portion of the earth’s surface layer, and the cold polar seawater flowing toward the equatorial region had high density due to temperature and salinity that allowed it to gradually replace bottom water.  The process continued taking thousands of years to have bottom water reach the surface, just as happens today.  Animals able to adapt to the abyss had such an extensive area to inhabit it enabled some to survive even the worst asteroid hits.

A second method was the selection of cysts and other survival mechanisms used as overwintering, drought resistant, and dispersal stages that can emerge from somewhat suspended animation when conditions improve.  Such stages were already being selected by the rigors of surviving dry periods in freshwater temporary ponds as well as in saltwater pools along ocean shores.  Gemmules of sponges and statoblasts of bryozoans are cystlike asexual reproductive bodies, common among freshwater species but missing in marine species, suggesting such survival mechanisms are now less valuable for marine species in their more stable environment.


A third method is direct uptake of nutrients dissolved in seawater.  The mortality of marine species may well have boosted the dissolved organic matter content of seawater following an asteroid extinction event.  It may be an important method in today’s oceans for the survival of larval stages during dispersal, especially in species providing little yolk for nutrient reserves for the larvae.  Stephens, Grover C., and Robert A. Schinske  (1961, Limnology and Oceanography, 6:175-181.)  found that in ten phyla tested only arthropods did not take up amino acids from very dilute solutions in seawater.  Manahan and Crisp (1982, American Zoologist, 22:635-646) found dissolved amino acids could be taken up by bivalves from egg to adult stages.

The pogonophora benefitted by their living in the abyss, a relatively protected location of such great extent that remnants of the population might survive.  They also benefitted from the third method of direct uptake of dissolved nutrients in sediment water.  The "degenerate" appearance of the pogonophorans is part of an evolutionary step backward that laid the groundwork for the advance of animals to today's vertebrates.

Joseph G. Engemann   Kalamazoo, Michigan    September 26, 2017


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