Supplemental Lecture (97/03/31 update) by Stephen T. Abedon (abedon.1@osu.edu)

1. Chapter title: History of Earth

1. A list of vocabulary words is found toward the end of this document
2. Our solar system (our star) formed approximately 4.6 billion years ago. Most of the matter found in the Earth probably had been formed by stars and then supernovae by about 7 billion years ago. We consider the Earth to have been formed by accretion approximately 4.4 to 4.6 billion years ago. The Earth has gone through many changes since it was formed. In fact, the seeming constancy of our planet "is an illusion produced by the human experience of time" (Allègre et al., 1994). From the beginning, Earth has been shaped, often catastrophically, by the effects of heat and gravity. Much later, the emergence of life on our planet came to play a significant role in the shaping of Earth.
3. For the past few thousand years, life on Earth has lived in an environment of relative climatic constancy. Today, however, the consequences of human overpopulation, industrialization, and random natural events (e.g., volcanoes) as well as cyclical natural events (e.g., ice ages) threaten this constancy. Imagine what would happen if a single year of global agricultural output were lost (no food!). It's happened before, at least locally, throughout the historical record. It will happen again.
4. The Earth is not the stable, life sustaining platform, nor human civilization nearly as robust as we like to think. Nevertheless, life in some form has managed to survive on this planet for the past 4.2 or so billion years.
5. Regardless, in about 15 billion years (i.e., a universe approximately twice its present age) the formation of sun-like stars and therefore Earth-like planets will be comparatively rare. Long before then (billions of years from now) our sun, in the course of normal stellar evolution, will have burned Earth to a cinder along with all the life upon it.
6. In this lecture we will consider certain properties of early earth that contrast with those of today and which allowed life as we know it to orginate and evolve.

2. Accretion [formation of planets]

1. Outer reaches of rotating disk:

i. During the gravitational collapse of clouds consisting of hydrogen and higher elements upon star formation (i.e., solar system formation), especially non-hydrogen elements can be found in the middle and outer reaches of the resulting rotating disk.

2. Dust seeds accretion:

i. Just as inhomogeneities in hydrogen clouds led to galaxy formation in the early universe, clumps of non-hydrogen matter (dust and larger) served (and serves) as gravity wells that increased in size as a consequence of accretion of similar clumps.

ii. That is, the high and low velocity "slamming" together of bits and pieces of matter which themselves reached their current collective mass as a consequence of previous collisions between smaller clumps.

iii. This concept of planetary development by accretion was first posited by Otto Schmidt in 1944 and came into favor following Apollo studies of the moon in the late 1960s, early 1970s (a plug for space/planetary science).

iv. "According to Schmidt, cosmic dust lumped together to form particles, particles became gravel, gravel became small balls, then big balls, then tiny planets, or planetesimals, and, finally, dust became the size of the moon. As the planetesimals became larger, their numbers decreased. Consequently, the number of collisions between planetesimals, or meteorites, decreased. Fewer items available for accretion meant that it took a long time to build up a large planet. (One) calculation . . . suggests that about 100 million years would pass between the formation of an object measuring 10 kilometers in diameter and an object the size of the Earth." (Allègre et al., 1994)

3. “A few million years after a star’s birth, the tiny particles of dust encircling it rapidly coalesce into larger bodies and eventually into a handful of full-blown planets. After a few hundred million years, the remaining debris crashes into the planets or is flung out of the system, ultimately leaving a relatively clean and dust-free system like our own… Imagine a vast intracellular cloud of molecular hydrogen and helium, a mere 10 degrees above absolute zero, more dilute than the best laboratory vacuum, but almost completely opaque because of a smattering of dust particles. This is a run-of-the-mill stellar nursery. Slow turbulence and magnetic pressure in the cloud tend to resist the force of gravity, but every now and then, in some particularly dense part of the cloud, gravity wins and a collapsing core of gas and dust heats up, signaling the imminent birth of a new star… Computer simulations suggest that within a few million years, the dust particles have already accreted through molecular forces into pebble-sized bodies. It takes another few million years for these dusty or icy golf balls to accrete into kilometer-sized bodies called planetesimals, which later collided to form planets…” Govert Schilling (1999, From a swirl of dust, a planet is born. Science 286:66-68)

3. Planet

1. Product of accretion:

i. The ultimate product of accretion is a planet.

ii. A planet, essentially, is a body orbiting a star which has swept up the majority of clumps of matter (meteors, asteroids, comets, dust, etc.) that overlap its orbit.

2. Stable gravity well/high energy potential:

i. A planet is a gravity well which can serve as a relatively stable outpost for the collection of fusion generated energy radiated from stars. That is, planets serve as stable platforms which are in physical contact with the high potential energy given off as photons (light) by local stars.

ii. As far as we know, life can exist only given the at least periodic contact with high energy potentials.

3. Hot genesis:

i. Because planets are formed by accretion, they begin as hot places (heated by the friction of infalling matter slamming together as well as by radioactive decay).

ii. By hot I mean temperatures at which rocks liquefy.

4. “In the earliest days of the nascent solar system, when dozens of Mars-sized protoplanets roamed the inner solar system and met in catastrophic collisions, tiny variations in trajectory made all the difference. These variations, as subtle and unpredicatable as the factors that control a roulette ball, ultimately determined the orbits of the four planets [Mercury, Venus, Earth, and Mars], how big they grew, and perhaps even what they were made of…” Richard A. Kerr (1999, Making new worlds with a throw of the dice. Science 286:68-69)

4. Illustration, Earth's crust

5. Atmosphere formation

1. The atmosphere represents a gaseous layer overlapping the Earth's crust and extending many kilometers up into space. The atmosphere is thus even less substantial than, for example, Earth's crust (see illustration above).
2. gravity retains atmosphere:

i. Because of the effects of gravity, the Earth's atmosphere is mostly retained (i.e., does not diffuse out into outer space thereby escaping from the Earth). Very light gasses (for example, hydrogen gas, H₂) when present, however, can and do escape into outer space, thus greatly depleting atmospheric and crustal stores.

ii. On planets less massive or dense than earth, atmospheric loss can be a bigger problem. For more massive or dense planets, atmospheric loss can be less of a problem.

3. The Earth's atmosphere was created by the offgassing of the Earth's mantle, 80-85% of this offgassing is thought to have occurred in the first million or so years of the Earth's history.

6. Early (reducing) atmosphere

1. The early atmosphere was likely dominated by carbon dioxide.
2. Other gasses:

i. Nitrogen was present in more than just trace amounts as was water vapor.

ii. Trace gasses included methane, ammonia, sulfur dioxide, and hydrochloric acid.

3. Little molecular oxygen was present (i.e., approaching 0%).
4. It is within this early reducing atmosphere that life first evolved.

7. Molecular oxygen [oxidizing atmosphere]

1. The presence of atmospheric molecular oxygen is unstable due to oxygen's propensity to react with (oxidize) all manor of elements as well as various more or less reduced molecules and compounds. For example, carbon dioxide is the fully oxidized form of carbon.
2. Given the absence of oxygen, the early Earth's atmosphere was said to be reducing (the opposite of oxidizing).
3. Oxidation-reduction:

i. By defintion, reduction is the donation of electrons to something (i.e., the recipient of the electrons is reduced) while oxidation is the "stealing" of electrons from something (i.e., the thing from which the electrons have been "stolen" is said to have been oxidized).

ii. Oxidation-reduction reactions always come in pairs (that is, if something has been reduced, something else must have been oxidized.

iii. An understanding of oxidation-reduction is crucial to understanding life and we will come back to these concepts repeatedly.

4. Poison:

i. In fact, it is highly likely that life could not have (cannot) originate and evolve de novo in an oxidizing atmosphere because the presence of molecular oxygen would be expected to destroy (oxidize) the many reduced organic compounds used by and making up life.

ii. Consistently, this problem persists today, with a large fraction of biomolecules (and other reduced carbon compounds) highly susceptible to oxidation and, therefore, to damage upon extended exposure to oxygen.

5. Photosynthesis:

i. Where did the Earth's molecular oxygen come from? Or, why doesn't the Earth retain a reducing atmosphere? The answers are water and photosynthesis.

ii. The former is the direct molecular source of free oxygen. The latter is the process by which water is converted, i.e., oxydized, into molecular oxygen (and by which carbon dioxide is subsequently reduced to form carbohydrate).

6. Oxygen sinks:

i. It is important to note that the invention of photosynthesis did not lead to a rapid increase in the molecular oxygen content of the atmosphere. Why not? For the same reason free, molecular oxygen was absent before the invention of photosynthesis: the presence of vast reserves of reduced elements in the Earth's crust served as molecular oxygen sinks. It wasn't until all of these reduced elements had been fully oxidized that molecular oxygen would stably exist in the Earth's atmosphere.

ii. Thus, the development (i.e., evolution) of necessary defenses by organisms against the poisoness consequences of prolonged exposure to molecular oxygen could have been a gradual process.

7. Not until approximately 2 billion years ago were significant quantities of oxygen present in the atmosphere (i.e., no longer reducing) and by 1.5 billion years ago the Earth's atmosphere was as oxidized as is today's (i.e., about 20%).
8. Today's aerobic organisms (more broadly including aerotolerant organisms, ones which can live in the presence of molecular oxygen) are descendants of organisms which long ago developed defense mechanisms (mostly molecular) against molecular oxygen.
9. Living in an oxidizing atmosphere remains a significant problem and, mechanistically at least, leads to all sorts of troubling consequences such as human aging (the choice is yours, die now for lack of oxygen or die later following a lifetime of breathing the stuff).

8. Oceans

1. The water making up Earth's oceans was liberated during the mantle offgassing which also created the Earth's atmosphere.
2. Most of Earth's hydrogen is locked into water molecules and thus is not free to escape to outer space. Water dissociation to molecular oxygen and hydrogen can and does occur. In a reducing atmosphere, loss of molecular hydrogen (and thus potential water) to outer space is a very real problem. This is of greater concern on small, less dense planets, and of lesser concern on large, more dense planets.
3. Paradoxically, perhaps, the existence of an oxidizing atmosphere protects Earth's hydrogen and therefor water reserves. That is, the existence of an oxidizing atmosphere on today's Earth prevents the inevitable loss of water, and thus protects the existence of the world's oceans, by rapidly reacting with liberated molecular hydrogen, converting it back to water.

9. Ozone layer

1. Ozone is O₃:

i. Another consequence of Earth developing an atmosphere consisting of significant quantities of molecular oxygen was the development of the Earth's ozone layer. Ozone is a ringed, trimeric form of oxygen that absorbs ultraviolet light (emitted by the sun) at the wavelengths harmful to DNA-based life forms

ii. Ultraviolet light also leads to the creation of ozone: O₂ + ultraviolet light --- O₃.

iii. Note: ozone is not a ringed molecule but instead a bent trimer with one double bond, one single bond, and resonance between the two bonds.

2. On the early Earth a significant layer of ocean water between organisms and the sun may have been necessary to effect protection from ultraviolet radiation.
3. Life on land:

i. Today 99% of the sun's ultraviolet radiation is absorbed by atmospheric ozone and, consequently, this ultraviolet radiation fails to reach Earth's surface. Thus, an oxidizing atmosphere with its consequent ozone layer and ultraviolet radiation protection was likely important to life's invasion of shallower waters and, especially, the land.

ii. (Press here for additional discussion of the sun and ultraviolet radiation, particularly as it applies to the human integumentary system.)

10. Crust

1. A thin layer (kilometers thick) of solid rock, called crust, covers the surface of our planet.
2. The formation of crust represents a sufficient cooling of the early Earth that surface rock solidification could occur.
3. It is possible that crust formation and melting occurred repeatedly prior to the formation of the ancestral crust of that upon which we live today. If so, the melting of crust was a consequence of the heat generated by accretion. It is certain that the global melting of crust would lead to the extinction of life. It is thus considered a possibility that life arose multiple times on planet Earth only to face repeated total extinctions before the ancestors arose of the crust and life that we have today. This would place the formation of the ancestors of life on Earth beginning subsequent to the last round of major (global crust melting) accretion, approximately 4.2 billion years ago.

11. Continental (and oceanic) crust

1. Also known as dry land. Continents have two characteristics that differentiate them from the land (crust) beneath oceans. One, they are lighter than oceanic crust (hence they rise to greater heights and are thus above sea level). Two, they are older than oceanic crust (this is because oceanic crust is repeatedly melted and reformed over time at their leading, subducting, and trailing edges, respectively).
2. The relative permanence of continental crust allows the retention of both rocks as well as rocks containing fossils. It is thus upon the convenience of dry land that a great deal of geology and paleontology is done. From this geological and paleontological work (as well as that of planetary science), an understanding of the development of the early Earth into the one existing today has been elucidated.

12. Plate tectonics

1. So long as we are mentioning continents (above), it makes sense to, as an aside, describe plate tectonics, a phenomenon that has played a key role in both the shaping of our planet's crust and in the evolution of life.
2. The theory of plate tectonics describes a process whereby continents literally act as rafts floating on a sea of molten rock of the Earth's mantle. Thus, continents move about on the Earth's surface relatively intact. They slam into and run over heavier oceanic crust on their leading edge. On their trailing edge they leave rifts which are filled with molten rock. Where two continents meet, mountains are raised.

13. Vocabulary

1. Accretion
2. Atmosphere formation
3. Earth’s crust, illustration
4. Molecular oxygen
5. Ocean formation
6. Ocean survival
7. Oxidizing atmosphere
8. Ozone layer
9. Planet
10. Reducing atmosphere

14. Practice questions

1. Which is the correct temporal order? (circle best answer) [PEEK]

i. accretion, inflation, supernova, condensation of matter

ii. fusion, supernova, accretion, singularity

iii. inflation, existence of gravity, formation of hydrogen, accretion

iv. life, inflation, supernova, fusion

2. True or False, the evolution of early life on earth was possible only because of the existence of an atmosphere dominated by molecules that were strong electron acceptors (circle correct answer). [PEEK]
3. The existence of the ozone layer is crucial particularly to [PEEK]

i. the survival of bottom dwelling ocean life

ii. the existence of liquid water on earth

iii. the colonization of land

iv. smog

v. all of the above

vi. none of the above

4. To the nearest billion, how long did the earth exist prior to the existence of an atmosphere in which molecular oxygen was plentiful? [PEEK]

i. 0

ii. 1

iii. 2

iv. 3

v. 4

vi. more than 4

5. Why couldn't early life have originated and evolved if an oxidizing atmosphere had existed on early Earth? [PEEK]
6. Today many scientists consider the origin of life given a reducing atmosphere and liquid water to be almost inevitable. Which of the following would you expect not to have been helpful to the survival of the very earliest organisms? (circle best answer) [PEEK]

i. lack of competition from modern organisms

ii. ozone blocking of harmful ultraviolet light

iii. a declining rate of accretion

iv. abundant resources

v. none of the above

vi. all of the above

7. Which is the correct temporal order? (circle best answer) [PEEK]

i. accretion, inflation, supernova, condensation of matter

ii. fusion, supernova, accretion, singularity

iii. inflation, existence of gravity, formation of hydrogen, accretion

iv. life, inflation, supernova, fusion

8. The atmosphere of early earth is thought to have been a "reducing atmosphere" specifically because there was an absence of oxygen. An absence of oxygen, nevertheless, is not a sufficient criteria to label an atmosphere reducing. However, if it could be proven that the early atmosphere contained all of the following, the presence of which one would convince you that the early atmosphere was indeed reducing? [PEEK]

i. water vapor

ii. molecular nitrogen (N₂)

iii. molecular hydrogen (H₂)

iv. carbon dioxide

9. One problem of living in an atmosphere dominated by oxygen is that by-products of the utilization of oxygen can lead to deadly DNA lesions. Other than than the utilization of oxygen during metabolism, name a positive consequence of living in such an atmosphere. [PEEK]
10. Life originated on this planet in an atmosphere consisting of __________ molecular oxygen. (circle one correct answer) [PEEK]

i. 40%

ii. 20%

iii. 10%

iv. 5%

v. all of the above (could be correct answers).

vi. none of the above (are correct answers).

11. Molecular oxygen _________. (circle one correct answer) [PEEK]

i. is, under some circumstances, harmful to life.

ii. is, under some circumstances, helpful to life.

iii. plays a role in the "survival" of water on planet Earth.

iv. is a waste product of photosynthesis.

v. all of the above (are correct).

vi. none of the above (are correct).

12. True or false, the world in which life evolved contained large numbers of compounds which possessed easily donated electrons. (circle one correct answer) [PEEK]

15. Practice question answers

1. iii, inflation, existence of gravity, formation of hydrogen, accretion
2. False, the opposite is true (absence of strong electron acceptors such as molecular oxygen).
3. iii, the colonization of land
4. iii, 2 but iv, 3 is also a reasonable enough answer. That is, 4.5 billion (approximate age of the earth) minus 2 or 1.5 (time before the present that the oxygen began to become plentiful and concentration of oxygen in our atmosphere was approximately equal to what it is today) equals 2.5 or 3 billion years (that the earth was in existence before molecular oxygen was plentiful).
5. Because the bulk of molecules making up and used by life would be destroyed (i.e., oxidized), or never formed in the first place, given an oxidizing atmosphere.
6. ii, ozone blocking of harmful ultraviolet light
7. iii, inflation, existence of gravity, formation of hydrogen, accretion
8. iii, molecular hydrogen
9. ozone layer blocking out UV, prevention of escape of hydrogen to space.
10. vi, none of the above
11. v, all of the above
12. True

16. References

1. Allègre, C.J., Schneider, S.H. (1994). The evolution of the Earth. Scientific American October:66-75.
2. Postlethwait, J.H. and Hopson, J.L. (1995). The Nature of Life. Third Edition. McGraw-Hill, Inc. pp. 908-909.