Since it is our home, we all know a great deal about Earth. Photos of the "blue" or "water" planet have appeared many times on TV and elsewhere in this Tutorial. So we are familiar with its appearance close-up as seen by a meteorological geostationary satellite.
But here is a full face view of Earth as taken from just above the Moon by the Clementine satellite:
As seen through a telescope from a distance of about 140 million kilometers, the Earth and its Moon would look something like a double star. Such a view was actually obtained by the Messenger satellite when it was on the other side of the Sun relative to Earth, so that it spots the terrestrial body because of reflected solar light, as seen here:
When seen in true color from the Voyager spacecraft near Mars, the Earth appears as a blue dot (tone largely from the oceans) of moderate brightness:
Seen from a distance by a more powerful telescope on the Cassini spacecraft as it orbits around Saturn, the best image of Earth from the outer Solar System once more shows it to resemble a star (the Moon was behind Earth at the time), with almost no details discernible - this gives an insight into how difficult it is to study the solar planets from earth-based telescopes and thus justifies why we have sent unmanned probes to all the planets to obtain close-up views:
But the Cassini image was taken through its telescope and is thus magnified. How would Earth appear to the naked eye from, say, Mars. Here is the first such unmagnified image made by the Mars rover Spirit. Our planet is nearly invisible in the twilight night glow that pervades space from the Sun outward , but the bright dot - Earth - has been enlarged in the inset. Under these conditions Earth is as small as Mars appears to us as we look up from our planet.
Earth is the largest of the four inner rocky planets. It almost certainly began to organize in the earliest days of the Solar System, along with its sister planets, even as the Sun itself came into being as a ball of hydrogen-helium gas mixed with heavier elements. Some of the gas and much solids - mostly dust size - remained outside the central region of the gas-dust "cloud" that comprised the protostar system that evolved into the present day Solar System. The best estimate of when this all began, based on meteorite age data (in which the primitive meteorites are assumed to record the accumulation of the dust within the local cloud that gathered into small objects), is between 4.55 and 4.6 billion years ago. The Earth started to organize soon thereafter.
A momentary digression to mention what will be covered in detail near the end of Section 20 - the conditions that led to the origin of the Solar System and the Sun. The overview in this paragraph is germane to the next few paragraphs: Present thinking considers the System to have formed from an organizing event that affected a cloud of hydrogen and dust. That event may have been a supernova explosion of a nearby star. This would account for the presence of considerable amounts of the heavier elements. The explosion could have intiated some compression of the cloud such that gravity began to bring matter together, with the Sun being the "sink" into which the dominant hydrogen moved, along with some of the heavier elements. Rotation commenced as the cloud organized into the Sun and solid and gaseous matter beyond. Clumps of matter in the extrasolar segments of the rotating cloud began to collide and build up into a few larger protoplanets. In time, much of the initial material in the space beyond the Sun that was influenced by the Sun's gravity as it built up was swept up to produce the present collection of planets, asteroids, comets, and interplanetary gas and dust.
This illustration follows one of several similar models that describe these formative phases, as applied both to Earth and to the planetary system as a whole:
The upper left panel shows the local gas-dust cloud as initially cold, but with some heating as it contracts (upper right). The middle left panel suggests that the cloud has now contracted into a protoplanetary disk, within which light H and He gases are drawn mainly to its center, where this material contracts rapidly into the early Sun (middle right). The bottom two panels indicate that material in the disk has separated into rings of denser materials which begin to heat up as their constituents are swept up into ever larger bodies that collect enough solids and gases to build into individual planets that gravitationally rework the hot solids (probably with partial to nearly complete melting) into spheres.
An actual example of a star-forming gas/dust cloud is shown in the Hubble Space Telescope image of a nearby nebula:
Beyond the forming Sun, solids start out mainly as dust-sized particles that collide and accrete into larger (meters range) bodies. Many of those further interact to grow into bodies in the kilometers range, which are called "planetesimals", some of which have been sampled as meteorites. The asteroids are an example.
Collisions persist, with some bodies growing larger than most others, until they reach sizes similar to present day planets or planetary cores. Thus, the bigger bodies use their growing gravitational force to attract smaller bodies that collide and accrete onto the increasingly larger bodies. The Sun, meanwhile, is organizing into a gas ball in which contraction raises internal temperatures to greater than 1,000,000 ░ C , to the extent that nuclear fusion commences. The early Sun had less energy output than it does today. Its surface temperatures then were less that at present. The fusion produces a steady solar wind that drives particles outward. Most of the original H and He retained at first around the inner planets is pulled off into space, in part because the lower gravity of these bodies cannot hold on to these low atomic weight gases. The outer planets, being further away, experience less solar wind and retain large amounts of H and He, giving them more mass and stronger gravitational pull that maintains their thick gas envelopes (including a mix with gases having other compositions).
All of this probably took less than 100 million years to accomplish, meaning that the planets and Sun are approximately contemporaneous. In the Solar System spatial realm, beyond the planets, a large quantity of gas and solids remained. Meteoroids developed from these solids, some remaining at a range of planetesimal sizes (these thus are primitive, carbon-rich and water-bearing, making up the carbonaceous chondrite type of meteorite falling on Earth) with others enlarging through collisions to 100s of kilometers. Examples of these planetesimals are still mostly in the Kuiper Belt beyond Pluto. While this was going on, much of the dust experienced some degree of melting (from solar energy bursts, electrical discharge, shock waves, radioactivity[?]) that produced small spheres of Fe-Mg silicates (chondrules; see previous page 19-2) that after being dispersed are recombined into newer planetesimals.
A few planetesimals appear to have reached sizes that led to complete melting and differentiation (by gravitational settling) into silicate bodies up to planet size, most likely with iron-nickel cores. Today interplanetary space is occupied by survivors of this early aggregation history as asteroids, comets (solid silicates mixed with frozen water and/or carbon dioxide ice [the so-called dirty "snowballs"]), and smaller meteoroids (many are broken pieces of small protoplanets).
Most of the bigger bodies underwent general (usually total) melting. The Earth is believed to have experienced almost complete melting in its early days. There are several sources of the heat needed to raise these bodies to temperatures in excess of 1000░ C. Heat from collisions (impact) during accretion can be large. Heat of compaction may be involved. But the main source is the heat released during radioactive element decay. Uranium and radioactive Potassium were important. In the earlier history of Earth, radioactive Al26, with a short half-life but in abundance then, probably was a significant contributor.
The larger bodies once molten adopt near-spherical shapes (the result of equal pull in all directions from internal gravity). The general arrangement of planet distribution in the Solar System - four "small" inner planets (the Rocky Group Mercury, Venus, Earth and Mars, the latter three with thin atmospheres) and four "giant" outer planets (the Gas Ball Group Jupiter, Saturn, Uranus, and Neptune with thick atmospheres)- may be unusual (in the light of recent observations of planetary systems around other stars). These planets have survived now for nearly 4.6 billion years. The giant planets influence the rocky planets in two vital ways. First, their gravitational pull counterbalances that of the Sun, thus helping to maintain the inner planet orbits. Second, the giants' large masses serve to preferentially attract small bodies such as asteroids and comets, pulling them into the gaseous envelopes around Jupiter, Saturn, Uranus, and Neptune, thus reducing the number that would otherwise hit the inner planets; early on they also attract gases in large quantities that formed the bulk of their volumes.
The position of Earth in the Solar System lineup seems fortuitously favored for its unique plethora of living creatures. It is far enough from the Sun to avoid having much of its atmosphere blown away by the solar wind. It is large enough to have retained that atmosphere (which continued to grow during terrestrial degassing and has evolved over time to support life). Its atmospheric-affected temperatures are conducive to life's origins and survival. And its surficial temperature conditions have permitted liquid water to be retained in large quantities, even in the early days of Earth's history.
Many of the meteorites that fall on Earth come from the Asteroid Belt between Mars and Jupiter. Some planetologists consider this belt to be the remnants of a fully formed planet (which on melting provided an iron core and iron-stony inner mantle that we observe in about 10% of the meteorite falls and finds). Others hold that this belt never organized into a larger planet because of the disrupting influence of Jupiter's gravity field.
More information on the above topics can be found on pages 20-11 and 20-12.
Let's zero in hereafter just on the Earth. It built up rapidly to a fraction of its present size. As it grew, it became hotter, owing to three sources of heat mentioned above. There is reason to believe that this proto-Earth underwent complete melting in its first 50 million years, with heavier iron and nickel, and smaller amounts of other heavy elements, sinking centripetally to form the Fe-Ni core, and the bulk of materials that are dominantly iron-magnesium silicates making up the outer Earth in what remains today as the Mantle. Various planetologists argue about the "completeness" of this melting - some restricting it to just the outer layers, forming the so-called "magma ocean", others accepting general melting. The indication that the core is almost all metallic iron + nickel, a deduction based in part on the nickel-iron meteorites that presumably were involved in another, now disrupted planet, would be most easily explained by complete melting. The next illustration is a conception of the early Earth surface as a partial magma ocean with "islands" of cooler solid bodies
It is reasonable to assume, in this model, that the magma oceans cooled sufficiently to form a thin solid crust of compositions similar to today's basic igneous rocks (principally basalts at and near the surface and gabbros and ultrabasics [e.g., pyroxenites] at greater depths). Parts of such a crust floundered, much like the crusts on the lake in Halemaumau Crater at the summit of today's Hawaii's Kilauea Iki volcano.
In the first billion years of Earth history (often called the Hadean Eon), the Earth's surface as it cooled continued to be bombarded by comets, asteroids, and meteoritic material at rates much greater than in more recent times thereafter. Here, again, is an artist's view of this process.
Another, less plausible variant of this model has both the Earth and Planet X mutually breaking each other apart; most planetologists now think in terms of the Earth remaining intact. Some of the incoming body's material was added to the Earth (which may have been smaller then but thus grew to its present size by collisional additions). The Earth may have largely remelted and resumed core formation. Much of the ejected material circled the Earth close in (initially the Moon was about a tenth its present distance from Earth, but it has gradually receded outward owing to tidal friction losses that lessened gravitational interactions). This orbiting material rapidly organized into terrestrially-derived planetesimals - these coalesced by accretion until the just-born Moon itself melted and organized into a spherical body. Chemical similarities (including diagnostic isotopic compositions) establish this Earth-Moon kinship and are consistent with an impact mode of origin. The Moon, as it melted completely to form a very small, now solid iron core, differentiated, and formed a feldspar-rich crust (the anorthositic Highlands discussed later in this Section).
One vital consequence attributed to this collision was that the Earth's rotation axis was tilted 23░: from the orbital ecliptic by the process. This gave rise to the variations of illumination of different parts of the planet during an Earth year. During the early Hadean the Earth day was just 6 hours long because of more rapid rotation; tidal interactions with the Moon have been slowing the rotation progressively, so that earth days are getting longer.
The early Earth melted during its first 100 million years, setting into motion the process called differentiation. This process spatially separates elements according to their atomic weights and chemical affinity. During this the iron (and nickel) collected in a large interior core, above which is the mantle consisting of silicates rich in iron and magnesium (the mafics), and in the Earth's outer exterior a crust of aluminum-rich silicates (the salics or felsics). These next two illustrations describe the structure of the Earth (as it is now but was largely developed in the first billion years) and the composition in terms of total Earth and outer crust.
Another major event in the first billion Hadean years was what is known as the "Second Bombardment", in which a notable fraction of larger objects near the Earth were involved in frequent impacts onto the planet and its satellite. The evidence for this is inferential: Study of lunar chronology has demonstrated a heavy bombardment about 3.9 billion years ago. The Earth, a much larger target, must also have experienced this bombardment but nearly all rocks that would have recorded these events have been destroyed.
A very important part of the story of early Earth involves the transition from solid crust over the entire globe to a stage in which the Earth became the "Water Planet". Evidence is mounting that this may have begun as soon as 150 to 200 million years after Earth's first formation. The extent of the water over the surface may have been regional or even more widespread. The source of the water was to some extent from the interior through venting from volcanoes and lava outpourings. Many planetologists believe that at least some (the proportion still debatable) water was added from comets, asteroids, and meteoroids, many of which contain significant but variable amounts of bound or free H2O. The critical condition behind water's accumulation is that the surface and atmospheric temperatures were between 0 and 100 degrees Centigrade. The primary consequences of this water buildup and its activity through rainfall and flow are that sedimentary rocks could form and, if other factors were operative, primitive organic molecules (possibly including some "life" at the bacterial level) might have originated and survived. The presence of this early water has been hypothesized based on the discovery of ancient rocks contain the mineral Zircon (see below).
Let us delve deeper into the history of Earth' first billion years, using as a guide this diagram.
Since we conventionally decipher terrestrial history using rocks and fossils, this would seem the "route to go" in describing this first Hadean Eon (which is followed by the beginning of Archean time about 3.8 billion years ago, an Eon that lasted until 2.5 b.y. ago, when the Proterozoic Eon [about 1.9 billion years in duration] was initiated). Unfortunately, Hadean rocks of any kind are extremely rare (Australia; Greenland) and discrete fossils from that Eon have yet to be found, although indirect evidence for simple microbes exists. Much of what is statable about this Eon is speculative.
If there was a magma ocean, the crust would have started to form from outlier "rafts" that eventually were enclosed by the first thin solid crust that survived remelting. From knowledge of other terrestrial planets, that crust was almost certainly basalt-rich (a more general term is simatic, which refers to igneous rocks low in silica and high in iron, magnesium, and calcium) in composition (the idea of a thin floated feldspar anorthositic analog to the Moon has been discounted by some planetologists [but remains an alternative]). As the crust thickened, parts of it also were remelted repeatedly by large impacts and probably also by internal thermal convection currents from the mantle. There is reason to believe that a simatic subcrust exists worldwide today within the lithosphere, as a modified survivor of the protocrust.
From knowledge of differentiation mechanisms occurring in the younger Earth, it seems plausible that here and there sialic (high in silica, aluminum, and sodium) crust formed regionally. This crust could have compositions described as rhyolitic, felsitic, andesitic. Conceivably cooling conditions might even have formed granites, which are a common host of the accessory mineral Zircon (see below). Another way to form sialic rocks is by metamorphism of sediments that have been enriched in Si, Al, Na. Regardless of mechanism, the end result was to develop clots of silica-enriched crust that rose above the general crustal elevations - these would form nuclei (analogous to the term craton in the Earth's present geology) that grew mainly by accretion to their boundaries (perhaps by obduction or terrane addition (see latter half of Section 17), especially if plate tectonics mechanisms developed early in Earth history. Remnants of these nuclei are probably buried within rocks making up the continental shields.
Evidence favoring some early non-mafic crustal rocks is found in the discovery of the mineral Zircon (a gemstone), ZrSiO4, at various localities, mainly Australia, where non-igneous rocks have been found to contain this mineral as an accessory component brought in from weathered older crystalline rocks. Zircon only rarely forms in mafic rocks, and certainly not directly in any sedimentary type, but is a common indicator of silicic igneous rocks, principally Granite. The oldest individual zircon crystal dated so far (Uranium/Lead decay data obtained using an ion microprobe) is from younger rocks, including conglomerates deposited between 3.3 and 3.7 billion years ago, found in the Jack Hills in Western Australia. This narrow belt of folded and old Precambrian metasedimentary rocks looks like this from space:
This minute zircon's age is ~4.4 billion years, making it the oldest known solid material derived from terrestrial rocks of very ancient ages. If so, this and other zircons of somewhat younger ages presumably formed in sialic igneous rocks, probably granite, that were then weathered, released their zircon crystals, which survived and was incorporated in sedimentary rocks of varying ages much later (the youngest only 2 billion years old). The Australian zircons vary in age, with many being 4.0 to 4.3 billion years old; the one shown below is the very crystal whose age is 4.4 billion years.
Here is the outcrop from which this zircon was recovered:
In zircon-bearing Granites worldwide, water in the magma is almost always involved in the formation of this rock type. This association is one of two lines of evidence for water in and perhaps on early Earth crust. The other is even more diagnostic: the O18/O16 ratio determined for these early Zircons. That ratio is as high as 7.4 for the oldest zircon yet found. The range of ages and their corresponding O18/O16 ratios for dated Hadean zircons is expressed in this graph. As discussed next, this implies that its parent igneous rock may have formed in the presence of water.
Notice that the Jack Hills zircons and those from several other locations have high O18/O16 ratios, commonly expressed as δO18. (The Oxygen isotopes are analyzed using the ion microprobe along with an instrument called SHRIMP.)
Some have interpreted this as of great significance. High O18/O16 ratios (above 5) are characteristic of water in several environments (processes selectively drive off the O16, being lighter and thus relatively enrich the heavier O18). The implication of this higher ratio is that liquid water was present in the environment in which the original (4.0+ b.y.) zircons crystallized. If this is ultimately proven, the conclusion is that the Earth's initial crust cooled rapidly (in less than 100 million years) and steam in the primitive atmosphere condensed to form bodies of water. That water may have been present mainly in hydrothermal solutions that affected the silicic igneous rocks (which would have formed as the end products of magmatic differentiation) containing the zircons. But a growing number of geoscientists take a larger leap and suggest that the water condensation at that time was large enough to form into bodies that would have been lakelike or even mini-oceanlike. Such waters became involved in igneous proceeses that led to granites. If that proves correct, then conditions might have existed for very primitive life to have started (see below) in surface/near surface settings (ponds to oceans) much earlier than now believed; but since no rocks from the early Hadean have been found, this remains a supposition.
Thus, the supposition from these zircon ages and their elevated δO18 values (as high as 7.4) is that sediments formed from erosion of the very early crust, collected in the protoseas, were buried, melted, and produced zircon-bearing sialic (silica-rich) igneous rocks including granite. If so, these rocks formed the nucleus of one or more ancient continents. They persisted above water long enough to erode over time while releasing the zircons of various ages that are now found in the sedimentary rocks of 3 to 3.5 b.y. age.
There are some surviving rocks that are about 3.8 to 3.95 billion years in age. These are metamorphic or metasedimentary. Nearly all rocks older than that - basalts, granites, probably also sedimentary, and metamorphic rocks - have seemngly all been destroyed (but geologists still seek representatives of such older rocks). Mechanisms for this destruction include bombardment, remelting, burial to now inaccessible depths, metamorphism (which resets the age-dating isotope clock), weathering/erosion, and possible subduction if the early crustal plates were set in motion (see below). A thorough review article, "A Cool Early Earth?", by John W. Valley, in the October 2005 issue of Scientific American summarizes the above ideas stemming from zircon analysis.
Canadian geologists from McGill University in Montreal claim to have found the oldest surviving rock formation on Earth. Located next to the eastern shore of Hudson Bay, it is greenstone, which is metamorphosed basalt. Its age is 4.28 billion years (some critics have challenged this result, citing uncertainties in the Nd/Sm age dating method). Archean greenstones are fairly common, as are Archean granites and metamorphic gneisses
A general model for continental crust, which had widespread development throughout the Archean over much of the Earth outer layers, is indicated in this diagram produced by Dr. Paul D. Lowman, Jr (author of Section 12 of this Tutorial).
The closest actual example of this type of primitive crust is in the Pilbara district of northern Australia (shown on page 6-15). We put up here a somewhat different version of that scene which shows granitic plutons intruded into greenstones in a block surrounded by younger rocks:
Dr. Lowman has also developed a three stage model highlighting the commonalities among the four inner terrestrial planets; read the annotation in the illustration for contextual applicability to the Earth.
The early continents were built around basalts (ultimately, metamorphosed into greenstone) into which magmas, some differentiated into sialic granites, intruded. Sedimentary rocks formed on and around continental nuclei; these could be metamorphosed by burial and heat into a variety of metamorphic rocks including gneiss and even granites formed by melting of those rocks. Thus, some number of individual continental masses gradually increased in size; these, being lighter in density could rise above the primitive oceans where their rocks were weathered, transported, and converted into sediments that hardened and were themselves weathered or subjected to forces that led to metamorphic rocks (the Rock Cycle; see page 2-1a). A crust composed of heavier basalt within which were "continental islands" was probably in place sometime in the first billion years. That crust was subjected to circulating convection currents in the mantle of the hot Earth. This brought about fracturing of the crust, upwelling of melted mantle, and lateral movements of the crustal blocks dragged along by convection currents, i.e., the plate tectonics described on page 2-1a.
It seems probable then that plate tectonics started in the first billion year period. This figure below depicts a hypothetical early Earth distribution of protocontinents which drifted by seafloor spreading until they collided into a supercontinent (Rondinia); that single supercontinent then broke up (like the later Pangaea) and its fragments made new individual continents - this process of coming together, splitting, moving in various directions over the global surface, recombining, resplitting, etc. probably has happened at least several times in Earth history.
Little has been said so far about the development of life on Earth. As mentioned above, oceans may have existed almost from the beginning but were definitely widespread by early Archean times. To appreciate the story of life's appearance and subsequent development, we need to talk briefly about the history of Earth's atmosphere.
At the very outset, hydrogen and helium were the dominant gases in the forming Earth. As said above, these were largely blown away by solar winds. As the Earth melted, it degassed volatiles from its interior through the hot or molten surface. CO, CO2, NH3, NH4, N2, methane (CH4) and water vapor were the likely principal gases that accumulated above the hot surface. Water vapor could have condensed to liquid but probably did not build up into an extensive early ocean, being more likely to revaporize as impacts continued. A dominant outlet was by volcanic vents, some being released into any water bodies, similar to the modern "black smokers" in today's oceans:
Most of the early Earth gases were released through volcanism, following a general pattern similar to modern conditions, except for the survival of any released oxygen:
The Earth's atmosphere for at least the first two billion years was very oxygen-poor and hence reducing. In time, N2 became the dominant constituent of the atmospheric envelope that extended as a thick shell around the solid Earth. Methane and carbon dioxide persisted for some time. The carbon dioxide was utilized in part by organisms that developed photosynthesis capability.
Any oxygen released by volcanism was quickly combined with other elements, so as to have only a fleeting residence in an atmosphere. Starting about 4 billion years ago, much of this oxygen combined with iron that precipitated along with silicates to form the banded iron formation (BIF) rock that is a mainstay of sedimentary deposits until about 2 billion years ago.
This rock resulted from accumulations of ferrous Iron (Fe+2) in oceans and lakes (which were more green in color than today; ferrous iron can produce that color as, for example, in a Coca-Cola glass bottle). The Iron readily combined with any available oxygen, so that the latter was always destined to be caught up in the iron precipitates (Fe2O3) and thus didn't remain in the atmosphere. While BIF is a hallmark of sedimentary rock formations during this extended period, other rocks also formed (shales; sandstones) but carbonates (limestones) were much less commmon. Starting about 2.3 billion years ago, oxygen levels and other factors led to common production of ferric oxides (Hematite) that made prominent red beds periodically to the present. One variety includes alternating chert layers, some rich in iron.
Not all BIF beds have the obvious red color from Hematite. This BIF outcrop shows alternating black (Magnetite?) and white (chert) layers (similar to the Taconite that the writer encountered in Minnesota while I was working (during a graduate summer school break) for Jones and Laughlin Steel as part of a field survey team:
As will be shown in Section 20, the origin of life is still uncertain but several models based on informed speculation have been proposed. In one (the Miller/Urey experiment), simple organic molecules were produced from atmospheric and dissolved constituents in the oceans that began to survive less than 4 billion years ago. Another cites organic molecules in carbonaceous meteorites. A third believes such molecules, and possibly even biologic organisms, were carried to Earth in comets (the "panspermia" model). Regardless, the first living organisms that have actually been observed are found in rocks formed about 3.55 billion years prior to the present. It is reasonable to surmise that rocks several hundred million years older hosted microorganisms but thermal processes have destroyed all traces; carbon isotope anomalies tend to support this postulate of life in these older rocks. They were microscopic bacteria in nature - these are numerically still the dominant form of life today. An abundant single-celled prokaryotic (without a nucleus) type is cyanobacteria (once misnamed "blue-green algae"). Colonies of trillions of these bacteria built up cabbage-like structures called stromatolites. The bulk of a stromatolite colony consists of layers of calcium carbonate interspersed with mattes deposited by the cyanobacteria. Stromatolites still exist on Earth but are rare (mainly at two localities in Australia).
The oldest fossil utilizing hard parts consisting of a tiny spherical shell dates at about 3.1 b.y.:
Ancient rocks have disclosed other life forms including those that represent different types of bacteria, along with primitive true algae. For about two billion years after Earth's start, these bacteria and algae gradually developed the capability of receiving their metabolic energy through sunlight, which evolved into the process of photosynthesis. One of the by-products of photosynthesis is O2. At first, this oxygen continued to combine with other elements but over time the accumulation of released oxygen exceeded the capacity to precipitate in the oceans and gradually built up a concentration in the atmosphere to its current 21% level. This had important consequences: O3, or ozone, developed by solar UV reactions with normal oxygen in the upper atmosphere, providing a shield that greatly reduced the amount of UV reaching the Earth's surface, so that organisms sensitive to damaging ultraviolet rays could now begin to survive, flourish, and diversify. Multicelled (eucaryotic and metazoic) organisms appeared in the last two billion years as the oxygen component of the atmosphere increased. Respiration became possible as a metabolic source of energy in more advanced life forms. These became prominent in the fossil record about 750 million years before the present and underwent an explosive evolution about 560 million years ago (Cambrian time).
Many of the topics covered above can be summarized in this diagram that shows the main events and changes in the Earth's history:
Two more illustrations expand on the ideas in the above figure. The first is a broadbrush overview of the Earth's history; the second uses the geologic time scale as a matrix in which key events in life's evolution are stated:
This next chart is similar both brings out more information on the first appearance of representative life forms.
Although some information is redundant in this next chart which concentrates on Precambrian history, there are other new entries which may add to your understanding of this time span which covers nearly 90% of Earth time.
One of the benchmarks of Phanerozoic life occurred near its beginning with the great explosion of animal life forms in the Cambrian (Burgess shale fauna, mentioned on page 20-12). A preview of this expansion was the Ediacaran fauna (most creatures with hard parts) around 700 million years ago, as preserved in Australian rocks.
Invertebrate and Plant Life in the Phanerozoic (all time after the Precambrian) is summarized in these two diagrams:
Vertebrates can be traced back to the Ordovician. The first vertebrates were fish, followed by amphibians, reptiles, mammals, and birds. The evolutionary history of vertebrates is encapsuled in this diagram:
The culmination of Earth history has been the appearance of creatures with reasoning intellects - the humans. Although only illustrated for now, the following diagram summarizes the main stages in the evolution of the hominids (humans) over the past 5 million years (the Australopithecines are ancestral to the hominids). Much more about the development of life on Earth and in the Solar System in general is covered on page 20-12.
There are many Web sites that supplement and reenforce some of the ideas presented on this page. Consult these select ones if you wish: 1) Malaspina University Geology group, Canada; 2) UCLA Astrobiology Center; and 3)The Evolution of Life site sponsored by the University of Waikato in New Zealand..
Many of the features and processes that commonly operate on today's Earth, and in the past, also have now been observed on other planets and their satellites - this is embodied in the comparative planetology approach mentioned on page 20-1. That, again, is why Planetology has Terrestrial Geology as its foundation and principal source of methodology. We usually identify a feature on another planet by comparing its to similar looking and acting features on Earth. So, the majority of Planetologists also are, or started as, Geologists; the others have learned a lot of Geology to prep them as planetary specialists.
Let us illustrate the value of comparative planetology, using the Earth as the reference base, by examining the "Snowball Earth" hypothesis - which asserts that most or even all of Earth, including the oceans, was covered with a kilometer or more of ice around 750 million years ago. This surmise has moved to the forefront in the last two decades owing to ideas put forth by Paul Hoffman of Harvard University and others. The essence of the hypothesis is summarized in this diagram (Stages 3 and 4 - the aftermath of maximum glaciation - have been omitted).
The evidence for this worldwide glaciation and ice cover is mainly the distribution of glacial tills and other deposits in most of today's continents, shown here as they were regrouped in the supercontinent Rodinia, at the time of the Grenville orogeny 1.1 billion years ago:
Associated Banded Iron Formation deposits and unusual carbonate deposits further support the Snowball model. But, the hypothesis is still not broadly accepted, as valid objections persist. You can read more about this subject at the Snowball Earth website produced by Wikipedia.
If indeed Earth was largely covered by ice then (and perhaps earlier, about 2.5 billion years ago), then it resembles (on a larger scale) many of the satellites of Jupiter, Saturn, and Uranus that will be examined later in this Section.
With this history of Earth as a planet, we can now turn our attention to the nearest large planetary body, our own Moon, which provides an excellent example of how scientists go about obtaining information about other solar objects and interpreting the nature and characteristics of planets.