Planet Earth is very old, about 4.5 billion years or more, according to recent scientific estimates. Most of the evidence for an ancient Earth is contained in the rocks that form the planet's crust. The rock layers themselves, like pages in a long and complicated history, record the surface-shaping events of the past, and buried within them are traces of life—that is, the plants and animals that evolved from the ancient organic structures that existed perhaps three billion years ago. Also contained in once molten rocks are radioactive elements whose isotopes provide Earth with an atomic clock. Within these rocks, parent isotopes decay at a predictable rate to form daughter isotopes. By determining the relative amounts of parent and daughter isotopes, scientists have determined the age of these rocks. Therefore, the results of studies of rock layers (stratigraphy) and of fossils (paleontology), coupled with the ages of certain rocks as measured by atomic clocks (geochronology), provide scientific evidence that humans' home planet is a very old place.
Up until now, scientists have not found a way to determine the exact age of Earth directly from terrestrial rocks because our planet's oldest rocks have been recycled and destroyed by the process of plate tectonics. If there are any of Earth's primordial rocks left in their original state, scientists have not yet found them. Nevertheless, scientists have been able to determine the probable age of the solar system and to calculate an age for Earth by assuming that Earth and the rest of the solid bodies in the solar system formed at the same time and are, therefore, of the same age.
The ages of Earth and Moon rocks and of meteorites are measured by the decay of long-lived radioactive isotopes of elements that occur naturally in rocks and minerals and that decay with half-lives of 700 million to more than 100 billion years into stable isotopes of other elements. Scientists use these dating techniques, which are firmly grounded in physics and are known collectively as radiometric dating, to measure the last time that the rock being dated was either melted or disturbed sufficiently to rehomogenize its radioactive elements. Ancient rocks that exceed 3.5 billion years in age are found on all of Earth's continents. The oldest terrestrial rocks found so far are the Acasta Gneisses in northwestern Canada near Great Slave Lake (4.03 billion years old) and the Isua Supracrustal rocks in West Greenland (about 3.7 to 3.8 billion years old), but well-studied rocks nearly as old are also found in the Minnesota River Valley and northern Michigan (some 3.5-3.7 billion years old), in Swaziland (about 3.4-3.5 billion years old), and in Western Australia (approximately 3.4-3.6 billion years old). Scientists have dated these ancient rocks using a number of radiometric dating methods, and the consistency of the results gives scientists confidence that the estimated ages are correct to within a few percent.
An interesting feature of these ancient rocks is that they are not from any sort of "primordial crust" but are lava flows and sediments deposited in shallow water—an indication that Earth history began well before these rocks were deposited. In Western Australia, single zircon crystals found in younger sedimentary rocks have radiometric ages of as much as 4.3 billion years, making these tiny crystals the oldest materials to be found on Earth so far. Scientists have not yet found the source rocks for these zircon crystals.
The ages measured for Earth's oldest rocks and oldest crystals show that humans' home planet is at least 4.3 billion years in age, but do not reveal the exact age of Earth's formation. The best age for Earth is currently estimated as 4.54 billion years. This value is based on old, presumed single-stage leads (Pb) coupled with the Pb ratios in troilite from iron meteorites, specifically the Canyon Diablo meteorite. In addition, mineral grains (zircon) with uranium-lead (U-Pb) ages of 4.4 billion years have recently been reported from sedimentary rocks found in west-central Australia.
The Moon is a more primitive planetary body than Earth because it has not been disturbed by plate tectonics. Therefore, some of the Moon's more ancient rocks are more plentiful. The six American Apollo Project human missions and three Russian Luna robotic spacecraft missions returned only a small number of lunar rocks to Earth. The returned lunar rocks vary greatly in age—an indication of their different ages of formation and their subsequent geologic histories. The oldest dated Moon rocks, however, have ages between 4.4 and 4.5 billion years and provide a minimum age for the formation of Earth's nearest planetary neighbor.
Thousands of meteorites, which are fragments of asteroids that fell on Earth, have been recovered. These primitive extraterrestrial objects provide the best ages for the time of formation of the solar system. There are more than 70 meteorites (of different types)
whose ages have been measured using radio-metric-dating techniques. The results show that the meteorites, and, by extrapolation, the solar system, formed between 4.53 and 4.58 billion years ago. The best age for Earth comes not from dating individual rocks but by considering Earth and meteorites as part of the same evolving system in which the isoto-pic composition of lead, specifically the ratio of lead-207 to lead-206, changes over time, owing to the decay of radioactive uranium-235 and uranium-238, respectively. Scientists have used this approach to determine the time required for the isotopes in Earth's oldest lead ores, of which there are only a few, to evolve from its primordial composition, as measured in uranium-free phases of iron meteorites, to its compositions at the time that these lead ores separated from their mantle reservoirs.
According to scientists at the U.S. Geological Survey (USGS), these calculations result in an age for Earth and meteorites, and therefore the solar system, of 4.54 billion years with an uncertainty of less than 1 percent. To be precise, this age represents the last time that lead isotopes were homogeneous throughout the inner solar system and the last time that lead and uranium were incorporated into the solid bodies of the solar system. The age of 4.54 billion years found for the solar system and Earth is consistent with current estimates of 11 to 13 billion years for the age of the Milky Way galaxy (based on the stage of evolution of globular cluster stars) or the estimated age of 10 to 15 billion years (based on the recession rate of distant galaxies).
links the chemistry of interstellar clouds with the prebiotic evolution of organic matter in the solar system and on early Earth. There is also compelling evidence that cellular life existed on Earth some 3.56 billion years ago (3.56 Gy). This implies that the cellular ancestors of contemporary terrestrial life emerged rather quickly (on a geologic time scale). These ancient creatures may have also survived the effects of large impacts from comets and asteroids in those ancient, chaotic times when the solar system was evolving.
The accompanying figure summarizes some of the factors that scientists now believe are important in the evolution of complex life. These factors include (A) endogenous factors stemming from physical-chemical properties of Earth and those of eukaryotic organisms; (B) factors associated with properties of the Sun and of Earth's position with respect to the Sun; (C) factors originating within the solar system, including Earth as a representative planet; and (D) factors originating in space far from humans' solar system.
The word eukaryotic refers to cells whose internal construction is complex, consisting of organelles (e.g., nucleus, mitochondria, etc.), chromosomes, and other structures. All higher terrestrial organisms are built of eukaryotic cells, as are many single-celled organisms (called protists). The evolution of complex life apparently had to await the evolution of eukaryotic cells—an event that is believed to have occurred on Earth about one billion years ago. A eukaryote is an organism built of eukaryotic cells.
And where does all this lead to in the cosmic evolution scenario? The reader should first recognize that all living things are extremely interesting pieces of matter. Life forms that have achieved intelligence and have developed technology are especially interesting and valuable in the cosmic evolution of the universe. Intelligent creatures with technology, including human beings here on Earth, can exercise conscious control over matter in progressively more effective ways as the level of their technology grows. Ancient cave dwellers used fire to provide light and warmth. Modern humans harness solar energy, control falling water, and split atomic nuclei to provide energy for light, warmth, industry, and entertainment. People later in this century will most likely "join atomic nuclei" (controlled fusion) to provide energy for light, warmth, industrial applications, and entertainment here
These factors are considered important in the evolution of complex life. (NASA)
Changes in luminosity
Asteroid and comet bombardments c
Asteroid and comet bombardments a i- r"'T perturbations Accretion of r
Earth effects Orbital a i- r"'T perturbations Accretion of r interplanetary material
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Sun on Earth, as well as for interplanetary power and propulsion systems for emerging human settlements on the Moon and Mars. The trend should be obvious.
Some scientists, while contemplating the ultimate fate of the universe, have boldly suggested an interesting destiny for intelligent species throughout the universe. If such (postulated) intelligent alien creatures throughout the Milky Way galaxy can learn to live with the awesome technical powers of their advanced civilizations, then it may be the destiny of advanced intelligent life (including hopefully humans) to share information and to cooperate in the exercise of (benevolent) control over all the matter and energy within the universe.
According to modern scientific theory, living organisms arose naturally on the primitive Earth through a lengthy process of chemical evolution of organic matter. This process began with the synthesis of simple organic compounds from inorganic precursors in the atmosphere; continued in the oceans, where these compounds were transformed into increasingly more complex organic substances; and then culminated with the emergence of organic microstructures that had the capability of rudimentary self-replication and other biochemical functions.
Human interest in the origins of life extends back deep into antiquity. Throughout history, each society's creation myth seemed to reflect that particular people's view of the extent of the universe and their place within it. Today, in the space age, the scope of those early perceptions has expanded well beyond the reaches of humans' solar system to other star systems, to the vast interstellar clouds, and to numerous galaxies that populate the seemingly limitless expanse of outer space. Just as the concept of biological evolution implies that all living organisms have arisen by divergence from a common ancestry, so too the concept of cosmic evolution implies that all matter in humans' solar system has a common origin. Following this line of reasoning, scientists now postulate that life may be viewed as the product of countless changes in the form of primordial stellar matter—changes brought about by the interactive processes of astro-physical, cosmochemical, geological, and biological evolution.
If scientists use the even larger context of cosmic evolution, they can further conclude that the chain of events, which led to the origins of life here on Earth, extends well beyond planetary history: to the origin of the solar system itself, to processes occurring in ancient interstellar clouds that spawned stars like the Sun, and ultimately to the very birth within these stars (through nucleosynthesis and other processes) of the elements that make up living organisms—the biogenic elements. The biogenic elements are those that are generally judged to be essential for all living systems. Scientists currently place primary emphasis on the elements hydrogen (H), carbon (C), nitrogen (N), oxygen (O), sulfur (S) and phosphorous
(P). The compounds of major interest are those normally associated with water and with organic chemistry, in which carbon is bonded to itself or to other biogenic elements. The essentially universal presence of these compounds throughout interstellar space gives exobiologists the scientific basis for forming the important contemporary hypothesis that the origin of life is inevitable throughout the cosmos wherever these compounds occur and suitable planetary conditions exist. Present-day understanding of life on Earth leads scientists to the conclusion that life originates on planets and that the overall process of biological evolution is subject to the often chaotic processes that are associated with planetary and solar-system evolution—for example, the random impact of a comet on a planetary body or the unpredictable breakup of a small moon.
Scientists now use four major epochs in describing the evolution of living systems and their chemical precursors. These are:
1. The cosmic evolution of biogenic compounds—an extended period corresponding to the growth in complexity of the biogenic elements from nucleosynthesis in stars to interstellar molecules to organic compounds in comets and asteroids.
2. Prebiotic evolution—a period corresponding to the development (in planetary environments) of the chemistry of life from simple components of atmospheres, oceans, and crustal rocks to complex chemical precursors to initial cellular life-forms.
3. The early evolution of life—a period of biological evolution from the first living organisms to the development of multicellular species.
4. The evolution of advanced life—a period characterized by the emergence of progressively more advanced life-forms, climaxing perhaps with the development of intelligent beings capable of communicating, using technology and exploring and understanding the universe within which they live.
As scientists unravel the details of the intriguing process for the chemical evolution of terrestrial life, they should also ask themselves another very intriguing question: If it happened here, did it or could it happen elsewhere? In other words, what are the prospects for finding extraterrestrial life—in this solar system or perhaps on Earthlike planets around distant stars?
According to the principle of mediocrity (a concept frequently invoked by exobiologists), there is nothing "special" about the solar system or planet Earth. Within this speculation, therefore, if similar conditions have existed or are now present on "suitable" planets around alien suns, the chemical evolution of life will also occur.
Contemporary planetary formation theory strongly suggests that objects similar in mass and composition to Earth may exist in many
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