Geologic Time

Note the thumbnail sketch of Earth History available at http://bearspace.baylor.edu/Vince_Cronin/www/PhysGeol/EarthHist.html, and refer to the answers to frequently-asked questions at the Baylor Geology Department web site: http://www.baylor.edu/Geology

Q: Don't all true Christians believe that Earth and all the rest of creation is ~6000 years old?
A: No. For example, the Catholic Church and most main-line Protestant denominations do not teach young-Earth creationism. As the leader of the world's largest Christian organization, Pope John Paul II, told the Pontifical Academy of Sciences in 1981 (Cosmology and Fundamental Physics),
"Cosmogony and cosmology have always aroused great interest among peoples and religions. The Bible itself speaks to us of the origin of the universe and its make-up, not in order to provide us with a scientific treatise, but in order to state the correct relationships of man with God and with the universe. Sacred Scripture wishes simply to declare that the world was created by God, and in order to teach this truth it expresses itself in the terms of the cosmology in use at the time of the writer. The Sacred Book likewise wishes to tell men that the world was not created as the seat of the gods, as was taught by other cosmogonies and cosmologies, but was rather created for the service of man and the glory of God. Any other teaching about the origin and make-up of the universe is alien to the intentions of the Bible, which does not wish to teach how heaven was made but how one goes to heaven."
It is difficult to answer this type of question in general, because of the embedded assertion that some people can be classified as true or authentic Christians while others are not true/authentic Christians. That is not an issue that requires input from science.

Q: Don't all Christians believe that all types of organisms, including humans, were specially created at the same time?
A: No. See the previous response.

Q: Don't all Christians believe that Darwinian evolution is a secular, atheistic theory that is inconsistent with Christianity?
A: No. For example, evolutionary biology is taught at all Catholic universities that have biology departments, to my knowledge. The science of evolution has also been taught at Baylor for about a century, according to the Dean of the College of Arts and Science.

Q: Isn't evolution just a theory, no better or worse than any other?
A: No.

1. That organisms evolve over time is a reproducibly observed fact. The theoretical aspect of the problem involves how evolution occurs, not whether it occurs.
2. A scientific theory is a specific testable explanation of how scientific facts relate to one another that has been tested and has not been falsified/disproven.
   This is a much more specific and restricted definition of the word theory than is used by the non-scientific general public, for whom a theory is the same as a guess.
   A scientific theory is significantly more reliable than a speculative or unsubstantiated idea that has not withstood rigorous testing, no matter how clever the speculation may be.

Q: Isn't it true that the dinosaurs are still alive, but they live in uninhabited places like the deep jungles of Africa?
A: No. With the exception of birds and crocodilians, dinosaurs were extinct by the end of the Cretaceous Period, 65 million years ago. Modern humans (Homo sapiens) first appear in the fossil record around 250 thousand years ago, missing coexistence with dinosaurs by almost 65 million years.

Q: Isn't it true that there are human footprints in the same sedimentary layers as dinosaur footprints, proving that humans and dinosaurs lived together?
A: No.
Sometimes "human footprints" are manufactured in a dinosaur trackway using acid or chisles, but these frauds are rather easily exposed. Sometimes, the upper layer of the footprint of a "3-toed" theropod dinosaur is eroded, removing the more shallow imprints of the toes and leaving the deeper imprint of the dinosaur's foot. Some people who are not vertebrate paleontologists have mistaken these eroded imprints as human footprints, even though they lack the size or shape of human feet.
No fossils of modern humans have been reported in strata of between 65 million to around 250 thousand years in age, and no dinosaur fossils (other than birds or crocodilians) have been found in strata of less than 65 million years in age. Dinosaurs and humans did not live at the same time.

Q: Isn't it true that many scientists think that Darwin's theory of evolution is not correct?
A: No, but this is a misleading question on several fronts.

1. While some scientists (and some non-scientists who call themselves scientists) are not convinced by the evidence that Darwin was substantially correct, a very very significant majority are convinced. The largest list of self-identified scientists who do not accept the basic ideas of Darwinian evolution that I have seen has around 500 names on it, including many who would not be objectively considered scientists or people with advanced knowledge of biology or paleobiology (e.g., lawyers, engineers, etc.) In contrast, Project Steve has thus far collected the endorsement of 972 scientists named Steve (or some variant thereof) who have endorsed a basic statement of Darwinian evolution. That statement follows:
   "Evolution is a vital, well-supported, unifying principle of the biological sciences, and the scientific evidence is overwhelmingly in favor of the idea that all living things share a common ancestry. Although there are legitimate debates about the patterns and processes of evolution, there is no serious scientific doubt that evolution occurred or that natural selection is a major mechanism in its occurrence. It is scientifically inappropriate and pedagogically irresponsible for creationist pseudoscience, including but not limited to "intelligent design," to be introduced into the science curricula of our nation's public schools."
   It is estimated that around 1% of scientists have the first name Steve. From this, one might infer that perhaps 99% of living scientists accept the basic ideas of Darwinian evolution.
   It is probably more relevant to ask how many scientists working in fields that would give them an expert's knowledge of the issue are convinced that Darwinian evolution is substantially valid. My guess is that the acceptance level among scientists with appropriate expertise (biologists, paleobiologists, paleontologists, geologists) is nearly unanimous.
2. Current models of biological evolution include many (but not all) of Darwin's ideas, along with more recent ideas based on molecular biology, genetics, evolutionary developmental biology, and so on. Evolutionary theory does not begin and end with Darwin and his work.
3. You should refer to the Baylor Biology Department's "statement on evolution" on their web site: http://www.baylor.edu/Biology. It states:
   "Evolution, a foundational principle of modern biology, is supported by overwhelming scientific evidence and is accepted by the vast majority of scientists. Because it is fundamental to the understanding of modern biology, the faculty in the Biology Department at Baylor University, Waco, TX, teach evolution throughout the biology curriculum. We are in accordance with the American Association for Advancement of Science's statement on evolution. We are a science department, so we do not teach alternative hypotheses or philosophically deduced theories that cannot be tested rigorously."

Q: Isn't it true that fossils are direct evidence of a worldwide flood: Noah's flood?
A: No. There is no geological evidence that the entire Earth has been flooded during the time that modern humans (Homo sapiens) have been on Earth: within the last 250 thousand years. Fossils range in age from around 3.5 billion years to less than 1 million years -- they were not all deposited at once.

Q: How old is the Earth?
A: Earth is around 4.5 to 4.6 billion years old. For more information and references, see the response to this question posted by the Baylor Geology Department via the FAQ section of its web site http://www.baylor.edu/Geology.

Q: How old is the universe?
A: The current best estimate for the age of the universe is ~13.7 billion years. For more information and references, see the response to this question posted by the Baylor Geology Department via the FAQ section of its web site http://www.baylor.edu/Geology.

Q: Are the decay constants used in isotopic dating actually constant, or do they vary over time naturally or in response to external forces?
A: With rare well-documented exceptions, the decay constants for radioactive isotopes used in geological dating are actually quite constant. For more information and references, see the response to this question posted by the Baylor Geology Department via the FAQ section of its web site http://www.baylor.edu/Geology.

Q: What is a fossil?
A: A fossil is evidence of prior life that is preserved in the rock record. There are many types of fossils, including petrified bones and shells, casts, imprints, and chemical markers. For more information and references, see the response to this question posted by the Baylor Geology Department via the FAQ section of its web site http://www.baylor.edu/Geology.

Q: How old are the oldest fossils?
A: The oldest unambiguous fossils are around 3.5 billion years old, although there are some chemical indicators of life in rock dated at ~3.8 billion years. For more information and references, see the response to this question posted by the Baylor Geology Department via the FAQ section of its web site http://www.baylor.edu/Geology.

Q: What are mass extinctions, and when have they occurred?
A: Mass extinctions are situations in which a very large number of types of organisms become extinct over a very short time period. Around 5 great mass extinctions have been recognized during the past 600 million years. For more information and references, see the response to this question posted by the Baylor Geology Department via the FAQ section of its web site http://www.baylor.edu/Geology.

Q: Does the fossil record support the idea of biological change over time (biological evolution)?
A: Yes, abundantly. For more information and references, see the response to this question posted by the Baylor Geology Department via the FAQ section of its web site http://www.baylor.edu/Geology.

1. Our personal concepts of time are scaled by how long we have been alive, extended by the transmitted memories of the older people with whom we have interacted and, in a less direct manner, by books and other cultural artifacts. We are most comfortable with the time interval since we were ~3 years old -- the time of personal remembrance.
   • Tree rings give us information back a few thousand years, including valuable climate information and an independent means to calibrate C14 isotopic dating
   • Written history and artifacts within a full cultural context extend the story back perhaps 10,000 years BP. For example...
     • The Parthenon on the Acropolis in Athens, built circa 448-432 BC,
     • Moses and the first 5 books of the Bible, circa 1250-1200 BC,
     • Petroglyphs in New Mexico, circa 9,000 BP
   • More ancient cultural artifacts indicate the artistic nature of humans (among the many other conclusions that can be drawn from them)
     • ~14,000-25,000 year old hut made of mastodon/mammoth bones, Mazhirich near Kiev
     • Cave paintings from southern France, from ~70,000 to ~15,000 years old
     • Bead-covered Homo sapiens skeleton buried ~23,000 years ago near Moscow
     • Formally buried Homo sapiens circa 28,000 years ago from Australia
2. Fossil remnants of Homo sapiens date from ~250 ka BP (250,000 years before present)
   (Information from C.B. Stringer, 2001, Dating the origin of modern humans: Geological Society of London, Special Publication 190, p. 265-274.)
   • Florisbad, South Africa: 259 ka, dated by electron spin resonance (ESR) and optically stimulated luminescence
   • Jebel Qafzeh, Israel: 92-120 ka, dated by thermoluminescence (TR), ESR, uranium series isotopic dating
   • Jebel Irhoud, Morocco: 87-190 ka by ESR
   • Skhul, Israel: 81-119 ka by ESR, TR
   • Singa, Sudan: =/>133 ka by ESR, uranium series isotopic dating
3. Geologists use a variety of techniques to establish the time order of geological events -- so-called "relative dating"
   • Principle of original horizontality: Sedimentary strata are deposited in near-horizontal beds (inclined less than ~6 degrees). The foreset laminae of cross beds may be inclined more steeply (to ~ 30 degrees); however, the cross-beds are bound above and below by near-horizontal beds.
   • Superposition: In an undeformed sequence of sedimentary strata, the oldest rocks are below the youngest rocks.
   • Inclusion: A geological object (clast, xenolith) that is included or enclosed or surrounded by other geological material is older than the material that surrounds it. The material in the clast is older than the sedimentary rock that contains the clast. The material in a xenolith is older than the igneous rock that contains the xenolith.
   • Cross-cutting relationships: A structure (fault, crack, dike) that cuts across a geological material is younger than the material it cuts. For example, a tear in a piece of paper must be younger than the piece of paper, so a fault must be younger than the rock that it cuts.
   • Faunal succession: Fossil forms are known to change over time, so two separate strata that each contain the same set of index fossils are interpreted to be the same age.
4. The geologic time scale was initially developed based on fossil evidence, with divisions that generally represent significant changes in fossil assemblages. Many if not all of the divisions are now known to represent extinction events or significant environmental/climatological changes. The geologic time scale was initially developed using relative dating techniques. Thanks to the work of Arthur Holmes of the University of Edinburg (who is your academic great-grandfather through Cronin and his teacher Donald McIntyre) and the advent of isotopic dating techniques, the ages (in years) of the various geological time periods began to be established in the 1920s.
   Note: Ga = billion years = 1,000,000,000 years; Ma = million years
   1. The length of geologic time scales
      1. How much is a million?
         • If I walk 1 mile every 20 minutes, it would take me approximately 38 years to walk 1 million miles.
         • If the length of geologic time is compared to a football field, Precambrian time represents the first 87 yards, and all events since the beginning of the Paleozoic are compressed into the last 13 yards. Dinosaurs first appeared 5 yards from the goal line. The glacial epoch occurred in the last inch, and historic time is so short that is only as long as the thickness of the last blade of grass before the goal line.
      2. How old is Earth?
         1. Early estimates
            1. Anglican Archbishop Ussher of Ireland (circa 1660): Creation started on October 23, 4004 BC -- based on analysis of time as referenced in the Bible
            2. Lord Kelvin (William Thompson) circa 1862: Earth and the Sun formed between 25 and 400 million years ago
            3. Clarence King (circa 1893): Earth's crust is 25 million years old (1893)
            4. John Joly (circa 1899): Earth is ~90 to 100 million years old, based upon the salinity of the oceans
            5. Bertram Boltwood at Yale (circa 1907): Earth is 2 billion years old
            6. Arthur Holmes (circa 1927): similar to current estimates -- ~4.5 billion years old
            7. Claire Patterson (1953, 1956): ~4.55 billion years
         2. Types of evidence available:
            1. Radiometric dates from rock on Earth (oldest zircon U-Pb ages ~4.4 Ga (Wilde et al., 2001); oldest rocks, 3.96 Ga; lead ages of ~4.55 Ga (Patterson, 1953, 1956))
            2. Radiometric dates from rock on Moon (4.51-4.56 Ga, (Halliday and Lee, 1999; Alibert et al., 1994; Hanan and Tilton, 1987))
            3. Radiometric dates from chondritic meteorites (4.566 Ga)
         3. Current best estimate: just over 4.5 to 4.56 billion years (Dalrymple, 2001; Hofman, 2001)
   2. Absolute dating
      1. Radioactive decay: spontaneous changes in the nuclei of large, unstable isotopes
         1. Half life: time it takes for half a given amount of the isotope to decay into its daughter isotope
            half life = ( (ln(0.5)) / lambda),
            where ln (0.5) = 0.693 = natural logarithm of 1/2
            lambda = the decay constant (ratio of number of atoms decaying each second to the number present)
            
         2. If the original number of atoms is No, the number of atoms ( N ) remaining after a time interval t will be
            N = No * [ e raised to the power (lambda * t )]
            where e is the natural number ( e = 2.718...). If a mineral contains a radioactive isotope (for instance, 238U), its age can be established by measuring the number of 238U atoms present (N238U) as well as the number of atoms of 206Pb (N206Pb), which is the isotope into which 238U commonly decays by successive a emissions and b decays. The two numbers (N238U + N206Pb) added give No, the number of atoms of 238U originally present. Since lambda is known, the equation can be solved for t, which is the age of the mineral (from Emiliani, 1988).
      2. Useful Radioactive Isotopes Used To Date Rocks
         Note: Ga = billion years = 1,000,000,000 years; Ma = million years
         

         Isotope	Half-Life	Daughter
         40K	1.25 Ga	40Ar
         238U	4.47 Ga	206Pb
         235U	704 Ma	207Pb
         232Th	14.0 Ga	208Pb
         87Rb	48.8 Ga	87Sr
         14C	5730 years	14N

5. The list of dates you should commit to memory for this course (thus far):
   • 13.7 billion years (13.7 Ga) The big bang initiates time and space as we know it
   • ~4.55 Ga Earth develops by gravitational attraction of the early-formed solid matter in this part of the solar nebula, and grows by accretion of meteorites to its current size. This is the answer to the question "how old is the Earth"
   • 4 Ga Oldest rock material yet found on Earth. The oldest individual mineral grain is a detrital zircon found in Australia with an age of 4.4 Ga.
   • 3.8 Ga Oldest chemical evidence of life on Earth (ambiguous).
   • 542 million years (543 Ma) End of the Precambrian and beginning of the Paleozoic. Marked by the first occurrence of the remains of organisms that had hard parts (shells, bones, teeth, etc.) in the fossil record
   • 251 Ma End of the Paleozoic and beginning of the Mesozoic. Largest mass extinction in the past half-billion years. Siberian flood basalts occurred at the same time -- perhaps there is a link.
   • 65 Ma End of the Mesozoic, beginning of the Cenozoic. Significant mass extinction that includes most of the dinosaurs; has been variously linked with the impact of a large meteorite in Yucatan, Mexico (Chicxulub crater) and with the eruption of the Deccan Traps flood basalts in India.
     The extinction mechanisms for both the large-meteorite-impact model and the massive-volcanism model are similar. Both would insert dust into the atmosphere that would tend to shroud Earth in clouds over a long time period. This would reduce the sunlight that reached Earth's surface and thereby suppress photosynthetic life, which would place extreme stress on herbivores, which would in turn affect carnivores. Rapid environmental change can lead to extinction.
   • ~250 thousand years (~250 ka) First fossil evidence of Homo sapiens
6. Some notes from the history of life on Earth
   • Biological evolution is a change in gene frequency within a population over time, resulting in changes in the form or function of a type of organism.
   • Biological evolution is observed directly among organisms that reproduce rapidly (e.g., flu virus, HIV, bacteria). Biological evolution is indicated for larger, slower-reproducing organisms in the fossil record.
   • The mechanisms leading to changes in gene frequencies include...
     • Selection (Darwin's "natural selection")
     • Migration of subgroups to/from larger populations
     • Random mutation (replication errors in DNA copying during cellular division)
     • Recombination through sexual reproduction
     • Genetic drift
   • The fossil record includes some species that are no longer alive (e.g., trilobites, dinosaurs, Irish elk) -- extinction happens
   • The fossil record indicates that new species have originated through changes in pre-existing species -- speciation happens
     For example, there are no fossils of modern horses in Paleozoic strata. All fossils of mammals have been found in rocks that are younger than ~220 Ma, and all larger mammals like horses are Cenozoic in age (<65 Ma).
7. Some key events in the history of the human family tree, based on the fossil record
   • ~220 Ma First fossil evidence of mammals
   • ~70 Ma First fossil evidence of primates
   • ~15 Ma The ancestral line that would lead to humans diverged from the line that led to Ramapithecus, Sivapithecus, and modern orangutans
   • ~8 Ma The ancestral line that would lead to humans diverged from the line that led to modern apes (gorillas, etc.)
   • ~5 Ma The ancestral line that would lead to humans diverged from the line that led to chimpanzees and bonobos
   • ~4.2 Ma First occurrence in the fossil record of Australopithecines, who were able to walk upright on two legs most or all of the time. (Mary Leakey found 3.6 million year old Australopithecine footprints in volcanic ash at Laetoli, Tanzania.)
   • ~2.3 Ma First member of genus Homo
     • Homo habilis, ~2.3 to ~1.6 Ma; a tool-maker who lived in Africa
     • Homo erectus, ~1.8 to ~0.3 Ma; expanded out of Africa into Asia
     • Homo heidelbergensis, ~0.6 to ~0.1 Ma; probable common ancestor for Homo sapiens and Homo neanderthalensis
     • Homo neanderthalensis, ~500,000 to ~30,000 years ago; best current evidence is that H. neanderthalensis and H. sapiens were separate (non-interbreeding) species evolved from a common ancestor
     • Homo sapiens (modern humans), ~250,000 years to present; best current evidence is that H. sapiens developed in Africa and spread worldwide from there.

Resources concerning geologic time and evolution, maintained by WGBH Public Television of Boston.

Resources concerning evolution, maintained by the Geological Society of America (the largest association of geologists in the world).

National Center for Science Education

Some references and recommended reading:
(Also refer to the separate list of recommended books for your continuing education about science, geology, and life on Earth.)

• C. Alibert, M. D. Norman, and M. T. McCulloch, 1994, An ancient Sm-Nd age for a ferroan noritic anorthosite clast from lunar breccia 67016: Geochimica et Cosmochimica Acta, v. 58, p. 2921-2926.
• G. Brent Dalrymple, 1991, The Age of the Earth: Stanford, Stanford University Press
• G. Brent Dalrymple, 2001, The age of the Earth in the twentieth century -- a problem (mostly) solved, in C.L.E. Lewis and S.J. Knell, editors, 2001, The Age of the Earth--From 4004 BC to AD 2002: London, The Geological Society, Geological Society Special Publication No. 190, p. 205-222.
• Stephen Jay Gould, 1999, Rock of Ages -- Science and Religion in the Fullness of Life: Library of Contemporary Thought, The Ballantine Publishing Group, New York, 241 pp., ISBN 0-345-43009-3.
• A. N. Halliday and D. C. Lee, 1999, Tungsten isotopes and the early development of the Earth and Moon: Geochimica et Cosmochimica Acta, v. 63, p. 4157-4179.
• B. B. Hanan and G. R. Tilton, 1987, 60025--Relict of primitive lunar crust? Earth and Planetary Science Letters, v. 84, p. 15-21.
• A. W. Hofman, 2001, Lead isotopes and the age of the Earth -- a geochemical accident, in C.L.E. Lewis and S.J. Knell, editors, 2001, The Age of the Earth--From 4004 BC to AD 2002: London, The Geological Society, Geological Society Special Publication No. 190, p. 223-236.
• Donald Johanson & Blake Edgar, 1996, From Lucy to Language: Simon & Schuster Editions, New York, 272 pp., ISBN 0-684-81023-9.
• Roger Lewin, 1988, In the Age of Mankind -- A Smithsonian Book of Human Evolution: Smithsonian Institution, Washington, D.C., 255 pp., ISBN 0-89599-022-9.
• Cherry Lewis, 2000, The Dating Game--One Man's Search for the Age of the Earth: Cambridge University Press, 216 p., ISBN: 0521790514
• C.L.E. Lewis and S.J. Knell, editors, 2001, The Age of the Earth--From 4004 BC to AD 2002: London, The Geological Society, Geological Society Special Publication No. 190, 288 p.
• C. C. Patterson, 1956, Age of meteorites and the earth: Geochimica et Cosmochimica Acta, v. 10, p. 230-237.
• Ian Tattersall, 1998, Becoming Human -- Evolution and Human Uniqueness: Harcourt Brace & Company, New York, 258 pp., ISBN 0-15-100340-8.
• S. A. Wilde, J. W. Valley, W. H. Peck and C. M. Graham, 2001, Evidence from detrital zircons for the existence of continental crust and oceans on the Earth 4.4 Gyr ago: Nature, v. 409, p. 175-178.

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