Biology
Chapter 15: 1-13 Geological Ages and Evolution
Lecture
Evolution: The Origins of Life
WebLecture Topics
A timeline for the early development of life
Based on astronomical, geological and fossil evidence, this seems to be the most common scientific model of a possible sequence of events leading to life on earth.
| Time Frame | Event | ||||||||
|---|---|---|---|---|---|---|---|---|---|
| 11 - 16 Billion Years Ago | Big Bang: energy and matter form; hydrogen atoms form. | ||||||||
| 11 - 16 Billion Years Ago | Big Bang: energy and matter form; hydrogen atoms form. | ||||||||
| 10 - 5 BYA | First generation stars form (primarily hydrogen); fuse heavier elements | ||||||||
| 5.5 BYA | Sun condenses from mixed-gas clouds | ||||||||
| 4.5 BYA | Condensing cloud of heavier elements form earth with solid surface | ||||||||
| 4-3.5 BYA | Single-cell life emerges, possibly using the following steps:
|
This reconstruction is based on experimental evidence which only proves that certain individual steps in the proposed sequence of events are possible. For some steps, we have not been able to recreate an actual reaction that matches the model (but it is important to note that much of the research in this field is very recent, so there hasn't been time to do everything). However, the evidence that a particular step is possible does not prove that it did occur, not can we yet show that the entire sequence could occur. On the other hand, every experiment that shows the model is possible is a kind of support for the self-consistency of the model. Given the impossibility of direct observational evidence for events in the past, this is the only kind of "support" one can have for this kind of scientific investigation. If we could prove that a particular step could not possibly occur, we would have to substitute one or more other possibilities for those steps in order to get a consistent model that addresses all the issues.
Geological Time Spans
Biologists have attempted to organize the fossil evidence into some kind of order, from which they can then deduce something about evolutionary patterns. The time scale which they use combines evidence of geological events with the fossils found in the surrounding deposits. The chart below is similar to the one in your text, but has some additional geological information.
| Geological Era
MYA = million years ago |
Events | ||||||||||||||||||||||||
|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|---|
Precambrian 570 MYA |
Microfossils in precambrian rocks dated to 3.5 billion years again. Major groups of bacteria, fungi, protists, some animals invertebrates) represented in deposits from the bottom of the Grand Canyon (Arizona), and from the Ediacaran Hills of South Australia which have been dated as 3.5 million years old. |
||||||||||||||||||||||||
Paleozoic |
248 MYA: MASS EXTINCTIONS: Cause unknown?
|
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Mesozoic "Age of Reptiles" |
65 MYA: MASS EXTINCTIONS: due to radical climate change? |
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Cenozoic |
Current period: MASS EXTINCTIONS due to ice ages, environmental changes introduced by humans. |
There are several interesting points to note about the time scale.
- Most types of organisms are represented in fossil evidence at the end of the Precambrian period; almost all animal phyla have fossil records by the time of Devonian period.
- Changes in the geological record reflect changes in the biological record. For example, the formation of Pangaea at the end of the Permian Period coincides with mass extinctions.
- There are no periods in the Precambrian era ( biologists don't have enough details to break down the fossil evidence into finer groups by event).
- There are epochs only in the Cenozoic era, where biologists think we have sufficient details to break down periods by specific events.
- Throughout the later periods, the tendency is toward increasing dominance of angiosperms (flowering plants), and mammals.
One problem with the geological evidence is that the nice "column" of geological strata portrayed in the above table does not exist as such. Geologists readily admit that catastrophic and slow tectonic activity of volcanoes, earthquakes, and continental drift is constantly burying, folding, and lifting the layers of rock, making it difficult to interpret strata at any given location. They use evidence from many sites and compare their findings to give the a likely sequence capable of accounting for the most observations.
One technique used by geologists to assist with strata labelling is to date the strata from specific fossils found in it which have already been identified as produced within a particular era or period. Some fossils are considered marker fossils. Like DNA markers, which are always found near but not part of specific gene sequences, marker fossils are specific fossil types which always found in particular types of sedimentary rock. Once the sedimentary rocks are dated by independent, radiometric methods, the marker fossils found in these layers (and not in any other type of layer) can be used to date new strata samples.
Radiometry and the age of the earth
Various radiometric (based on radioactivity of included elements such as carbon and potassium) dating techniques are used to establish the age of a specimen rock (and by association, the geological strata in which it is found). Wherever possible, geologists base their estimate of the age of a rock sample (and hence the earth) on the radioactive decay patterns of several different elements.
Radioactive isotopes are atoms of a particular element which tend to decay, throwing off pieces of their nuclei and changing into different elements, which may also be radioactive isotopes. Ultimately, the chain reaction stops with a stable, non-radioactive isotope of some element. The length of time it takes one-half of a sample of a particular isotope to decay to its own product (not the end product) is called the half-life of the isotope. Some isotopes of elements cannot be formed by the radioactive decay of other elements, and so are considered primordial isotopes. All atoms of a primordial isotope already existed at the time of the creation of the earth.
When a mineral forms, it incorporates elements in a specific ratio depending upon the availability of the elements in the earth's crust at that time; this includes any radioactive and non-radioactive isotopes of a given element and any end-product isotopes and primordial isotopes of the final product of the decay sequence. For example, uranium-235 decays to lead-207 with a half life of 713 million years. On the other hand, no decay series creates the primordial isotope lead-204. By comparing the amounts of uranium-235 with non-radioactive uranium-238 and the amounts of lead-204 with lead-207 in a sample rock, geologists try to determine the amount of time which has passed since the sample was formed. The oldest ages calculated are in the 4 to 5 billion year old range. This method requires that the geologist make some assumption about the original concentrations of isotopes in the original rock—which makes the outcome suspect if the assumptions are wrong.
There are, however, some minerals which must have a particular chemical composition in order to form crystal lattices. In the case of these minerals, we do not have to make assumptions about the original amount of the radioactive material available. In particular, in crystals formed with potassium-40 (which decays to argon-40), all the argon now present is a result of the decay of the potassium originally present. By measuring the ratios of K-40 and Ar-40 in various samples, geologists calculate a period of 4 to 5 billion years, which supports the time frame of the uranium decay series.
A third method involves the decay of rubidium-87 to strontium-87. Because strontium-86 and strontium-87 are chemically identical, there is no reason for a sample to contain one over the other during formation, and a sample containing several different minerals with strontium as a component will have the same strontium-86 to strontium-87 ratio in each mineral; that ratio will be whatever was available at the time of formation. However, each mineral will have different amounts of rubidium in it (depending on its chemical composition), and over time, each will acquire an additional amount of strontium-87 from the decaying rubidium-87 which the amount of strontium-86 remains constant. By comparing the current strontium-87 to strontium-86 ratios with the current rubidum 87- strontium-86 ratios, geologists can calculate the length of time required to produce the additional strontium-87 without making assumptions about the original concentrations. These calculations place rock ages at about 4.2 billion years, again, within the range predicted by the uranium-to-lead and potassium-to-argon decay sequences.
Note that while all three methods do make the assumption that the currently observed rates of radioactive decay for each series have remained constant, only the first requires assumptions about the original concentrations. Geologists use these more rigorous methods to validate their assumptions about the uranium/lead ratios, then use those ratios to date rocks where the potassium/argon or rubidium/strontium sequences cannot be used.
Geologists also consider the entire range of radioactive elements in determining the age of the earth. There are 64 known isotopes of various elements with half-lives greater than 1000 years, and 47 of these have half-lives between 1000 years and 50 million years. Of these latter, 7 can be formed by bombardment from cosmic rays or by radioactive decay of other elements; the current amount in the earth's crust is constantly replenished. But none of the other 40 are found as naturally-occurring isotopes, indicating that the earth is much older than 50 million years. Of the 17 isotopes with half-lives greater than 50 million years, all are found naturally occurring in the earth's crust--they haven't had time to completely decay to non-radioactive forms. So geologists conclude that while the earth is much older than 50 million years, it is not infinitely old.
Plate tectonics
The study of the motions of the earth's crust has consequences for both geology and biology. The more important points of modern plate tectonics are as follows
- The continents are several distinct land masses or plates that "float" on top of a liquid magma layer of the earth.
- There are also plates which form the suboceanic floors; these are also floating and in motion.
- Currents in the magma force the various plates to move, sometimes separating from each other (as Europe and North America are doing now) and sometimes colliding (as the Pacific Plate and the North American plate are doing).
- Shifts in the magma flows are responsible as well for changes in the direction and magnitude of the earth's magnetic field over time.
- Earthquakes are the result of shock waves sent out as the plates collide or buckle in response to stresses of compression or expansion.
- Volcanoes occur where magma breaches the crust.
- Both earthquakes and volcanoes are more likely along the borders of the plates than within them.
By comparing geological formations which are found on multiple continents, and identifying geological formations which are unique to a particular area, geologists have put together a history of the movement of the earth's crust and continents over time. Their observations have been interpreted to show that the current continental plates date back to about 248 million years ago (the end of the Paleozoic era), but these plates have moved apart and come together several different times since.
The resulting shifts have produced seas and mountains, deserts and new land masses. While there is no conclusive geological evidence for a world-wide flood at any particular point in time, most areas show signs of inundation at some point in the past. For example, geologists and biologists think that much of southern Alberta and northern Montana were covered by a gigantic lake (Lake Missoula) near the end of the last ice age, about 11000 years ago. A glacial dam held the lake in place, but broke several times during the ice age period. Each time the lake emptied across the eastern Washington plains. The theory of Lake Missoula's dambreaks accounts for the "scoured" surface of the Washington Palouse area, the sinkhole lakes which were created, the carving of the Columbia River Gorge (and the levels of sedimentary carving along the gorge), and the presence of rocks native to northern Montana mountains found as river deposits along the Willamette in Oregon.
Such massive changes in surface orientation and location—even when they are carried out over long periods of time—would affect climates, which would affect the ability of species to survive and could account for some mass extinctions observed in the fossil record as well as genetic drift, genetic bottleneck, and genetic flow situations. Short-term violent tectonic events, such as volcanic eruptions or a series of earthquakes, could also change local habitats. Biologists have had ample opportunity to study their models of repopulation, natural selection, and species adaptation to changed habitats with the formation of Surtsey Island (off the coast of Iceland) and the eruptions of Mt. St. Helens in 1980. A surprising outcome in both cases was the rapidity with which both areas of volcanic "sterilization" were converted to complex ecological systems supporting many species.
Mass Extinctions and Key Adaptations
One of the more fascinating areas of speculation in evolutionary history is the problem of mass extinctions. Many biologists think there were six periods of massive extinctions, during which many species disappeared. Such extinctions occurred at the end of the Permian period (possible as a result of the formation of the supercontinent Pangaea), at the end of the Mesozoic era with the extinction of the dinosaurs, and during several ice-age periods of the more recent Holocene epoch. Besides tectonic events such as plate drift or a period of intense volcanic activity, biologists have proposed cataclysmic events such as comet or meteor impacts to account for mass extinctions. Any of these would create dust clouds which would lower temperatures planet-wide and result in severe changes in environment, weeding out all species which did not possess the ability to survive the new temperature range.
Species possessing key adaptations which fitted them for survival under both the old and new conditions (or which were preadapted to the new conditions) would not only survive, but gain new dominance as their former competitors for resources and habitats were wiped out. As the survivors increased in population, there would be greater genetic diversity and a greater range of characteristics within a species. This would account for the new types of fossils found in the post-extinction periods.
One possibility is that such new traits are the result of changes to the pattern of differentiation in the development of an organism.
Consider the salamander, a type of amphibian (which also includes frogs and toads). During the juvenile, aquatic stage, salamanders have gills (as do tadpoles). In most salamander species, metamorphosis to an adult form produces a gill-less, land-dwelling adult. However, some salamander species go through an incomplete metamorphosis; the adults of these species are mature enough to reproduce, but retain their gills, several of their other adolescent features--and remain aquatic for their entire lives. One speculation is that a mutation (what we might consider a defect) caused the metamorphic process to abort; but since the resulting individuals were still able to survive and ultimately inherit an aquatic habitat in which they didn't have to compete with as many other salamanders.
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