"The geological record is extremely imperfect."
-- Charles Darwin
Thoughts on evolution through the ages:
Plato - There is a Great Chain of Beings from simplest to most complex. Complexity reflects the more perfect form. All life imbued with an immutable essence. The chain of beings ranges (with no sense of time or evolution implied) from inanimate matter to algae to higher plants, sponges jellyfish, echinoderms, worms, clams/snails, insects, crustaceans, squids, octopus, egg-laying vertebrates, whales, 4 legged mammals to Humans. Humans are nearest (although short of?) to the perfect life form.
Jean-Baptise Lamarck (1744-1829). Presented the first general theory of evolution. Life spontaneously generates from dirt to form lower plants. No extinction occurs, so all forms that have arisen exist today. Traits are inherited from parents to offspring, and there is a long-term response to use or non-use of organs. Humans, which are complex, have progressed further from simplest life, and this implies longer time. Thus, humans are the oldest of organisms. Algae are relatively young.
George Cuvier (1769-1832). An expert in comparative anatomy and the 'father' of Catastrophism. He argued that extinction does take place. This is largely a consequence of catastrophic environmental disruption (of unspecified nature). Evolution is not possible, as species are stable and immutable. There are a great diversity of life forms and diverse body designs. Complexity and sequence is not explained.
Charles Darwin. Was a medical student at Edinburg, then a divinity student at Cambridge. His interested turned increasingly to naturalism (sort of synoptic science of the day), and he was profoundly influenced by the book "Principles of Geology" written by Charles Lyell in 1830. The notions of uniformitarianism and great spans of time stemming from Hutton and Lyell began to form in his mind a notion of biological gradualism. In 1831-36 he went on naturalist expedition of the HMS Beagle. Saw many features in South America and the Galapagos that reinforced his conviction that the geological notions were sound (including the effects of a mammoth, magnitude 9+ earthquake in southern Chile, which uplifted coastlines and inundated some regions). Careful study of finches and other species populations on the Galapagos framed by the notion of long time catalyzed his thinking about evolution. He formulated a theory of evolution and natural selection upon his return to England, but waited about 20 years to publish it as he saw similar conclusions emerging in others (and he had sought to avoid some of the inevitable controversy he knew would descend upon him). In 1859 published Origin of Species, Descent with Modification.
Basic notions: Evolution occurs by very gradual transformation upon descent from earlier organisms. The relationship between parent and offspring is key to all diversity (genetic nature of reproduction was not yet known). Variation in species or subspecies lead to divergence. Was aware that this occurs with spatial isolation of subpopulations (as in the progressive patterns seen across the Galapagos). He amassed a great body of knowledge to formulate his argument for evolution, including Morphology (similarity of active and vistigial organs and bones in diverse animals), Embryology (similarity of very young life forms, pre-diversification of body parts, amongst very different adult species), Geographic Distribution (isolation of groups such as the marsupials found only in Australia), and the overall classification of life based on organized relationships (similarities of function/form in body parts; energy harvesting mechanisms, etc., that enable description as a branching tree of life).
But, how does evolution work? Natural selection provides a mechanism:
IF certain conditions are met:
Variation: Populations must vary in traits
Fitness Difference: There must be a relationship between variation in traits and mating ability, fertility, fecundity, and/or survivorship that affects viability of offspring
Inheritance: Offspring acquire (somehow) traits from their parents that will activate fitness differences.
THEN
Different ages classes in the population will differ in mean value of a given trait. This allows a favored trait to become a majority trait; the system is not in equilibrium. This is EVOLUTION WITH ADAPTATION, and is a very gradual process with small imperceptible steps. Mutation plays a key role in inducing variation.
Subsequent work by Gregor Mendel (using pea plants) established that inheritance has discrete particulate nature (ultimately explained as due to Genes providing heridatary particles. Mutation is largely attributed to radiation (solar/cosmic X-Rays and Gamma Rays) and chemical damage. About 1 out of 100,000 sex cells may be affected in a given population. Most damage is bad, and leads to non-viable offspring. A fraction lead to diverse neutral or positive traits. Resulting diversity operates in a system of fitness selection.
Through gradual shift of dominant traits due to adaptation/competition in a fixed environment, one develops a population that cannot breed with its earlier form; SPECIATION has taken place. Darwin felt that EXTINCTION took place with competing forms, with traits that had become more suitable for the environment win out and take-over. He seems to have believed this was not likely a catastrophic process by one of steady increase in diversity with time with continuous speciation and extinction.
While the fossil record documents an expanding sequence of life forms, one cannot find all intermediate transitional forms between seemingly related species. Only a few well recorded gradual transitions are evident in the fossil record. Darwin argued that this is to be expected, as the "geologic record is extremely imperfect".
Darwin's theory radically affected biological thinking, and over 150 years the evidence for evolution and speciation has been extensively tested and documented. The role of selection and inference about the continually radiating cone of life (the only figure in Darwin's book), have been the subject of much debate.
Most important of the subsequent perspectives is the belief that speciation occurs in subpopulations that are isolated (genetically) from the parent population. Only by isolation in a separate environment can traits evolve substantially without being blended back into the large population and stymied. This is the notion of Allopatric Speciation (speciation in a separate location). Darwin seems to have had a sense for this, but geologists were slow to apply the notion to the geological record of life, even as allopatric speciation gained great conviction in the biological community dealing with modern life forms and experiments.
The major boundaries in the geological record are defined primarily by major transitions in the life assemblages as recorded in fossils. Some of these transitions appear to be relatively gradual, while others are abrupt and dramatic. But, is the apparent abruptness an artifact of incomplete geological preservation of the history of time and life? A strong requirement of the Huttonian theory of geological evolution is that large gaps in time exist in the rock record, from the moment at which a region stops receiving new rock deposition and is elevated and eroded before sinking again and being overlain by rock. The gap in time may be immense, and almost must be immense, for the rates of vertical motion are presently observed to be relatively slow (1 cm/thousand years is pretty fast). The nature of unconformities predicts that there will be huge gaps in the rock, and hence in the fossil records.
From the work of Charles Lyell (Principles of Geology, 1833) to Charles Darwin (Origin of Species, 1859), the notion of great gouts of time, incomplete preservation of the rock record, and Uniformitarianism became so ingrained in Earth Science thought that Gradualism dominated the field for 150 years. Invoking uniformity of Law, Process and Rate, there was a strong dogma that no special catastrophic processes need be introduced to understand either geological history of evolution.
This was invoked repeatedly by Darwin, to explain the absence of gradual transitions in evolutionary history. The many 'missing links' between successive fossil lifeforms was attributed to an incomplete geological record. Darwin assumed that the environment changed only very gradually, and that competition drives the process of speciation and evolution. In a uniform environment, each organism will thrive by widening its niche, at the expense of other organisms sharing a common pie of life. This requires mutation and gradual adaptation which leads to the concept of survival of the fittest.
However, in the 1970's this dogma came under scrutiny by paleontologists, who were looking at much more comprehensive rock and fossil records than available to Lyell or Darwin. Steven Gould articulated the notion of Punctuated Equilibrium in a series of works from 1972-1977, arguing that speciation occurs in subpopulations, largely in response to competition with a changing environment, rather than under uniform conditions. While accepting some aspects of gradualism, this notion invokes external triggers which change the environment abruptly to perturb the system, allowing some organisms to flourish and others to abruptly go extinct, by an accelerated natural selection.
Presently, Earth Sciences involve a merging of the ideas of Gradualism and Punctuated Change, recognizing that both play a role in both geological processes and evolution of life. In particular, mass extinctions prompted by rapid environmental changes have modulated the history of life.
Let us now consider one of the most intensively studied mass extinctions. This is the Cretaceous/Tertiary (K-T) extinction event which took place about 65 million years ago. This is long ago, but only a tiny fraction of the Earth's lifetime, and the rock records are quite good for this time interval. As with any extinction event, the first issue to address is what lifeforms were involved in the extinction process that defines the transition. In this case, the K-T event involved extinction of 64% of all species on Earth, and included organisms in both Marine and Terrestrial environments:
Marine:
Terrestrial:
Equally important are the survivors: small mammals, crocodiles
These facts suggest a global extinction that was not confined to just oceans or land, and which involved a wide range of organisms with different life needs.
The next issue is how abrupt was the extinction process? This involves the imperfection of the rock record. Precision in timing requires a well-documented, continuous record. Given the statistical vagaries of fossil preservation to begin with, sparse sampling of a gradual extinction may give the false impression of a catastrophic abrupt extinction.
For the K-T boundary, there has been extensive analysis of extinctions before the boundary, which show about 5-8 million years of reducing diversity in some groups such as the Rudists and Inoceramids. These well-preserved fossil records clearly indicate that there was a period of slow environmental change preceding the boundary which was responsible for some extinctions. The general cause is believed to have been the gradual lowering of sea level and recession of the huge interior seaways, such as had inundated much of North America. The question then arise; were the gradual changes responsible for some run-away mechanism that caused more abrupt extinction events at the K-T boundary? Or are there two independent processes operating?
The way to address the abruptness of any of the extinction events is to improve the record quality for the time interval of interest. One seeks appropriate age rock formations that were deposited in sites with continuous sedimentation (no intervals of uplift and erosion), which points toward marine environments. In addition one wants high sedimentation rates, so that a thick layer of rocks is deposited across the time interval. The faster the rate of sedimentation, the better the resulting time resolution. This points toward marine environments adjacent to continents, where there is good sediment supply.
This search to find a good record section led Walter Alvarez, a U.C. Berkeley geologist to the Gubbio rock formation in Italy. In this formation, a high rate of continuous sedimentation straddles the K-T boundary, and there were good characteristic fossils deposited throughout the section. At the 'top', or most recent part of the Cretaceous, the Gubbio formation involved pelagic marls, or carbonate rich sedimentary rocks, with stable Cretaceous age marine planktic (floating) foraminifera. This marl is disrupted by an layer of clay, dated right at 65 million years, indicating a major change in sediment type, that slowly was restored to pelagic marls, but now with Tertiary assemblages of foraminifera. The clay layer is very distinctive, and being bracketed by Cretaceous fossils and Tertiary fossils, it is taken to lie right at the boundary. What caused the change in sediment? Is it related to the change in lifeforms?
Walter asked the question, is there anything unusual about this clay? He benefited from the fact that his father, a professor of Physics at Berkeley, Luis Alvarez, had set up an analysis procedure to determine the composition of heavy metals in substances. Some of the boundary clay was tested, and revealed an unusually high concentration of the Platinum group element IRIDIUM. Iridium is a heavy metal very sparsely found on Earth, and with no known mechanism for concentrating it in a geological process. This suggested an extraterrestrial origin, as many iron-rich meteorites are much enriched in Iridium relative to the Earth's rocks (most of the Earth's Iridium is concentrated into its core). Thus, the Alvarez father and son advanced the idea that a large meteorite, enriched in Iridium by its own prior history of metal separation hit the Earth and the debris deposited a global layer.
This was immediately testable by examining the K-T transition at other regions with continuous sediment deposition across the boundary. Again and again, the materials, often involving a clay layer, but sometimes not, showed enriched Iridium relative to the background level of normal rock. In detail, the abundance of the Iridium relative to other heavy metals was very close to that for meteorite samples. Systematic mapping of this Iridium anomaly ensued. Further examination of K-T record sections revealed an equally unusual attribute in 1984. This was the discovery of quartz grains with shock lamella, or bands. Shocked quartz requires very high pressures and temperatures (it was previously only observed near underground nuclear explosions), and is very difficult to account for by anything other than a large impact. The K-T sections also revealed a large amount of microspherules, small glass beads, which appear to be drops of molten rock that cooled rapidly while flying through the air.
Mapping of the Iridium anomaly, shocked quartz, and microspherules, indicated a worldwide event at the end of the Cretaceous. Additional field studies revealed very thick, complex deposits around the Caribbean, some in Haiti, enriched in microspherules and quartz, and some in Texas and Louisiana which involved jumbled piles of debris that appears to be the deposit of a large tsunami wave. These thick deposits suggest that the impact took place in the Gulf of Mexico. In 1989, subsurface imaging revealed the existence of a 180 km diameter crater underlying the northern Yucatan peninsula, buried by the last 65 million years of sediment. The rock type there is rich in carbonates and evaporites and there is a thick deposit of broken rock overlying the subsurface crater. This is the Chicxulub crater, now the most likely candidate for the primary impact of the K-T period (there is actually evidence for two impacts, perhaps one year apart). Other candidate craters for later impacts include the smaller Manson crater in Iowa and the Popigai crater in Siberia, both of which have ages of 65 million years. Perhaps the asteroid broke up and their were multiple impacts, just as happened with the Shoemaker-Levy comet that hit Jupiter recently.
If we accept this evidence for a large impact, we must ask the question, how plausible is such an event? Is it a likely or highly improbable event? Studies of the crater density on Earth and the Moon provide some estimate of the rate of impact of different size objects. The results are startling:
|
Diameter |
Frequency |
Energy (TNT) |
|---|---|---|
|
several meters |
1/yr |
20 Kilotons (Hiroshima=14Kt) |
|
10's of meters |
1/100yr |
several Megatons (1908 Tunguska) |
|
>1 km |
1/1000000yr |
1 million Megatons |
There should be a 100 m object hitting the Earth about every 10,000 years, which is the size of impactor that is associated with Meteor Crater in Arizona. A 10 km object, such as needed to account for the volume of Iridium at the K-T boundary should hit Earth about once every 50-100 million years. Thus, in a way, meteor falls are common phenomena, and are indeed to be expected with time.
Could such an impact actually kill so many forms of life, as observed at the K-T boundary? What are the kill mechanisms? Several are postulated:
Direct Hit: The explosion produced by impact of a 10 km diameter projectile is about 100 million megatons of TNT, or 5 billion times the strength of the Hiroshima bomb. It would produce a 100+ km crater, like that found in the Yucatan. Over 100 x 10exp(12) tons of pulverized rock would be ejected into the atmosphere.
Since the impact site was under water, there was a huge wave generated.
The Dust in the atmosphere would produce a darkening for at least one year, sometimes called the Impact Winter scenario. This is long enough for most plants to die, disrupting the food chain on both land and sea.
The fireball from the impact would set the world's forests burning, and indeed in the carbon isotope record for the K-T boundary there is evidence for massive fires.
The impact site was rich in carbonates, and this would vaporize to produce carbonic acid, with a short duration of acid rain.
Carbon dioxide released from the impact would add to a short-term Greenhouse effect, causing planetary warming.
These effects could have combined to eliminate many of the species that became extinct at the end of the Cretaceous.
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