IN THE BEGINNING. Welby Thomas Cox, Jr.

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СКАЧАТЬ failure of evolutionary theory. The fossil record was incomplete in Darwin's time, but many of the important gaps that existed then have been filled by subsequent paleontological research. Perhaps the most persuasive fossil evidence for evolution is the consistency of the sequence of fossils from early to recent. Nowhere on Earth do we find, for example, mammals in Devonian (the age of fishes) strata, or human fossils coexisting with dinosaur remains. Undisturbed strata with simple unicellular organisms predate those with multicellular organisms, and invertebrates precede vertebrates; nowhere has this sequence been found inverted. Fossils from adjacent strata are more similar than fossils from temporally distant strata. The most reasonable scientific conclusion that can be drawn from the fossil record is that descent with modification has taken place as stated in evolutionary theory.

      Special creationists argue that "no one has seen evolution occur." This misses the point about how science tests hypotheses. We don't see Earth going around the sun or the atoms that make up matter. We "see" their consequences. Scientists infer that atoms exist, and Earth revolves because they have tested predictions derived from these concepts by extensive observation and experimentation.

      Furthermore, on a minor scale, we "experience" evolution occurring every day. The annual changes in influenza viruses and the emergence of antibiotic-resistant bacteria are both products of evolutionary forces. Indeed, the rapidity with which organisms with short generation times, such as bacteria and viruses, can evolve under the influence of their environments is of great medical significance. Many laboratory experiments have shown that, because of mutation and natural selection, such microorganisms can change in specific ways from those of immediately preceding generations.

      On a larger scale, the evolution of mosquitoes resistant to insecticides is another example of the tenacity and adaptability of organisms under environmental stress. Similarly, malaria parasites have become resistant to the drugs that were used extensively to combat them for many years. As a consequence, malaria is on the increase, with more than 300 million clinical cases of malaria occurring every year.

      Molecular evolutionary data counter a recent proposition called "intelligent design theory." Proponents of this idea argue that structural complexity is proof of the direct hand of God in specially creating organisms as they are today. These arguments echo those of the 18th century cleric William Paley who held that the vertebrate eye, because of its intricate organization, had been specially designed in its present form by an omnipotent Creator. Modem-day intelligent design proponents argue that molecular structures such as DNA, or molecular processes such as the many steps that blood goes through when it clots, are so irreducibly complex that they can function only if all the components are operative at once. Thus, proponents of intelligent design say that these structures and processes could not have evolved in the stepwise mode characteristic of natural selection.

      However, structures and processes that are claimed to be "irreducibly" complex typically are not on closer inspection. For example, it is incorrect to assume that a complex structure or biochemical process can function only if all its components are present and functioning as we see them today. Complex biochemical systems can be built up from simpler systems through natural selection. Thus, the "history" of a protein can be traced through simpler organisms. Jawless fish have a simpler hemoglobin than do jawed fish, which in turn have a simpler hemoglobin than mammals.

      The evolution of complex molecular systems can occur in several ways. Natural selection can bring together parts of a system for one function at one time and then, at a later time, recombine those parts with other systems of components to produce a system that has a different function. Genes can be duplicated, altered, and then amplified through natural selection. The complex biochemical cascade resulting in blood clotting has been explained in this fashion.

      Similarly, evolutionary mechanisms are capable of explaining the origin of highly complex anatomical structures. For example, eyes may have evolved independently many times during the history of life on Earth. The steps proceed from a simple eye spot made up of light-sensitive reticula cells (as is now found in the flatworm), to formation of individual photosensitive units (ommatidia) in insects with light focusing lenses, to the eventual formation of an eye with a single lens focusing images onto a retina. In humans and other vertebrates, the retina consists not only of photoreceptor cells but also of several types of neurons that begin to analyze the visual image. Through such gradual steps, very different kinds of eyes have evolved, from simple light-sensing organs to highly complex systems for vision.

      Chapter 4

      Life's Origins by Land or Sea?

      Volcanic springs and deep-ocean vents get new evidence

      A laboratory-created 'chemical garden' made of a combination of black iron sulfide and orange iron hydroxide/oxide is shown in this photo. Chemical gardens are a nickname for chimney-like structures that form at bubbling vents on the seafloor. Credi

      The question ‘How did life begin?’ is closely linked to the question ‘Where did life begin?’ Most experts agree over ‘when’: 3.8–4 billion years ago. But there is still no consensus as to the environment that could have fostered this event. Since their discovery, deep sea hydrothermal vents have been suggested as the birthplace of life, particularly alkaline vents, like those found at ‘the Lost City’ field in the mid-Atlantic. But not everyone is convinced that life started in the sea – many say the chemistry just won’t work and are looking for a land-based birthplace. With several hypotheses in play, the race is on to replicate the conditions that allowed life to emerge.

      In 1977, the first deep sea hydrothermal vent was discovered in the East Pacific Rise mid-oceanic ridge. Named ‘black smokers’, the vents emit geothermally heated water up to 400°C, with high levels of sulfides that precipitate on contact with the cold ocean to form the black smoke. This was followed in 2000 by the discovery of a new type of alkaline deep-sea hydrothermal vent found a little off axis from mid-ocean ridges. The first field, known as the Lost City, was discovered on the sea floor Atlantis Massif mountain in the mid-Atlantic.

      The vents are formed by a process known as serpentinization. Seabed rock, in particular olivine (magnesium iron silicate) reacts with water and produces large volumes of hydrogen. In the Lost City, when the warm alkaline fluids (45–90°C and pH 9–11) are mixed with seawater, they create white calcium carbonate chimneys 30–60m tall.

      In 1993, before alkaline vents were actually discovered, geochemist Michael Russell from Nasa’s Jet Propulsion Laboratory (JPL) in California, US, suggested a mechanism by which life could have started at such vents.1 His ideas, updated in 2003, suggest life came from harnessing the energy gradients that exist when alkaline vent water mixes with more acidic seawater (the early oceans were thought to contain more carbon dioxide than now).

      This mirrors the way that cells harness energy. Cells maintain a proton gradient by pumping protons across a membrane to create a charge differential from inside to outside. Known as the proton-motive force, this can be equated to a difference of about 3 pH units. It’s effectively a mechanism to store potential energy and this can then be harnessed when protons are allowed to pass through the membrane to phosphorylate adenosine diphosphate (ADP), making ATP.

      Russell’s theory suggests that pores in the hydrothermal vent chimneys provided templates for cells, with the same 3 pH unit difference across the thin mineral walls of the interconnected vent micropores that separate the vent and sea water. This energy, along with catalytic iron nickel sulfide minerals, allowed the reduction of carbon dioxide and production of organic molecules, then self-replicating molecules, and eventually true cells with their own membranes.

       CHEMICAL GARDENS СКАЧАТЬ