Chapter 25 - The History of Life on Earth

Chapter 25: The History of Life on Earth

• Fossils of microbes that existed 3.5 billion years ago provide direct proof of life on early Earth.

• However, how did the first living cells emerge? Observations and experiments in chemistry, geology, and physics have prompted scientists to suggest one scenario, which we will look at in this article.

• They propose that chemical and physical processes might have resulted in basic cells via a four-stage process:

  1. The abiotic (nonliving) synthesis of small organic molecules, such as amino acids and nitrogenous bases

  2. The joining of these small molecules into macromolecules, such as proteins and nucleic acids.

  3. The packaging of these molecules into protocells, droplets with membranes that maintained internal chemistry different from that of their surroundings

  4. The origin of self-replicating molecules that eventually made inheritance possible

• Synthesis of amino acids during a simulated volcanic eruption. Miller conducted an experiment mimicking a volcanic explosion in addition to his classic 1953 research. Researchers discovered that considerably more amino acids were generated under simulated volcanic circumstances than under the original 1953 experiment settings in a 2008 reanalysis of those data.

• By simulating laboratory circumstances similar to those expected to prevail on early Earth at the time (as shown in the image attached).

• His equipment produced a variety of amino acids that are still found in organisms today, as well as other chemical substances

• Many laboratories have since replicated Miller's famous experiment using other atmospheric formulas, some of which also generated organic molecules.

• However, some evidence shows that the early atmosphere was largely composed of nitrogen and carbon dioxide and that it was neither reducing nor oxidizing (electron removing).

• Recent Miller/Urey experiments employing such "neutral" atmospheres have also yielded organic compounds. Furthermore, tiny pockets of the early atmosphere, such as those around volcanic vents, may have been decreasing.

• The earliest organic compounds may have originated around volcanoes. In 2008, researchers utilized cutting-edge technology to reanalyze molecules preserved by Miller from one of his studies.

• The 2008 research discovered that many amino acids synthesized under circumstances resembling a volcanic explosion (as shown in the image attached above).

• Another theory holds that organic molecules were initially created at deep-sea hydrothermal vents, which are placed on the seafloor where hot water and minerals rush from the Earth's core into the ocean.

• Some of these vents, dubbed "black smokers," emit water that is so hot (300–400°C) that organic molecules produced there may be unstable.

• Other deep-sea vents, known as alkaline vents, emit water with a high pH (9–11) and is warm (40–90°C) rather than hot, creating an environment that may have been more conducive to the beginning of life (as shown in the image attached).

• According to research connected to the volcanic-atmosphere and alkalinevent theories, abiotic synthesis of organic molecules is feasible under a variety of circumstances. Meteorites might also have been a source of organic compounds. For example, remnants of the Murchison meteorite, a 4.5 billion-year-old rock that fell in Australia in 1969, contain more than 80 amino acids, some in significant quantities.

  • The term macroevolution refers to the sweeping changes in life on Earth as revealed by fossils of which the broad pattern of evolution is above the species level.

  • Examples of macroevolutionary change include the emergence of terrestrial vertebrates through a series of speciation events, the impact of mass extinctions on biodiversity, and the origin of key adaptations such as flight.

• The fossil record traces the evolution of life. The fossil record, which is primarily based on fossils discovered in sedimentary rocks, records the rise and collapse of many groups of creatures across time.

• The relative ages of fossils are revealed through sedimentary layers. Radiometric dating and other methods can be used to estimate the ages of fossils. The fossil record demonstrates how new groupings of creatures can emerge through the progressive alteration of preexisting ones.

• Plate tectonics describes the slow movement of continental plates through time, affecting the physical geography and climate of the Earth, resulting in extinctions in some groups and speciation in others.

• Five major extinctions have occurred throughout evolutionary history, dramatically altering the course of life. Continental drift, volcanic activity, and comet impacts are all possible causes of these extinctions.

• Adaptive radiations that occurred after catastrophic extinctions resulted in significant increases in biological variety. Adaptive radiations have also occurred in groups of species that possessed significant evolutionary advances or colonized new areas with minimal competition from other creatures.

• Developmental genes influence species morphological differences through altering the rate, timing, and spatial patterns of change in an organism's shape as it develops into an adult.

• Changes in nucleotide sequences or the control of developmental genes can lead to the evolution of new forms. Novel and sophisticated biological structures can emerge as a result of a succession of gradual changes, each of which benefits the organism that has it.

• Natural selection in a changing environment or species selection as a consequence of interactions between organisms and their present habitats can both generate evolutionary trends.

• Key events in life’s history include the origins of unicellular and multicellular organisms and the colonization of land as shown in the image attached.

• The rise and fall of groups of organisms reflect differences in speciation and extinction rates.

• You've seen numerous examples of form-fitting functions at all levels of the living process. We may, however, conceive structures that might work better than certain ones seen in nature. For example, if a bird's wings were not created from its forelimbs, such a bird might fly while still holding items with its forelimbs.

• When horse ancestors reached the grasslands that expanded throughout the mid-Cenozoic, there was a strong selection favoring grazers that could avoid predators by racing faster.

• This tendency would not have developed in the absence of open grasslands. An evolutionary tendency, whatever its source, does not indicate that there is an underlying urge toward a specific phenotype.

• Evolution is the outcome of organisms' interactions with their existing surroundings; if environmental conditions change, an evolutionary tendency may halt or even reverse itself. The sum of these continuous interactions between creatures and their surroundings is enormous: It is via them that the mind-boggling divers are reached.