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The History of Life on Earth

Biodiversity, the diversity of living things on earth, is changing constantly. Species arise and go extinct over time, and overall diversity on earth waxes and wanes.

The Origin of Life

Life originated via four major phases

Abiogenesis: Life from Non-living Matter

Abiogenesis is the origin of life from inorganic precursors.

  • Aleksandr Oparin
    In 1924, Russian biologist Aleksandr (Alexander) Oparin published a paper entitled The Origin of Life, in which he suggested that chemical reactions in the primitive oceans could have eventually "created" life. The work was never translated from Russian, it had little impact at the time.

    In Russia, Oparin was working with another aspect of the origin of early life.

    Recall how shaking oil and water together can (at least briefly) create a colloid.

    Other substances can be agitated together to form colloids.

  • Oparin mixed gelatin (a protein) and gum arabic (a polysaccharide/carbohydrate) and stirred them with a motion mimicking that of the early seas.

  • The results: tiny, stable, globular structures. He called these coacervates.

  • Adding additional substances to the reaction mixture yielded different results:

  • A coacervate with working enzyme systems inside can be considered a protocell/protobiont.

    A protobiont has some of the properties of life, depending on what was added to the mix, but not all.
    To be considered truly alive, a thing must have The Properties of Life

    What was Primordial Earth like?
    The early atmosphere was composed mostly of

  • Conventional Wisdom: Earth's primordial atmosphere was reducing, and conducive to formation of complex molecules.

  • Or: earth's early atmosphere was only weakly reducing, or even neutral.

  • Or: earth's early atmosphere was rich in either CO2 or H2 (also conducive to complex organic molecule formation)

  • Or: Life may have its origin in places other than earth's surface:

    The First Biological Molecules

    You already should be familiar with the Miller-Urey apparatus and its significance.

    (Click on the picture for more detailed information)

    Experiments by Joan Oro with a similar apparatus yielded

    Other experiments showed that dripping amino acid solutions onto hot clay or sand--as might have been found on primordial earth--caused the amino acids to polymerize into (highly cross-linked) oligopeptides (short proteins).

    Thomas Maimone of Scripps University has compiled an overview of experiments (with literature cited) that have contributed to our understanding of prebiotic chemistry.

    The Earliest Genetic Material
    Ribonucleic acid (RNA), not deoxyribonucleic acid (DNA), was the original genetic material

    John Sutherland, et al. (Manchester, UK) demonstrated that making RNA polymers from inorganic precursors was much simpler than previously believed.

    The evolution of comparatively stable DNA from an RNA precursor may have conferred a tremendous selective advantage.
    All known living organisms use DNA as a permanent genetic blueprint.

    Chlorophyll a and the Origin of the Oxidizing Atmosphere

    Earth's atmosphere contained little O2 until photosynthetic organisms appeared.

  • chlorophyll a (the most primitive form of chlorophyll) first appeared about three billion years ago, in the early ancestors of the cyanobacteria.

  • ~ 1 bya - eukaryotic autotrophs appear

  • For 1.2 billion years, these autotrophs produced oxygen that was taken up by exposed iron in the earth's crust (rust).

  • Once the rust "sink" was filled, oxygen began to enter the atmosphere.
  • ~ 600mya - atmospheric O2 levels were about 1% of PAL (PAL = "Present Atmospheric Level"). At this point:

  • ~ 400 mya - land plants were established. Atmospheric oxygen levels were about 10% PAL, and continuing to creep upwards.

  • Today's atmosphere is about 21% oxygen, thanks to photosynthesis.

    The Origin of Eukaryotes

    Two processes likely contributed to eukaryotic origin:


    Sequential Endosymbiosis

    After the first eukaryotic cells had formed, further endosymbioses occurred to produce various eukaryotic lineages.
  • Primary endosymbiosis - a larger cell engulfs a smaller cell, which then takes up residence to the benefit of both cells
    (as described above).

  • Secondary endosymbiosis - the product of primary endosymbiosis is engulfed by a larger cell, and then takes up residence to the benefit of both cells.

    Secondary endosymbiosis has given rise to a vast array of eukaryotic lineages.

    The typical cladogram we saw earlier could be modified to account for this combining of ancestral lineages:

    Transfer of genes from one species to another in this manner (i.e., not from parent to offspring) is known as HORIZONTAL GENE TRANSFER. (<--You don't have to read the entire link, but you should know what this term means.)

    YOU are a product of viral horizontal gene transfer. (OPTIONAL: Read more HERE.)

    Climate Change and Continental Drift: Driving Natural Selection

    As the land masses moved across the globe, their climates changed with their position on the globe, since climate of any given region is largely determined by its annual exposure to sunlight.

    Changing climate adds another dimension to natural selection.

    Global Patterns of Climate

    The ultimate source of climate is the sun, which provides not only the majority of energy on earth, but also creates climatic events when its randomizing energy interacts with the earth.

    The Tilt of Earth's Axis is the Reason for the Season(s)

    Flora and fauna are profoundly affected by environmental and seasonal changes in solar intensity and spectral distribution.

    The 23.5o tilt of he earth's axis results in annual changes in solar irradiation (sunlight hitting the earth's surface), and that creates the seasons in the northern and southern hemispheres:

    The living passengers on the moving land masses were thus subjected to changing ecosystems as the continents moved, resulting in differences in natural selection on each continent.

    Related species were gradually separated, and began to evolve in isolation from one another.

    Gradual change in climate of the moving continents was a major factor driving both speciation and extinctions. We can trace those changes through the fossil record. As you know, matching organismal phylogeny to continental drift and other geographical changes is the essence of biogeography

    Local climatic and geographic events (smaller than continental drift) have a similar, if more localized effect: rising mountain ranges, changes in humidity, water level, nutrient content and other physical factors can dramatically affect the evolutionary processes.

  • Continental drift caused allopatric speciation on a grand scale.
  • Fossil organisms now found on separated continents are the remnants of once-contiguous populations.

    The Cambrian Explosion: Evolution's Big Bang

    About 540 million years ago, the fossil record reveals a rather sudden appearance of a vast array of different types of organisms.

    All living phyla (and many more that are now extinct) had evolved by the end of the Cambrian Explosion.

    A quote you'll often hear is that all these taxa appeared in "the blink of an eye, geologically speaking".
    But geological time is not the same as biological time.
    As we have seen, sudden genetic changes can have very rapid evolutionary consequences if those changes are adaptive.

    Speciation and Extinction

    Most of the species that have existed on earth are now extinct.

    Just as speciation can be driven by

    So extinction can be driven by

    Mass Extinctions

    A particularly interesting case of amensalism nearly wiped out life on earth just as it barely started about 2.5 billion years ago.

    This is known as the Great Oxygenation Event, or the Oxygen Catastrophe.

    Mass Extinctions: Natural Disasters

    Natural catastrophes--terrestrial or cosmic--also have driven extinctions.
    The fossil record contains evidence of at least five global extinction cycles more recent than the Oxygen Catastrophe.