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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
The generation of small organic molecules from abiotic (non-living) precursors
The joining of these smaller subunits into macromolecules
The packaging of these macromolecules into protocells consisting of a membrane and an internal space filled with fluid that was different from the surrounding medium
The origin of self-replicating molecules that made inheritance possible
Abiogenesis: Life from Non-living Matter
Abiogenesis is the origin of life from inorganic precursors.
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
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
Lipids created a membrane-like sheet that conferred greater stability.
Enzymes and substrates could be incorporated into coacervates, and began to function normally inside them.
Or: Life may have its origin in places other than earth's surface:
deep sea thermal vents
space debris colliding with earth (Joan Oro's idea)
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
amino acids (from hydrogen cyanide + ammonia in aqueous solution)
adenine (from HCN)
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).
The Earliest Genetic Material
Ribonucleic acid (RNA), not deoxyribonucleic acid (DNA), was the original
it is simpler (than DNA) to construct from raw materials
it has enzymatic properties (ribozyme activity), and can modify its own structure as well as self-replicate
extant viruses (retroviruses) demonstrate that RNA can be reverse transcribed into its more stable cousin, DNA
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
~ 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:
Cellular Respiration replaced fermentation as the most common metabolic pathway.
An ozone (O3) layer started to form in the upper
Ozone shielded microorganisms from lethal ultraviolet (UV) and gamma radiation.
Upper levels of the ocean (and possibly wet areas of earth's surface) could now be colonized by living things.
~ 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:
Extensive inpocketing of the external plasma membrane formed a complex internal network of membranes.
First proposed in 1910 by Russian botanist Konstantin Mereschkowski, this theory was confirmed via genetic evidence by Lynn Margulis in 1967. It describes how small, energy-transducing prokaryotes were either
ingested as prey
...inside larger prokaryotes, where they survived and thrived.
Eventually, host and symbiont became inextricably linked
in a symbiotic relationship that gave rise to the first eukaryotic cells.
such symbioses exist today (e.g. Giardia, a basal (i.e., very primitive)
flagellated protist has symbiotic, energy-transducing bacteria instead of
mitochondria and two haploid nuclei.)
A few extant species of cyanobacteria and heterotrophic bacteria
strongly resemble chloroplasts and mitochondria, respectively.
Bacterial DNA is circular. Mitochondrial and chloroplast DNA are also circular.
Neither bacterial, mitochondrial, nor chloroplast DNA are associated with histones.
Mitochondria and chloroplasts synthesize their own DNA and proteins independently of the rest of the cell.
Like bacteria, mitochondria and chloroplasts begin translation of all proteins with a tRNA carrying N-formyl methionine (not plain methionine, as in proteins encoded by the nucleus).
Mitochondria and chloroplasts reproduce via fission, as bacteria do.
If all mitochondria or chloroplasts are removed from a cell, the cell cannot replace them by making new ones.
Mitochondrial and chloroplast ribosomes are more similar to bacterial ribosomes than to the ribosomes in the eukaryotic cytoplasm.
Mitochondria and chloroplasts are compromised by many of the same antibiotics that compromise bacteria, and for the same reasons. (For example, chloramphenicol interferes with bacterial and mitochondrial ribosomes, but not cytoplasmic ribosomes.)
nucleus, mitochondria and chloroplasts all have a double membrane.
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.
Red algae (products of primary endosymbiosis) took up residence/were ingested by heterotrophic eukaryotes and became the precursors of modern taxa
Green algae (products of primary endosymbiosis) took up residence/were ingested by heterotrophic eukaryotes and became the precursors of modern taxa
Chlorarachniophyta (green algae and plants)
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 tropics lie between the Tropic of Cancer (23.5o N) and the Tropic of Capricorn (23.5o S).
The tropics receive the highest annual input of solar energy on earth. The tropics are the only place on earth where the sun ever shines directly overhead. This happens on
March 21 (vernal)
September 21 (autumnal)
the subtropics lie between the Tropic of Cancer and
30oN in the northern hemisphere, and between the
Tropic of Capricorn and
30oS in the southern hemisphere. (Miami is in the
the temperate regions lie between 30oN and
60oN in the Northern Hemisphere and between 30oS and
60oS in the Southern Hemisphere.
the polar regions lie above 60oN (arctic) and S (antarctic)
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.
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.
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.
Ordovician-Silurian Extinction - 440 mya
Mass extinction of small marine organisms.
Devonian Extinction - 365 mya
Large-scale extinction of tropical marine species.
Permian-Triassic Extinction - 250 mya
The largest mass extinction event in Earth's history affected a range of species, including many vertebrates. Over the course of about 500,000 years, approximately 96% of all marine animals died off, and terrestrial animals also suffered massive extinctions.
This occurred close to the time of a huge series of volcanic eruptions in what is now Siberia.
Extinctions may have been due to
Permian Mass Extinction -
direct effects of lava and ash
excess CO2 caused global temperatures to rise ~6oF
More uniform temperature across the globe reduced ocean water cycling, reducing oxygen content (ocean anoxia)
anaerobic bacterial overgrowth in oceans may have increased H2S concentrations: toxic, and ozone-destroying
Triassic-Jurassic Extinction - 210 mya
The extinction of many vertebrate species on land allowed dinosaurs and plants to flourish.
(In paleontology, K is the abbreviation used for the Cretaceous, since the letter C is already used as an abbreviation for the Cambrian.)
Mass extinction of about 75% of terrestrial plant and animal species, including the non-avian dinosaurs. The event defined the end of the Cretaceous and the beginning of the Tertiary.
More than half of all marine species, and huge numbers of terrestrial species died off.
Possibly caused by the Chixulub Comet, whose crater lies at the bottom of the ocean near Mexico's Yucatan peninsula.
If you want to hear a recent theory about what actually happened when the comet hit (and never sleep again), listen to Radiolab's Diopocalypse: Science as Performance Art.
And if you just can't get enough Mass Extinction...