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How did life begin?

After the earth cooled, there was little free oxygen in the atmosphere and lots of energy available (lightening, volcanoes, UV light)
Organic molecules could form spontaneously in the oceans
History of life is a series of speciation and extinction events


The first cells (types, genetic molecules, energy)

Prokaryotes (bacteria/archaea)
Used RNA for functions now performed by DNA and proteins (RNA world)
Obtained energy by absorbing and breaking down organic molecules and later by engulfing and digesting smaller cells


Why did the increase in atmospheric oxygen occur and why is it considered the most important environmental change in the history of life?



Evolutionary Milestones

1. Photosynthesis
2. Evolution of eukaryotes
3. Multicellularity
4. Internal skeleton
5. Eukaryotes move on to land
6. Reptiles evolved from early tetrapods
7. Asteroid impact/ Environmental change
8. Mammals diversified
9. Human evolution


1. Photosynthesis

Evolved 1st in prokaryotes (eg cyanobacteria)
Released oxygen into atmosphere
O2 initially reacted with iron and minerals, but eventually built up in the oceans and atmosphere


The Oxygen Catastrophe

Newly "oxic" environment caused mass extinction of anaerobic life
Reacted with methane (a greenhouse gas), which removed greenhouse gas from the atmosphere and precipitating a snowball ice age
Organisms that could withstand the catastrophe survived, allowing for eventual evolution of aerobic respiration and formation of the ozone layer which facilitated colonization of land


2. Evolution of Eukaryotes

Key: Much greater metabolic efficiency
Evolved from prokaryotes that engulfed/ were invaded by other cells
Engulfed cell lived and reproduced inside host cell (endosymbiosis)
The 2 cells became co-dependent
The engulfed cell's descendants became organelles (chloroplasts, mitochondria)


Evidence that chloroplasts and mitochondria used to be free-living prokaryotes

1. Both contain DNA
2. Both reproduce independently of the rest of the cell
3. Both contain ribosomes resembling those of bacteria
4. Both have double membranes


3. Multicellularity

Key: enabled organisms to grow larger and enabled specialized structures to form
2 types: simple and complex multicellularity


Simple multicellularity

Aggregation of cells


Complex multicellularity

Communication between cells, specialized parts


What hurdles need to be overcome for multicellularity to evolve?

Preventing overpopulation of cells (controlling cell division)
*Cancer is a disease that represents a breakdown in the ability to overcome this hurdle


4. Internal skeleton

Key: better support and allows greater range of body shapes
Better locomotion
Precursor to jaws


5. Eukaryotes move onto land

Algae and plants moved first
Then invertebrates (worms, insects)
Then 1st tetrapods (4 legged animals) evolved from lobe-finned fish but required moisture


What is evidence that limbs evolved while animals were still living in water?

Fish fossil found with fingers, gills, and fish-like tail
Also tetrapod arm found in streambed


What was the advantage for moving onto land?

Escape predators in water
Access to more resources


6. Reptiles evolved from early tetrapods

Able to live in dryer habitats
Some evolved to large size (dinosaurs)


7. Asteroid impact/ Environmental change

Drove dinosaurs to extinction
Smaller animals survived
Birds and mammals both evolved from reptiles prior to extinction of dinosaurs, and they diversified following the extinction


Mammal-like reptiles

Mammals descended from pre-dinosaur reptiles
Upright stance
Transition to mammalian teeth, jaws, and ears
Mammals have variety of teeth, reptiles have all one type of tooth
Reptiles have 3 bones on each side of the lower jaw and 1 bone in each ear, while mammals have 1 bone on each side of the lower jaw and 3 bones in each ear


What is the advantage of using marsupials over rodents for studying development?

Have development outside the uterus


Why do embryos sometimes illustrate the same changes during development that occurred over millions of years of evolution?

Early embryonic development is highly conserved over time


Key points of life history

1st tetrapods evolved from fish
Ancestors of reptiles, birds, and mammals branched off next
Both birds and mammals evolved from reptiles


8. Mammals diversified

Primates evolved from insectivores
Had forward-facing eyes, which is an advantage for binocular vision and depth perception
Had opposable thumb
Most recent common ancestor of humans and chimps was about 6 mya


9. Human evolution

Human and chimp lineages diverged about 6 mya
Many different hominin species have existed, often simultaneously
Human family tree is a twiggy bush rather than a linear progression
Few older hominin fossils have been found because they died in forests that do not fossilize well and are hard to find fossils in


Important transitions in hominin evolution

1st bipeds (walk on 2 legs) appeared early in hominin evolution (advantage= can see farther, keep cooler, carry food, and better for long-distance travel)
1st manufactured stone tools about 2 mya
Fire about .5 mya


Why is it hypothesized that the first genetic material was RNA rather than DNA?

When modern cells make proteins, they first copy genes from DNA into RNA and then use the RNA as a blueprint to make proteins. This last stage could have existed independently at first. Later on, DNA could have appeared as a more permanent form of storage, thanks to its superior chemical stability.

Also, the RNA versions of enzymes, called ribozymes, also serve a pivotal role in modern cells. The structures that translate RNA into proteins are hybrid RNA protein machines, and it is the RNA in them that does the catalytic work. Thus, each of our cells appears to carry in its ribosomes “fossil” evidence of a primordial RNA world


Modern RNA and DNA molecules contain a nucleobase, ribose sugar, and phosphate, all of which combine into a nucleotide. What were the difficulties with understanding how both ribose and phosphate originally became available and incorporated into the genetic material? What are the current hypotheses concerning both the ribose and the phosphate? How do scientists think the three subunits began to assemble into nucleotides (or at least the C and U nucleotides)?

Ribose is unstable and rapidly breaks down in an even mildly alkaline solution. Phosphorus is abundant in the earth’s crust but mostly in minerals that do not dissolve readily in water, where life presumably originated. So it is not obvious how phosphates would have gotten into the prebiotic mix. The high temperatures of volcanic vents can convert phosphate-containing minerals to soluble forms of phosphate, but the amounts released, at least near modern volcanoes, are small. A completely different potential source of phosphorus compounds is schreibersite, a mineral commonly found in certain meteors. The corrosion of schreibersite in water releases its phosphorus component. This pathway seems promising because it releases phosphorus in a form that is both much more soluble in water than phosphate and much more reactive with organic (carbon-based) compounds.

Simply mixing the three components in water does not lead to the spontaneous formation of a nucleotide—largely because each joining reaction also involves the release of a water molecule, which does not often occur spontaneously in a watery solution. For the needed chemical bonds to form, energy must be supplied. The new proposed pathway does not generate exclusively the “correct” nucleotides. But amazingly, exposure to ultraviolet light destroys the “incorrect” nucleotides and leaves behind the “correct” ones. The end result is a remarkably clean route to the C and U nucleotides.


How is it hypothesized that the first cells copied their genetic information, given that they probably lacked enzymes made of protein? There are two hypotheses, one that includes a catalyst (i.e. a molecule that functions like an enzyme) and one that does not.

RNA sequences in the very first organisms could have directed its own replication by having catalytic RNA sequences.

Or, small changes to the chemical structure of the sugar component in DNA/RNA—changing one oxygen-hydrogen pair to an amino group (made of nitrogen and hydrogen)—made the polymerization hundreds of times faster, so that complementary strands formed in hours instead of weeks.


What were the earliest cell membranes probably like?

Primitive membranes were probably made of simpler molecules, such as fatty acids. These membranes would still pose a formidable barrier to the entry of nucleotides and other complex nutrients into the cell. So, vesicles probably brought nutrients in and out


What role might a ‘volcanic area’ have play in the origin of life?

In one possible scenario, the protocells circulated between the cold and warm sides of a pond, which may have been partially frozen on one side (the early earth was mostly cold) and thawed on the other side by the heat of a volcano. The temperature differences would cause convection currents, so that every now and then protocells in the water would be exposed to a burst of heat as they passed near the hot rocks, but they would almost instantly cool down again as the heated water mixed with the bulk of the cold water. The sudden heating would cause a double helix to separate into single strands. Once back in the cool region, new double strands—copies of the original one—could form as the single strands acted as templates