1. History of Life On Earth
AP Biology

2. Essential properties of shared, core life processes
All organisms share core life processes:
1. DNA & RNA as carriers of genetic information
2. A universal genetic code
3. Many metabolic pathways
These shared, core life processes are shared by all domains of life and give evidence for common ancestry.

3. Eukaryotes share core life processes
In eukaryotes, conserved core elements include:
1. presence of cytoskeleton
2. nucleus
3. membrane-bound organelles
4. linear chromosomes
5. endomembrane systems
Supports common ancestry for all eukaryotes

4. Formation of Earth
There is scientific evidence that Earth and other planets of the solar system formed about 4.6 bya, condensing from a vast cloud of dust and rocks that surrounded the young sun.

5. Conditions on early earth
As the bombardment of early Earth by collisions of huge chunks of rock and ice began to slow down (about 3.9bya), conditions on early Earth were extremely different than today.
First atmosphere: very thick with water vapor, along with various compounds released from volcanic eruptions (N, NOx, CO2, CH4, NH3, H2, and HS).

6. A. I. Oparin and J.B.S. Haldane’s Hypothesis
In the 1920s, Russian chemist A.I. Oparin and British scientist J.B.S. Haldane hypothesized that Earth’s early atmosphere was a reducing (electron-adding) environment, in which organic compounds could have formed from simple molecules.
The energy for this synthesis came from lightning and intense UV radiation.
Haldane suggested that the early oceans were a solution of organic molecules, a “primitive soup” from which life arose.

7. Can organic molecules form under conditions believed to simulate those on early Earth?
In 1953, Stanly Miller set up a closed system to simulate conditions thought to have existed on the earth Earth.
A flask of water simulated the primeval sea.
The water was heated so that some vaporized and moved into a second, higher flask containing the “atmosphere”—a mixture of gases.
Sparks were discharged in the synthetic atmosphere to mimic lightning.

8. Miller Urey Experiment results
Miller identified a variety of organic molecules that are common in organisms.
These included simple compounds such as formaldehyde (CH2O), hydrogen cyanide (HCN) and more complex molecules including amino acids and hydrocarbons.

9. Meteorites bring organic compounds
Miller-Urey experiments show that abiotic synthesis of organic molecules is possible given certain conditions of the early Earth.
Support for this idea also comes from analyses of the chemical composition of meteorites.
Among the meteorites that land on Earth are carbonaceous chondrites—rocks that are 1-2% carbon compounds.
Fragments of a fallen 4.5 billion year old chondrite found in Australia in 1969 contain more than 80 amino acids. The proportions of these match those produced in the Miller-Urey experiment.

10. Abiotic synthesis of macromolecules
The presence of small organic molecules doesn’t mean life.
Every cell has a vast assortment of macromolecules such as proteins and DNA.
Could such macromolecules have formed on early Earth?
Scientists have shown that when organic monomers (such as amino acids) are heated and splashed onto hot rocks or sand, the heat vaporizes the water and binds the amino acids into polymers called “proteinoids”.

11. Protobionts
Protobionts are collections of abiotically produced molecules surrounded by a membrane-like structure.
Protobionts may exhibit some properties of life, including simple reproduction and metabolism, as well as maintaining an internal chemical environment different from that of their surroundings.

12. Protobiont formation may start with liposomes
Laboratory experiments demonstrate that protobionts could have formed spontaneously from abiotically produced organic compounds.
Certain small membrane- bound droplets called liposomes can form when lipids or other organic molecules are added to water.
The hydrophobic molecules form into bilayers at the surface of the droplet.
Liposomes can “reproduce”. Their bilayers are selectively permeable. They can undergo osmotic swelling and shrinking . Some can perform simple metabolic reactions.

13. Self-replicating rna
The first genetic material was likely RNA, not DNA.
Thomas Cech (U of Co) and Sidney Altman (Yale) found that RNA can carry out a number of enzyme-like catalytic functions.
Cech called these RNA catalysts ribozymes. Some ribozymes can make complementary copies of RNA, provided they have the nucleotide building blocks.

14. Ability to self-replicate rna led to natural selection
Natural selection on the molecular level has produced ribozymes capable of self-replication in the laboratory.
Single-stranded RNA molecules assume a variety of 3-dimensional shapes.
In a particular environment, RNA molecules with certain base sequences are more stable and replicate faster and with fewer errors than other sequences.

15. Protobionts with self-replicating, catalytic rna
A protobiont with self- replicating, catalytic RNA would differ from its many neighbors that did not carry RNA.
If that protobiont could grow, split, and pass its RNA molecules to its “daughters”, the daughters would have some of the properties of their “parent”.

16. Probable appearance of an early protobiont

17. Protobiont evolution

18. Conditions on the early earth made the origin of life possible
The earliest evidence of life on Earth comes from fossils of microorganisms that are about 3.5 billion years old.
Stromatolites living on coast of Australia (left) and rocks showing stromatolites from 3.5bya.

19. Early prokaryotic cells
The first cells were simple prokaryotic cells. Evidence of prokaryotic cells appear in rocks 3.5 billion years old.
Photosynthetic prokaryotes appeared bya. Rocks in Greenland show evidence of the appearance of atmospheric oxygen (called banded rock formations)
from photosynthetic bacteria.

20. Oxygen levels increase in atmosphere
Over time, as photosynthetic bacteria produce oxygen, it begins to build up on the atmosphere.
This eventually paves the way for the evolution of eukaryotes.

21. Photosynthesis and the oxygen revolution
Most atmospheric oxygen gas (O2) is of biological origin, produced during photosynthesis.
At first, the O2 dissolved in the surrounding water, reacting with iron to form iron oxides, which began to accumulate in sediments. These sediments compressed into banded rock formations.
Once all of the water was saturated with O2, it began to enter the atmosphere.
The increase in O2 had a tremendous impact on life.
In certain of its chemical forms, Oxygen attacks
chemical bonds and can inhibit enzymes and
damage cells. As a result, many types of prokaryotes
became extinct. Survivors, however, evolved many
adaptations.

22. Endosymbiotic theory—the first eukaryotes
The oldest fossils of eukaryotes are about 2.1 billion years old. How did eukaryotic cells evolve from prokaryotic?

23. Endosymbiotic theory Evidence
1. The inner membranes of both organelles (mitochondria and chloroplasts) have enzymes and transport systems that are homologous to those found in the plasma membranes of living prokaryotes.
2. Mitochondria and plastids replicate by a splitting process that is similar to that of certain prokaryotes.
3. Each of these organelles contains a single, circular DNA molecule that, like the chromosomes of bacteria, is not associated with histones or other proteins. They have the capability of producing proteins.
4. Their ribosomes are similar to those of prokaryotes.