Origin of cells

Conditions on early Earth and prebiotic formation of carbon compounds (simplier vr)

Lack of the ozone layer (free oxygen)

High temperatures (>100 degrees Celsius)

No solid land mass → surface was water/ocean

The inner core was liquid and hotter

Ultraviolet light penetration

Electrical storms

Volcanic eruptions

Higher concentrations of methane, CO2, ammonia, hydrogen, and water vapor

Smaller magnetic field and higher lvl of radiation

Cells as the smallest units of self-sustaining life

Cell theory

Smallest unit of life

All living things are composed of cells

Cells only arise from pre-existing cells

Characteristics of living organisms: Mrs. Gren

Movement

Reproduction

Sensitivity

Growth

Respiration

Excretion

Nutrition

In cases such as viruses, they are regarded as non-living, even though they possess genetic material (DNA and RNA); they do not possess most of the characteristics of life

Viruses do not have metabolic reactions such as respiration

Cannot excrete metabolic wastes

Unable to respond to stimuli, therefore cannot grow

They are not capable of reproducing by themselves (need a host to do so)

Spontaneous origin of cells

Cells are highly complex structures that can currently be produced by the division of pre-existing cells

Requirement for the development of the first cells:

Catalysis

Self-replication of molecules

Self-assembly

Emergence of compartmentalization

Evidence for the origin of carbon compounds: Miller-Urey experiment:

Purpose: To test Haldane and Oparin’s hypothesis that the macromolecules of life could be spontaneously generated on a prebiotic Earth

Attempted to replicate the conditions of prebiotic Earth in a closed laboratory environment. Includes → low oxygen, high radiation lvl, high temperature and electrical storm

Methane. Ammonia, hydrogen, and water vapor represented the prebiotic atmosphere

Water = represents the ocean → boiled to form vapor, then mixed with methane, ammonia, and hydrogen

A mixture of gases is then exposed to electrical discharge (sparks) to stimulate lightning

The condenser allowed water vapor to return to the liquid water.

Mixture was then left to rest, after 1 week = it turns brownish black

Demonstrates that water contains many complex organic molecules → amino acids, hydrocarbons, etc

Conclusion: Organic molecules can spontaneously form from inorganic molecules

Evaluation of Miller-Urey experiment:

Strength

Weakness

Modelled prebiotic Earth and its atmosphere

Demonstrated that organic molecules such as amino acids can spontaneously formed from inorganic molecules under certain conditions

The experiment can be replicated by other scientists

Did not produce all of the organic molecules required for life

Simulation could not account for all conditions on the prebiotic earth

Spontaneous formation of vesicles

Formation of a membrane-bound compartment is needed to allow internal chemistry to be separated from external compartments

Formation of bilayers: Isolate internal environment = optimal conditions for replication or catalysis can be maintained

Cell membranes: Cell metabolism needs to be separated from the cytoplasm’s external environment, which cell membranes are responsible for (modern cells)

Fatty acids form spontaneously, forming spherical bilayers (vesicles) when mixed with water

Hypothesize that the first genetic material was trapped within a phospholipid vesicle, forming a protocell

RNA

Can be replicated and have some catalytic activity = which could initially act as both genetic material and the enzymes of the earliest cells.

Ribozymes in ribosomes are still used to catalyze peptide bonds during protein synthesis

RNA is hypothesized to be the first genetic material bc:

Can store genetic information

Is capable of self-replication

Can catalyze reactions

Evidence for the last universal common ancestor (LUCA)

The universal genetic code and shared genes across all organisms. Leading to the belief that other forms of life evolved but became extinct due to competition from the LUCA and its descendants

LUCA

Most recent common ancestor for all organisms on Earth

LUCA and its descendants may outcompete other forms of life that exist in the same period.

Evidences

All living things share several genes that are assumed to have been inherited from LUCA

Cladistics allows scientists to estimate the age of common ancestors by analyzing changes in the genetic code

Estimate dates of the first living cells and LUCA

Earth → 4.6 b yrs, LUCA → approx appeared 4 b yrs ago

Life has continued to evolve for the last 4 b yrs

Cyanobacteria: Oldest fossils of cells, and are approx 3.5 b yrs old. Have been discovered in ancient seafloor hydrothermal vent precipitates.

Minerals and rocks around the earliest fossils have been dated thru radioactive decay of uranium (decays @ a steady rate)

Molecular clocks: Comparing the # of mutations in genes between species, allowing scientists to estimate when two species diverged. Also used this technique to estimate when LUCA existed

Evidence for the evolution of LUCA in hydrothermal vents

Ancient seafloor hydrothermal vent precipitates + evidence of sequences from genomic analysis

Phylogenetic analysis and evidence suggest that LUCA evolved in hydrothermal vents. As the vents are thought to physically protect LUCA from external oceans

Hydrothermal vents

Rich in organic compounds → methane, carbon dioxide, hydrogen, etc

Similar to Miller-Urey’s experiment, it demonstrated that proteins and nucleotides can form from these chemicals