- The earliest cells emerged around 3.8 billion years ago from non-living organic molecules on early Earth. Experiments in the 1950s showed that amino acids could form from basic atmospheric molecules through lightning and heat. - It is believed that RNA was the first genetic material and could self-replicate, in what is called the 'RNA world'. Later, DNA replaced RNA as the genetic material and protein synthesis evolved. - The first cells likely had a phospholipid membrane enclosing self-replicating RNA and proteins. Photosynthesis and aerobic respiration later evolved, allowing cells to more efficiently harness energy from sunlight and oxygen. Modern eukaryotic cells acquired mitochondria and chloroplasts through endosymbiosis of ancient

1. Origin of Cells
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2. Origins of Life
There is an unbroken chain of life from the first cells on Earth to all cells in organisms alive today.
We know this because cells can only be formed by division of pre- existing cells.
It is estimated that life first emerged at least 3.8 billion years
ago, approximately 750 million years after Earth was formed

4. Can Life Arise Spontaneously?
It was hypothesised in the 1920s that organic molecules (like amino acids) could arise under the right conditions
Miller’s experiments in the 1950s supported the hypothesis
With electricity (i.e. lightning), heat (from the sun), water and atmospheric molecules like CO2and NH3, the organic molecules began to form over time.

5. From Organic Molecules to Cells
The first cells must have arisen from non-living material.
Miller’s experiments show that the materials to make proteins were present.
The 64 codons in the genetic code have the
same meanings in nearly all organisms, but that
there are some minor variations that are likely to
have accrued since the common origin of life
on Earth.

6. The RNA World
In the 1980s, Altman and Cechdiscovered that RNA is capable of catalysingsome chemical reactions, including the polymerization of nucleotides. RNA is uniquely able both to serve as a templatefor and to catalyse its own replication.
Consequently, RNA is generally believed to have been the initial genetic system, and an early stage of chemical evolution is thought to have been based on self- replicating RNA molecules—a period of evolution known as theRNA world.
Ordered interactions between RNA and amino acids
then evolved into the present-day genetic code, and
DNA eventually replaced RNA as the genetic material.

7. Cell Membrane
The first cell is assumed to have had a phospholipid bilayer membrane, enclosing a cytoplasm and DNA.
The enclosure of self-replicatingRNA and associated molecules in a phospholipid membrane would thus have maintained them as a unit, capable of self-reproduction and further evolution.
RNA-directed protein synthesis may already have evolved by this time, in which case the first cell would have consisted of self-replicating RNA and its encodedproteins.

8. Metabolism
 It is believed that cells initially used anaerobic
respiration (in the absence of oxygen) in a process like
glycolysis: the breakdown of glucose to lactic acid
 Then two things must have happened:
 Origin of photosynthesis
 Aerobic respiration

9. Photosynthesis
Photosynthesis allows cells to use light and CO2to make glucose for energy
The first photosynthetic bacteria, which evolved about3 billion years ago, probably used H2S to convert CO2to organic molecules: a pathwaystill used by some bacteria today.
H2O as an electron donor and hydrogen for the conversion of CO2to organic compounds evolved later and led to the oxygenation of Earth's atmosphere.
The use of H2O in photosynthetic reactions produces the by- product free O2; this mechanism is thought to have been responsible for making O2abundant in Earth's atmosphere.

10. Aerobic Respiration
Once oxygen was abundant in the atmosphere, respiration began to use oxygen: aerobic respiration.
O2is a highly reactive molecule, and oxidative metabolism, utilizing this reactivity, has provided a mechanism for generating energy from organic molecules that is much more efficient than anaerobicglycolysis.

11. Endosymbiotic Theory
Photosynthesis and respiration initially took place in the cytoplasm.
However, when we look at eukaryotic cells today (like our own), we see that many have chloroplasts and mitochondria: organelles where these reactions take place.
They also have a nuclear membrane encasing the DNA
The organelles are thought to have been acquired as a result of the association ofprokaryotic cellswith the ancestor of eukaryotes.

12. Evidence for Endosymbiosis
The hypothesis thateukaryotic cellsevolved from a symbiotic association of prokaryotes—endosymbiosis—is particularly well supported by studies ofmitochondriaand chloroplasts, which are thought to have evolved from bacteria living in large cells.
Both mitochondria and chloroplasts are similar to bacteria in size, and like bacteria, they reproduce by dividing in two.
Most importantly, both mitochondria and chloroplasts contain their ownDNA, which encodes some of their components.

13. What is Endosymbiosis?
Endosymbiotic theory suggests that chloroplasts, mitochondria and perhaps even other organelles, were prokaryotic cells which lived inside larger cells.
Mitochondria and chloroplasts originated from the endosymbiotic association of aerobic bacteria andcyanobacteria, respectively, with the ancestors of eukaryotes.

14. Pasteur’s Experiments
Pasteur’s experiments were evidence that spontaneous generation of cells and organisms does not now occur on Earth.
In his day, people did not realise that bacterial infections were transmitted from person to person, so hand washing and treatment of food was not commonplace
Pasteur showed that bacterial growth on foods like milk, and the infections in hospitals, were from pre-existing cells replicating and being transported.
Thus, he developed the germ theory of disease

15. Pasteur’s Work
Using germ theory, Louis Pasteur was then able to develop a cure for anthrax (in cattle) and rabies.
He developed the process of pasteurisation to make milk safe to drink