The earliest life on Earth were simple prokaryotic cells that existed around 3.5-3.9 billion years ago. These included stromatolites, primitive photosynthetic bacteria that formed along coastlines. As these prokaryotes became more widespread, they changed the atmosphere to include more oxygen. The first eukaryotic cells evolved around 2.1 billion years ago and included protists like protozoa and algae. Eukaryotic cells developed in two stages: infolding of the cell membrane to form internal structures like the nucleus, and endosymbiosis where mitochondria and chloroplasts originated from engulfed prokaryotic cells and became cellular organelles.

2. CELL EVOLUTION

3. Fossil evidence indicates that prokaryotic
cells existed before eukaryotic cells
Prokaryotic cells are simpler and have less
structural organization when compared to
eukaryotic cells
It appears that the first organism on Earth
were simple prokaryotes that existed
around 3.5 – 3.9 billion years ago =
stromatolites
Stromatolites are primitive
photosynthetic prokaryotes

4. They were primitive bacteria which formed
along the coastline and preserved as
fossils.
These were autotrophic organisms , able to
make their own food and produce oxygen.
As they became more widespread they
were able to change the composition of the
atmosphere.
An increase in the amount of oxygen
enabled the development of heterotrophic
organisms
The oldest eukaryotic organisms seem to
have evolved around 2.1 billion years ago.

5. The oldest and most primitive eukaryotes
are called protists and are divided into two
groups. Many forms still exist and live
mainly in aquatic habitats
1. Protozoa
2. Algae
Their development occurred in two stages
1. Infolding of the plasma membrane to
form an endo-membrane system. This
process is thought to have created the
endoplasmic reticulum, golgi body and
nucleus

6. 2. The second process is called
endosymbiosis. Chloroplasts and
mitochondria appear to have evolved from
smaller prokaryotes which became
incorporated, possibly after being “eaten”
by a larger cell.
Mitochondria are thought to have been
originally heterotrophic cells, able to use
oxygen in the process of aerobic respiration
to release energy.
Chloroplasts are thought to have evolved
from autotrophic cells, able to use light
energy to produce food and release oxygen

8. Evidence supporting the
endosymbiotic theory
Mitochondria and chloroplasts have inner
membranes that are similar to the plasma
membrane of living prokaryotes
Both replicate by splitting in a similar
fashion to prokaryotic cells
Both have circular DNA like prokaryotes
Both have their own protein manufacturing
system including ribosomes