Evidence suggests that life first evolved around 3.5 billion years ago. This evidence takes the form of microfossils (fossils too small to be seen without the aid of a microscope) and ancient rock structures in South Africa and Australia called stromatolites. Stromatolites are produced by microbes (mainly photosynthesizing cyanobacteria) that form thin microbial films which trap mud; over time, layers of these mud/microbe mats can build up into a layered rock structure — the stromatolite.

Stanley Miller and Arnold Urey

Life could have arisen from simpler compounds present on primitive Earth.

Experiments:
Flask filled with water, hydrogen, methane, and ammonia and added electricity. Produced amino acids using a setup of test tubes and flasks to simulate conditions of early Earth.

Scientists at the University of Sheffield believe they have found evidence that life on Earth originated in space.

The research suggests that Earth is constantly bombarded by microbes from outer space, which arrive on comets and meteors.

Therefore life on Earth began when the planet became habitable enough for the microbes to survive and evolve.

Recently some scientists have narrowed in on the hypothesis that life originated near a deep sea hydrothermal vent. The chemicals found in these vents and the energy they provide could have fueled many of the chemical reactions necessary for the evolution of life.

Using the DNA sequences of modern organisms, biologists have tentatively traced the most recent common ancestor of all life to an aquatic microorganism that lived in extremely high temperatures — a likely candidate for a hydrothermal vent inhabitant!

Microspheres are not cells, but they have some characteristics of living systems: selectively permeable membrane, energy storage/release. Maybe microspheres acquired more and more characteristics of living cells!

Experiments simulating conditions of early earth suggest that small sequences of RNA might have formed and replicated on their own. From the relatively simple RNA-based life several steps could have led to the system of DNA-directed protein synthesis that exists now.

Abiotic stew --> simple organic molecules --> RNA nucleotides --> RNA replication/syntesis --> DNA directed protein synthesis

About 2 bya prokaryotic cells began evolving internal cell membranes. Then, some smaller prokaryotic organisms entered other prokaryotic organisms! Instead of infecting them, like a parasite, they started living inside the larger cell. Over time, they formed a special relationship whereby different prokaryotes developed different abilities. For example, one group evolved the ability to use oxygen to generate ATP. Other prokaryotes carried out photosynthesis. These eventually became the modern day mitochondria and chloroplasts.

Lynn Margulis
1. Mitochondria and chloroplasts contain DNA similar to bacterial DNA.
2. Mitochondria and chloroplasts have ribosomes whose size and structure closely resemble that of bacteria.
3. Like bacteria, mitochondria and chloroplasts divide by binary fission like bacteria do!

Origin of Earth
First one-celled organisms
First multicelled organisms


It is not known when life originated, but carbon in 3.8 billion year old rocks from islands off western Greenland may be of organic origin. Well-preserved bacteria older than 3.46 billion years have been found in Western Australia.[5] Probable fossils 100 million years older have been found in the same area. There is a fairly solid record of bacterial life throughout the remainder of the Precambrian.

First organisms with shells
Trilobites dominant
First fishes

First land plants
Fishes dominant
First insect fossils

Large Amphibians abundant.
Large coal swamps
First reptiles
Extinction of trilobites and many other marine animals

Dinosaurs dominant.
First birds
First flowering plants

Humans develop

"Age of mammals"

Extinction of dinosaurs and
many other species.