No-one is certain, but the current hypothesis is in the depths of the ocean, near sources of geothermal energy. These are the most hospitable environments, providing an environment for a huge diversity of organisms.
There are some biologists who believe that life(or its building blocks) may have been brought here by comets. It's not so outlandish when you consider that bacteria and viruses can survive space, and that amino acids have been detected on comets.In the case of Earth,both hypotheses may be true.Neither would disprove the other.It is also possible that there were primitive bacteria here before the oceans formed,when the earth was still hot, and water was only able to condense into pools at high altitudes.Maybe the first life evolved in these.
Life started when Earth started to cool down (When Earth was born, it was red hot and incapable of supporting any life form we know today). The steam from the Volcanoes formed clouds and it started to rain. Disregard that previous sentence, volcanic clouds are completely different than rain clouds. After month of raining oceans started to form ( other theories suggest that a meteorite containing frozen H2O might have hit earth) basic chemicals started to react with each other. Basically ammonia, Phosphoric salt, light, heat, electricity (lightning), and etc made life possible. This is what they call the "soup" when primitive RNA and DNA was in the ocean. It would be easy for organisms to evolve after they take a basic life form.
Then there's always these theories about life being "seeded" from an extra terrestrial environment. Because there's such LITTLE chance of having life formed on a planet. Some alien species might have put us here on Earth, for what reason, I do not know. But this theory came up because just shortly after Earth was formed (roughly a billion years) there is a diversity of life already in the ocean.
So let me sum it up for you, however life started we do not know, and we probably will never know, it may be god or some sort of consciousness that exists outside of space and time, but we do know that there was a big explosion of evolution within a short amount of time. (one billion year is very short for life to take up such amount of diversity).
The current theories indicate that Earth was formed around 4.6 Billion years ago.
The first cell-like organisms evolved just over 4 Billion years ago.
Note that bacteria are considered the simplest form of life, but actually have some very complex structures with DNA replication cell walls, flagella, and etc. These likely took millions or hundreds of millions of years to evolve.
Eukaryotic cells (cells in plants, animals, yeasts (but still single cellular)) appeared about 2 billion years ago.
Multicellular organisms first appeared about 1.2 billion years ago.
Movement of life onto land was about 600 million years ago, and coincided with the Cambrian Explosion and a major diversification of species.
See Wikipedia links.
Life began very early in the history of the earth, some 3 to 4 billion years ago. There would likely have been very little surface land at that time. It has been speculated that life may have begun in some shallow sea, fed by nutrients washed from a shore. We know the oceans teemed with life hundreds of millions of years before the first multicellular organisms (plants and insects) invaded the land.
I believe life most likely originated near deep ocean thermal vents. We know amino acids are naturally produced in abundance in such locations. There early cells would have been protected, with rich supplies of energy and nutrients from which to feed.
Life on Earth predates humans so we cannot accurately answer either part of this question.
When
The earliest evidence we have of life is from fossils of stromatolites in Australia that are about 3.4 billion years old.
How
This is still in dispute, but it is generally felt that a meteor crashing into the earth might have caused reactions which created first amino acids, which are the building blocks of all life forms.
It is currently believed that life on Earth began in the oceans at hydrothermal vents, mid-ocean ridges, and/or in the photic zone.
The first life on Earth were probably of bacterial origin, more specifically archaebacteria. These bacterias are simple and can survive in harsh conditions such as those of the early Earth.
It is disputed, but it was probably a microbe that metabolized sulfate and lived in the deep sea near hydrothermal vents over 3.5 billion years ago.
God created the earth.
in the seas and oceans
4.6 BYA
Life which does not originate from planet Earth may very well have developed on other planets. There is really no other explanation for where it would have come from.
earth origin
mantle
Sedimentary
4.6 BYA
Not at present. All life on Earth appears to have evolved here, and it's all related.
Life which does not originate from planet Earth may very well have developed on other planets. There is really no other explanation for where it would have come from.
An exabiologist is one who studies alien life forms; creatures that do not originate from Earth. :)
On Earth One must remember that God made all things spiritually before He made them physically and placed them upon the earth.
Earth
Alien, is the term used to refer to life which does not originate from Earth. Though no hard evidence has ever been found to confirm presence of any life form outside earth.
There are many places where weather could potentially originate on Earth. Most of the weather on Earth originates from the atmosphere.
From the goddess
The definition of alien is a "person in a country who is not a citizen of that country". They are also referred to as Extraterrestrial life (from the Latin words: extra ("beyond", or "not of") and‎ terrestris ("of or belonging to Earth")) is defined as life that does not originate from Earth.
earth origin
Underground - from radioactive decay.