This question comes from my own son, Rohan who is nine and, like most children his age, he is full of lots of what, why and when queries.
The Earth is about 4.5 billion years old. The start of any type of life form is, naturally, a topic of much debate.
Some scientists estimate that ‘life’ began on our planet as early as four billion years ago.
And the first living things were simple, single-celled, micro-organisms called prokaryotes (they lacked a cell membrane and a cell nucleus).
There are still lots of prokaryotes around today, bacteria are just one example of these types of cell.
Trying to look back over time is never an easy study, as you can imagine.
Where do you begin to look for signs of early life?
Many scientists look for clues in ancient rock formations and there are reports of fossil remains of living organisms found in rock formations (in Greenland) dated back 3.7 billion years.
Other fossil remains were also found in rock formations in Australia, dating to around the same period.
More recent fossil studies carried out on rock samples from Canada suggest they have found fossil evidence of life that may date back as much as 4.28 billion years.
These rock formations are linked to ancient hydrothermal vents (fissures on the seabed from which hot, mineral-rich waters spew up).
Even if fossil evidence of living micro-organisms are found in these rock formations, we cannot say with certainty that the fossils are actually as old as the rocks themselves, and not simply a later addition, maybe carried into crack in the rocks by penetrating fluids, like water.
There is also the possibility that life began even earlier than these findings suggest, before these ancient rock were formed, before rock records began.
A recent study found the possible evidence of life in 4.1 billion-year-old zircon crystals, again found in Canada.
The study reports finding graphite (a form of carbon) in arrangements that would suggest an organic origin.
In simple terms this means that the type of graphite the found would have been the type expected to be present in early life.
If life did begin at these earlier times on earth (4.1 to 4.2 billion years ago) it would have had to survive a very hash environment.
This is known as the Hadean eon and, as the name suggests, it would have been a pretty hellish place to live.
The Earth would have been very hot and pretty unstable, its crust would only recently have formed.
On top of all that it would have been subject to frequent bombardment from asteroids and meteorites.
The atmosphere was mainly carbon dioxide although there was some hydrogen and water vapour also.
There were liquid water oceans on the planet, despite the high temperatures.
So like many questions asked in science, the answers are not given with 100% certainty.
It seems likely there was life on Earth not too long after it was formed, probably at least 3.5 billion years ago.
These simple life forms, either directly or indirectly, are part of our evolutionary history; from land, to sea, to air dwellers, all life ultimately came from similar, simplistic origins.