Without any free oxygen in the early atmosphere, there was no ozone layer to protect the surface from the intense bombardment of UV radiation from the Sun. The only places on Earth that were protected from lethal UV rayswere underwater. At the bottom of the oceans, in the darkest deep, volcanic vents erupted lava and chemically active gases into the cold water. These volcanic chemicals reacted with seawater to produce all kinds of organic compounds which further reacted to produce yet more complex chemicals.
Over millions of millennia, organic compounds developed near deep sea vents that could make copies of themselves from the ingredients around them. There was nobody there to notice, but at some point we would say these chemicals were alive! The energy for this earliest form of life was derived from the chemistry of the hot seepage from the vents. The chemicals that grew fastest and were best protected became most abundant. Eventually, the reactive stuff was sealed off inside a membrane made of fatty, oily material. These blobs of isolated chemicals were the first cells.
A billion years had passed since the oceans condensed when some cells began to manufacture energy-rich carbohydrates from dissolved CO2 using the energy of sunlight near the ocean surface. This earliest photosynthesis probably emerged around 3.5 billion years ago. We would probably call these earliest photosynthetic cells bacteria. They had a huge advantage in being able to produce their own food, so they could grow faster than their neighbors which harvested energy-rich molecules from the ocean. They reacted inorganic dissolved CO2 with seawater to make simple sugars, but they produced free oxygen as a byproduct. By 3 billion years ago, the first traces of dissolved oxygen began to accumulate in the primordial oceans.
Oxygen is incredibly reactive! In the oceans, it acts a lot like bleach. Oxygen combines aggressively with almost anything, breaking up chains of atoms in organic molecules. The advent of photosynthesis was a global crisis which probably killed most of the proto-life on the planet. Dissolved oxygen slowly built up as a waste product for hundreds of millions of years, so that by 2.5 billion years ago the only living things in the oceans either made oxygen by photosynthesis or could tolerate its burn.
By 2 billion years ago, photosynthesis had transformed not just the oceans, but even the rocks and the air. Rocks began to rust, forming a new class of oxide minerals. Seawater became saturated with oxygen, which then began to degas into the atmosphere. Eventually free oxygen levels in the atmosphere were high enough that the hard UV sunshine blasted it into ozone, shielding the bare continents from the deadly radiation.
Once the atmosphere was oxidized, many of its gases reacted away. This was especially true of methane (CH4), a reduced greenhouse gas that had kept the Earth warm for billions of years. As the methane oxidized to CO2 and H2O, the Earth cooled dramatically. At least three times, the oceans froze all the way to the Equator, plunging the Earth into a stable ice-ball state that locked up all the liquid water at the surface. Each time, the CO2 from volcanos then built up over many millions of years to re-thaw the planet. The most recent of these “Snowball Earth” episodes was about 700 million years ago.