Once life arose in the form of simple, single celled organisms, it continued as single celled organisms for at least a billion years. Two very different types of life developed, archaea and bacteria. Both are single celled organisms with no nucleus or organelles. For many years they were thought to just be very different types of bacteria, but have since been discovered to have very distinct biochemistry and are considered to be separate domains in the three-domain system of classification. Together bacteria and archaea are referred to as prokaryotes.
At some point, around 2 billion years ago, archaea and bacteria found a way to share genes or merge some of their material and a third kingdom of life, eukaryotes, was born. Today eukaryotes are much larger and more complex than either bacteria or archaea, but when they first emerged, they were probably not very different from their cousins. It is difficult for scientists to tell exactly when they emerged because such small organisms did not fossilize very frequently or very well. They were certainly already in existence by 1.8 billion years ago.
Photosynthesis was a major development that substantially changed the environment on Earth about 500 million years ago. Many existing organisms likely became extinct during this time, while others evolved to take advantage of the more oxygen-rich atmosphere.
Around this time, multicellular organisms began to develop, and life as we know it took off. The invention of meiosis, a type of reproduction unique to eukaryotes, developed and within 200 million years, there were more than 100 different orders of plants and animals, which is almost as many as today!
There are two different hypotheses about how multicellular life developed, and both are difficult to prove because the first multicellular organisms were very small, and perhaps also very soft, and probably didn’t fossilize well.
Some scientists believe that what we see in the fossil record is actually how life developed. There were very few or no multicellular organisms until about 500 million years ago, and then very quickly, a variety of them began to develop. Others argue that multicellular organisms could have developed much earlier than what we see in the fossil record. These organisms were likely very small and very soft (no hard shells or bones to make their fossils obvious) and we simply aren’t seeing much evidence for them, but they must have been there.