The Earth is about 4.6 billion years old. Its early atmosphere came from gases released by volcanic activity, and it was very different from the air we breathe today. Nobody was there to measure it, so scientists can only say what the atmosphere was thought to contain.
It was thought to contain a large amount of carbon dioxide, water vapour and small amounts of other gases such as nitrogen, methane and ammonia. It contained little or no oxygen.
The evidence is limited because it is so old. Scientists use clues from very old rocks, and they compare with other planets: Mars and Venus today have atmospheres that are mostly carbon dioxide with almost no oxygen, which fits the idea that the early Earth was similar. Because the evidence is indirect, the ideas can change if new evidence is found.