How thick was Earth's primordial atmosphere? Earth's first atmosphere was made of hydrogen and helium accumulated before fusion started in the Sun. As soon as this happened it was stripped away by the Sun. How many atm of hydrogen and helium had Earth accumulated at the peak pressure of the first atmosphere?
We have no way of knowing how much hydrogen and helium was in the Earth's primordial atmosphere. What is more important is what the atmosphere consisted of when the primeval soup from which life arose was formed, just over 4 billion years ago. We have a fairly good idea what this atmosphere consisted of: mainly carbon dioxide and nitrogen, rather like Mars at the present time, but with substantial amounts of ammonia and methane. Also very small amounts of hydrogen and helium, most of which had by then been blown away by solar radiation. There was hardly any oxygen. As for pressure, all we can say for sure is that on Earth it was a good deal higher than it is today. Conditions at that time were similar, though not identical, on Venus, Earth and Mars, perhaps on Titan as well, so it is possible that all three terrestrial planets developed a version of the Earth's primeval soup, which was essential for the creation of life. Whether life emerged on any planet other than Earth, we don't yet know. Because all three planets had oceans, water vapour would also have been an important atmospheric constituent.