Scientists have long postulated that the early Earth's atmosphere must have had considerably higher pressure than it does today, since the sunlight was fainter. However, researchers have made a discovery which suggests that this theory is way off the mark. By studying the size of bubbles in 2.7 billion-year-old lava flows, they've found that the air pressure was less than half what it is now -- in other words, the atmosphere was much lighter back then. As there's evidence of liquid water at that time, the Earth must have had less nitrogen and more greenhouse gases (such as carbon dioxide and methane) preventing heat from escaping into space.
The researchers will need to find other samples to double-check their data. If it holds up, though, it'll do more than just prompt a rethink of Earth's primordial history. The breakthrough will also show that microbes can survive on planets with thin atmospheres, which raises the hope of finding exoplanets that harbor life.