I am writing this from the Phanerozoic eon, and you are presumably reading it from the Phanerozoic eon, as well.
When describing geological time (and writing pop songs), different terms are used to describe different scales. For example, an eon is the largest unit and contains all the others, such as era, period, epoch, and age. Those are the “geochronological” units in order, from largest to smallest.
We are currently in the Meghalayan age in the Holocene epoch in the Quarternary period in the Phanerozoic era. I, for one, am disappointed to learn that the age of Aquarius is nowhere to be seen from here.
As mentioned, we are currently in the Phanerozoic era, but it hasn’t always been like this. Never mind roads and bridges, in prior eras, the land and the air were very different than they are today. Our current era was preceded by the Proterozoic era, which — if I’m understanding Ma (megaannus) properly — was from 2,500,000,000 – 538,000,000 years ago. The Proterozoic era is the longest of the four eras so far.
Following… no, preceding the Proterozoic was the Archaen eon, which ran from 4,031 to 2,500 Ma (4,031,000,000 to 2,500,000,000 years ago!). When we look back this far, we see a planet that essentially doesn’t support life as we know it. Back at the beginning of the Archaen eon, if there was any life at all, it was likely comprised of single-cell organisms that consumed sulfur.
But let’s go back even further! Let’s go back to the beginning, to the Hadean era — Hades, as you may know — is Greek for hell or the underworld. The geological era has that term because the physical conditions are thought to have been very inhospitable to life. Ok, now we are way way way back. It is theorized a planet called Theia that was ~1/4 the size of Earth collided with Earth, creating both the oceans and Earth’s moon. This collision is theorized to have happened about four and a half billion years ago. I guess that’s not really the beginning — as we define it. However, this is so far back that the Earth is thought to have been a protoplanetary disk at this point.