The black lines on the granite setts around the edge of the Rock Circus show the evolution of life on Earth condensed into one calendar year. This is based on fossil evidence and on radiometric dating of the rocks. The origin of the Earth, 4.6 billion years ago, is shown by the black line behind the obelisk.
The red lines on this diagram show five mass extinctions of life. Only the two largest mass extinctions are shown around the edge of the Circus.