I think this picture is badly put together. It is labelled as if the centre is the past, and left and right are the present, but that's not actually how it's drawn, since molluscs (snails, claims and octopuses) lie right in the middle at the four billion year ago mark.
Those light lines that arch across the diagram are lines of equal age. Decreasing age radiates outward in all the directions from the 4 billion years ago marking at the bottom center. Instead of displaying the diversity of life on a straight line, they are displaying it on a clockwise outward spiral (or something like that).
The diagram is still pretty confusing.
Added by edit: I think the origin age of a major group of specific life-forms is at the inner boundary of the colored area. That is why so much boundary of the colored area is at the Cambrian explosion time line.
Added by edit 3: Note where lines end within the shaded area. That is that particular branch of the bush going extinct. The extinction times include those where a specific critter is illustrated.
Moose
Edited by Minnemooseus, : See above.
Edited by Minnemooseus, : Modify previous ABE.
Edited by Minnemooseus, : See above. Also changed subtitle.
Edited by Minnemooseus, : Modify edit 3.