Set Your Watch

This image came out a couple months ago in Nature, but I just came across it today. I quite like the way it sums up the history of life--something that's maddening hard to do, since the time scales are so vast. It shows how life's diversity has been accumulating for billions of years. This chart shows the timing of the earliest paeolontological evidence for different kinds of life, ranging from fossils to chemical markers. A few definitions may help. Phototrophic bacteria can harness sunlight to grow. Cyanobacteria are also known as blue-green algae (aka pond scum). Eukaryotes are species such as amoebae, plants, fungi, and animals. Algal kindoms include red algae and green algae (closely related to land plants). Some of these bars may need to be pushed back in time when earlier evidence is discovered. Some studies on DNA suggests that a number of such "ghost lineages" remain to be discovered.