Macroevolution is the study of evolution over geological rather than ecological time scales. It’s goal is to understand the modern biosphere as just one ‘time slice’ of a vastly more complex Tree of Life, with roots going back three billion years or more. Living biodiversity represents perhaps just 1% of all the species that have ever existed, with many of the extinct forms appearing bizarre or even implausible to neontologists. Only by understanding the long and complex history of Life can we truly understand the present.
There are essentially two, independent narratives on the history of Life. The first of these derives from living organisms, and takes the form of inferences about the past, made by extrapolation. The data from extant forms is very rich and diverse. Living animals and plants can be studied in great anatomical detail, we can investigate their physiology and behaviour, and their genes can be sequenced. However, no amount of extrapolation from the present could have revealed the existence of feathered dinosaurs (or dinosaurs of any kind), pelycosaurs, or the arthropods of the Cambrian Burgess Shale. For revelations of this kind, we must rely exclusively on the second narrative: the fossil record. Frustratingly incomplete at times, it offers us the only direct glimpses onto the past, and the only means to root the Tree of Life in absolute time.