The modern use of the word 'fossil' refers to the physical evidence of former life from a period of time prior to recorded human history. This prehistoric evidence includes the fossilised remains of living organisms, impressions and moulds of their physical form, and marks/traces created in the sediment by their activities. There is no universally agreed age at which the evidence can be termed fossilised, however it's broadly understood to encompass anything more than a few thousand years. Such a definition includes our prehistoric human ancestry and the ice age fauna (e.g. mammoths) as well as more ancient fossil groups such as the dinosaurs, ammonites and trilobites. The earliest reported fossil discoveries date from 3.5 billion years ago, however it wasn't until approximately 600 million years ago that complex multi-cellular life began to enter the fossil record, and for the purposes of fossil hunting the majority of effort is directed towards fossils of this age and younger. Fossils occur commonly around the world although just a small proportion of life makes it into the fossil record. Most living organisms simply decay without trace after death as natural processes recycle their soft tissues and even hard parts such as bone and shell. Thus, the abundance of fossils in the geological record reflects the frequency of favourable conditions where preservation is possible, the immense number of organisms that have lived, and the vast length of time over which the rocks have accumulated.