But we should not despair. There is real order in all this apparent chaos. Life has had a long and complex, but ultimately comprehensible, history. There are patterns repeated over and over again as new species come and go, and as ecosystems form and fall apart. These organizing principles of life's history are the processes of evolution. We apply them to fossils to render that order.
-- Niles Eldredge
Life on Earth has been changing in many ways since the first living organisms started to invade marine and terrestrial habitats, and the fossil record, like a great script, tells us some part of what happened. From those few recovered pages we have to reconstruct the whole masterpiece.
For a vast time, our planet's biosphere was dominated by bacteria and other simple single-celled organisms. These 3900 millions years are known as the Pre-Cambrian Period, and the remains of life forms from that period are fragmented and scarce. Between 530 and 520 million years ago most of the basic life forms emerged, defining the beginning of the Cambrian Period. Whether this explosion was really so brief or the result of a slower process is a matter of debate, but there was a time when multicellular life forms had little diversity of form, and a time afterwards when animals displayed an amazing range of morphological patterns.
There is an extraordinary gold mine of organisms that first emerged during the Cambrian explosion: 1 the Burgess Shale in the Canadian Rockies (Figure 9.1). These old remains of life's history in the Rocky