The Burgess Shale

The Tree of Life

All life forms that inhabit our planet share a common origin billions of years ago, and are part of the same tree of life. On this tree, the branches representing different groups of organisms are called taxa – the process of evolution – discrete changes that accumulate from generation to generation – combined with complex interactions with other life forms and environments has diversified and expanded life into the myriad species we know today.

Block diagram showing divisions of life
The main subdivisions of life.

The first hints of life on Earth are four billion years old – and appear just 500 million years after the Earth formed. Cells evolved and organized themselves into multicellular forms; but it took a long time to reach large organisms and for billions of years life was mostly microscopic and simple. The evolution of animals – the Metazoans – as shown by the fossils of the Burgess Shale is a comparatively recent phenomenon.

Diagram of timeline showing history of life on Earth

Sponges and radially-symmetrical animals probably evolved during the Ediacaran Period (635-542 million years ago).

The first fossil remains of unambiguous true animals with complex tissues and organs – the bilaterians – are found in rocks dating to the earliest Cambrian Period. This marks the beginning of the Cambrian Explosion, starting about 542 million years ago, in which the vast majority of animal groups still alive today first appear in the fossil record. These fossils form the basis for reconstructing the early branches of the tree of animal life, but it is possible that many groups originated earlier than the first fossils tell us.

The genetic information stored in the cells of modern organisms can be used to reconstruct how various groups are related to one another and the order in which they arose. Results of such analyses are typically displayed in “phylogenetic trees” (like your common family tree, but at a much broader scale). In the context of the Cambrian Explosion, genetic data have proved to be important in reconstructing the relationships between the main branches at the base of the animal tree of life.

These main branches represent fundamental animal body plans. By using fossils as reference points for calibration, it is possible to estimate the geological time when the different animal body plans first appeared. Both genetic and fossil data strongly suggest that the first animals originated during the Ediacaran Period. The earliest Ediacaran forms were likely primitive members of very large groups (e.g., bilaterians). It wasn’t until the Cambrian Explosion that these diversified to give rise to smaller groups (e.g., individual Phyla).

Graphic showing times when major phyla appear in fossil record
The evolutionary tree of animals in the context of the Cambrian Explosion. Dotted lines represent the probable range of particular groups of animals. Solid lines represent fossil evidence. Extinct groups (taxa) are represented by a circled cross. Cones represent the approximate origin and diversification of the modern phyla (the crown groups). The basic body plan of major groups of animals (today's phyla) had already evolved by the time of the Burgess Shale. (Modified after Xiao and Laflamme, Peterson et al and Dunn et al.).