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.).

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Taxonomy and the Species Concept

The science of classification is called taxonomy. A taxon (plural taxa) is a group of organisms classified together because they are more closely related to each other than to any other organism. If such a taxon is made up of an ancestor together with all of its descendants, it is called a clade. Clades are used for reconstructing evolutionary relationships and the results are displayed into a cladogram or evolutionary tree. The following ranks of taxonomic groups, each more inclusive than the one below it, are used when classifying an organism:

  • Kingdom
  • Phylum
  • Class
  • Order
  • Family
  • Genus
  • Species

The most fundamental and exclusive rank is the species, which forms the basis of classification for all organisms living today and in the past. Among living animals, a species is generally regarded as a group of closely-related individuals that can interbreed and successfully produce fertile offspring. It is not always possible, however, to test this definition, so a species is often defined on morphological traits (the form and shape of an organism) alone; in the case of fossils, this is usually the only way to define a species.

Similar species (those that share traits because they are closely related) may be classified together within the next higher taxonomic rank - the genus (plural genera). For example, modern humans are referred to as the species Homo sapiens. In this case, the genus name is Homo and the specific species name is sapiens. Homo habilis is another species of humans within the genus Homo, but is now extinct. The genus and species names are always italicized - the genus name starts with a capital letter and the species name with a lower case letter. Genus and species names are usually derived from classical Latin or Greek roots, but there are exceptions: for example, names of people or geographic features can also be used (learn the etymology of Burgess Shale fossil names in the Taxonomy sections of the Fossil Gallery and see what sources Walcott used).

Above the level of genus, each taxonomic rank contains larger and larger sets of species up to the level of a Kingdom. The higher you go on this taxonomic ladder, the more dissimilar (distantly related) two organisms from two different groups usually are. For example, fruit flies and humans belong to the kingdom Animalia because they share some fundamental traits common to all complex animals (for example, possession of a gut). But humans and fruit flies are too dissimilar to be classified together in any lower taxonomic group (humans are part of the phylum Chordata, while fruit flies are part of the phylum Arthropoda).

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Prokaryotes and Eukaryotes

Life can be divided into two large groups (called superdomains) of cellular organisms, the prokaryotes and the eukaryotes, based on differences in cell types.

Prokaryotes are organisms represented mostly by very small, single-celled forms (Bacteria and Archaea). They lack a nucleus and other internal structures found in eukaryotes.

Eukaryotes include single-celled protists as well as all multicellular animals and plants. They have large, complex cells with a nucleus and a range of internal structures.

The origin and evolutionary relationship of these two superdomains is still debated. The standard view suggests eukaryotes evolved through a process in which some prokaryotes were incorporated into others as "symbionts", producing a more complex internal cell structure (this is known as the Endosymbiotic Theory).

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Perhaps one of the most important evolutionary developments after the evolution of cells was the capacity of different cells to function together collectively: multicellularity. Several grades of multicellularity are known in both prokaryotes and eukaryotes representing a wide range of inter-cellular relationships. In prokaryotes, multicellularity remains relatively simple (for example, filaments in microbial mats are composed of chains of linked cells, but each cell still functions as an individual). Multicellular eukaryotes (fungi, plants, and animals) are more integrated and able to grow to a larger size. Multicellularity at its most complex level sees cells differentiating to form specialized tissues and organs in the "true" animals (Eumetazoans) and land plants.

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Metazoans and the Bilaterians

Metazoans (animals) are eukaryotes that acquire energy by ingesting other organisms (heterotrophy) - as opposed to obtaining energy from chemicals (chemoautotrophy) or light (phototrophy). Most animals, representing the vast majority of metazoan phyla, are "bilaterians": they are multicellular, have bilateral symmetry (i.e., show approximately mirror-image halves), and possess specialized cells organized into tissues and organs, such as muscles, nerves, and a digestive tube. Bilaterians are considered "true" animals (Eumetazoans), with a front and a back end, and a lower (ventral) and upper (dorsal) side - at least at some stage of their development. Some radially-symmetrical organisms (i.e., with bodies arranged in spoke-like fashion about a central point), including jellyfish and corals (cnidarians) and comb-jellies (ctenophores), have a simpler organization with limited organ development. Sponges lack true tissues and organs.

Body Plans, Disparity and Diversity

A body plan is a set of fundamental traits - a basic structural blueprint - shared among a vast number of related organisms. We only see a limited range of body plans among all living animals (expert opinions vary between 30 and 40). By contrast, the number of different species alive today on our planet is enormous - even the most conservative estimates lie in the millions. (And if we were to try to account for all species that have gone extinct over the span of the last 3,500 million years, the number would be staggering!).

The morphological difference between body plans is known as disparity, as opposed to diversity which refers to numbers of individual species. The concepts of disparity and diversity can be explained simply in the following analogy. Imagine two houses - one built in Europe and one in North America. Materials used for the basic framework of these houses would differ on the two continents: concrete in Europe, and wood in North America. Both provide comparable solutions to the same functional requirements (a rigid supporting framework), but come from different sources and are assembled in different ways. The two construction approaches are equivalent to two animal body plans in which skeletal elements are of different composition, anatomical origin, and arrangement.

To carry the analogy further, if additional materials were superimposed on the basic framework of the two houses (electrical wiring and plumbing pipes, for example), more detailed differences in materials and methods would accumulate. The end result of the construction projects would be two houses, built for similar purposes, but of markedly disparate appearance and substance but underneath these differences, the houses will have much in common: bedrooms, kitchens, and so on. This is analogous to different species belonging to two different body plans.

Each of the houses, however, would be comparable to others built on their respective continents, albeit with differences in size and layout. These latter variations would be analogous to distinctions between different species belonging to each of the two disparate body plans. Regional, provincial or more local differences between houses would correspond to lower taxonomic ranks, such as Class, Order, or Family. The total number of different house types at each level would represent the species diversity within that rank. Within a suburb, houses might be essentially identical, even in the details - corresponding to a single species.

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Stem Group and Crown Group Concepts

The crown group concept was initially proposed for differentiating living organisms from extinct taxa. It has recently been adopted in the study and classification of the animals of the Burgess Shale. A crown group consists of the last common ancestor of a living group of organisms (i.e., the most immediate ancestor shared by at least two species), and all its descendants. By definition, all living members of phyla (representing all the major animal groups today) are members of their respective crown groups. A crown group can also contain extinct animals. For example, the extinct Tasmanian wolf still belongs to the crown group of the class Mammalia within the phylum Chordata.

Graphic showing relationship between stem and crown groups

Diagram representing crown groups and stem groups showing extinct taxa.

A stem group consists entirely of extinct organisms that display some, but not all, the morphological features of their closest crown group. Studying stem group animals reveals clues to the timing and order that different features were acquired within particular lineages. For example, while the Burgess Shale animal Opabinia possesses some characteristics of an arthropod (e.g., jointed feeding claws), it does not belong to the arthropod crown group because it is not descended from the last common ancestor of all living arthropods.

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